Apical caps cover the lung apices and often bilateral. They can be chronic, due to fibrosis or acute due to pleural or extra-pleural fluid.
Epidemiology
The frequency of chronic apical pleural thickening increases with age 3. There may be a slightly greater male predilection 7.
Pathology
Apical caps increase in frequency with age and are usually caused by pleural and/or pulmonary fibrosis which displaces extrapleural fat. Chronic ischemia may cause plaque-like elastotic fibrosis of the visceral pleura/subpleural lung 9.
Upper lobe fibrosis such as pleuroparenchymal fibroelastosis can cause thick apical pleural caps 9.
Acutely, infection, neoplasm and trauma can cause pleural or extra-pleural fluid collections which are more likely to be unilateral. Aortic rupture can be associated with a left apical cap due to hematoma 9.
Causes and associations include:
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pleural thickening/scarring
idiopathic: chronic ischemic etiology is favored for most cases 4
secondary to previous apical infection: typically pulmonary tuberculosis
may be present in up to 10% of radiographs 2
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hematoma
upper thoracic spine injury
fractured 1st rib
lymphoma: extending from neck/mediastinum
abscess within the neck/mediastinum
Distribution
It is often bilateral but if unilateral may be more common on the right 7.
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