Septal lines, or Kerley lines, are seen when the interlobular septa in the pulmonary interstitium become prominent. It may be because of lymphatic engorgement or edema of the connective tissues of the interlobular septa. They usually occur when pulmonary capillary wedge pressure reaches 20-25 mmHg.
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Radiographic features
Kerley A lines
These are 2-6 cm long oblique lines that are <1 mm thick and course towards the hila 2. They represent the thickening of the interlobular septa that contain lymphatic connections between the perivenous and broncho-arterial lymphatics deep within the lung parenchyma. On chest radiographs, they are seen to cross normal vascular markings and extend radially from the hilum to the upper lobes. Kerley A lines are less frequent than Kerley B and C lines and are usually not seen in the absence of the other two. HRCT is the best modality for the demonstration of Kerley A lines.
Kerley B lines
These are thin lines 1-2 cm in length in the periphery of the lung(s). They are perpendicular to the pleural surface and extend out to it. They represent thickened subpleural interlobular septa and are usually seen at the lung bases.
Kerley C lines
Kerley C lines are short lines which do not reach the pleura (unlike B or D lines) and do not course radially away from the hila (unlike A lines).
Kerley D lines
Kerley D lines are the same as Kerley B lines, except they are seen on lateral chest radiographs in the retrosternal air gap 3.
Pathology
Causes
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neoplasm
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lymphangitic spread of cancer (e.g. lymphangitic carcinomatosis): Kerley lines with a delicate peripheral reticular pattern
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pneumonia
interstitial pulmonary fibrosis
History and etymology
Kerley lines are named after Sir Peter James Kerley (1900-1979), an Irish radiologist who, in addition to describing the interstitial lines now known as Kerley lines, was a co-founder of the Faculty of Radiology (later to become the Royal College of Radiologists), and also attended to King George VI 3,4.
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