Endometriosis can attach to the lung, forming chocolate-like cysts. Generally the parietal pleura is involved, but the lung itself, the visceral layer, the diaphragm, and more rarely the tracheobronchial tree may also be afflicted. How this endometrial tissue reaches the thorax remains enigmatic, although defects in the diaphragm can often be found.
The cysts can release blood; the endometrial cyst "menstruates" in the lung. Air can move in by an unknown mechanism. The blood and air cause the lung to collapse (i.e. catamenial hemopneumothorax).
Catamenial pneumothorax pathophysiology
Hemothorax pathophysiology
Hemothorax differential diagnosis
Catamenial pneumothorax epidemiology and demographics
Pneumomediastinum differential diagnosis
Air embolism pathophysiology
Hemothorax risk factors
Tracheitis differential diagnosis