Spontaneous pneumomediastinum is related to the appearance of free air in the mediastinum without predeceasing trauma, surgery or other medical procedures.

Associated with sudden chest pain, neck pain, dyspnea or signs of subcutaneous emphysema.

Commonly associated with retro sternal chest pain that radiates to the neck and back, cough and dysphagia.

Concomitant pneumothorax may or may not be present.

Important clinical findings include subcutaneous emphysema, rhinolalia,

hoarseness, and neck swelling.
Pathophysiology suggested to result from alveolar hyperinflation causing alveolar damage with leakage of air into the interstitial spaces followed by proximal migration of air toward the hilum and mediastinum alongside the pulmonary blood vessels (Macklin).

The Macklin’s effect of alveolar rupture due to differentials between intra-alveolar and perialveolar pressures, causing air leakage into the mediastinum cavity and formation of a spontaneous pneumomediastinum in the absence of trauma or esophageal rupture.

Other mechanisms including air arriving from the neck soft tissue, esophagus, tracheal or abdominal cavity.

A rare complication of anorexia nervosa, postulated to be due to thinner alveolar walls in starvation.

Associated with a relatively benign clinical process.

May be associated with a crunching sound on auscultation of the chest indicating air trapped between the chest wall and the anterior parietal pericardium-known as that Hamman crunch.

Associated with blunt and penetrating chest trauma, post-neck/thoracic/retroperitoneal  surgery, esophageal perforation, tracheobronchial perforation, vigorous exercise, asthma, barotrauma, infection, interstitial lung disease, and connective tissue disorder.
Diagnosis is typically established with plain anterior chest x-rays, 
which show lucent  streaks, bubbles of air outlining  mediastinal structures, and visible mediastinal pleura.
Chest x-rays can yield a diagnosis in almost 60-90% of cases.
Neck and chest CT scans can help confirm diagnosis if chest X-ray is inconclusive.

Average hospital stay 2 to 4 days, with full resolution within approximately 1 to 3 weeks.

Often associated with pneumothorax.

Men in their 20s are at particular risk for this process, and other risk factors include: pre-existing lung disease such as asthma, bronchiectasis, COPD, cystic fibrosis, excessive vomiting, coughing or Valsalva maneuver, illicit drug use, scuba diving, and rigorous physical activity.

The vast majority of cases are treated conservatively including: bedrest, oxygen therapy, and adequate analgesia.
The air in the mediastinum is gradually absorbed, administering high concentration of oxygen enhances the absorption.
antibiotics are reserved for patients with underlying infection for evidence of mediastinitis.
A chest tube may be required if there is an accompanying pneumothorax.
If pneumomediastinum is associated with tension, tamponade, airway compression, a surgical decompression with video assisted thoracic surgery or even thoracotomy can be performed.

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