Evidence of focal small-fiber axonal degeneration in complex regional pain syndrome-I (reflex sympathetic dystrophy)

Abstract

CRPS-I consists of post-traumatic limb pain and autonomic abnormalities that continue despite apparent healing of inciting injuries. The cause of symptoms is unknown and objective findings are few, making diagnosis and treatment controversial, and research difficult. We tested the hypotheses that CRPS-I is caused by persistent minimal distal nerve injury (MDNI), specifically distal degeneration of small-diameter axons. These subserve pain and autonomic function. We studied 18 adults with IASP-defined CRPS-I affecting their arms or legs. We studied three sites on subjects' CRPS-affected and matching contralateral limb; the CRPS-affected site, and nearby unaffected ipsilateral and matching contralateral control sites. We performed quantitative mechanical and thermal sensory testing (QST) followed by quantitation of epidermal neurite densities within PGP9.5-immunolabeled skin biopsies. Seven adults with chronic leg pain, edema, disuse, and prior surgeries from trauma or osteoarthritis provided symptom-matched controls. CRPS-I subjects had representative histories and symptoms. Medical procedures were unexpectedly frequently associated with CRPS onset. QST revealed mechanical allodynia (P<0.03) and heat-pain hyperalgesia (P<0.04) at the CRPS-affected site. Axonal densities were highly correlated between subjects' ipsilateral and contralateral control sites (r=0.97), but were diminished at the CRPS-affected sites of 17/18 subjects, on average by 29% (P<0.001). Overall, control subjects had no painful-site neurite reductions (P=1.00), suggesting that pain, disuse, or prior surgeries alone do not explain CRPS-associated neurite losses. These results support the hypothesis that CRPS-I is specifically associated with post-traumatic focal MDNI affecting nociceptive small-fibers. This type of nerve injury will remain undetected in most clinical settings.