The double diaphragm sign is one of several radiological signs seen with a pneumothorax in a supine patient.
Supine films are commonly performed in unwell patients, particularly in the ICU. In a supine patient with a pneumothorax, air may outline the anterior portions of the hemidiaphragm and cause visualization of the anterior costophrenic sulcus.
When both the diaphragmatic dome and anterior portions of the diaphragm are visualized (being outlined by lung and pleural air respectively) the double diaphragm sign is produced.
It should not be confused with the continuous diaphragm sign.