Image Findings and Background Characteristics of Extrapleural Air Collection: A Pneumothorax Mimicker.

Objective: The extrapleural space (EPS) is a potential space external to the thoracic cavity. Extrapleural air collection (EAC) refers to the accumulation of air within the EPS. Differentiating EAC from pneumothorax using imaging is challenging but crucial for accurate diagnosis and effective management. This retrospective study aimed to identify imaging and clinical findings that aid in distinguishing EAC from pneumothorax.

Materials and methods: Two radiologists reviewed 2771 cases of pneumothorax identified in computed tomography reports, focusing on web-like linear septa within air collection around the lung (described as a web appearance). Twenty-two patients met the inclusion criteria. Additional imaging findings, including pneumomediastinum, dependent distribution (air localized dorsally relative to the ventral margin of the descending aorta), and perivascular distribution (air surrounding the internal thoracic artery), were evaluated. Patient background information was analyzed using medical records. We also investigated CT images of a control group to determine whether these imaging characteristics could be observed in age/sex-matched patients with typical pneumothorax.

Results: Pneumomediastinum was observed in all cases. Dependent distribution and perivascular distribution were present in 59% (13/22) and 50% (11/22) of cases, respectively.Concurrent interstitial lung disease and steroid use were identified in 77% (17/22) and 73% (16/22) of patients, respectively. In contrast, pneumomediastinum, dependent distribution and perivascular distribution were present in 4.5% (1/22), 55% (12/22), and 0% (0/22) of cases in the control group. In particular, pneumomediastinum ( P =3.71×10 -10 ) and perivascular distribution ( P =1.53×10 -4 ) were statistically more frequent in EAC patients than in the control group.

Conclusions: Pneumomediastinum is a critical diagnostic feature of EAC. Dependent distribution and perivascular distribution are valuable imaging findings for diagnosing EAC. Careful interpretation is warranted when pneumomediastinum is observed in patients with interstitial lung disease or those on steroid therapy.