Thoracic endometriosis presenting as a catamenial hemothorax with discordant video-assisted thoracoscopic surgery.

Thoracic endometriosis is uncommon and may be overlooked, resulting in a delay in diagnosis. We describe the case of a 47-year-old woman presenting with acute onset pleuritic pain and hemothorax secondary to this rare entity. The diagnosis of thoracic endometriosis is driven by a compatible clinical history coupled with supportive imaging and immunohistochemical findings. Imaging features lack specificity, however, computed tomography and magnetic resonance imaging play an important role in identifying pleural/diaphragmatic involvement and excluding other more common diseases. Immunohistochemical pleural fluid analysis can confirm the presence of hormone receptor-positive endometrial glands and stroma. We illustrate a few potential diagnostic pitfalls, specifically the inconsistency in diagnostic yield of video-assisted thoracoscopic surgery/thoracentesis and the variable temporal association of patients' symptoms and pathology with menstruation. Prompt identification of thoracic endometriosis is important as it enables early institution of therapy and limits future complications.