Expiratory air trapping on thoracic computed tomography. A diagnostic subclassification.

RATIONALE: Multiple causes for air trapping as identified by expiratory computed tomography (CT) have been reported, but a unified evaluation schema has never been proposed.

OBJECTIVES: It was our purpose to identify imaging features that would help distinguish etiologies of mosaic air trapping.

METHODS: Cases with the term "air trapping" in the radiology report in 2010 were identified by searching the Radiology Information System of an academic tertiary care center and associated community hospital. Medical records and CT examinations were reviewed for the causes of air trapping.

MEASUREMENTS AND MAIN RESULTS: Causes for moderate to severe air trapping could be identified in 201 of 230 (87.4%) cases and could be subdivided into those associated with bronchiectasis (76 of 201, 38%), those associated with interstitial lung disease (62 of 201, 31%), those associated with tree-in-bud opacities (5 of 201, 2%), and those with air trapping alone (58 of 201, 29%). When found with bronchiectasis, nontuberculous mycobacteria, cystic fibrosis, idiopathic bronchiectasis, and transplant-related bronchiolitis obliterans were the most common causes of air trapping. When found with interstitial lung disease, sarcoidosis, hypersensitivity pneumonitis, or unspecified interstitial lung disease were the most common cause of air trapping. When found in isolation, chronic bronchitis, asthma, bronchiolitis obliterans, and unspecified small airways disease were the most common causes of air trapping. Unusual conditions causing isolated air trapping included vasculitis and diffuse idiopathic neuroendocrine cell hyperplasia.

CONCLUSION: A variety of conditions can cause air trapping. Associated imaging findings can narrow the differential diagnosis.