Extrapleural Fat Sign
Explanation: Extrapleural fat, endothoracic fascia, and the innermost layers of intercostal muscle are superficial to the parietal pleura. Extrapleural fat is usually seen as a 1 mm thick low attenution line on chest CT and is almost imperceptible in normal individuals.2 It may be thicker in obese patients.
Accumulation of blood in the extrapleural space results in an extrapleural hematoma (EPH), which displaces the overlying extrapleural fat centrally and is most commonly the result of injury to intercostal arteries or veins.3 EPH is an underreported complication of blunt chest trauma. Less common causes of EPH include aortic rupture, penetrating trauma, and iatrogenic injury.3–6
Discussion: A rapidly expanding biconvex extrapulmonary opacity on chest radiography or high attenuation extrapleural collection on CT should raise the suspicion of an EPH, particularly in the context of trauma. Frequently associated with rib fractures, intercostal vessel injury is considered the most common etiology of EPH (Fig. 1).3 Direct bleeding from rib fractures and paravertebral muscle trauma may also cause EPH.7 Other signs of trauma, such as pulmonary contusion or laceration, hemothorax, and pneumothorax, often coexist.
In a retrospective review of 13 cases of EPH by Chung et al, the majority of patients (11/13) had a traumatic etiology.7 Nine of the 11 trauma patients had rib fractures, 4 of whom required surgical intervention. The authors divided EPH into two groups: biconvex and nonconvex. Biconvex EPHs have a partial or complete biconvex margin, often forming an oval or round shape similar to the appearance of an intracranial epidural hematoma. Biconvex EPH tends to be larger than nonconvex EPH. The biconvex morphology suggests arterial bleeding and a higher likelihood of surgical intervention.7 Nonconvex EPH more often has lobulated margins that conform to the shape of the inner chest wall, likely results from low pressure venous bleeding, and usually does not require surgical management.7
The presence of the extrapleural fat sign should alert the radiologist to the extrapleural location of the fluid collection or mass in question. Extrapleural hematoma, the prototypical cause for an extrapleural collection, is frequently misdiagnosed as hemothorax and cannot be drained with pleural tube placement.7 Drainage of a large EPH most often requires surgery.