Intrathecal, but not intravenous, clonidine reduces experimental thermal or capsaicin-induced pain and hyperalgesia in normal volunteers

Clonidine is approved for intraspinal administration in the treatment of neuropathic cancer pain. Some studies have suggested an analgesic effect after systemic clonidine administration. The purpose of this study was to compare the analgesic effects of intrathecal and TV clonidine with acute noxious stimulation and with hyperalgesia from intradermal capsaicin injection in volunteers. Sixteen healthy volunteers received intradermal injections of capsaicin (100 mu g) before and after the IV or intrathecal injection of clonidine 50 or 150 mu g in a randomized, double-blind manner. Pain and areas of mechanical hyperalgesia and allodynia were determined at specified intervals. In addition, pain to noxious heat stimulation was determined. The capsaicin injection produced pain, followed by hyperalgesia and allodynia. The intrathecal, but not TV, injection of 150 mu g of clonidine reduced capsaicin-induced pain and area of hyperalgesia. Intrathecal clonidine (150 mu g) reduced pain to heat stimulation, whereas TV clonidine did not. The groups did not differ in hemodynamic or sedative effects from clonidine. These data support the value of intraspinal administration of clonidine for the treatment of acute pain and of pain states associated with hyperalgesia. Similarly, they suggest that analgesia from the systemic administration of this ct, adrenergic agonist, if any, is weak in doses that produce sedation and reduce blood pressure. Implications: To the extent that the experimental pain conditions used in this study reflect those in patients with acute and chronic pain, these data support the spinal rather than IV injection of clonidine for analgesia.