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Kerley A, Kerley B and Kerley C lines

As pulmonary venous pressure rises, the upper lobe veins distend. They initially reach the size of, and eventually become larger than, the lower lobe vessels (thus reversing the normal ‘gravity-dependent’ pattern). This is described as ‘upper lobe venous diversion’ and is often the first recognized radiological sign of pulmonary venous hypertension . 

If the pulmonary venous pressure continues to rise and exceeds the plasma oncotic pressure, fluid will begin to accumulate in the lung interstitium. This is known as interstitial pulmonary oedema. Radiologically this is associated with the appearance of interstitial (Kerley B) lines . These lines were first described in 1933 and represent thickening of interlobular septa (as a result of fluid accumulation) within the lung. They were originally classified into three groups:

Kerley A lines are approximately 4 cm in length and are most conspicuous in the upper and mid portions of the lung. They are deep septal lines (lymphatic channels) that radiate from the hila into the central portions of the lungs but do not reach the pleura. Their presence normally indicates a more acute or severe degree of oedema.

Kerley B lines are shorter (1 cm or less) interlobular septal lines, found predominantly in the lower zones peripherally, and parallel to each other but at right angles to the pleural surface.
The originally described Kerley C lines are now designated as due to overlapping  Kerley C lines. The term is no longer used

Septal lines can be differentiated from blood vessels as the latter are not visible in the outer 1 cm of the lung. In addition, deep septal lines do not branch uniformly (as is the case for blood vessels) and are seen with a greater clarity (as they represent a sheet of tissue) than a blood vessel of similar calibre. Under normal circumstances septal lines caused by interstitial fluid overload would be expected to disappear after suitable reduction in pulmonary venous pressure. Exceptionally, however, they may persist, e.g. in long-standing PVH, where haemosiderin deposition or fibrosis has occurred. Other causes of persistent septal lines include idiopathic interstitial fibrosis, lymphangitis carcinomatosa and pneumoconiosis. 

Differentiation between the causes on plain radiography may be helped by ancillary signs (e.g. cardiomegaly and calcification of the mitral valve which both favour PVH as the diagnosis). Other signs of interstitial fluid overload include perihilar haze (loss of visible clarity of the lower lobe and hilar vessels) and peribronchial cuffing (apparent thickening of proximal bronchial walls as a result of interstitial fluid accumulating around their walls). As the pulmonary venous pressure continues to increase fluid begins to accumulate in the alveolar spaces. This is termed alveolar oedema ( Fig. 26.3 ). Kerley B lines, airspace nodules, bilateral symmetric consolidation in the mid and lower lung zones and pleural

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