CRPS is a chronic pain disorder characterized by pain, swelling and trophic change in the skin overlying the affected area. CRPS may be seen following trauma, stroke, MI, musculoskeletal disorder, or malignancy, or it may be idiopathic.

There are two types: CRPS Type I and Type II as defined by International Association for the Study of Pain (IASP). Diagnostic criteria (described by Merskey, 1994) are as follows:

CRPS Type I (reflex sympathetic dystrophy or RSD): In CRPS I, an initiating noxious event or immobilization is often present. There is continuing pain, allodynia, or hyperalgesia with the pain being disproportionate to the inciting event. There is often evidence of edema, changes in skin blood flow, or abnormal sudomotor (sweat gland) activity in the painful region at some point in the disease progression. Generally, there is an absence of other conditions that would explain the degree of pain and dysfunction.

CRPS Type II (causalgia) has same criteria as CRPS Type I, except the pain occurs after a definable nerve injury. There are no specific laboratory tests to diagnose CRPS. EMG and nerve conduction studies are typically normal. Imaging studies may show asymmetric skin temperatures (difference >0.6° C) on thermography. X-rays may demonstrate trophic changes such as patchy demineralization. The triple-phase bone scan is considered the most sensitive and specific, especially in the early stages. Sudomotor function test (e.g., sweat test), laser doppler imaging, and a response to diagnostic sympathetic ganglion block are other tests, with the caveat that a diagnostic sympathetic block is not pathognomonic for CRPS, and not all CRPS may respond to sympathetic ganglion block (Singh, 2008).