ACUTE HERPES ZOSTER & POSTHERPETIC NEURALGIA
During an initial childhood infection (chickenpox), the varicella-zoster virus (VZV) infects dorsal root ganglia, where it remains latent until reactivation. Acute herpes zoster, which represents VZV reac-tivation, manifests as an erythematous vesicular rash in a dermatomal distribution that is usually associated with severe pain. Dermatomes T3–L3 are most commonly affected. The pain often pre-cedes the rash by 48–72 h, and the rash usually lasts 1–2 weeks. Herpes zoster is most common in elderly and immunocompromised patients but may occur at any age. It is typically a self-limited disorder in younger, healthy patients (<50 years old). Treat-ment is primarily supportive, consisting of oral analgesics and oral acyclovir, famciclovir, ganciclo-vir, or valacyclovir. Antiviral therapy reduces the duration of the rash and speeds healing. Immuno-compromised patients with disseminated infection (nondermatomal distribution of vesicles) require intravenous acyclovir therapy. Epidural steroid injections have not been proven to prevent posther-petic neuralgia (PHN).
Older patients may continue to experience severe, radicular pain from PHN even after the rash resolves. The incidence of PHN following acute her-pes zoster is estimated to be 50% in patients older than 50 years of age. Moreover, PHN is often very difficult to treat. An oral course of corticosteroids during acute zoster may decrease the incidence ofPHN but remains controversial and may increase the likelihood of viral dissemination in immuno-compromised patients. Sympathetic blocks per-formed during the acute episode of herpes zoster often produce excellent analgesia and may decrease the incidence of PHN, although this is controversial. Some studies suggest that when sympathetic blocks are initiated within 2 months of the rash, PHN resolves in up to 80% of patients. Once the neuralgia is well established, however, sympathetic blocks, like other treatments, are generally ineffective. Antide-pressants, anticonvulsants, opioids, and TENS may be useful in some patients. Tricyclic antidepressants may be particularly effective, though their use is often limited by anticholinergic side effects. Appli-cation of a transdermal lidocaine 5% patch (Lido-derm) over the most painful area may help relieve symptoms, presumably by decreasing peripheral sensitization of nerve endings and receptors. Appli-cation of capsaicin cream or a transdermal capsa-icin 8% patch (Qutenza) may be helpful; however, Qutenza must be administered in a monitored set-ting. Administration of EMLA ( euctectic mixture of local anesthetic) cream 1 h before application of thetransdermal capsaicin patch may decrease the inci-dence and severity of pain from the capsaicin in the patch.