Last revised by Rohit Sharma on 3 Jan 2024

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. Tension pneumomediastinum is potentially fatal.

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 8.

Although it is rare, pneumomediastinum can occur spontaneously (spontaneous pneumomediastinum). This is considered benign and generally affects young adult males 9,10.

Tension pneumomediastinum is a life-threatening condition which compromises venous return to the heart by selectively compressing the lower pressure right heart chambers (tamponade). The trachea and bronchi can also be compressed and this can affect speech. Gas typically also spreads into the chest wall (and beyond), decreasing compliance and restricting lung ventilation. Any tracheobronchial injury can cause this phenomenon. Alternatively, alveolar rupture with a pressure gradient causing dissection of gas along the peribronchovascular interstitium to the mediastinum (Macklin effect) can cause tension pneumomediastinum. The pressure gradient is increased by coughing, asthma or mechanical ventilation. Tension pneumomediastinum can be fatal, causing cardiovascular collapse 17.

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 14:

  • 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 15

    • cyclic appearance of a dense band of horizontal reverberation artifacts which were discontinuous in the far field 

Most pneumomediastinum requires no treatment, with the air being gradually absorbed on the following days. 

Tension pneumomediastinum can cause fatal cardiac tamponade and may require urgent decompression through mediastinal drains 15

Must be distinguished most importantly from:

For small gas collections on a CT scan, also consider 3:

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