DEAR DR. DONOHUE: The family had always assumed that mother had developed Alzheimer's until our bright young family doctor detected abnormal brain activity from another cause. It was called normal pressure hydrocephalus, and treatment has helped her immensely. Can you discuss this problem and what brings it on? Mother is 67 and in really great health - especially now that this scare is over. - Mrs. J.T.

ANSWER: Normal pressure hydrocephalus (NPH) is an example of an illness that can masquerade as Alzheimer's disease, a tragedy of sorts, since NPH can be treated, as your family has learned.NPH has to do with the brain and its fluid. It is this cerebrospinal fluid circulating in interconnected caverns that nourishes and protects brain tissue. The body has ways of maintaining this fluid volume, balancing production against absorption.

In NPH, this balance has been lost. Fluid absorption is not keeping up with production, so the channels and caverns enlarge.

As brain tissue comes under new pressures, the symptoms arise - slow and awkward gait, loss of urinary control, a general deterioration of mental function.

When diagnosed, NPH responds to standard therapy, including rerouting of the fluid into either a nearby vein or the abdominal cavity. It's done by way of a tube inserted into a brain cavern.

Too often, we leap at the Alzheimer's diagnosis when other forces, most of them treatable, are at fault.

DEAR DR. DONOHUE: I have read recently about something called the sick building syndrome. Do you have anything on this? - M.K.

ANSWER: This is a relatively new phenomenon in public health. Apparently, a few of the newly constructed or newly remodeled buildings suffer from misdirected efforts to establish energy efficiency at the expense of proper ventilation. At least this is what some believe lies behind sick building syndrome.

Such buildings are tightly sealed, some to the point where windows themselves do not open. If you add less than ideal ventilation to this setting, the results are predictable. Workers report nose and eye irritation, dry skin, headaches and unexplained fatigue, symptoms that tend to subside when they are away from the building, as at home or on vacation.

The answer to sick building syndrome, of course, lies in correction of the engineering and practices like banning smoking on the premises. The sick building syndrome can be worsened by tricks of the imagination, fortifying the notion that if you expect to be sick, you very well might become sick.

DEAR DR. DONOHUE: Please help me out with a description of mittelshmerz. Thank you. - J.B.

ANSWER: Mittelschmerz is one-sided lower abdominal pain, lasting a day or two in the middle of a woman's menstrual cycle. It happens when the ovum is being released from the ovary. Mild pain relievers usually suffice to take care of the discomfort. Mittelschmerz does not signify anything amiss. Not all women have mittelschmerz.

DEAR DR. DONOHUE: I think you wrote that people with Parkinson's on L-Dopa should not take certain vitamins. Please explain. - J.F.

ANSWER: The warning was for vitamin B6 (pyridoxine), and only when taken in amounts higher than 5 mg or more a day. That is almost three times the recommended daily amount for health. That might undo the action of L-Dopa (levodopa).

PARKINSON'S DISEASE AFFLICTS more than 200,000 Americans, with 36,000 new cases reported annually. Dr. Donohue's pamphlet No. 24 is a guide to treatment, therapies and drugs used for control. For a copy, send your request to Dr. Donohue/No. 24, P.O. Box 830, Gibbstown, NJ 08027-9909. Please enclose a long, self-addressed, stamped (52 cents) envelope and $2.

Dr. Donohue welcomes reader mail but regrets that due to the tremendous volume received daily he is unable to answer individual letters. Readers' questions are incorporated in his column whenever possible.