Heterogenous patterns of sensory dysfunction in postherpetic neuralgia suggest multiple pathophysiologic mechanisms

Background Postherpetic neuralgia (PHN) is considered by some investigators to be predominantly a deafferentation-type central pain syndrome; others s uggest that activity of remaining peripheral nociceptors plays a critical r ole. The authors investigated the sensory dysfunction in subjects with PHN of varying duration and at different sites to gain further insight into the mechanisms responsible for the clinical features of neuropathic pain. in a ddition, the relationships between ongoing pain and pain evoked by mechanic al and thermal stimuli were compared in patients with trigeminal and trunca l PHN, to determine if the pathophysiologic mechanisms differed among subje cts. Methods: In 63 subjects with PHN, quantitative sensory testing was performe d in the region of maximum allodynia or ongoing pain and the corresponding contralateral site. The intensity of ongoing pain was recorded. Sensory thr esholds for warmth, coolness, heat pain, and cold pain were determined. Pai n, induced by various mechanical stimuli (dynamic, static, punctate) was ra ted using a numerical rating scale of 0-10. Results: The mean rating of ongoing PHN pain was 7.3 +/- 2.0 (mean +/- SD). Allodynia induced by one or more mechanical stimuli was observed in 78% of subjects. A smaller subset (40%) had hyperalgesia to heat or cold stimuli. In subjects with duration of PHN of less than or equal to 1 yr duration, b ut not in those with duration of >1 yr, the intensity of ongoing pain corre lated with intensity of allodynia induced by dynamic stimuli. Deficits in t hresholds for heat and cold pain were observed in the affected region of su bjects with PHN in the thoracic dermatomes (P < 0.005), but not in the trig eminal distribution. No relationship was observed between the thermal defic its and ongoing pain or mechanical allodynia in the groups of subjects with either trigeminal or thoracic PHN, Conclusion Despite a common cause, the patterns of sensory abnormalities di ffer between subjects. Particular differences were noted between groups wit h facial or truncal PHN and between groups with recent or more chronic PHN. The observations suggest that the relative contributions of peripheral and central mechanisms to the pathophysiology of pain differ among subjects an d may vary over the course of PHN.