II. Pathophysiology

  1. Interstitial Infiltrates occur within the connective tissue surrounding the air spaces
  2. Lung interstitial space is only visible in disease state (highlighted by fluid, fibrosis or tumor)
  3. Contrast with Alveolar Infiltrates which occur within the air spaces (Bronchioles, alveoli)

III. Approach

  1. Distinguish Interstitial Infiltrate pattern from Alveolar Infiltrate pattern to identify primary process
  2. Findings more suggestive of interstial infiltrate (contrasted with Alveolar Infiltrate)
    1. Diffuse infiltrate
    2. Follow distribution of pulmonary vessels
    3. Lung base appears more radiodense than apex

IV. Types: Linear Interstitial Infiltrates (Kerley A and B Lines)

  1. General: Interlobular thickened septae or congested lymphatics
    1. Pulmonary edema (most common, e.g. Left Congestive Heart Failure)
    2. Mitral Stenosis
    3. Lymphangitic carcinoma
    4. Lymphoma
    5. Mycoplasma pneumonia or Viral Pneumonia
    6. Sarcoidosis
    7. Idiopathic Pulmonary Fibrosis
    8. Hemosiderosis (late findings)
  2. Kerley A Lines
    1. Long (2-6 cm), linear upper lobe white lines radiating out from the hilum (central lung)
    2. Oriented perpendicular to pleura
  3. Kerley B Lines
    1. Short (1-2 cm) linear horizontal white lines in periphery, perpendicular and in contact with pleura
    2. Occur near costophrenic angles of lower lobes

V. Types: Reticular Interstitial Infiltrate

  1. Spider web network of lines anywhere in lung
  2. Overlapping short linear and curved white lines
  3. Irregular distribution (may be patchy or diffuse)
  4. Typically caused by reversible Interstitial Lung Disease
    1. Viral Pneumonia
    2. Sarcoidosis
    3. Hypersensitivity pneumonitis

VI. Types: Peripheral reticular interstitial nfiltrate (honeycomb)

  1. Honeycomb appearance in the peripheral lung spaces (bibasilar and lateral lung, sparing the perihilar and apical regions)
  2. Honeycombs represent patchwork of lines surrounding small cystic lung spaces
  3. Typically caused by advanced, irreversible Interstitial Lung Disease (e.g. interstitial Pneumonia)

VII. Types: Nodular Interstitial Infiltrate

  1. Discrete, round small, similarly sized densities (<1 cm)
  2. Localized to upper and middle lobes (typically spares the lower lobes)
  3. Causes
    1. Sarcoidosis
    2. Langerhan Cell Histiocytosis
    3. Silicosis
    4. Coal Worker's Lung

VIII. Types: Reticulo-Nodular Interstitial Infiltrate

  1. Combination of both the reticular and nodular patterns

IX. References

  1. Collins and Stern (2008) Chest Radiology, Lippincott Williams Wilkins, Philadelphia, p. 34-5

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