Pneumomediastinum

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.

Terminology

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.

Pathology

Etiology

• blunt or penetrating chest trauma

• secondary to neck, thoracic, or retroperitoneal surgery

• esophageal perforation ¹⁶

  • Boerhaave syndrome

  • endoscopic intervention

  • esophageal carcinoma

• tracheobronchial perforation

  • laceration

  • bronchial stump dehiscence

  • bronchoscopy

  • tracheostomy

  • laryngeal fracture

• vigorous exercise (see: pulmonary interstitial emphysema)

  • childbirth (see: Hamman syndrome)

  • weightlifting ¹⁶

  • Valsalva maneuver

• asthma ¹⁶

• barotrauma ¹⁶

  • diving

  • ventilator: most commonly secondary to ARDS with positive pressure ventilation

• infection

  • tuberculosis

  • histoplasmosis

  • dental or retropharyngeal infection

  • mediastinitis

• interstitial lung disease ¹²

• connective tissue disorders ¹¹

  • polymyositis/dermatomyositis, uncertain if related to vasculopathy or associated interstitial lung disease ¹²

• idiopathic

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

Radiographic features

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

• subcutaneous emphysema

• gas anterior to pericardium: pneumopericardium

• gas around pulmonary artery and main branches: ring around artery sign

• gas outlining major aortic branches: tubular artery sign

• gas outlining bronchial wall: double bronchial wall sign

• continuous diaphragm sign: due to gas trapped posterior to pericardium

• gas between parietal pleura and diaphragm: extrapleural sign

• gas in pulmonary ligament

• Naclerio V sign

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)

Ultrasound

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 

Treatment and prognosis

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

Tension pneumomediastinum can be fatal and usually requires intervention with insertion of drains through the anterior thoracic wall aiming decompression ¹⁵. 

Differential diagnosis

Must be distinguished most importantly from:

• medial pneumothorax

• pneumopericardium

• pneumoperitoneum

• pneumoretroperitoneum

• Mach bands

• subcutaneous emphysema

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

• subcarinal air cyst

• paratracheal air cyst

• tracheal diverticulum