Complex regional pain syndrome (CRPS) is an uncommon chronic pain condition that usually affects the arms or legs. Patients may experience symptoms such as burning or aching pain along with swelling, skin discoloration, altered skin temperature, abnormal sweating, decreased function and hypersensitivity of the affected area. In rare cases, CRPS may affect other parts of the body.

The cause of CRPS is not completely understood. Some scientists believe it may result from disturbances in the sympathetic nervous system, which controls blood flow and sweat glands. It may also be caused by injury to the peripheral nervous system (nerves outside the brain and spinal cord) or the central nervous system (brain and spinal cord). In other cases it may result from an immune system response.

As with other poorly understood pain conditions such as fibromyalgia and chronic fatigue syndrome, patients may sometimes be told their condition is psychosomatic or be suspected of faking symptoms for attention or other gains. However, U.S. researchers in 2006 reported discovering what they called the first physiological evidence of nerve damage in CRPS. They found injury to small-fiber nerve endings, which is also seen in other causes of nerve pain, such as diabetic neuropathy.

CRPS often occurs after trauma to an arm or a leg. The source of trauma can vary widely, such as a blunt injury, fracture, sprain, animal bite, knife wound or even an injection. CRPS may develop months to years after the initial damage to the nervous or immune system. The disease often gets worse over time, causing pain that is much greater than the initial damage. Conversely, spontaneous remission from symptoms occurs in some patients.

CRPS tends to affect women more frequently than men. It is most common in people between the ages of 40 and 60, though it can affect people of any age, including children.

There is no known cure for CRPS, but treatments are available. Patients who have symptoms of CRPS should seek medical attention early. Treatment is most effective if started earlier after symptoms first appear. Left untreated, CRPS can progress to more disabling signs and symptoms and can progress to the other side of the body as well.