Researchers at Massachusetts General Hospital (MGH) have found the first evidence of a physical abnormality underlying the chronic pain condition called reflex sympathetic dystrophy or complex regional pain syndrome-I (CRPS-I).
In the February issue of the journal Pain, they describe finding that skin affected by CRPS-I pain appears to have lost some small-fiber nerve endings, a change characteristic of other neuropathic pain syndromes.
"This sort of small-fiber degeneration has been found in every nerve pain condition ever studied, including postherpetic neuralgia and neuropathies associated with diabetes and HIV infection," says Anne Louise Oaklander, MD, PhD, director of the MGH Nerve Injury Unit, who led the study. "The nerve damage in those conditions has been much more severe, which may be why it's been so hard to detect CRPS-I-related nerve damage."
Complex regional pain syndrome is the current name for a baffling condition first described in the 19th century in which some patients are left with severe chronic pain and other symptoms - swelling, excess sweating, change in skin color and temperature - after what may be a fairly minor injury. The fact that patients' pain severity is out of proportion to the original injury is a hallmark of the syndrome, and has led many to doubt whether patients' symptoms are caused by physical damage or by a psychological disorder. Pain not associated with a known nerve injury has been called CRPS-I, while symptoms following damage to a major nerve has been called CRPS-II.
Because small-fiber nerve endings transmit pain messages and control skin color and temperature and because damage to those fibers is associated with other painful disorders, the MGH research team hypothesized that those fibers might also be involved with CRPS-I. To investigate their theory they studied 18 CRPS-I patients and 7 control patients with similar chronic symptoms known to be caused by arthritis. Small skin biopsies were taken under anesthesia from the most painful area, from a pain-free area on the same limb and from a corresponding unaffected area on the other side of the body.
The skin biopsies showed that, the density of small-fiber nerve endings in CRPS-I patients was reduced from 25 to 30 percent in the affected areas compared with unaffected areas. No nerve losses were seen in samples from the control participants, suggesting that the damage was specific to CRPS-I, not to pain in general. Tests of sensory function performed in the same areas found that a light touch or slight heat was more likely to be perceived as painful in the affected areas of CRPS-I patients than in the unaffected areas, also indicating abnormal neural function.