Treatment of Postherpetic Neuralgia
Postherpetic neuralgia (PHN) is a serious complication of herpes zoster that has a predilection for older individuals. PHN is often associated with significant morbidity, and it can cause insomnia, fatigue, depression and interference with daily activities in affected individuals. Treatment for PHN is initiated with anitvirals during the acute herpes zoster outbreak. Acyclovir (Zoviraxr, GlaxoSmithKline), valacyclovir (Valtrex, GlaxoSmithKline) or famciclovir (Famvir, Novartis) can be used to treat herpes zoster, and all three have been shown to reduce the duration of the herpetic rash and zoster-associated pain. These antivirals are most effective when used within the first 72 hours of the onset of the rash. Side-effects of these antivirals are low and include nausea, vomiting, abdominal pain and headache. Other treatment options for PHN include topical analgesics, opioid analgesics, tricyclic antidepressants and gabapentin. Because of the complexity of PHN, most patients require a combination of treatment modalities for adequate pain relief.

Herpes zoster is initially characterized by a prodromal phase that is associated with pain and paresthesia in the affected dermatome. Hours to days later, a papular rash appears and progresses to vesicles, then pustules and finally crusts and heals 3-4 weeks later. In some patients, the pain persists weeks to months, or years after the rash has healed, hence the term postherpetic neuralgia (PHN). Studies have demonstrated that there are three phases of PHN: acute, subacute, and chronic. The acute phase occurs with the onset of the rash and lasts for approximately 30 days, the subacute phase lasts 1-3 months after the onset of the rash, and the chronic phase, or PHN, lasts 3 months or longer after the onset of the rash.

Risk factors for PHN include prodromal symptoms and severity of pain at the onset of the rash. The most significant risk factor for the development of PHN is age, as the incidence of PHN increases with age. While studies have demonstrated the overall incidence ranges from 10% to 27%, the incidence for individuals over the age of 50 is 40%, and 75% for those over the age of 75.

The persistent pain associated with herpes zoster is variable in nature, and can be characterized as any of the following: (1) burning background pain with fluctuating severity; (2) sudden, sharp shooting pain; (3) mechanical or thermal allodynia (pain produced by non-noxious stimulus). As a result of this severe, often debilitating pain, a patient's quality of life is often adversely affected. In addition to interfering with activities of daily living, PHN may lead to fatigue, insomnia, anxiety, and depression.11 Because of the severity and complexity of the disease, treatment is initiated at the onset of the rash and may be necessary months to years later.