Complex regional pain syndrome (CRPS), also known as reflex sympathetic dystrophy (CRPS1) or causalgia (CRPS2), is a chronic pain disorder that results from abnormal healing after an injury—either minor or major— to a bone, muscle, or nerve. The development of symptoms is commonly disproportionate to the severity of the injury and seems to result from abnormal functioning of the sympathetic nervous system, the part of the nervous system that controls the diameter of blood vessels. One or more extremities and other parts of the body may be affected. CRPS may also be seen in postoperative patients and in patients with diseases that can cause chronic pain, such as cancer and arthritis.