By Mayo Clinic News Network

Complex regional pain syndrome is an uncommon form of chronic pain that usually affects an arm or a leg. Complex regional pain syndrome typically develops after an injury, surgery, stroke or heart attack, but the pain is out of proportion to the severity of the initial injury.

The cause of complex regional pain syndrome isn't clearly understood. Treatment for complex regional pain syndrome is most effective when started early. In such cases, improvement and even remission are possible.

Source: http://www.mayoclinic.org/diseases-conditions/complex-regional-pain-syndrome/basics/definition/con-20022844