Pneumomediastinum: concept, aetiological classification, and pathogenic mechanisms

Pneumomediastinum is the presence of air in the mediastinum, outside the tracheal and oesophageal lumina. There are two major aetiological groups of pneumomediastinum: spontaneous and traumatic (or simply secondary or acquired), depending on the absence or presence of an external physical force or element (traumatic agent) that may cause the pneumomediastinum, respectively. Spontaneous pneumomediastina are subdivided into primary spontaneous and secondary spontaneous subtypes, depending on the absence or presence of underlying bronchopulmonary disease (which acts as a predisposing factor because it causes bronchopulmonary fragility that facilitates rupture of the alveolar wall), respectively. There are also precipitating or triggering factors (coughing fits, respiratory effort, vomiting, etcetera) which, by increasing the alveolar-interstitial pressure gradient, may cause rupture of the alveolar wall with the consequent leakage of intra-alveolar air into the peribronchovascular interstitium; this air then travels through the peribronchovascular sheaths until it reaches the ipsilateral pulmonary hilum and the mediastinum, ultimately causing a pneumomediastinum (Macklin effect). Traumatic pneumomediastina are subclassified as iatrogenic and non-iatrogenic. Iatrogenic cases are caused by a diagnostic or therapeutic medical or surgical procedure. Non-iatrogenic cases are caused by external thoracic trauma in the strict sense, either open (penetrating wounds due to puncture or incision) or closed (contusions). The aetiological classification of pneumomediastinum outlined above is based on its clinical utility (prognostic-diagnostic-therapeutic approach) and on its exact similarity to the aetiological classification of pneumothorax.