Dynamics of diaphragmatic endometriosis with thoracic endometriosis-related pneumothorax

Background: Pneumothorax that occurs during menstruation is called catamenial pneumothorax; however, it is also called thoracic endometriosis-related pneumothorax (TERP) because of its etiology. Because of its high postoperative recurrence rate, TERP often remains unresolved. Diaphragmatic endometriosis is often observed with TERP, but its detailed mechanism has not yet been elucidated. This study aimed to clarify the dynamics and mechanisms of endometrial tissue engraftment on the diaphragm observed with TERP. Methods: A total of 140 patients with TERP who underwent the initial video-assisted thoracic surgery (VATS) at our hospital between January 2018 and December 2023 were eligible for enrollment in this study; however, 10 patients were subsequently excluded. Diaphragmatic lesions were morphologically classified as hiatus, hematoma, and scar lesions. The diaphragmatic regions were divided into four equal areas by virtually drawing lines parallel and perpendicular to the coronary ligament in the membrane area ventral to the coronary ligament of the diaphragm. The ventral lateral part was designated as area A, and the other parts (proceeding in a counterclockwise direction) were designated as areas B, C, and D. The area dorsal to the coronary ligament was designated as area E. Endometrial tissue distribution was determined based on surgical reports and intraoperative images. Results: All diaphragmatic endometrial lesions were located in area A (n=130 patients; 100%), whereas the others were located in area B (52 patients; 40%) and area D (21 patients; 16%). Hiatus, hematoma, and scar lesions were found in area A in more than 60% of patients, as well as in areas B and D. Three lesions were found in area C (2%). Hematoma and scar lesions were found in area E (2%). Conclusions: Diaphragmatic endometriosis frequently invades the thoracic cavity at the ventral side of diaphragm membrane. Hiatus lesions are unique to diaphragmatic endometriosis. The synergistic effect of physical traction during the respiratory movement of the diaphragm and the protein dissolution phenomenon of endometrial tissue engraftment may possibly form hiatus lesions.