Case of Pneumothorax with Deep Sulcus Sign

Findings

Supine radiograph shows increased lucency at the left base, but no pleural line is seen. The left costophrenic sulcus extends more inferiorly than the right. Wires represent ECG leads overlying the patient.
Follow-up abdominal CT (performed because of the history of trauma) and upright radiograph confirm the small pneumothorax (black arrows).

Deep sulcus sign of a left pneumothorax and a right pleural effusion are the two main considerations.

Because the most nondependent portion of the pleural space is at the base, a pneumothorax may be largest at the base on a supine radiograph. The base of the affected hemothorax may be more lucent and the costophrenic angle more apparent (deep sulcus sign).
Care should be made to avoid confusion with blunting of the contralateral costophrenic angle from a pleural effusion on the other side.
In patients on positive end-expiratory pressure mechanical ventilation (PEEP), a deep sulcus may prompt thoracostomy tube placement because of the potential for conversion to a larger pneumothorax.

Upright radiographs and decubitus films (side of suspected pneumothorax up) can be used to confirm that the deep sulcus is secondary to a pneumothorax.
Although placement of a thoracostomy tube depends on clinical status, communication with the clinical team is essential because the deep sulcus sign is often quite subtle and often overlooked if images are viewed outside of a reading room.