Complex sentence: Pneumomediastinum is the presence of air in the mediastinum, which can lead to pneumothorax, pneumoperitoneum, and pneumopericardium if left untreated in some cases, though that does not always happen and sometimes those conditions actually are the cause, not the result, of pneumomediastinum.
Split into simpler sentences: Pneumomediastinum is the presence of air in the mediastinum, which can lead to pneumothorax, pneumoperitoneum, and pneumopericardium if left untreated in some cases. However, that does not always happen and sometimes those conditions actually are the cause, not the result, of pneumomediastinum.