Chest X-ray: Alveolar vs Interstitial Disease

Interstitium is the scaffolding that supports the alveolar walls and surrounds both the alveoli and the terminal bronchioles. Neither alveoli nor interstitium is visible on a chest X-ray when normal. It is necessary to analyze whether the pattern of diffuse opacification in the lung field is alveolar or interstitial.

Terms:

1. Fine or small nodules: tiny opacities
2. Reticular: mesh or basket-like – fine or coarse lines.
3. Reticulo-nodular: a combination of both reticular and nodular pattern
4. Septal lines: fine thread-like lines produced by fluid or thickening of the septa between the lobules of the lung. Kerley B lines are one of the commonest septal lines mentioned around in the rounds and textbooks.
5. Kerley B lines: fine horizontal lines approximately 1 cm long, situated perpendicular to the lateral pleural surface – commonly seen just above the costophrenic angles on a frontal CXR
6. Air bronchogram: air-filled bronchi (dark) being made visible by the opacification of surrounding alveoli (grey/white)

Difference between alveolar vs interstitial shadow:

Differential diagnosis:

These two entities may be present simultaneously but generally, one of them is present dominantly.

Dominant alveolar pattern

1. Adults:

• Pulmonary edema
• Lobar pneumonia
• Hemorrhage
• Lymphoma
• Bronchioloalveolar cell carcinoma
• Adult respiratory distress syndrome (early)
• Aspiration pneumonia

2. Infants:

• Hyaline membrane disease
• Transient tachypnoea of the newborn

Dominant Interstitial pattern:

• Pulmonary oedema
• Pneumonia: viral or Pneumocystis carinii (early)
• Tuberculosis
• Sarcoidosis
• Idiopathic pulmonary fibrosis
• Rheumatoid lung
• Sclerodema
• Lymphangitis carcinomatosa
• Crack smoking