Complex regional pain syndrome (CRPS), also known as reflex sympathetic dystrophy or RSD, is one of the conditions a doctor will look for in a standard medical workup for chronic pain. Millions of people in the United States suffer from CRPS. The main symptoms are various degrees of severe pain, swelling, and hypersensitivity to touch, usually in an arm or leg. CRPS usually happens after a triggering event, such as an injury, surgery, stroke, or heart attack that causes nerve irritation and pain in the affected area. The pain continues past the time it should go away and is often out of proportion to the severity of the initial injury. A patient may feel burning, muscle or joint stiffness, rapid hair and nail growth, and constriction of blood vessels at the site. The pain then becomes more severe, often associated with weakening of the muscles, stiffening of the joints, and thinning of the bone (osteoporosis), until the joints can be fixed in a contracted position. Although CRPS usually follows an injury or surgery, sometimes no cause can be found. If you have unusual pain, discuss the possibility of CRPS with your doctor.