II. Pathophysiology
- Interstitial Infiltrates occur within the connective tissue surrounding the air spaces
- Lung interstitial space is only visible in disease state (highlighted by fluid, fibrosis or tumor)
- Pulmonary vessels are still visible but with fuzzy margins ("trees in fog")
- Contrast with Alveolar Infiltrates which occur within the air spaces (Bronchioles, alveoli) and obscure the vessels
III. Approach
- Distinguish Interstitial Infiltrate pattern from Alveolar Infiltrate pattern to identify primary process
- Findings more suggestive of interstial infiltrate (contrasted with Alveolar Infiltrate)
- Diffuse infiltrate
- Follow distribution of pulmonary vessels
- Lung base appears more radiodense than apex
IV. Types: Linear Interstitial Infiltrates (Kerley A and B Lines)
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General: Interlobular thickened septae or congested Lymphatics
- Pulmonary Edema (most common, e.g. Left Congestive Heart Failure)
- Mitral Stenosis
- Lymphangitic carcinoma
- Lymphoma
- Mycoplasma pneumonia or Viral Pneumonia
- Sarcoidosis
- Idiopathic Pulmonary Fibrosis
- Hemosiderosis (late findings)
- Kerley A Lines
- Long (2-6 cm), linear upper lobe white lines radiating out from the hilum (central lung)
- Oriented perpendicular to pleura
- Kerley B Lines
- Short (1-2 cm) linear horizontal white lines in periphery, perpendicular and in contact with pleura
- Occur near costophrenic angles of lower lobes
V. Types: Reticular Interstitial Infiltrate
- Spider web network of lines anywhere in lung
- Overlapping short linear and curved white lines
- Irregular distribution (may be patchy or diffuse)
- Typically caused by reversible Interstitial Lung Disease
- Viral Pneumonia
- Sarcoidosis
- Hypersensitivity pneumonitis
VI. Types: Peripheral reticular interstitial nfiltrate (honeycomb)
- Honeycomb appearance in the peripheral lung spaces (bibasilar and lateral lung, sparing the perihilar and apical regions)
- Honeycombs represent patchwork of lines surrounding small cystic lung spaces
- Typically caused by advanced, irreversible Interstitial Lung Disease (e.g. interstitial Pneumonia)
VII. Types: Nodular Interstitial Infiltrate
- Discrete, round small, similarly sized densities (<1 cm)
- Localized to upper and middle lobes (typically spares the lower lobes)
- Causes
- Sarcoidosis
- Langerhan Cell Histiocytosis
- Silicosis
- Coal Worker's Lung
VIII. Types: Reticulo-Nodular Interstitial Infiltrate
- Combination of both the reticular and nodular patterns
IX. References
- Collins and Stern (2008) Chest Radiology, Lippincott Williams Wilkins, Philadelphia, p. 34-5