CAPSAICIN-EVOKED MECHANICAL ALLODYNIA AND HYPERALGESIA CROSS NERVE TERRITORIES - EVIDENCE FOR A CENTRAL MECHANISM

Background: The finding in some patients with neuropathic pain that me chanical allodynia (pain evoked by light touch) and hyperalgesia (supr anormal pain evoked by painful stimuli) extend beyond the territory of a single nerve or spinal sensory root (extraterritorial pain) often p rompts a diagnosis of psychiatric illness. The hypothesis that focal n ociceptive input in a single nerve territory can result in allodynia a nd hyperalgesia in a nerve territory adjacent to the input was investi gated in normal human subjects. Methods: On separate days, 13 healthy volunteers each received left radial and ulnar nerve blocks. After blo ck of either nerve, sensation remaining for three classes of afferents (A beta low-threshold mechanoreceptors, A delta nociceptors, and C po lymodal nociceptors) allowed inference of the nerve territory of the a djacent nerve, and the area of overlapping innervation. On a third day , 1,000 mu g intradermal capsaicin was administered into a site such t hat C-nociceptor input was confined to the ulnar nerve territory. Area s of brush allodynia and pinprick hyperalgesia were determined. Result s: Spread of brush allodynia beyond all three borders of the ulnar ner ve territory occurred in 9 of 13 patients (for these subjects, range 5 -28 mm), whereas spread of pinprick hyperalgesia beyond all borders of the ulnar nerve territory occurred in 12 of 13 subjects (range 1-31 m m). Spread of brush allodynia beyond the A beta border of the ulnar ne rve territory occurred in 10 of 13 subjects (range 4-35 mm); and sprea d of pinprick hyperalgesia beyond the A delta border of the ulnar nerv e territory occurred in 12 of 13 subjects (range 1-31 mm). Conclusions : It is concluded that activation of C-nociceptors evokes a state of c entral sensitization that may manifest itself by the appearance of ext raterritorial pain abnormalities.