Treatment of Postherpetic Neuralgia

Treatment of Postherpetic Neuralgia

Analgesics
Capsaicin, an extract from hot chili peppers, is currently the only drug labeled by the U.S. Food and Drug Administration for the treatment of postherpetic neuralgia.19 Trials have shown this drug to be more efficacious than placebo but not necessarily more so than other conventional treatments.20

Substance P, a neuropeptide released from pain fibers in response to trauma, is also released when capsaicin is applied to the skin, producing a burning sensation. Analgesia occurs when substance P is depleted from the nerve fibers. To achieve this response, capsaicin cream must be applied to the affected area three to five times daily. Patients must be counseled about the need to apply capsaicin regularly for continued benefit. They also need to be counseled that their pain will likely increase during the first few days to a week after capsaicin therapy is initiated. Patients should wash their hands thoroughly after applying capsaicin cream in order to prevent inadvertent contact with other areas.

Patches containing lidocaine have also been used to treat postherpetic neuralgia. One study found that compared with no treatment, lidocaine patches reduced pain intensity, with minimal systemic absorption. Although lidocaine was efficacious in relieving pain, the effect was temporary, lasting only four to 12 hours with each application.21

Over-the-counter analgesics such as acetaminophen (e.g., Tylenol) and nonsteroidal anti-inflammatory drugs have not been shown to be highly effective in the treatment of postherpetic neuralgia. However, these agents are often useful for potentiating the pain-relieving effects of narcotics in patients with severe pain. Because of the addictive properties of narcotics, their chronic use is discouraged except in the rare patient who does not adequately respond to other modalities.
Tricyclic antidepressants or anticonvulsant medications given in low dosages can be effective adjuncts in controlling the pain of postherpetic neuralgia.

Tricyclic Antidepressants
Tricyclic antidepressants can be effective adjuncts in reducing the neuropathic pain of postherpetic neuralgia. These agents most likely lessen pain by inhibiting the reuptake of serotonin and norepinephrine neurotransmitters.22

Tricyclic antidepressants commonly used in the treatment of postherpetic neuralgia include amitriptyline (Elavil), nortriptyline (Pamelor), imipramine (Tofranil) and desipramine (Norpramin). These drugs are best tolerated when they are started in a low dosage and given at bedtime. The dosage is increased every two to four weeks to achieve an effective dose.

The tricyclic antidepressants share common side effects, such as sedation, dry mouth, postural hypotension, blurred vision and urinary retention. Nortriptyline and amitriptyline appear to have equal efficacy; however, nortriptyline tends to produce fewer anticholinergic effects and is therefore better tolerated. Treatment with tricyclic antidepressants can occasionally lead to cardiac conduction abnormalities or liver toxicity. The potential for these problems should be considered in elderly patients and patients with cardiac or liver disease.

Because tricyclic antidepressants do not act quickly, a clinical trial of at least three months is required to judge a patient's response. The onset of pain relief using tricyclic antidepressants may be enhanced by beginning treatment early in the course of herpes zoster infection in conjunction with antiviral medications.20

Anticonvulsants
Phenytoin (Dilantin), carbamazepine (Tegretol) and gabapentin (Neurontin) are often used to control neuropathic pain. A recent double-blind, placebo-controlled study showed gabapentin to be effective in treating the pain of postherpetic neuralgia, as well as the often associated sleep disturbance.23

The anticonvulsants appear to be equally effective, and drug selection often involves trial and error. Lack of response to one of these medications does not necessarily portend a poor response to another. The dosages required for analgesia are often lower than those used in the treatment of epilepsy.

Anticonvulsants are associated with a variety of side effects, including sedation, memory disturbances, electrolyte abnormalities, liver toxicity and thrombocytopenia. Side effects may be reduced or eliminated by initiating treatment in a low dosage, which can then be slowly titrated upward.

There are no specific contraindications to using anticonvulsants in combination with antidepressants or analgesics. However, the risk of side effects increases when multiple medications are used.

Effective treatment of postherpetic neuralgia often requires multiple treatment approaches. In addition to medications, modalities to consider include transcutaneous electric nerve stimulation (TENS), biofeedback and nerve blocks.

Corticosteroids
Orally administered corticosteroids are commonly used in the treatment of herpes zoster, even though clinical trials have shown variable results. Prednisone used in conjunction with acyclovir has been shown to reduce the pain associated with herpes zoster.15 The likely mechanism involves decreasing the degree of neuritis caused by active infection and, possibly, decreasing residual damage to affected nerves.

Some studies designed to evaluate the effectiveness of prednisone therapy in preventing postherpetic neuralgia have shown decreased pain at three and 12 months.16,17 Other studies have demonstrated no benefit.15,18

If the use of orally administered prednisone is not contraindicated, adjunctive treatment with this agent is justified on the basis of its effects in reducing pain, despite questionable evidence for its benefits in decreasing the incidence of postherpetic neuralgia. Given the theoretic risk of immunosuppression with corticosteroids, some investigators believe that these agents should be used only in patients more than 50 years of age because they are at greater risk of developing postherpetic neuralgia.15 The recommended dosage for prednisone is given in Table 1.

Analgesics
The pain associated with herpes zoster ranges from mild to excruciating. Patients with mild to moderate pain may respond to over-the-counter analgesics. Patients with more severe pain may require the addition of a narcotic medication. When analgesics are used, with or without a narcotic, a regular dosing schedule results in better pain control and less anxiety than "as-needed" dosing.

Lotions containing calamine (e.g., Caladryl) may be used on open lesions to reduce pain and pruritus. Once the lesions have crusted over, capsaicin cream (Zostrix) may be applied. Topically administered lidocaine (Xylocaine) and nerve blocks have also been reported to be effective in reducing pain.