ABSTRACT
Neither theory is universally accepted and neither has been fully substantiated. The humoral theory suggests that thoracic diseases cause an opening of arteriovenous shunts in the lung that permit the release of vasoactive substances into the arterial circu lation. These vasoactive substances, normally destroyed by the lung, cause increased peripheral blood flow in the limbs resulting in HO. Peripheral vasodilation has been experimentally induced with arteiovenous shunts, but HO has not been pro duced. The neuronal theory suggests that neural reflexes produced within thoracic tumors stimulate afferent fibers of the vagus nerve that subsequently stimulate efferent fibers of the distal limbs resulting in increased blood flow and the production of HO. The origin of these neural reflexes is open to question, but HO has been induced by stimu lation of afferent nerve fibers believed to originate in the pulmonary hilus, mediastinum and parietal pleura. Additionally, HO will resolve when the vagus nerve is severed near the pulmonary hilus. HO also quickly regresses with appropriate treat ment of the primary disease.
