POSTHERPETIC NEURALGIA - IRRITABLE NOCICEPTORS AND DEAFFERENTATION

Postherpetic neuralgia (PHN) is a common and often devastatingly painf ul condition. It is also one of the most extensively investigated of t he neuropathic pains. Patients with PHN have been studied using quanti tative testing of primary afferent function, skin biopsies, and contro lled treatment trials. Together with insights drawn from an extensive and growing literature on experimental models of neuropathic pain thes e patient studies have provided a preliminary glimpse of the pain-gene rating mechanisms in PHN. It is clear that both peripheral and central pathophysiological mechanisms contribute to PHN pain. Some PHN patien ts have abnormal sensitization of unmyelinated cutaneous nociceptors ( irritable nociceptors). Such patients characteristically have minimal sensory loss. Other patients have pain associated with small fiber dea fferentation. In such patients pain and temperature sensation are prof oundly impaired but light moving mechanical stimuli can often produce severe pain (allodynia). In these patients, allodynia may be due to th e formation of new connections between nonnociceptive large diameter p rimary afferents and central pain transmission neurons. Other deaffere ntation patients have severe spontaneous pain without hyperalgesia or allodynia and presumably have lost both large and small diameter fiber s. In this group the pain is likely due to increased spontaneous activ ity in deafferented central neurons and/or reorganization of central c onnections. These three types of mechanism may coexist in individual p atients and each offers the possibility for developing new therapeutic interventions. (C) 1998 Academic Press.