Intrathecal clonidine and adenosine: effects on pain and sensory processing in patients with chronic regional pain syndrome.

Chronic pain may be accompanied by hyperalgesia and allodynia, and analgesic interventions may reduce these hypersensitivity phenomena. Preclinical data suggest that intrathecal clonidine and adenosine reduce hypersensitivity, but only clonidine reduces pain; therefore, we tested the effects of these interventions in patients with chronic pain. Twenty-two subjects with pain and hyperalgesia in a lower extremity from complex regional pain syndrome were recruited in a double-blind crossover study to receive intrathecal clonidine, 100 μg, or adenosine, 2 mg. Primary outcome measure was proportion with ≥30% reduction in pain 2 hours after injection, and secondary measures were pain report, areas of hypersensitivity, and temporal summation to heat stimuli. Treatments did not differ in the primary outcome measure (10 met success criterion after clonidine administration and 5 after adenosine administration), although they did differ in pain scores over time, with clonidine having a 3-fold greater effect (P = 0.014). Both drugs similarly reduced areas of hyperalgesia and allodynia by approximately 30% and also inhibited temporal summation. The percentage change in pain report did not correlate with the percentage change in areas of hyperalgesia (P = 0.09, r = 0.08) or allodynia (P = 0.24, r = 0.24) after drug treatment. Both intrathecal clonidine and adenosine acutely inhibit experimentally induced and clinical hypersensitivity in patients with chronic regional pain syndrome. Although these drugs do not differ in analgesia by the primary outcome measure, their difference in effect on pain scores over time and lack of correlation between effect on pain and hypersensitivity suggest that analgesia does not parallel antihyperalgesia with these treatments.