InternalMed_Harrison

InternalMed_Harrison. Two mechanisms exist for cobalamin absorption. One is passive, occurring equally through buccal, duodenal, and ileal mucosa; it is rapid but extremely inefficient, with <1% of an oral dose being absorbed by this process. The normal physiologic mechanism is active; it occurs through the ileum and is efficient for small (a few micro-grams) oral doses of cobalamin, and it is mediated by gastric intrinsic factor (IF). Dietary cobalamin is released from protein complexes by enzymes in the stomach, duodenum, and jejunum; it combines rapidly with a salivary glycoprotein that belongs to the family of cobalaminbinding proteins known as haptocorrins (HCs). In the intestine, the haptocorrin is digested by pancreatic trypsin and the cobalamin is transferred to IF. Cobalamin deficiency or abnormalities of cobalamin metabolism (see Tables 128-3, 128-4) Folate deficiency or abnormalities of folate metabolism (see Table 128-5) Therapy with antifolate drugs (e.g., methotrexate)


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InternalMed_Harrison. Folate deficiency or abnormalities of folate metabolism (see Table 128-5) Therapy with antifolate drugs (e.g., methotrexate) Independent of either cobalamin or folate deficiency and refractory to cobalamin and folate therapy: Some cases of acute myeloid leukemia, myelodysplasia Therapy with drugs interfering with synthesis of DNA (e.g., cytosine arabinoside, hydroxyurea, 6-mercaptopurine, azidothymidine [AZT]) Orotic aciduria (responds to uridine)


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InternalMed_Harrison. Therapy with drugs interfering with synthesis of DNA (e.g., cytosine arabinoside, hydroxyurea, 6-mercaptopurine, azidothymidine [AZT]) Orotic aciduria (responds to uridine) IF (gene at chromosome 11q13) is produced in the gastric parietal cells of the fundus and body of the stomach, and its secretion parallels that of hydrochloric acid. Normally, there is a vast excess of IF. The IF-cobalamin complex passes to the ileum, where IF attaches to a specific receptor (cubilin) on the microvillus membrane of the enterocytes. Cubilin also is present in yolk sac and renal proximal tubular epithelium. Cubilin appears to traffic by means of amnionless (AMN), an endocytic receptor protein that directs sublocalization and endocytosis of cubilin with its ligand IF-cobalamin complex. The cobalamin-IF complex enters the ileal cell, where IF is destroyed. After a delay of about 6 h, the cobalamin appears in portal blood attached to transcobalamin (TC) II.


InternalMed_Harrison

InternalMed_Harrison. Between 0.5 and 5 μg of cobalamin enter the bile each day. This binds to IF, and a major portion of biliary cobalamin normally is reabsorbed together with cobalamin derived from sloughed intestinal cells. Because of the appreciable amount of cobalamin undergoing enterohepatic circulation, cobalamin deficiency develops more rapidly in individuals who malabsorb cobalamin than it does in vegans, in whom reabsorption of biliary cobalamin is intact.


InternalMed_Harrison

InternalMed_Harrison. Two main cobalamin transport proteins exist in human plasma; they both bind cobalamin—one molecule for one molecule. One HC, also known as TC I, is closely related to other cobalamin-binding HCs in milk, gastric juice, bile, saliva, and other fluids. The gene TCNL is at chromosome 11q11-q12.3. These HCs differ from each other only in the carbohydrate moiety of the molecule. TC I is derived primarily from the specific granules in neutrophils. Normally, it is about two-thirds saturated with cobalamin, which it binds tightly. TC I does not enhance cobalamin entry into tissues. Glycoprotein receptors on liver cells are involved in the removal of TC I from plasma, and TC I may play a role in the transport of cobalamin analogues (which it binds more effectively than IF) to the liver for excretion in bile.


InternalMed_Harrison

InternalMed_Harrison. The other major cobalamin transport protein in plasma is transcobalamin, also known as TC II. The gene is on chromosome 22q11-q13.1. As for IF and HC, there are nine exons. The three proteins are likely to have a common ancestral origin. TC II is synthesized by liver and by other tissues, including macrophages, ileum, and vascular endothelium. It normally carries only 20–60 ng of cobalamin per liter of plasma and readily gives up cobalamin to marrow, placenta, and other tissues, which it enters by receptor-mediated endocytosis involving the TC II receptor and megalin (encoded by the LRP-2 gene). The TC II cobalamin is internalized by endocytosis via clathrin-coated pits; the complex is degraded, but the receptor probably is recycled to the cell membrane as is the case for transferrin. Export of “free” cobalamin is via the ATP-binding cassette drug transporter alias multidrug resistance protein 1.


InternalMed_Harrison

InternalMed_Harrison. Folic (pteroylglutamic) acid is a yellow, crystalline, water-soluble substance. It is the parent compound of a large family of natural folate compounds, which differ from it in three respects: (1) they are partly or completely reduced to dior tetrahydrofolate (THF) derivatives, (2) they usually contain a single carbon unit (Table 128-2), and (3) 70–90% of natural folates are folate-polyglutamates.


InternalMed_Harrison

InternalMed_Harrison. Most foods contain some folate. The highest concentrations are found in liver, yeast, spinach, other greens, and nuts (>100 μg/100 g). The total folate content of an average Western diet is ~250 μg daily, but the amount varies widely according to the type of food eaten and the method of cooking. Folate is easily destroyed by heating, particularly in large volumes of water. Total body folate in the adult is ~10 mg, with the liver containing the largest store. Daily adult requirements are ~100 μg, and so stores are sufficient for only 3–4 months in normal adults and severe folate deficiency may develop rapidly.


InternalMed_Harrison

InternalMed_Harrison. Folates are absorbed rapidly from the upper small intestine. The absorption of folate polyglutamates is less efficient than that of monoglutamates; on average, ~50% of food folate is absorbed. Polyglutamate forms are hydrolyzed to the monoglutamate derivatives either in the and dG(guanine)TP (purines), dT(thymine)TP and dC(cytosine) lumen of the intestine or within the mucosa. All dietary folates are con-TP (pyrimidines). In deficiencies of either folate or cobalamin, verted to 5-methylTHF (5-MTHF) within the small intestinal mucosa there is failure to convert deoxyuridine monophosphate (dUMP) to before entering portal plasma. The monoglutamates are actively trans-deoxythymidine monophosphate (dTMP), the precursor of dTTP ported across the enterocyte by a proton-coupled folate transporter (Fig 128-1). This is the case because folate is needed as the coenzyme (PCFT, SCL46A1). This is situated at the apical brush border and is 5,10-methylene-THF polyglutamate for conversion of dUMP to


InternalMed_Harrison

InternalMed_Harrison. (Fig 128-1). This is the case because folate is needed as the coenzyme (PCFT, SCL46A1). This is situated at the apical brush border and is 5,10-methylene-THF polyglutamate for conversion of dUMP to most active at pH 5.5, which is about the pH of the duodenal and dTMP; the availability of 5,10-methylene-THF is reduced in either jejunal surface. Genetic mutations of this protein underlie hereditary cobalamin or folate deficiency. An alternative theory for megaloblastic malabsorption of folate (see below). Pteroylglutamic acid at doses >400 anemia in cobalamin or folate deficiency is misincorporation of uracil μg is absorbed largely unchanged and converted to natural folates in into DNA because of a buildup of deoxyuridine triphosphate (dUTP) the liver. Lower doses are converted to 5-MTHF during absorption at the DNA replication fork as a consequence of the block in converthrough the intestine. sion of dUMP to dTMP.


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InternalMed_Harrison. 641glycine. Methionine, the other product of the methionine synthase reaction, is the precursor for S-adenosylmethionine (SAM), the universal methyl donor involved in >100 methyltransferase TABLE 128-2 BIOCHEMICAL rEACTIONs Of fOLATE COENzYMEs Reaction Coenzyme Form of Folate Involved Single Carbon Unit Transferred Importance About 60–90 μg of folate enters the bile each day and is excreted into the small intestine. Loss of this folate, together with the folate of COBALAMIN-FOLATE RELATIONS sloughed intestinal cells, accelerates the speed with which folate defi-Folate is required for many reactions in mammalian tissues. Only two ciency develops in malabsorption conditions. reactions in the body are known to require cobalamin. Methylmalonyl


InternalMed_Harrison

InternalMed_Harrison. CoA isomerization requires adocobalamin, and the methylation of homocysteine to methionine requires both methylcobalamin and Folate is transported in plasma; about one-third is loosely bound to 5-MTHF (Fig. 128-1). This reaction is the first step in the pathway byalbumin, and two-thirds is unbound. In all body fluids (plasma, cerewhich 5-MTHF, which enters bone marrow and other cells from plasma,brospinal fluid, milk, bile), folate is largely, if not entirely, 5-MTHF is converted into all the intracellular folate coenzymes. The coenzymes in the monoglutamate form. Three types of folate-binding protein are all polyglutamated (the larger size aiding retention in the cell), but are involved. A reduced folate transporter (RFC, SLC19A1) is the the enzyme folate polyglutamate synthase can use only THF, not MTHF,major route of delivery of plasma folate (5-MTHF) to cells. Two folate as substrate. In cobalamin deficiency, MTHF accumulates in plasma,receptors, FR2 and FR3 embedded in the cell


InternalMed_Harrison

InternalMed_Harrison. only THF, not MTHF,major route of delivery of plasma folate (5-MTHF) to cells. Two folate as substrate. In cobalamin deficiency, MTHF accumulates in plasma,receptors, FR2 and FR3 embedded in the cell membrane by a glycosyl and intracellular folate concentrations fall due to failure of formation phosphatidylinositol anchor, transport folate into the cell via receptor-of THF, the substrate on which folate polyglutamates are built. This hasmediated endocytosis. The third protein, PCFT, transports folate at been termed THF starvation, or the methylfolate trap.


InternalMed_Harrison

InternalMed_Harrison. low pH from the vesicle to the cell cytoplasm. The reduced folate This theory explains the abnormalities of folate metabolism thattransporter also mediates uptake of methotrexate by cells. occur in cobalamin deficiency (high serum folate, low cell folate, positive purine precursor aminoimidazole carboxamide ribonucleotide [AICAR] excretion; Table 128-2) and also why the anemia of cobala-Folates (as the intracellular polyglutamate derivatives) act as coenzymes min deficiency responds to folic acid in large doses.


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InternalMed_Harrison. in the transfer of single-carbon units (Fig. 128-1 and Table 128-2). Two of these reactions are involved in purine synthesis and one in pyrimi- dine synthesis necessary for DNA and RNA replication. Folate is also a coenzyme for methionine synthesis, in which methylcobalamin is also Many symptomless patients are detected through the finding of a raised involved and in which THF is regenerated. THF is the acceptor of single mean corpuscular volume (MCV) on a routine blood count. The main carbon units newly entering the active pool via conversion of serine to clinical features in more severe cases are those of anemia. Anorexia is


InternalMed_Harrison

InternalMed_Harrison. Methylated product Folic acid Folic acid Deoxythymidine monophosphate Purines Serine Glycine S-Adenosylhomocysteine (SAH) 5-Methyl tetrahydrofolate 5, 10-Methylene tetrahydrofolate 5-Methyl tetrahydrofolate (monoglutamate) Plasma Cell Tetrahydrofolate Dihydrofolate S-Adenosylmethionine (SAM) Homocysteine Methionine Cystathionine Pyruvate GSH Cysteine THE METHYLATION CYCLE Substrate ATP (e.g., methylated lipids, myelin basic protein, DOPA, DNA) Methyltransferases Methionine synthase methylcobalamin Cystathionine synthase vitamin B6 5,10-Methylene-tetrahydrofolate reductase DHF reductase Polyglutamate synthase + glutamates 10-Formyl tetrahydrofolate DNA CYCLE (CELL REPLICATION) Formate Deoxyuridine monophosphate FIGURE 128-1 The role of folates in DNA synthesis and in formation of S-adenosylmethionine (SAM), which is involved in numerous meth-ylation reactions. DHF, dihydrofolate; GSH, glutathione. (Reprinted from AV Hoffbrand et al [eds]: Postgraduate Haematology, 5th ed. Oxford, UK,


InternalMed_Harrison

InternalMed_Harrison. (SAM), which is involved in numerous meth-ylation reactions. DHF, dihydrofolate; GSH, glutathione. (Reprinted from AV Hoffbrand et al [eds]: Postgraduate Haematology, 5th ed. Oxford, UK, Blackwell Publishing, 2005; with permission.) usually marked, and there may be weight loss, diarrhea, or constipation. Glossitis, angular cheilosis, a mild fever in more severely anemic patients, jaundice (unconjugated), and reversible melanin skin hyper-pigmentation also may occur with a deficiency of either folate or cobalamin. Thrombocytopenia sometimes leads to bruising, and this may be aggravated by vitamin C deficiency or alcohol in malnourished patients. The anemia and low leukocyte count may predispose to infections, particularly of the respiratory and urinary tracts. Cobalamin deficiency has also been associated with impaired bactericidal function of phagocytes.


InternalMed_Harrison

InternalMed_Harrison. GENERAL TISSUE EFFECTS OF COBALAMIN AND FOLATE DEFICIENCIES Epithelial Surfaces After the marrow, the next most frequently affected tissues are the epithelial cell surfaces of the mouth, stomach, and small intestine and the respiratory, urinary, and female genital tracts. The cells show macrocytosis, with increased numbers of multinucleate and dying cells. The deficiencies may cause cervical smear abnormalities. Complications of Pregnancy The gonads are also affected, and infertility is common in both men and women with either deficiency. Maternal folate deficiency has been implicated as a cause of prematurity, and both folate deficiency and cobalamin deficiency have been implicated in recurrent fetal loss and neural tube defects, as discussed below.


InternalMed_Harrison

InternalMed_Harrison. Neural Tube Defects Folic acid supplements at the time of conception and in the first 12 weeks of pregnancy reduce by ~70% the incidence of neural tube defects (NTDs) (anencephaly, meningomyelocele, encephalocele, and spina bifida) in the fetus. Most of this protective effect can be achieved by taking folic acid, 0.4 mg daily, at the time of conception. The incidence of cleft palate and harelip also can be reduced by prophylactic folic acid. There is no clear simple relationship between maternal folate status and these fetal abnormalities, although overall the lower the maternal folate, the greater the risk to the fetus. NTDs also can be caused by antifolate and antiepileptic drugs.


InternalMed_Harrison

InternalMed_Harrison. An underlying maternal folate metabolic abnormality has also been postulated. One abnormality has been identified: reduced activity of the enzyme 5,10-methylene-THF reductase (MTHFR) (Fig. 128-1) caused by a common C677T polymorphism in the MTHFR gene. In one study, the prevalence of this polymorphism was found to be higher than in controls in the parents of NTD fetuses and in the fetuses themselves: homozygosity for the TT mutation was found in 13% of cases compared with 5% of control subjects. The polymorphism codes for a thermolabile form of MTHFR. The homozygous state results in a lower mean serum and red cell folate level compared with control subjects, as well as significantly higher serum homocysteine levels. Tests for mutations in other enzymes possibly associated with NTDs, e.g., methionine synthase and serine–glycine hydroxymethylase, have been negative. Serum vitamin B12 levels are also lower in the sera of mothers of NTD infants than in controls. In addition, maternal TC


InternalMed_Harrison

InternalMed_Harrison. methionine synthase and serine–glycine hydroxymethylase, have been negative. Serum vitamin B12 levels are also lower in the sera of mothers of NTD infants than in controls. In addition, maternal TC II receptor polymorphisms are associated with increased risk of NTD births. There are, however, no studies showing dietary fortification with vitamin B12 reduces the incidence of NTDs.


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InternalMed_Harrison. Cardiovascular Disease Children with severe homocystinuria (blood levels ≥100 μmol/L) due to deficiency of one of three enzymes, methionine synthase, MTHFR, or cystathionine synthase (Fig. 128-1), have vascular disease, e.g., ischemic heart disease, cerebrovascular disease, or pulmonary embolus, as teenagers or in young adulthood. Lesser degrees of raised serum homocysteine and low levels of serum folate and homozygous inherited mutations of MTHFR have been found to be associated with cerebrovascular, peripheral vascular, and coronary heart disease and with deep vein thrombosis. Prospective randomized trials of lowering homocysteine levels with supplements of folic acid, vitamin B12, and vitamin B6 against placebo over a 5-year period in patients with vascular disease or diabetes have not, however, shown a reduction of first event fatal or nonfatal myocardial infarction, nor have these supplements reduced the risk of recurrent cardiovascular disease after an acute myocardial infarct.


InternalMed_Harrison

InternalMed_Harrison. however, shown a reduction of first event fatal or nonfatal myocardial infarction, nor have these supplements reduced the risk of recurrent cardiovascular disease after an acute myocardial infarct. Meta-analysis showed an 18% reduction in strokes but no significant prevention of death from any cause. Venous thrombosis has been reported to be more frequent in vitamin B12–deficient subjects than in controls. This was ascribed to raised plasma homocysteine levels in vitamin B12 deficiency.


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InternalMed_Harrison. Malignancy Prophylactic folic acid in pregnancy has been found in some but not all studies to reduce the subsequent incidence of acute lymphoblastic leukemia (ALL) in childhood. A significant negative association has also been found with the MTHFR C677T polymorphism and leukemias with mixed lineage leukemia (MLL) translocations, but a positive association with hyperdiploidy in infants with ALL or acute myeloid leukemia or with childhood ALL. A second polymorphism in the MTHFR gene, A1298C, is also strongly associated with hyperdiploid leukemia. There are various positive and negative associations between polymorphisms in folate-dependent enzymes and the incidence of adult ALL. The C677T polymorphism is thought to lead to increased thymidine pools and “better quality” of DNA synthesis by shunting one-carbon groups toward thymidine and purine synthesis. This may explain its reported association with a lower risk for colorectal cancer. Most but not all studies suggest that prophylactic


InternalMed_Harrison

InternalMed_Harrison. shunting one-carbon groups toward thymidine and purine synthesis. This may explain its reported association with a lower risk for colorectal cancer. Most but not all studies suggest that prophylactic folic acid also protects against colon adenomas. Other tumors that have been associated with folate polymorphisms or status include follicular lymphoma, breast cancer, and gastric cancer. A meta-analysis of 50,000 individuals given folic acid or placebo in cardiovascular or colon adenoma prevention trials found that folic acid supplementation did not substantially increase or decrease the incidence of site-specific cancer during the first 5 years of treatment. Because folic acid may “feed” tumors, it probably should be avoided in those with established tumors unless there is severe megaloblastic anemia due to folate deficiency.


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InternalMed_Harrison. Neurologic Manifestations Cobalamin deficiency may cause a bilateral peripheral neuropathy or degeneration (demyelination) of the posterior and pyramidal tracts of the spinal cord and, less frequently, optic atrophy or cerebral symptoms. The patient, more frequently male, presents with paresthesias, muscle weakness, or difficulty in walking and sometimes dementia, psychotic disturbances, or visual impairment. Long-term nutritional cobalamin deficiency in infancy leads to poor brain development and impaired intellectual development. Folate deficiency has been suggested to cause organic nervous disease, but this is uncertain, although methotrexate injected into the cerebrospinal fluid may cause brain or spinal cord damage.


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InternalMed_Harrison. An important clinical problem is the nonanemic patient with neurologic or psychiatric abnormalities and a low or borderline serum cobalamin level. In such patients, it is necessary to try to establish whether there is significant cobalamin deficiency, e.g., by careful examination of the blood film, tests for serum gastrin level and for antibodies to IF or parietal cells, along with serum methylmalonic acid (MMA) measurement if available. A trial of cobalamin therapy for at 643 least 3 months will usually also be needed to determine whether the symptoms improve. The biochemical basis for cobalamin neuropathy remains obscure. Its occurrence in the absence of methylmalonic aciduria in TC II deficiency suggests that the neuropathy is related to the defect in homocysteine-methionine conversion. Accumulation of S-adenosylhomocysteine in the brain, resulting in inhibition of transmethylation reactions, has been suggested.


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InternalMed_Harrison. Psychiatric disturbance is common in both folate and cobalamin deficiencies. This, like the neuropathy, has been attributed to a failure of the synthesis of SAM, which is needed in methylation of biogenic amines (e.g., dopamine) as well as that of proteins, phospholipids, and neurotransmitters in the brain (Fig. 128-1). Associations between lower serum folate or cobalamin levels and higher homocysteine levels and the development of decreased cognitive function and dementia in Alzheimer’s disease have been reported. A meta-analysis of randomized, placebo-controlled trials of homocysteine-lowering B-vitamin supplementation of individuals with and without cognitive impairment, however, showed that supplementation with vitamin B12, vitamin B6, and folic acid alone or in combination did not improve cognitive function. It is unknown whether prolonged treatment with these B vitamins can reduce the risk of dementia in later life.


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InternalMed_Harrison. Oval macrocytes, usually with considerable anisocytosis and poikilo cytosis, are the main feature (Fig. 128-2A). The MCV is usually >100 fL unless a cause of microcytosis (e.g., iron deficiency or thalassemia trait) is present. Some of the neutrophils are hypersegmented (more than five nuclear lobes). There may be leukopenia due to a reduction in granulocytes and lymphocytes, but this is usually >1.5 × 109/L; the platelet count may be moderately reduced, rarely to <40 × 109/L. The severity of all these changes parallels the degree of anemia. In a non-anemic patient, the presence of a few macrocytes and hypersegmented neutrophils in the peripheral blood may be the only indication of the underlying disorder.


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InternalMed_Harrison. In a severely anemic patient, the marrow is hypercellular with an accumulation of primitive cells due to selective death by apoptosis of more mature forms. The erythroblast nucleus maintains a primitive appearance despite maturation and hemoglobinization of the cytoplasm. The cells are larger than normoblasts, and an increased number of cells with eccentric lobulated nuclei or nuclear fragments may be present (Fig. 128-2B). Giant and abnormally shaped metamyelocytes and enlarged hyperpolyploid megakaryocytes are characteristic. In severe cases, the accumulation of primitive cells may mimic acute myeloid leukemia, whereas in less anemic patients, the changes in the marrow may be difficult to recognize. The terms intermediate, mild, and early have been used. The term megaloblastoid does not mean mildly megaloblastic. It is used to describe cells with both immature-appearing nuclei and defective hemoglobinization and is usually seen in myelodysplasia.


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InternalMed_Harrison. Bone marrow cells, transformed lymphocytes, and other proliferating cells in the body show a variety of changes, including random breaks, reduced contraction, spreading of the centromere, and exaggeration of secondary chromosomal constrictions and overprominent satellites. Similar abnormalities may be produced by antimetabolite drugs (e.g., cytosine arabinoside, hydroxyurea, and methotrexate) that interfere with either DNA replication or folate metabolism and that also cause megaloblastic appearances. There is an accumulation of unconjugated bilirubin in plasma due to the death of nucleated red cells in the marrow (ineffective erythropoiesis). Other evidence for this includes raised urine urobilinogen, reduced haptoglobins and positive urine hemosiderin, and a raised


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InternalMed_Harrison. FIGURE 128-2 A. The peripheral blood in severe megaloblastic anemia. B. The bone marrow in severe megaloblastic anemia. (Reprinted from AV Hoffbrand et al [eds]: Postgraduate Haematology, 5th ed. Oxford, UK, Blackwell Publishing, 2005; with permission.) serum lactate dehydrogenase. A weakly positive direct antiglobulin test due to complement can lead to a false diagnosis of autoimmune hemolytic anemia. Cobalamin deficiency is usually due to malabsorption. The only other cause is inadequate dietary intake.


InternalMed_Harrison

InternalMed_Harrison. Cobalamin deficiency is usually due to malabsorption. The only other cause is inadequate dietary intake. INADEQUATE DIETARY INTAKE Adults Dietary cobalamin deficiency arises in vegans who omit meat, fish, eggs, cheese, and other animal products from their diet. The largest group in the world consists of Hindus, and it is likely that many millions of Indians are at risk of deficiency of cobalamin on a nutritional basis. Subnormal serum cobalamin levels are found in up to 50% of randomly selected, young, adult Indian vegans, but the deficiency usually does not progress to megaloblastic anemia since the diet of most vegans is not totally lacking in cobalamin and the enterohepatic circulation of cobalamin is intact. Dietary cobalamin deficiency may also arise rarely in nonvegetarian individuals who exist on grossly inadequate diets because of poverty or psychiatric disturbance.


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InternalMed_Harrison. Infants Cobalamin deficiency has been described in infants born to severely cobalamin-deficient mothers. These infants develop megaloblastic anemia at about 3–6 months of age, presumably because they are born with low stores of cobalamin and because they are fed breast milk with low cobalamin content. The babies have also shown growth retardation, impaired psychomotor development, and other neurologic sequelae. See Tables 128-3 and 128-4.


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InternalMed_Harrison. See Tables 128-3 and 128-4. Pernicious Anemia Pernicious anemia (PA) may be defined as a severe lack of IF due to gastric atrophy. It is a common disease in north Europeans but occurs in all countries and ethnic groups. The overall incidence is about 120 per 100,000 population in the United Kingdom (UK). The ratio of incidence in men and women among whites is ~1:1.6, and the peak age of onset is 60 years, with only 10% of patients being <40 years of age. However, in some ethnic groups, notably black individuals and Latin Americans, the age at onset of PA is generally lower. The disease occurs more commonly than by chance


InternalMed_Harrison

InternalMed_Harrison. Nutritional Vegans Malabsorption Pernicious anemia Gastric causes Congenital absence of intrinsic factor or functional abnormality Total or partial gastrectomy Intestinal causes Intestinal stagnant loop syndrome: jejunal diver ticulosis, ileocolic fistula, anatomic blind loop, intestinal stricture, etc. Ileal resection and Crohn’s disease Selective malabsorption with proteinuria Tropical sprue Transcobalamin II deficiency Fish tapeworm Gastric causes Simple atrophic gastritis (food cobalamin malabsorption) Zollinger-Ellison syndrome Gastric bypass surgery Use of proton pump inhibitors Deficiencies of cobalamin, folate, protein, ?riboflavin, ?nicotinic acid


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InternalMed_Harrison. Therapy with colchicine, para-aminosalicylate, neomycin, slow-release potassium chloride, anticonvulsant drugs, metformin, phenformin, cytotoxic drugs Alcohol in close relatives and in persons with other organ-specific autoimmune diseases, e.g., thyroid diseases, vitiligo, hypoparathyroidism, and Addison’s disease. It is also associated with hypogammaglobulinemia, with premature graying or blue eyes, and persons of blood group A. An association with human leukocyte antigen (HLA) 3 has been reported in some but not all series and, in those with endocrine disease, with HLA-B8, -B12, and -BW15. Life expectancy is normal in women once regular treatment has begun. Men have a slightly subnormal life expectancy as a result of a higher incidence of carcinoma of the stomach than in control subjects. Gastric output of hydrochloric acid, pepsin, and IF is severely reduced. The serum gastrin level is raised, and serum pepsinogen I levels are low.


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InternalMed_Harrison. Gastric Biopsy A single endoscopic examination is recommended if PA is diagnosed. Gastric biopsy usually shows atrophy of all layers of the body and fundus, with loss of glandular elements, an absence of parietal and chief cells and replacement by mucous cells, a mixed inflammatory cell infiltrate, and perhaps intestinal metaplasia. The infiltrate of plasma cells and lymphocytes contains an excess of CD4 cells. These are directed against gastric H/K-ATPase. The antral mucosa is usually well preserved. Helicobacter pylori infection occurs infrequently in PA, but it has been suggested that H. pylori gastritis occurs at an early phase of atrophic gastritis and presents in younger patients as iron-deficiency anemia but in older patients as PA. H. pylori is suggested to stimulate an autoimmune process directed against parietal cells, with the H. pylori infection then being gradually replaced, in some individuals, by an autoimmune process.


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InternalMed_Harrison. Serum Antibodies Two types of IF immunoglobulin G antibody may be found in the sera of patients with PA. One, the “blocking,” or type I, antibody, prevents the combination of IF and cobalamin, whereas the “binding,” or type II, antibody prevents attachment of IF to ileal mucosa. Type I occurs in the sera of ~55% of patients, and type II in 35%. IF antibodies cross the placenta and may cause temporary IF deficiency in a newborn infant. Patients with PA also show cell-mediated immunity to IF. Type I antibody has been detected rarely in the sera of patients without PA but with thyrotoxicosis, myxedema, Hashimoto’s disease, or diabetes mellitus and in relatives of PA patients. IF antibodies also have been detected in gastric juice in ~80% of PA patients. These gastric antibodies may reduce absorption of dietary cobalamin by combining with small amounts of remaining IF.


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InternalMed_Harrison. Parietal cell antibody is present in the sera of almost 90% of adult patients with PA but is frequently present in other subjects. Thus, it occurs in as many as 16% of randomly selected female subjects age >60 years. The parietal cell antibody is directed against the α and β subunits of the gastric proton pump (H+,K+-ATPase). This usually occurs in older children and resembles PA of adults. Gastric atrophy, achlorhydria, and serum IF antibodies are all present, although parietal cell antibodies are usually absent. About one-half of these patients show an associated endocrinopathy such as autoimmune thyroiditis, Addison’s disease, or hypoparathyroidism; in some, mucocutaneous candidiasis occurs.


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InternalMed_Harrison. An affected child usually presents with megaloblastic anemia in the first to third year of life; a few have presented as late as the second decade. The child usually has no demonstrable IF but has a normal gastric mucosa and normal secretion of acid. The inheritance is autosomal recessive. Parietal cell and IF antibodies are absent. Variants have been described in which the child is born with IF that can be detected immunologically but is unstable or functionally inactive, unable to bind cobalamin or to facilitate its uptake by ileal receptors. After total gastrectomy, cobalamin deficiency is inevitable, and prophylactic cobalamin therapy should be commenced immediately after the operation. After partial gastrectomy, 10–15% of patients also develop this deficiency. The exact incidence and time of onset are most influenced by the size of the resection and the preexisting size of 645 cobalamin body stores.


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InternalMed_Harrison. Failure of release of cobalamin from binding proteins in food is believed to be responsible for this condition, which is more common in the elderly. It is associated with low serum cobalamin levels, with or without raised serum levels of MMA and homocysteine. Typically, these patients have normal cobalamin absorption, as measured with crystalline cobalamin, but show malabsorption when a modified test using food-bound cobalamin is used. The frequency of progression to severe cobalamin deficiency and the reasons for this progression are not clear. INTESTINAL CAUSES OF COBALAMIN MALABSORPTION Intestinal Stagnant Loop Syndrome Malabsorption of cobalamin occurs in a variety of intestinal lesions in which there is colonization of the upper small intestine by fecal organisms. This may occur in patients with jejunal diverticulosis, enteroanastomosis, or an intestinal stricture or fistula or with an anatomic blind loop due to Crohn’s disease, tuberculosis, or an operative procedure.


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InternalMed_Harrison. Ileal Resection Removal of ≥1.2 m of terminal ileum causes malabsorption of cobalamin. In some patients after ileal resection, particularly if the ileocecal valve is incompetent, colonic bacteria may contribute further to the onset of cobalamin deficiency. Selective Malabsorption of Cobalamin with Proteinuria (Imerslund’s


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InternalMed_Harrison. Syndrome; Imerslund-Gräsbeck Syndrome; Congenital Cobalamin Malabsorption; Autosomal Recessive Megaloblastic Anemia; MGA1) This autosomally recessive disease is the most common cause of megaloblastic anemia due to cobalamin deficiency in infancy in Western countries. More than 200 cases have been reported, with familial clusters in Finland, Norway, the Middle East, and North Africa. The patients secrete normal amounts of IF and gastric acid but are unable to absorb cobalamin. In Finland, impaired synthesis, processing, or ligand binding of cubilin due to inherited mutations is found. In Norway, mutation of the gene for AMN has been reported. Other tests of intestinal absorption are normal. Over 90% of these patients show nonspecific proteinuria, but renal function is otherwise normal and renal biopsy has not shown any consistent renal defect. A few have shown aminoaciduria and congenital renal abnormalities, such as duplication of the renal pelvis.


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InternalMed_Harrison. Tropical Sprue Nearly all patients with acute and subacute tropical sprue show malabsorption of cobalamin; this may persist as the principal abnormality in the chronic form of the disease, when the patient may present with megaloblastic anemia or neuropathy due to cobalamin deficiency. Absorption of cobalamin usually improves after antibiotic therapy and, in the early stages, folic acid therapy. Fish Tapeworm Infestation The fish tapeworm (Diphyllobothrium latum) lives in the small intestine of humans and accumulates cobalamin from food, rendering the cobalamin unavailable for absorption. Individuals acquire the worm by eating raw or partly cooked fish. Infestation is common around the lakes of Scandinavia, Germany, Japan, North America, and Russia. Megaloblastic anemia or cobalamin neuropathy occurs only in those with a heavy infestation.


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InternalMed_Harrison. Gluten-Induced Enteropathy Malabsorption of cobalamin occurs in ~30% of untreated patients (presumably those in whom the disease extends to the ileum). Cobalamin deficiency is not severe in these patients and is corrected with a gluten-free diet. Severe Chronic Pancreatitis In this condition, lack of trypsin is thought to cause dietary cobalamin attached to gastric non-IF (R) binder to be unavailable for absorption. It also has been proposed that in pancreatitis, the concentration of calcium ions in the ileum falls below the level needed to maintain normal cobalamin absorption. HIV Infection Serum cobalamin levels tend to fall in patients with HIV infection and are subnormal in 10–35% of those with AIDS. 646 Malabsorption of cobalamin not corrected by IF has been shown in some, but not all, patients with subnormal serum cobalamin levels. Cobalamin deficiency sufficiently severe to cause megaloblastic anemia or neuropathy is rare.


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InternalMed_Harrison. Zollinger-Ellison Syndrome Malabsorption of cobalamin has been reported in the Zollinger-Ellison syndrome. It is thought that there is a failure to release cobalamin from R-binding protein due to inactivation of pancreatic trypsin by high acidity, as well as interference with IF binding of cobalamin. Radiotherapy Both total-body irradiation and local radiotherapy to the ileum (e.g., as a complication of radiotherapy for carcinoma of the cervix) may cause malabsorption of cobalamin. Graft-versus-Host Disease This commonly affects the small intestine. Malabsorption of cobalamin due to abnormal gut flora, as well as damage to ileal mucosa, is common. Drugs The drugs that have been reported to cause malabsorption of cobalamin are listed in Table 105-4. However, megaloblastic anemia due to these drugs is rare.


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InternalMed_Harrison. Drugs The drugs that have been reported to cause malabsorption of cobalamin are listed in Table 105-4. However, megaloblastic anemia due to these drugs is rare. ABNORMALITIES OF COBALAMIN METABOLISM Congenital Transcobalamin II Deficiency or Abnormality Infants with TC II deficiency usually present with megaloblastic anemia within a few weeks of birth. Serum cobalamin and folate levels are normal, but the anemia responds to massive (e.g., 1 mg three times weekly) injections of cobalamin. Some cases show neurologic complications. The protein may be present but functionally inert. Genetic abnormalities found include mutations of an intra-exonic cryptic splice site, extensive deletion, single nucleotide deletion, nonsense mutation, and an RNA editing defect. Malabsorption of cobalamin occurs in all cases, and serum immunoglobulins are usually reduced. Failure to institute adequate cobalamin therapy or treatment with folic acid may lead to neurologic damage.


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InternalMed_Harrison. Congenital Methylmalonic Acidemia and Aciduria Infants with this abnormality are ill from birth with vomiting, failure to thrive, severe metabolic acidosis, ketosis, and mental retardation. Anemia, if present, is normocytic and normoblastic. The condition may be due to a functional defect in either mitochondrial methylmalonyl CoA mutase or its cofactor adocobalamin. Mutations in the methylmalonyl CoA mutase are not responsive, or only poorly responsive, to treatment with cobalamin. A proportion of infants with failure of adocobalamin synthesis respond to cobalamin in large doses. Some children have combined methylmalonic aciduria and homocystinuria due to defective formation of both cobalamin coenzymes. This usually presents in the first year of life with feeding difficulties, developmental delay, microcephaly, seizures, hypotonia, and megaloblastic anemia. Acquired Abnormality of Cobalamin Metabolism: Nitrous Oxide Inhalation


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InternalMed_Harrison. Acquired Abnormality of Cobalamin Metabolism: Nitrous Oxide Inhalation Nitrous oxide (N2O) irreversibly oxidizes methylcobalamin to an inactive precursor; this inactivates methionine synthase. Megaloblastic anemia has occurred in patients undergoing prolonged N2O anesthesia (e.g., in intensive care units). A neuropathy resembling cobalamin neuropathy has been described in dentists and anesthetists who are exposed repeatedly to N2O. Methylmalonic aciduria does not occur as adocobalamin is not inactivated by N2O.


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InternalMed_Harrison. Dietary folate deficiency is common. Indeed, in most patients with folate deficiency a nutritional element is present. Certain individuals are particularly prone to have diets containing inadequate amounts of folate (Table 128-5). In the United States and other countries where fortification of the diet with folic acid has been adopted, the prevalence of folate deficiency has dropped dramatically and is now almost restricted to high-risk groups with increased folate needs. Nutritional Particularly in: old age, infancy, poverty, alcoholism, chronic invalids, and the psychiatrically disturbed; may be associated with scurvy or kwashiorkor Major causes of deficiency Tropical sprue, gluten-induced enteropathy in children and adults, and in association with dermatitis herpetiformis, specific malabsorption of folate, intestinal megaloblastosis caused by severe cobalamin or folate deficiency Minor causes of deficiency


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InternalMed_Harrison. Minor causes of deficiency Extensive jejunal resection, Crohn’s disease, partial gastrectomy, congestive heart failure, Whipple’s disease, scleroderma, amyloid, diabetic enteropathy, systemic bacterial infection, lymphoma, sulfasalazine (Salazopyrin) Pregnancy and lactation, prematurity Hematologic diseases: chronic hemolytic anemias, sickle cell anemia, thalassemia major, myelofibrosis Malignant diseases: carcinoma, lymphoma, leukemia, myeloma Inflammatory diseases: tuberculosis, Crohn’s disease, psoriasis, exfoliative dermatitis, malaria Metabolic disease: homocystinuria Excess urinary loss: congestive heart failure, active liver disease Hemodialysis, peritoneal dialysis Anticonvulsant drugs (phenytoin, primidone, barbiturates), sulfasalazine Nitrofurantoin, tetracycline, antituberculosis (less well documented)


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InternalMed_Harrison. Hemodialysis, peritoneal dialysis Anticonvulsant drugs (phenytoin, primidone, barbiturates), sulfasalazine Nitrofurantoin, tetracycline, antituberculosis (less well documented) Liver diseases, alcoholism, intensive care units aIn severely folate-deficient patients with causes other than those listed under Dietary, poor dietary intake is often present. bDrugs inhibiting dihydrofolate reductase are discussed in the text. folate deficiency occurs in kwashiorkor and scurvy and in infants with repeated infections or those who are fed solely on goats’ milk, which has a low folate content.


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InternalMed_Harrison. Malabsorption of dietary folate occurs in tropical sprue and in gluten-induced enteropathy. In the rare congenital recessive syndrome of selective malabsorption of folate due to mutation of the proton-coupled folate transporter (PCFT), there is an associated defect of folate transport into the cerebrospinal fluid, and these patients show megaloblastic anemia, which responds to physiologic doses of folic acid given parenterally but not orally. They also show mental retardation, convulsions, and other central nervous system abnormalities. Minor degrees of malabsorption may also occur after jejunal resection or partial gastrectomy, in Crohn’s disease, and in systemic infections, but in these conditions, if severe deficiency occurs, it is usually largely due to poor nutrition. Malabsorption of folate has been described in patients receiving sulfasalazine (Salazopyrin), cholestyramine, and triamterene.


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InternalMed_Harrison. EXCESS UTILIZATION OR LOSS Pregnancy Folate requirements are increased by 200–300 μg to ~400 μg daily in a normal pregnancy, partly because of transfer of the vitamin to the fetus but mainly because of increased folate catabolism due to cleavage of folate coenzymes in rapidly proliferating tissues. Megaloblastic anemia due to this deficiency is prevented by prophylactic folic acid therapy. It occurred in 0.5% of pregnancies in the UK and other Western countries before prophylaxis with folic acid, but the incidence is much higher in countries where the general nutritional status is poor.


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InternalMed_Harrison. Prematurity A newborn infant, whether full term or premature, has higher serum and red cell folate concentrations than does an adult. However, a newborn infant’s demand for folate has been estimated to be up to 10 times that of adults on a weight basis, and the neonatal folate level falls rapidly to the lowest values at about 6 weeks of age. The falls are steepest and are liable to reach subnormal levels in premature babies, a number of whom develop megaloblastic anemia responsive to folic acid at about 4–6 weeks of age. This occurs particularly in the smallest babies (<1500 g birth weight) and those who have feeding difficulties or infections or have undergone multiple exchange transfusions. In these babies, prophylactic folic acid should be given.


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InternalMed_Harrison. Hematologic Disorders Folate deficiency frequently occurs in chronic hemolytic anemia, particularly in sickle cell disease, autoimmune hemolytic anemia, and congenital spherocytosis. In these and other conditions of increased cell turnover (e.g., myelofibrosis, malignancies), folate deficiency arises because it is not completely reutilized after performing coenzyme functions. Inflammatory Conditions Chronic inflammatory diseases such as tuberculosis, rheumatoid arthritis, Crohn’s disease, psoriasis, exfoliative dermatitis, bacterial endocarditis, and chronic bacterial infections cause deficiency by reducing the appetite and increasing the demand for folate. Systemic infections also may cause malabsorption of folate. Severe deficiency is virtually confined to the patients with the most active disease and the poorest diet.


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InternalMed_Harrison. Homocystinuria This is a rare metabolic defect in the conversion of homocysteine to cystathionine. Folate deficiency occurring in most of these patients may be due to excessive utilization because of compensatory increased conversion of homocysteine to methionine. Long-Term Dialysis Because folate is only loosely bound to plasma proteins, it is easily removed from plasma by dialysis. In patients with anorexia, vomiting, infections, and hemolysis, folate stores are particularly likely to become depleted. Routine folate prophylaxis is now given. Congestive Heart Failure, Liver Disease Excess urinary folate losses of >100 μg per day may occur in some of these patients. The explanation appears to be release of folate from damaged liver cells.


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InternalMed_Harrison. Congestive Heart Failure, Liver Disease Excess urinary folate losses of >100 μg per day may occur in some of these patients. The explanation appears to be release of folate from damaged liver cells. A large number of epileptics who are receiving long-term therapy with phenytoin or primidone, with or without barbiturates, develop low serum and red cell folate levels. The exact mechanism is unclear. Alcohol may also be a folate antagonist, as patients who are drinking spirits may develop megaloblastic anemia that will respond to normal quantities of dietary folate or to physiologic doses of folic acid only if alcohol is withdrawn. Macrocytosis of red cells is associated with chronic alcohol intake even when folate levels are normal. Inadequate folate intake is the major factor in the development of deficiency in spirit-drinking alcoholics. Beer is relatively folate-rich in some countries, depending on the technique used for brewing.


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InternalMed_Harrison. The drugs that inhibit DHF reductase include methotrexate, pyrimethamine, and trimethoprim. Methotrexate has the most powerful action against the human enzyme, whereas trimethoprim is most active against the bacterial enzyme and is likely to cause megaloblastic anemia only when used in conjunction with sulfamethoxazole in patients with preexisting folate or cobalamin deficiency. The activity of pyrimethamine is intermediate. The antidote to these drugs is folinic acid (5-formyl-THF). Some infants with congenital defects of folate enzymes (e.g., cyclohydrolase or methionine synthase) have had megaloblastic anemia. The diagnosis of cobalamin or folate deficiency has traditionally depended on the recognition of the relevant abnormalities in the peripheral blood and analysis of the blood levels of the vitamins.


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InternalMed_Harrison. COBALAMIN DEFICIENCY 647 Serum Cobalamin This is measured by an automated enzyme-linked immunosorbent assay (ELISA) or competitive-binding luminescence assay (CBLA). Normal serum levels range from 118–148 pmol/L (160–200 ng/L) to ~738 pmol/L (1000 ng/L). In patients with megaloblastic anemia due to cobalamin deficiency, the level is usually <74 pmol/L (100 ng/L). In general, the more severe the deficiency, the lower is the serum cobalamin level. In patients with spinal cord damage due to the deficiency, levels are very low even in the absence of anemia. Values between 74 and 148 pmol/L (100 and 200 ng/L) are regarded as borderline. They may occur, for instance, in pregnancy, in patients with megaloblastic anemia due to folate deficiency. They may also be due to heterozygous, homozygous, or compound heterozygous mutations of the gene TCN1 that codes for haptocorrin (transcobalamin I). There is no clinical or hematologic abnormality. The serum cobalamin level is sufficiently robust,


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InternalMed_Harrison. or compound heterozygous mutations of the gene TCN1 that codes for haptocorrin (transcobalamin I). There is no clinical or hematologic abnormality. The serum cobalamin level is sufficiently robust, cost-effective, and most convenient to rule out cobalamin deficiency in the vast majority of patients suspected of having this problem. However, problems have arisen with commercial


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InternalMed_Harrison. CBLA assays involving intrinsic factor in PA patients with intrinsic antibodies in serum. These antibodies may cause false normal serum vitamin B12 levels in up to 50% of cases tested. Where clinical indications of PA are strong, a normal serum vitamin B12 does not rule out the diagnosis. Serum MMA levels will be elevated in PA (see below).


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InternalMed_Harrison. Serum Methylmalonate and Homocysteine In patients with cobalamin deficiency sufficient to cause anemia or neuropathy, the serum MMA level is raised. Sensitive methods for measuring MMA and homocysteine in serum have been introduced and recommended for the early diagnosis of cobalamin deficiency, even in the absence of hematologic abnormalities or subnormal levels of serum cobalamin. Serum MMA levels fluctuate, however, in patients with renal failure. Mildly elevated serum MMA and/or homocysteine levels occur in up to 30% of apparently healthy volunteers, with serum cobalamin levels up to 258 pmol/L (350 ng/L) and normal serum folate levels; 15% of elderly subjects, even with cobalamin levels >258 pmol/L (>350 ng/L), have this pattern of raised metabolite levels. These findings bring into question the exact cutoff points for normal MMA and homocysteine levels. It is also unclear at present whether these mildly raised metabolite levels have clinical consequences.


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InternalMed_Harrison. Serum homocysteine is raised in both early cobalamin and folate deficiency but may be raised in other conditions, e.g., chronic renal disease, alcoholism, smoking, pyridoxine deficiency, hypothyroidism, and therapy with steroids, cyclosporine, and other drugs. Levels are also higher in serum than in plasma, in men than in premenopausal women, in women taking hormone replacement therapy or in oral contraceptive users, and in elderly persons and patients with several inborn errors of metabolism affecting enzymes in trans-sulfuration pathways of homocysteine metabolism. Thus, homocysteine levels must be carefully interpreted for diagnosis of cobalamin or folate deficiency.


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InternalMed_Harrison. Tests for the Cause of Cobalamin Deficiency Only vegans, strict vegetarians, or people living on a totally inadequate diet will become vitamin deficient because of inadequate intake. Studies of cobalamin absorption once were widely used, but difficulty in obtaining radioactive cobalamin and ensuring that IF preparations are free of viruses has made these tests obsolete. Tests to diagnose PA include serum gastrin, which is raised; serum pepsinogen I, which is low in PA (90–92%) but also in other conditions; and gastric endoscopy. Tests for IF and parietal cell antibodies are also used, as well as tests for individual intestinal diseases.


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InternalMed_Harrison. FOLATE DEFICIENCY Serum Folate This is also measured by an ELISA technique. In most laboratories, the normal range is from 11 nmol/L (2 μg/L) to ~82 nmol/L (15 μg/L). The serum folate level is low in all folate-deficient patients. It also reflects recent diet. Because of this, serum folate may be low before there is hematologic or biochemical evidence of deficiency. Serum folate rises in severe cobalamin deficiency because of the block in conversion of MTHF to THF inside cells; raised levels have also 648 been reported in the intestinal stagnant loop syndrome due to absorption of bacterially synthesized folate.


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InternalMed_Harrison. Red Cell Folate The red cell folate assay is a valuable test of body folate stores. It is less affected than the serum assay by recent diet and traces of hemolysis. In normal adults, concentrations range from 880–3520 μmol/L (160–640 μg/L) of packed red cells. Subnormal levels occur in patients with megaloblastic anemia due to folate deficiency but also in nearly two-thirds of patients with severe cobalamin deficiency. False-normal results may occur if a folate-deficient patient has received a recent blood transfusion or if a patient has a raised reticulocyte count. Serum homocysteine assay is discussed earlier. Tests for the Cause of Folate Deficiency The diet history is important. Tests for transglutaminase antibodies are performed to confirm or exclude celiac disease. If positive, duodenal biopsy is needed. An underlying disease causing increased folate breakdown should also be excluded.


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InternalMed_Harrison. It is usually possible to establish which of the two deficiencies, folate or cobalamin, is the cause of the anemia and to treat only with the appropriate vitamin. In patients who enter the hospital severely ill, however, it may be necessary to treat with both vitamins in large doses once blood samples have been taken for cobalamin and folate assays and a bone marrow biopsy has been performed (if deemed necessary). Transfusion is usually unnecessary and inadvisable. If it is essential, packed red cells should be given slowly, one or two units only, with the usual treatment for heart failure if present. Potassium supplements have been recommended to obviate the danger of the hypokalemia but are not necessary. Occasionally, an excessive rise in platelets occurs after 1–2 weeks of therapy. Antiplatelet therapy, e.g., aspirin, should be considered if the platelet count rises to >800 × 109/L.


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InternalMed_Harrison. It is usually necessary to treat patients who have developed cobalamin deficiency with lifelong regular cobalamin injections. In the UK, the form used is hydroxocobalamin; in the United States, cyanocobalamin. In a few instances, the underlying cause of cobalamin deficiency can be permanently corrected, e.g., fish tapeworm, tropical sprue, or an intestinal stagnant loop that is amenable to surgery. The indications for starting cobalamin therapy are a well-documented megaloblastic anemia or other hematologic abnormalities and neuropathy due to the deficiency. Patients with borderline serum cobalamin levels but no hematologic or other abnormality may be followed to make sure that the cobalamin deficiency does not progress (see below). If malabsorption of cobalamin or rises in serum MMA levels have been demonstrated, however, these patients also should be given regular maintenance cobalamin therapy. Cobalamin should be given routinely to all patients who have had a total gastrectomy or


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InternalMed_Harrison. have been demonstrated, however, these patients also should be given regular maintenance cobalamin therapy. Cobalamin should be given routinely to all patients who have had a total gastrectomy or ileal resection. Patients who have undergone gastric reduction for control of obesity or who are receiving longterm treatment with proton pump inhibitors should be screened and, if necessary, given cobalamin replacement.


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InternalMed_Harrison. Replenishment of body stores should be complete with six 1000-μg IM injections of hydroxocobalamin given at 3to 7-day intervals. More frequent doses are usually used in patients with cobalamin neuropathy, but there is no evidence that they produce a better response. Allergic reactions are rare and may require desensitization or antihistamine or glucocorticoid cover. For maintenance therapy, 1000 μg hydroxocobalamin IM once every 3 months is satisfactory. Because of the poorer retention of cyanocobalamin, protocols generally use higher and more frequent doses, e.g., 1000 μg IM, monthly, for maintenance treatment.


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InternalMed_Harrison. Because a small fraction of cobalamin can be absorbed passively through mucous membranes even when there is complete failure of physiologic IF-dependent absorption, large daily oral doses (1000– 2000 μg) of cyanocobalamin have been used in PA for replacement and maintenance of normal cobalamin status in, e.g., food malabsorption of cobalamin. Sublingual therapy has also been proposed for those in whom injections are difficult because of a bleeding tendency and who may not tolerate oral therapy. If oral therapy is used, it is important to monitor compliance, particularly with elderly, forgetful patients. This author prefers parenteral therapy for initial treatment, particularly in severe anemia or if a neuropathy is present, and for maintenance.


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InternalMed_Harrison. For treatment of patients with subnormal serum vitamin B12 (B12) levels with a normal MCV and no hypersegmentation of neutrophils, a negative IF antibody test in the absence of tests of B12 absorption is problematic. Some (perhaps 15%) cases may be due to TC I (HC) deficiency. Homocysteine and/or MMA measurements may help, but in the absence of these tests and with otherwise normal gastrointestinal function, repeat serum B12 assay after 6–12 months may help one decide whether to start cobalamin therapy.


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InternalMed_Harrison. Vitamin B12 injections are used in a wide variety of diseases, often neurologic, despite normal serum B12 and folate levels and a normal blood count and in the absence of randomized, double-blind, controlled trials. These conditions include multiple sclerosis and chronic fatigue syndrome/myalgic encephalomyelitis (ME). It seems probable that any benefit is due to the placebo effect of a usually painless, pink injection. In ME, oral B12 therapy, despite providing equally large amounts of B12, has not been beneficial, supporting the view of the effect of the injections being placebo only.


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InternalMed_Harrison. Oral doses of 5–15 mg folic acid daily are satisfactory, as sufficient folate is absorbed from these extremely large doses even in patients with severe malabsorption. The length of time therapy must be continued depends on the underlying disease. It is customary to continue therapy for about 4 months, when all folate-deficient red cells will have been eliminated and replaced by new folate-replete populations. Before large doses of folic acid are given, cobalamin deficiency must be excluded and, if present, corrected; otherwise cobalamin neuropathy may develop despite a response of the anemia of cobalamin deficiency to folate therapy. Studies in the United States, however, suggest that there is no increase in the proportion of individuals with low serum cobalamin levels and no anemia since food fortification with folic acid, but it is unknown if there has been a change in incidence of cobalamin neuropathy.


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InternalMed_Harrison. Long-term folic acid therapy is required when the underlying cause of the deficiency cannot be corrected and the deficiency is likely to recur, e.g., in chronic dialysis or hemolytic anemias. It may also be necessary in gluten-induced enteropathy that does not respond to a gluten-free diet. Where mild but chronic folate deficiency occurs, it is preferable to encourage improvement in the diet after correcting the deficiency with a short course of folic acid. In any patient receiving long-term folic acid therapy, it is important to measure the serum cobalamin level at regular (e.g., once-yearly) intervals to exclude the coincidental development of cobalamin deficiency. Folinic Acid (5-Formyl-THF) This is a stable form of fully reduced folate. It is given orally or parenterally to overcome the toxic effects of methotrexate or other DHF reductase inhibitors, e.g., trimethoprim or cotrimoxazole.


InternalMed_Harrison

InternalMed_Harrison. Prophylactic folic acid is used in chronic dialysis patients and in parenteral feeds. Prophylactic folic acid has been used to reduce homocysteine levels to prevent cardiovascular disease and for cognitive function in the elderly, but there are no firm data to show any benefit.


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InternalMed_Harrison. Pregnancy In over 70 countries (but none in Europe), food is fortified with folic acid (in grain or flour) to reduce the risk of NTDs. Nevertheless, folic acid, 400 μg daily, should be given as a supplement before and throughout pregnancy to prevent megaloblastic anemia and reduce the incidence of NTDs, even in countries with fortification of the diet. The levels of fortification provide up to 400 μg daily on average in Chile, but in most countries, it is nearer to 200 μg, so periconceptual folic acid is still needed. Studies in early pregnancy show significant lack of compliance with the folic acid supplements, emphasizing the benefit of food fortification. Supplemental folic acid reduces the incidence of birth defects in babies born to diabetic mothers. In women who have had a previous fetus with an NTD, 5 mg daily is recommended when pregnancy is contemplated and throughout the subsequent pregnancy.


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InternalMed_Harrison. Infancy and Childhood The incidence of folate deficiency is so high in the smallest premature babies during the first 6 weeks of life that folic acid (e.g., 1 mg daily) should be given routinely to those weighing <1500 g at birth and to larger premature babies who require exchange transfusions or develop feeding difficulties, infections, or vomiting and diarrhea. The World Health Organization currently recommends routine supplementation with iron and folic acid in children in countries where iron deficiency is common and child mortality, largely due to infectious diseases, is high. However, some studies suggest that in areas where malaria rates are high, this approach may increase the incidence of severe illness and death. Even where malaria is rare, there appears to be no survival benefit.


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InternalMed_Harrison. This may occur with many antimetabolic drugs (e.g., hydroxyurea, cytosine arabinoside, 6-mercaptopurine) that inhibit DNA replication. Antiviral nucleoside analogues used in treatment of HIV infection may also cause macrocytosis and megaloblastic marrow changes. In the rare disease orotic aciduria, two consecutive enzymes in purine synthesis are defective. The condition responds to therapy with uridine, which bypasses the block. In thiamine-responsive megaloblastic anemia, there is a genetic defect in the high-affinity thiamine transport (SLC19A2) gene. This causes defective RNA ribose synthesis through impaired activity of transketolase, a thiamine-dependent enzyme in the pentose cycle. This leads to reduced nucleic acid production. It may be associated with diabetes mellitus and deafness and the presence of many ringed sideroblasts in the marrow. The explanation is unclear for megaloblastic changes in the marrow in some patients with acute myeloid leukemia and myelodysplasia. Lucio


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InternalMed_Harrison. and the presence of many ringed sideroblasts in the marrow. The explanation is unclear for megaloblastic changes in the marrow in some patients with acute myeloid leukemia and myelodysplasia. Lucio Luzzatto DEFINITIONS 129


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InternalMed_Harrison. Hemolytic Anemias and Anemia Due to Acute Blood Loss A finite life span is a distinct characteristic of red cells. Hence, a logical, time-honored classification of anemias is in three groups: (1) decreased production of red cells, (2) increased destruction of red cells, and (3) acute blood loss. Decreased production is covered in Chaps. 126, 128, and 130; increased destruction and acute blood loss are covered in this chapter. All patients who are anemic as a result of either increased destruc tion of red cells or acute blood loss have one important element in common: the anemia results from overconsumption of red cells from the peripheral blood, whereas the supply of cells from the bone marrow is normal (indeed, it is usually increased). On the other hand, these two groups differ in that physical loss of red cells from the bloodstream or from the body itself, as in acute hemorrhage, is fundamentally different from destruction of red cells within the body, as in hemolytic anemias.


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InternalMed_Harrison. aHereditary causes correlate with intracorpuscular defects, because these defects are due to inherited mutations; the one exception is PNH, because the defect is due to an acquired somatic mutation. Similarly, acquired causes correlate with extracorpuscular factors, because mostly these factors are exogenous; the one exception is familial hemolytic-uremic syndrome (HUS; often referred to as atypical HUS), because here an inherited abnormality allows complement activation to be excessive, with bouts of production of membrane attack complex capable of destroying normal red cells. Therefore, the clinical aspects and pathophysiology of anemia in these two groups of patients are quite different, and they will be considered separately.


InternalMed_Harrison

InternalMed_Harrison. Therefore, the clinical aspects and pathophysiology of anemia in these two groups of patients are quite different, and they will be considered separately. With respect to primary etiology, anemias due to increased destruction of red cells, which we know as hemolytic anemias (HAs), may be inherited or acquired; from a clinical point of view, they may be more acute or more chronic, and they may vary from mild to very severe; the site of hemolysis may be predominantly intravascular or extravascular. With respect to mechanisms, HAs may be due to intracorpuscular causes or to extracorpuscular causes (Table 129-1). But before reviewing the individual types of HA, it is appropriate to consider what they have in common.


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InternalMed_Harrison. The clinical presentation of a patient with anemia is greatly influenced in the first place by whether the onset is abrupt or gradual, and HAs are no exception. A patient with autoimmune HA or with favism may be a medical emergency, whereas a patient with mild hereditary spherocytosis or with cold agglutinin disease may be diagnosed after years. This is due in large measure to the remarkable ability of the body to adapt to anemia when it is slowly progressing (Chap. 77).


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InternalMed_Harrison. What differentiates HAs from other anemias is that the patient has signs and symptoms arising directly from hemolysis (Table 129-2). At the clinical level, the main sign is jaundice; in addition, the patient may report discoloration of the urine. In many cases of HA, the spleen is enlarged, because it is a preferential site of hemolysis; and in some cases, the liver may be enlarged as well. In all severe congenital forms of HA, there may also be skeletal changes due to overactivity of the bone marrow (although they are never as severe as they are in thalassemia). Abbreviations: LDH, lactate dehydrogenase; MCH, mean corpuscular hemoglobin; MCV, mean corpuscular volume.


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InternalMed_Harrison. Hemolytic Anemias and Anemia Due to Acute Blood Loss 650 The laboratory features of HA are related to hemolysis per se and the erythropoietic response of the bone marrow. Hemolysis regularly produces an increase in unconjugated bilirubin and aspartate aminotransferase (AST) in the serum; urobilinogen will be increased in both urine and stool. If hemolysis is mainly intravascular, the telltale sign is hemoglobinuria (often associated with hemosiderinuria); in the serum, there is hemoglobin, lactate dehydrogenase (LDH) is increased, and haptoglobin is reduced. In contrast, the bilirubin level may be normal or only mildly elevated. The main sign of the erythropoietic response by the bone marrow is an increase in reticulocytes (a test all too often neglected in the initial workup of a patient with anemia). Usually the increase will be reflected in both the percentage of reticulocytes (the more commonly quoted figure) and the absolute reticulocyte count (the more definitive parameter). The


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InternalMed_Harrison. with anemia). Usually the increase will be reflected in both the percentage of reticulocytes (the more commonly quoted figure) and the absolute reticulocyte count (the more definitive parameter). The increased number of reticulocytes is associated with an increased mean corpuscular volume (MCV) in the blood count. On the blood smear, this is reflected in the presence of macrocytes; there is also polychromasia, and sometimes one sees nucleated red cells. In most cases, a bone marrow aspirate is not necessary in the diagnostic workup; if it is done, it will show erythroid hyperplasia. In practice, once an HA is suspected, specific tests will usually be required for a definitive diagnosis of a specific type of HA.


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InternalMed_Harrison. The mature red cell is the product of a developmental pathway that brings the phenomenon of differentiation to an extreme. An orderly sequence of events produces synchronous changes, whereby the gradual accumulation of a huge amount of hemoglobin in the cytoplasm (to a final level of 340 g/L, i.e., about 5 mM) goes hand in hand with the gradual loss of cellular organelles and of biosynthetic abilities. In the end, the erythroid cell undergoes a process that has features of apoptosis, including nuclear pyknosis and actual loss of the nucleus. However, the final result is more altruistic than suicidal; the cytoplasmic body, instead of disintegrating, is now able to provide oxygen to all cells in the human organism for some remaining 120 days of the red cell life span.


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InternalMed_Harrison. As a result of this unique process of differentiation and maturation, intermediary metabolism is drastically curtailed in mature red cells (Fig. 129-1); for instance, cytochrome-mediated oxidative phosphorylation has been lost with the loss of mitochondria (through a process of physiologic autophagy); therefore, there is no backup to anaerobic glycolysis, which in the red cell is the only provider of adenosine triphosphate (ATP). Also the capacity of making protein has been lost with the loss of ribosomes. This places the cell’s limited metabolic apparatus at risk, because if any protein component deteriorates, it cannot be replaced, as it would be in most other cells; and in fact the activity of most enzymes gradually decreases as red cells age. At the same time, during their long time in circulation, various red cell components inevitably accumulate damage; in senescent red cells, the membrane protein band 3 molecules (see below and Fig. 129-1), having bound hemichromes on their


InternalMed_Harrison

InternalMed_Harrison. in circulation, various red cell components inevitably accumulate damage; in senescent red cells, the membrane protein band 3 molecules (see below and Fig. 129-1), having bound hemichromes on their intracellular domains, tend to cluster. Now they bind anti–band 3 IgG antibodies (present in most people) and C3 complement fragments; thus they become opsonized and are eventually removed by phagocytosis in the reticuloendothelial system.


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InternalMed_Harrison. Another consequence of the relative simplicity of red cells is that they have a very limited range of ways to manifest distress under hardship; in essence, any sort of metabolic failure will eventually lead either to structural damage to the membrane or to failure of the cation pump. In either case, the life span of the red cell is reduced, which is the definition of a hemolytic disorder. If the rate of red cell destruction exceeds the capacity of the bone marrow to produce more red cells, the hemolytic disorder will manifest as HA.


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InternalMed_Harrison. Thus, the essential pathophysiologic process common to all HAs is an increased red cell turnover; and in many HAs, this is due at least in part to an acceleration of the senescence process described above. The gold standard for proving that the life span of red cells is reduced (compared to the normal value of about 120 days) is a red cell survival study, which can be carried out by labeling the red cells with 51Cr and measuring residual radioactivity over several days or weeks: however, this classic test is now available in very few centers, and it is rarely necessary.


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InternalMed_Harrison. glucose phosphate isomerase glucose hexokinase ATP ADP glucose-6-phosphate fructose-6-phosphate ATP ADP phosphofructokinase fructose-1, 6-diphosphate aldolase glyceraldehyde-3-phosphate NAD+ glyceraldehyde 3-phosphate dehydrogenase 1,3-bisphosphoglycerate ADP ATP phosphoglycerate kinase 3-phosphoglycerate 3-phosphoglycerate mutase 2-phosphoglycerate NADH GSH GSSG NADPHNADP+ 6-phosphogluconate 2,3-bisphosphoglycerate mutase 2,3-bisphosphoglycerate phosphatase reductase G6PD 2,3-bisphosphoglycerate


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InternalMed_Harrison. FIGURE 129-1 Red blood cell (RBC) metabolism. The Embden-Meyerhof pathway (glycolysis) generates ATP for energy and membrane maintenance. The generation of NADPH maintains hemoglobin in a reduced state. The hexose monophosphate shunt generates NADPH that is used to reduce glutathione, which protects the red cell against oxidant stress. Regulation of 2,3-bisphosphoglycerate levels is a critical determinant of oxygen affinity of hemoglobin. Enzyme deficiency states in order of prevalence: glucose 6-phosphate dehydrogenase (G6PD) > pyruvate kinase > glucose-6-phosphate isomerase > rare deficiencies of other enzymes in the pathway. The more common enzyme deficiencies are encircled.


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InternalMed_Harrison. If the hemolytic event is transient, it does not usually cause any longterm consequences, except for an increased requirement for erythropoietic factors, particularly folic acid. However, if hemolysis is recurrent or persistent, the increased bilirubin production favors the formation of gallstones. If a considerable proportion of hemolysis takes place in the spleen, as is often the case, splenomegaly may become increasingly a feature, and hypersplenism may develop, with consequent neutropenia and/or thrombocytopenia.


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InternalMed_Harrison. The increased red cell turnover also has metabolic consequences. In normal subjects, the iron from effete red cells is very efficiently recycled by the body; however, with chronic intravascular hemolysis, the persistent hemoglobinuria will cause considerable iron loss, needing replacement. With chronic extravascular hemolysis, the opposite problem, iron overload, is more common, especially if the patient needs frequent blood transfusions. Chronic iron overload will cause secondary hemochromatosis; this will cause damage particularly to the liver, eventually leading to cirrhosis, and to the heart muscle, eventually causing heart failure.


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InternalMed_Harrison. Compensated Hemolysis Versus Hemolytic Anemia Red cell destruction is a potent stimulus for erythropoiesis, which is mediated by erythropoietin (EPO) produced by the kidney. This mechanism is so effective that in many cases the increased output of red cells from the bone marrow can fully balance an increased destruction of red cells. In such cases, we say that hemolysis is compensated. The pathophysiology of compensated hemolysis is similar to what we have just described, except there is no anemia. This notion is important from the diagnostic point of view, because a patient with a hemolytic condition, even an inherited one, may present without anemia; and it is also important from the point of view of management, because compensated hemolysis may become “decompensated,” i.e., anemia may suddenly appear, in certain circumstances, for instance in pregnancy, folate deficiency, or renal failure interfering with adequate EPO production. Another general feature of chronic HAs is seen when


InternalMed_Harrison

InternalMed_Harrison. suddenly appear, in certain circumstances, for instance in pregnancy, folate deficiency, or renal failure interfering with adequate EPO production. Another general feature of chronic HAs is seen when any intercurrent condition, such as an acute infection, depresses erythropoiesis. When this happens, in view of the increased rate of red cell turnover, the effect will be predictably much more marked than in a person who does not have hemolysis. The most dramatic example is infection by parvovirus B19, which may cause a rather precipitous fall in hemoglobin—an occurrence sometimes referred to as aplastic crisis.


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InternalMed_Harrison. There are three essential components in the red cell: (1) hemoglobin, (2) the membrane-cytoskeleton complex, and (3) the metabolic machinery necessary to keep hemoglobin and the membrane-cytoskeleton complex in working order. Diseases caused by abnormalities of hemoglobin, or hemoglobinopathies, are covered in Chap. 127. Here we will deal with diseases of the other two components.


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InternalMed_Harrison. Hemolytic Anemias due to Abnormalities of the Membrane-Cytoskeleton Complex The detailed architecture of the red cell membrane is complex, but its basic design is relatively simple (Fig. 129-2). The lipid bilayer incorporates phospholipids and cholesterol, and it is spanned by a number of proteins that have their hydrophobic transmembrane domain(s) embedded in the membrane; most of these proteins also extend to both the outside (extracellular domains) and the inside of the cell (cytoplasmic domains). Other proteins are tethered to the membrane through a glycosylphosphatidylinositol (GPI) anchor; these have only an extracellular domain, and they include ion channels, receptors for complement components, and receptors for other ligands. The most abundant red cell membrane proteins are glycophorins and the so-called band 3, an anion transporter. The extracellular domains of many of these proteins are heavily glycosylated, and they carry antigenic 4.2 GPA determinants that correspond to


InternalMed_Harrison

InternalMed_Harrison. and the so-called band 3, an anion transporter. The extracellular domains of many of these proteins are heavily glycosylated, and they carry antigenic 4.2 GPA determinants that correspond to blood groups. Underneath the mem-651 brane, and tangential to it, is a network of other proteins that make up the cytoskeleton. The main cytoskeletal protein is spectrin, the basic unit of which is a dimer of α-spectrin and β-spectrin. The membrane is physically linked to the cytoskeleton by a third set of proteins (includ ing ankyrin and the so-called band 4.1 and band 4.2), which thus make these two structures intimately connected to each other.


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InternalMed_Harrison. The membrane-cytoskeleton complex is so integrated that, not surprisingly, an abnormality of almost any of its components will be disturbing or disruptive, causing structural failure, which results ultimately in hemolysis. These abnormalities are almost invariably inherited mutations; thus, diseases of the membrane-cytoskeleton complex belong to the category of inherited HAs. Before the red cells lyse, they often exhibit more or less specific morphologic changes that alter the normal biconcave disk shape. Thus, the majority of the dis eases in this group have been known for over a century as hereditary spherocytosis and hereditary elliptocytosis. Over the past 20 years, their tions can arise from mutations in several genes with considerable overlap (Fig. 129-3).


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InternalMed_Harrison. Hereditary spHerocytosis (Hs) This is a relatively common type of genetically determined HA, with an estimated frequency of at least 1 in 5000. Its identification is credited to Minkowksy and Chauffard, who, at the end of the nineteenth century, reported families who had the presence of numerous spherocytes in the peripheral blood (Fig 129-4A). In vitro studies revealed that the red cells were abnormally susceptible to lysis in hypotonic media; indeed, the presence of osmotic fragility became the main diagnostic test for HS. Today we know that HS, thus defined, is genetically heterogeneous; i.e., it can arise from a variety of mutations in one of several genes (Table 129-3). It has been also recognized that the inheritance of HS is not always autosomal dominant (with the patient being heterozygous); indeed, some of the most severe forms are instead autosomal recessive (with the patient being homozygous).


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InternalMed_Harrison. clinical presentation and diagnosis The spectrum of clinical severity of HS is broad. Severe cases may present in infancy with severe anemia, whereas mild cases may present in young adults or even later in life. The main clinical findings are jaundice, an enlarged spleen, and often gallstones; indeed, it may be the finding of gallstones in a young person that triggers diagnostic investigations. -Spectrin Dematin4.1R Hemolytic Anemias and Anemia Due to Acute Blood Loss


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InternalMed_Harrison. FIGURE 129-2 The red cell membrane. In this figure, one sees, within the lipid bilayer, several membrane proteins, of which band 3 (anion exchanger 1 [AE1]) is the most abundant; the α-β spectrin dimers that associate to form most of the cytoskeleton; and several proteins (e.g., ankyrin) that connect the membrane to the cytoskeleton. In addition, as examples of glycosylphosphatidylinositol (GPI)-linked proteins, one sees acetylcholinesterase (AChE) and the two complement-regulatory proteins CD59 and CD55. The (nonrealistic) shapes of the protein moieties of the GPI-linked proteins are meant to indicate that they can be very different from each other and that, unlike with the other membrane proteins shown, the entire polypeptide chain is extracellular. Branched lines symbolize carbohydrate moiety of proteins. The molecules are obviously not drawn to the same scale. Additional explanations can be found in the text. (From N Young et al: Clinical Hematology. Copyright Elsevier, 2006; with


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InternalMed_Harrison. moiety of proteins. The molecules are obviously not drawn to the same scale. Additional explanations can be found in the text. (From N Young et al: Clinical Hematology. Copyright Elsevier, 2006; with permission.)


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InternalMed_Harrison. FIGURE 129-3 Hereditary spherocytosis (HS), hereditary elliptocytosis (HE), and hereditary stomatocytosis (HSt) are three morphologically distinct forms of congenital hemolytic anemia. It has emerged that each one can arise from mutation of one of several genes and that different mutations of the same gene can give one or another form. (See also Table 129-3.)


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InternalMed_Harrison. The variability in clinical manifestations that is observed among patients with HS is largely due to the different underlying molecular lesions (Table 129-3). Not only are mutations of several genes involved, but also individual mutations of the same gene can also give very different clinical manifestations. In milder cases, hemolysis is often compensated (see above), and this may cause variation in time even in the same patient, due to the fact that intercurrent conditions (e.g., pregnancy, infection) may cause decompensation. The anemia is usually normocytic, with the characteristic morphology that gives the disease its name. An increased mean corpuscular hemoglobin concentration (MCHC) on an ordinary blood count report should raise the suspicion of HS, because HS is almost the only condition in which this abnormality occurs. It has been apparent for a long time that the spleen plays a special role in HS through a dual mechanism. On one hand, like in many other HAs, the spleen


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InternalMed_Harrison. condition in which this abnormality occurs. It has been apparent for a long time that the spleen plays a special role in HS through a dual mechanism. On one hand, like in many other HAs, the spleen itself is a major site of destruction; on the other hand, transit through the splenic circulation makes the defective red cells more spherocytic and, therefore, accelerates their demise, even though that may take place elsewhere.


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InternalMed_Harrison. When there is a family history, it is usually easy to make a diagnosis based on features of HA and typical red cell morphology. However, there may be no family history for at least two reasons. First, the patient may have a de novo mutation, i.e., a mutation that has taken place in a germ cell of one of his parents or early after zygote formation. Second, the patient may have a recessive form of HS (Table 129-3). In such cases, more extensive laboratory investigations are required, including osmotic fragility, the acid glycerol lysis test, the eosin-5′-maleimide (EMA)–binding test, and SDS-gel electrophoresis of membrane proteins; these tests are usually carried out in laboratories with special expertise in this area. Sometimes a definitive diagnosis can be obtained only by molecular studies demonstrating a mutation in one of the genes underlying HS (Table 129-3).


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InternalMed_Harrison. We do not have a causal treatment for HS; i.e., no way has yet been found to correct the basic defect in the membrane-cytoskeleton structure. Given the special role of the spleen in HS (see above), it has long been thought that an almost obligatory therapeutic measure was splenectomy. Because this operation may have more than trivial consequences, today we have more articulate recommendations, based on disease severity (having found out, whenever possible, about the outcome of splenectomy in the patient’s relatives with HS), as follows. In mild cases, avoid splenectomy. Delay splenectomy until puberty in moderate cases or until 4–6 years of age in severe cases. Antipneumococcal vaccination before splenectomy FIGURE 129-4 Peripheral blood smear from patients with membrane-cytoskeleton abnormalities. A. Hereditary spherocytosis. B. Hereditary elliptocytosis, heterozygote. C. Elliptocytosis, with both alleles of the α-spectrin gene mutated.


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InternalMed_Harrison. is imperative, whereas penicillin prophylaxis after splenectomy is controversial. Along with splenectomy, cholecystectomy should not be regarded as automatic; it should be carried out, usually by the laparoscopic approach, when clinically indicated. Abbreviations: HE, hereditary elliptocytosis; HS, hereditary spherocytosis.


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InternalMed_Harrison. Hereditary elliptocytosis (He) HE is at least as heterogeneous as HS, both from the genetic point of view (Table 129-3, Fig. 129-3) and from the clinical point of view. Again, it is the shape of the red cells (Fig. 129-4B) that gives the name to the condition, but there is no direct correlation between the elliptocytic morphology and clinical severity. In fact, some mild or even asymptomatic cases may have nearly 100% elliptocytes, whereas in severe cases, all kinds of bizarre poikilocytes can predominate. Clinical features and recommended management are similar to those outlined above for HS. Although the spleen may not have the specific role it has in HS, in severe cases, splenectomy may be beneficial. The prevalence of HE causing clinical disease is similar to that of HS. However, an in-frame deletion of nine amino acids in the SLC4A1 gene encoding band 3, causing the so-called Southeast Asia ovalocytosis, has a frequency of up to 7% in certain populations, presumably as a result


InternalMed_Harrison

InternalMed_Harrison. in-frame deletion of nine amino acids in the SLC4A1 gene encoding band 3, causing the so-called Southeast Asia ovalocytosis, has a frequency of up to 7% in certain populations, presumably as a result of malaria selection; it is asymptomatic in heterozygotes and probably lethal in homozygotes.


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InternalMed_Harrison. Disorders of Cation Transport These rare conditions with autosomal dominant inheritance are characterized by increased intracellular sodium in red cells, with concomitant loss of potassium; indeed, they are sometimes discovered through the incidental finding, in a blood test, of a high serum K+ (pseudohyperkalemia). In patients from some families, the cation transport disturbance is associated with gain of water; as a result, the red cells are overhydrated (low MCHC), and on a blood smear, the normally round-shaped central pallor is replaced by a linear-shaped central pallor, which has earned this disorder the name stomatocytosis (Fig. 129-3). In patients from other families, instead, the red cells are dehydrated (high MCHC), and their consequent rigidity has earned this disorder the name xerocytosis. One would surmise that in these disorders the primary defect may be in a cation transporter; indeed, xerocytosis results from mutations in PIEZO1. In other patients with stomatocytosis,


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InternalMed_Harrison. xerocytosis. One would surmise that in these disorders the primary defect may be in a cation transporter; indeed, xerocytosis results from mutations in PIEZO1. In other patients with stomatocytosis, mutations are found in other genes also related to solute transport (Table 129-3), including SLC4A1 (encoding band 3), the Rhesus gene RHAG, and the glucose transporter gene SLC2A1 responsible for a special form called cryohydrocytosis. Hemolysis can vary from relatively mild to quite severe. From the practical point of view, it is important to know that in stomatocytosis, splenectomy is strongly contraindicated because it has been followed in a significant proportion of cases by severe thromboembolic complications.


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InternalMed_Harrison. channel. Enzyme Abnormalities When there is an important defect in the membrane or in the cytoskeleton, hemolysis is a direct consequence of the fact that the very structure of the red cell is abnormal. Instead, when one of the enzymes is defective, the consequences will depend on the precise role of that enzyme in the metabolic machinery of the red cell, which, in first approximation, has two important functions: (1) to provide energy in the form of ATP and (2) to prevent oxidative damage to hemoglobin and to other proteins by providing sufficient reductive potential; the key molecule for this is NADPH.


InternalMed_Harrison

InternalMed_Harrison. abnormalities of tHe glycolytic patHway Because red cells, in the course of their differentiation, have sacrificed not only their nucleus and their ribosomes, but also their mitochondria, they rely exclusively on the anaerobic portion of the glycolytic pathway for producing energy in the form of ATP. Most of the ATP is required by the red cell for cation transport against a concentration gradient across the membrane. If this fails, due to a defect of any of the enzymes of the glycolytic pathway (Table 129-4), the result will be hemolytic disease.


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InternalMed_Harrison. pyruvate kinase deficiency Abnormalities of the glycolytic pathway are all inherited and all rare. Among them, deficiency of pyruvate kinase (PK) is the least rare, with an estimated prevalence in most populations of the order of 1:10,000. However, very recently, a polymorphic PK mutation (E277K) was found in some African populations, with heterozygote frequencies of 1–7%, suggesting that this may be another malaria-related polymorphism. The clinical picture of homozygous (or compound biallelic) PK deficiency is that of an HA that often presents in the newborn with neonatal jaundice; the jaundice persists, and it is usually associated with a very high reticulocytosis. The anemia is of variable severity; sometimes it is so severe as to require regular blood transfusion treatment, whereas sometimes it is mild, bordering on a nearly compensated hemolytic disorder. As a result, the diagnosis may be delayed, and in some cases, it is made, for instance, in a young woman during her first


InternalMed_Harrison

InternalMed_Harrison. sometimes it is mild, bordering on a nearly compensated hemolytic disorder. As a result, the diagnosis may be delayed, and in some cases, it is made, for instance, in a young woman during her first pregnancy, when the anemia may get worse. The delay in diagnosis may be also helped by the fact that the anemia is remarkably well tolerated, because the metabolic block at the last step in glycolysis causes an increase in bisphosphoglycerate (or DPG; Fig. 129-1), a major effector of the hemoglobin-oxygen dissociation curve; thus, the oxygen delivery to the tissues is enhanced, a remarkable compensatory feat.


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InternalMed_Harrison. Hemolytic Anemias and Anemia Due to Acute Blood Loss aThe numbers from (1) to (4) indicate the ranking order of these enzymopathies in terms of frequency. Abbreviations: AHA, acquired hemolytic anemia; CNS, central nervous system; NM, neuromuscular.


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InternalMed_Harrison. The management of PK deficiency is mainly supportive. In view of the marked increase in red cell turnover, oral folic acid supplements should be given constantly. Blood transfusion should be used as necessary, and iron chelation may have to be added if the blood transfusion requirement is high enough to cause iron overload. In these patients, who have more severe disease, splenectomy may be beneficial. There is a single case report of curative treatment of PK deficiency by bone marrow transplantation from an HLA-identical PK-normal sibling. This seems a viable option for severe cases when a sibling donor is available. Rescue of inherited PK deficiency through lentiviral-mediated human PK gene transfer has been successful in mice. Prenatal diagnosis has been carried out in a mother who had already had an affected child.


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InternalMed_Harrison. other glycolytic enzyme abnormalities All of these defects are rare to very rare (Table 129-4), and all cause hemolytic anemia with varying degrees of severity. It is not unusual for the presentation to be in the guise of severe neonatal jaundice, which may require exchange transfusion; if the anemia is less severe, it may present later in life, or it may even remain asymptomatic and be detected incidentally when a blood count is done for unrelated reasons. The spleen is often enlarged. When other systemic manifestations occur, they can involve the central nervous system (sometimes entailing severe mental retardation, particularly in the case of triose phosphate isomerase deficiency), the neuromuscular system, or both. This is not altogether surprising, if we consider that these are housekeeping genes. The diagnosis of hemolytic anemia is usually not difficult, thanks to the triad of normomacrocytic anemia, reticulocytosis, and hyperbilirubinemia. Enzymopathies should be considered in


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InternalMed_Harrison. genes. The diagnosis of hemolytic anemia is usually not difficult, thanks to the triad of normomacrocytic anemia, reticulocytosis, and hyperbilirubinemia. Enzymopathies should be considered in the differential diagnosis of any chronic Coombs-negative hemolytic anemia. Unlike with membrane disorders where the red cells show characteristic morphologic abnormalities, in most cases of glycolytic enzymopathies, these are conspicuous by their absence. A definitive diagnosis can be made only by demonstrating the deficiency of an individual enzyme by quantitative assays; these are carried out in only a few specialized laboratories. If a particular molecular abnormality is already known in the family, then one could test directly for that defect at the DNA level, thus bypassing the need for enzyme assays. Of course the time may be getting nearer when a patient will present with her or his exome already sequenced, and we will need to concentrate on which genes to look up within the file. The


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InternalMed_Harrison. assays. Of course the time may be getting nearer when a patient will present with her or his exome already sequenced, and we will need to concentrate on which genes to look up within the file. The principles for the management of these conditions are similar as for PK deficiency. In one case of phosphoglycerate kinase deficiency, allogeneic bone marrow transplantation (BMT) effectively controlled the hematologic manifestations but did not reverse neurologic damage.