Alpha-adrenergic supersensitivity of the sudomotor nerve in complex regional pain syndrome.

alpha-Adrenoreceptor supersensitivity in many tissues has been described in patients with complex regional pain syndrome type I (CRPS I). Because excessive sweating of the affected limb is an important feature of CRPS I, we investigated whether this supersensitivity also occurs in the sudomotor system. We compared the sweat response to iontophoresis of an alpha-adrenergic agent (phenylephrine) in 4 patients with acute CRPS I and 3 patients with resolved CRPS I with that in 9 control subjects using the methodology of the quantitative sudomotor axon reflex test (QSART). A significantly higher sweat response was observed in the affected limb of patients with acute CRPS I compared to their unaffected limb (p = 0.03), to control subjects (p > 0.018), and to the affected or unaffected limbs of patients with resolved CRPS I (p = 0.02), whose sweat response was not significantly different from that of control subjects. We conclude that the abnormal response in patients with acute CRPS I is most likely mediated by an axon reflex and that alpha-adrenoreceptor supersensitivity occurs in the presynaptic portion of the postganglionic sudomotor axon. This supersensitivity is reversed when CRPS I resolves.