Herpes zoster is caused by the varicella zoster virus which lies dormant in dorsal root ganglia and becomes active again during immunosuppression or as immunity declines with advancing age. Herpes zoster most commonly involves the thoracic and trigeminal dermatomes with the most common being the thoracic distribution , followed by the ophthalmic division of the trigeminal nerve. Postherpetic neuralgia is defined as pain in the affected dermatome persisting well beyond healing of the skin rash (4 weeks to 6 months). The incidence is considerably higher in older patients (16% in patients younger than 60 years old, 47% in patients older than 60 years). Pain can be spontaneous and constant, stabbing or shooting, aching, or dyesthetic provoked by light tactile stimulation (allodynia), which lasts longer than the duration of the stimulus (hyperpathia).
Risk Factors for Post-Herpetic Neuralgia
- Age: rare before age 40, near 50% in patients > 60 years ( incidence of herpes zoster is 1-2:1000 in general population, 5-10:1000 among older patients)
- Severe pain during acute phase: these patients are more likely to develop postherpetic neuralgia
- Sensory abnormalities during acute phase: most patients recover over several months but in 10-20% of patients the pain persists for more