Black pleural line | Radiology Signs

The black pleural line, also called the black pleura sign, is caused by pulmonary alveolar microlithiasis (PAM), a rare genetic lung disorder characterized by widespread accumulation of calcium phosphate microliths within the alveolar spaces.

This results in dense calcification of the lung parenchyma centrally in the secondary pulmonary lobules, while the subpleural regions remain relatively spared. This differential calcification pattern creates a thin, lucent (dark) line along the pleura beneath the ribs seen on imaging modalities such as chest X-ray and CT scan. PAM typically presents with slowly progressive respiratory symptoms and can lead to pulmonary fibrosis and respiratory failure.

Why is it called so?

It is called the black pleural line or black pleura sign because the spared subpleural region appears as a thin, dark (black/lucent) line compared to the surrounding diffusely dense, calcified lung parenchyma on imaging. The stark contrast between the heavily calcified lung and the relatively unaffected subpleural zone produces this distinct radiological appearance.

Pathophysiology

The black pleural line develops due to the pathologic accumulation of calcospherites (microliths) within the alveoli predominantly in the central portions of the secondary pulmonary lobules, sparing the peripheral subpleural alveoli.

This pattern of deposition leads to dense, diffuse calcifications throughout most of the lung parenchyma. Subpleural sparing, often accompanied by subpleural cystic changes, results in a narrow strip of relatively lucent lung just beneath the pleura. On imaging, this creates a visible line of decreased density juxtaposed against the surrounding heavily calcified lung tissue, which appears radiodense.

Alternative names: Black pleura sign

Other associated named signs: Sandstorm appearance (diffuse micronodular calcifications throughout lungs in PAM)