The complex regional pain syndrome (CRPS) is an uncommon condition that usually develops after injury to an arm or leg. The main symptom is chronic, severe pain in the affected limb, but there also are changes in skin, bone, blood vessels, and muscle power that lead to additional symptoms and disabilities. The cause is unknown. Early treatment with pain medicines, physical therapy, drugs that are injected into nerves to numb them, and other treatments can be effective, but medication is much less effective after 6 months (this early treatment enhances recovery, but most patients do get better with or without treatment).