Post herpetic neuralgia (PHN) is a sequelae of herpes zoster infection. Herpes zoster infection typically begins with flu like prodromal symptoms and progresses to a rash of grouped vesicles on an erythematous base as the reactivated virus replicates and spreads to dermoepidermal junction. Herpes zoster most often affects thoracic dermatomes in 50% of cases followed by the face in 10-20% of cases. Diagnosis of herpes zoster is made clinically with history and symptomatic dermatomal rash not crossing the midline. Risks factors for herpes zoster include older age and immunosuppression, greater severity of acute pain, presence of painful prodrome, greater rash severity. Treatment of herpes zoster rash with antiviral therapy has been shown to decrease acute and chronic pain and should be initiated within 72 hours of rash onset if possible. Herpes zoster treatment should also include effective multimodal pain therapy which may decrease the risk of PHN. Herpes zoster vaccination in adults is recommended in healthy adults 50 years and older with live attenuated vaccine or the new recombinant zoster, which is preferable.
Herpes zoster pain can precede or accompany the rash and mostly resolves as the rash resolves. When persistent pain results, it can be defined as post herpetic neuralgia if the pain that persists for more than 90 to 120 days after rash onset. Risk of PHN after herpes zoster varies from 5% to more than 30% worldwide depending on the study.
Treatment of postherpetic neuralgia is mostly pharmacological with a limited role of interventional and alternative modalities. Anticonvulsants, tricyclic antidepressants, serotonin norepinephrine reuptake inhibitors, opioids, and topical modalities are often used. Anticonvulsants are first line therapies and include gabapentin and pregabalin. Tricyclic antidepressants are also effective in PHN and act by inhibiting reuptake of norepinephrine and serotonin and sodium channel blockade. Opioids are effective, however, adverse effect profile must be considered including nausea, sedation, urinary retention, pruritis, constipation, immune suppression, hypogonadism, as well as tolerance, dependence and opioid induced hyperalgesia. Topical therapies including lidocaine and capsaicin are also attractive therapy in the older patients who are sensitive to side effects. Interventional therapies including nerve blocks, neuraxial blockade, dorsal root ganglion pulsed radiofrequency ablation, botulism injection, and sympathetic nerve blocks have been performed for PHN.