Postherpetic neuralgia: risk factors

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 (PHN) 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 PHN

• Age: rare before age 40, near 50% in patients older than 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

• Greater extent of the initial rash and the presence of a prodrome dermatomal sensation during the initial infection.

These risk factors may increase the risk of developing PHN by as much as 75%. Using antiviral agents to treat the initial infection was shown to significantly decrease the incidence and severity of PHN. Furthermore, administration of a zoster vaccine significantly decreases the incidence of both acute infection and PHN in adults older than 60 years.