Third, after alveolar rupture, gas may track along the perivascular interstitium in the lung and enter the mediastinum at the pulmonary hilum. This third pathway is the likely route of gas entry in patients subjected to barotrauma from mechanical positive pressure ventilation or in those who suffer pneumomediastinum as a complication of bronchoscopy. Pneumomediastinum occurring in the setting of mechanical ventilation may be associated with the more serious complications of barotrauma, which include pneumothorax, gas embolization, and pneumopericardium. Most often, however, pneumomediastinum occurs independently of traumatic or iatrogenic causes, in which case it is referred to as spontaneous pneumomediastinum. Spontaneous pneumomediastinum occasionally occurs as a complication of pneumonia or asthma, but most often, it is found in young and otherwise healthy individuals.
Symptoms are typically abrupt in onset and usually follow exaggerated respiratory efforts.
Thus, events that commonly precede spontaneous pneumomediastinum include parturition, cough, emesis, and straining at stool. In addition, numerous reports have appeared describing spontaneous pneumomediastinum after cocaine or marijuana use. Typical presenting symptoms are retrosternal chest pain and dyspnea. Because the free mediastinal gas often escapes cephalad into the subcutaneous tissues of the neck and supraclavicular area, patients may also complain of neck pain and sore throat, dysphagia, or a peculiar swelling and crepitation in the upper chest and neck. The last finding, which is detected by palpation of crepitations resulting from the formation of gas bubbles just below the surface of the skin, may in extreme cases of pneumomediastinum extend as far down as the arms, abdominal wall, and genitals.