Pneumomediastinum is the presence of extraluminal gas within the mediastinum. Gas may originate from the lungs, trachea, central bronchi, esophagus, and peritoneal cavity and track from the mediastinum to the neck or abdomen.


In the setting of trauma, if pneumomediastinum is visible on chest x-ray it is termed overt pneumomediastinum whereas if it is only visible on CT then it is termed occult pneumomediastinum .



Although it is rare, pneumomediastinum can occur spontaneously. This is considered benign and generally affects young adult males .

Radiographic features

Small amounts of gas appear as linear or curvilinear lucencies outlining mediastinal contours such as:

Pediatric pneumomediastinum may have slightly different appearances:

  • elevated thymus: thymic wing sign
  • gas crossing the superior mediastinum: haystack sign (the heart appears like a haystack in a Monet painting)

Sonography is not a diagnostic modality of choice but is often used in the initial workup of undifferentiated trauma patients, or in differentiating causes of dyspnea or chest pain. Sonographic features which may appear in pneumomediastinum include :

  • cervical subcutaneous emphysema
    • anterior cervical ultrasonography demonstrates hyperechoic foci with "dirty" posterior acoustic shadowing
    • typically found anterior or lateral to the thyroid and medial to the internal jugular vein
  • loss of the parasternal and apical views when performing transthoracic echocardiography
    • with preservation of the subxiphoid window
  • the air gap sign
    • originally described using M-mode, describes systolic obscuration of cardiac structures as viewed from a parasternal long axis view
    • cyclic appearance of a dense band of horizontal reverberation artifacts which were discontinuous in the far field 

Differential diagnosis

Must be distinguished most importantly from:

For small gas collections on a CT scan, also consider :

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