Postherpetic neuralgia: the never-ending challenge

Abstract

Postherpetic neuralgia (PHN) is defined as pain that persists 1 to 3 months following the rash of herpes zoster (HZ). PHN affects about 50% of patients over 60 years of age and 15% of all HZ patients. Patients with PHN may experience two types of pain: a steady, aching, boring pain and a paroxysmal lancinating pain, usually exacerbated by contact with the involved skin. Herpes zoster is initially a clinical diagnosis, based on the observation of a typical dermatomal distribution of rash and radicular pain. HZ is pathologically characterized by inflammatory necrosis of dorsal root ganglia, occasionally associated with evidence of neuritis, leptomeningitis, and segmental unilateral degeneration of related motor and sensory roots. Although acyclovir has been used successfully as standard therapy for varicella zoster virus (VZV) infection in the past decade, resistant strains of VZV are often recognized in immunocompromised patients. Therapy with acyclovir and the use of corticosteroids have been reported to prevent PHN in up to 60% of HZ patients. Management of chronic pain in PHN is more problematic. The only therapy proven effective for PHN in controlled study is the use of tricyclic antidepressants, including amitriptyline and desipramine. There is good evidence of efficacy from randomized trials that gabapentin and pregabalin (new anticonvulsant drugs) are of benefit in the reduction of pain from PHN. As alternative therapies, topical agents such as capsaicin, lidocaine or opioid analgesic treatment may give satisfactory results. Interventions with low risk, such as transcutaneous electrical nerve stimulation (TENS), are appropriate. Evidence is scant for the value of surgical and procedural interventions in general, although there are numerous, small studies supporting the use of specific interventions such as nerve blocks, neurosurgical procedures, and neuroaugmentation. Although antiviral agents are appropriate for acute HZ, and the use of neural blockade and sympathetic blockade may be helpful in reducing pain in selected patients with HZ, there is little evidence that these interventions will reduce the likelihood of developing PHN. Postherpetic neuralgia remains a difficult pain problem. This review describes the epidemiology and pathophysiology of PHN and discusses proposed mechanisms of pain generation with emphasis on the various pharmacological treatments and invasive modalities currently available.