Postherpetic neuralgia (PHN) is neuropathic pain that occurs due to damage to a peripheral nerve caused by the reactivation of the varicella zoster virus (herpes zoster, also known as shingles). Typically, the nerve pain (neuralgia) is confined to an area of skin innervated by a single sensory nerve, which is known as a dermatome. PHN is defined as dermatomal nerve pain that persists for more than 90 days after an outbreak of herpes zoster affecting the same dermatome. Several types of pain may occur with PHN including continuous burning pain, episodes of severe shooting or electric-like pain, and a heightened sensitivity to gentle touch which would not otherwise cause pain (mechanical allodynia) or to painful stimuli (hyperalgesia). Abnormal sensations and itching may also occur.The nerve pain of PHN is thought to result from damage in a peripheral nerve that was affected by the reactivation of the varicella zoster virus or troubles after chemotherapy. PHN typically begins when the herpes zoster vesicles have crusted over and begun to heal, but can begin in the absence of herpes zoster—a condition called zoster sine herpete.There is no treatment that modifies the disease course of PHN; therefore, controlling the affected persons symptoms is the main goal of treatment. Medications applied to the skin such as capsaicin or topical anesthetics (e.g., lidocaine) are used for mild pain and can be used in combination with oral medications for moderate to severe pain. Oral anticonvulsant medications such as gabapentin and pregabalin are also approved for treatment of PHN.
Lidocaine patches (5% concentration) are approved in the United States and Europe to treat PHN though evidence supporting their use is limited. A meta-analysis of multiple small placebo-controlled randomized controlled trials found that for every two people treated with topical lidocaine, one person experienced at least a 50% reduction in their PHN-associated pain (number needed to treat (NNT)=2).Low-dose capsaicin may be useful for reducing PHN-associated pain but is limited by side effects (redness and a burning or stinging sensation with application) and the need to apply it four times daily. Approximately three people must be treated with low-dose capsaicin cream for one person to experience significant pain relief (number needed to treat =3.3). A single topical application of a high-dose capsaicin patch over the affected area after numbing the area with a topical anesthetic has also been found to relieve PHN-associated pain. For every eleven people treated with a high-dose capsaicin patch for up to 12 weeks, one person experienced a significant improvement in their pain. (number needed to treat=11). Due to the need for topical anesthesia before application of the high-dose capsaicin patch, referral to a pain specialist is generally recommended if this approach is being considered. Oral medications
Multiple oral medications have demonstrated efficacy in relieving postherpetic neuralgia pain. Tricyclic antidepressants (TCAs), such as nortriptyline or desipramine, are effective in reducing postherpetic neuralgia pain but are limited by their numerous side effects. For every three people treated with a tricyclic antidepressant, one person is expected to have a clinically significant reduction in their pain (NNT=3).