Air in unusual places: some causes and ramifications of pneumomediastinum.

Five unusual cases of pneumomediastinum are described. In three the probable cause was thoraco-abdominal straining against a closed glottis during violent exercise, in criminal assault, or competitive sport. The resultant increase in intra-alveolar pressure produces an air leak which passes via the pulmonary interstitium into the mediastinum. It can then pass up into the neck to produce widespread subcutaneous emphysema and down through the diaphragmatic hiatuses to produce extraperitoneal emphysema. This may outline the lower surface of the diaphragm to stimulate intraperitoneal air, but it can also leak through the parietal peritoneum to result in actual intraperitoneal air. Therefore, in the patient who has been hospitalised after violent physical stress with or without blunt trauma, pneumomediastinum does not necessarily indicate tracheobronchial or oesophageal rupture and subdiaphragmatic air does not necessarily indicate bowel rupture. Probably any form of exercise in which the Valsalva manoeuvre is performed may cause pneumomediastinum, as may other causes of increased intra-alveolar pressure such as mechanical ventilation, bronchospasm, coughing and vomiting. Vomiting is a likely contributing cause in the pneumomediastinum of diabetic ketosis, of which a case is described. Another case is presented in which air passed in the opposite direction, from perforated extraperitoneal bowel up into the mediastinum.