Complex Regional Pain Syndrome, CRPS, formerly known as RSD Reflex Sympathetic Dystrophy, is a progressive disease of the Autonomic Nervous System, and more specifically, the Sympathetic Nervous System. The pain is characterized as constant, extremely intense, and out of proportion to the original injury. The pain is typically accompanied by swelling, skin changes, extreme sensitivity, and can often be debilitating. It usually affects one or more of the four limbs but can occur in any part of the body and in over 70% of the victims it spreads to additional areas.

CRPS is ranked as the most painful form of chronic pain that exists today by the McGill Pain Index.