Each year, approximately 1 million individuals in the United States develop shingles, or herpes zoster. Approximately 20% of these shingles patients, or 200,000 individuals, go on to suffer from the painful neuropathic condition, postherpetic neuralgia (PHN). Caused by reactivated varicella virus that remains “dormant” after the chicken pox, PHN is a painful condition that occurs in a dermatomal distribution (the area governed by a particular sensory nerve) after an attack of herpes zoster, usually manifesting after the vesicles of the rash have crusted over and begun to heal. For unknown reasons, in some patients the pain does not resolve when the rash heals, but may continue for months or years afterward. PHN is most prevalent in patients over the age of 50; approximately 75% of those who experience severe pain are over 70 years old.

PHN is thought to result after nerve fibers are damaged during a case of herpes zoster. These damaged fibers cannot transmit electrical signals from the skin to the brain as they normally do, and these signals may be erratic or even exaggerated, causing chronic, often excruciating pain, that may persist or recur for months or years in the area where shingles first occurred.^1,2 Most patients with PHN typically are hypersensitive to light mechanical stimulation which causes severe pain even from gentle touch or pressure (allodynia). Other patients with PHN may have severe, spontaneous pain without allodynia. With the forehead and thoracic region being most commonly affected, the pain associated with PHN is thought to represent a major health threat to the older population, and can often be disabling.

A recent major advance in the treatment and prevention of PHN is the development of the herpes zoster vaccine. It has the ability to reduce the likelihood of developing herpes zoster by approximately 50% in immuno-competent individuals who are 60 years of age and older, and who have had the chicken pox in the past.³ For patients who have not had the chicken pox, vaccination against it is thought to be an appropriate choice. Examples of inappropriate candidates for vaccination would be patients who:

• Have ever had a life-threatening allergic reaction to gelatin, the antibiotic neomycin, or any other component of the shingles vaccine
• Have a weakened immune system from HIV/AIDS or another disease that is immuno-compromised
• Are receiving medical treatments such as steroids, radiation and chemotherapy
• Have a history of bone marrow or lymphatic cancer
• Have active or untreated tuberculosis

Antiviral agents have been shown to decrease the severity of an attack of PHN, and are considered valuable components of treatment. Treatment of PHN pain, like other types of neuropathic pain, can sometimes be resistant to many types of pharmacologic and interventional therapies. However, recent data provides evidence for the analgesic efficacy of a number of pharmacologic therapies and systemic and topical therapies.⁴

Topical therapies:

• Lidocaine patch 5%
  • The available clinical trials in PHN patients with allodynia suggest that the lidocaine patch is effective in providing pain relief from PHN with minimal systemic absorption and few side effects

• Capsaicin
  • Has been shown to be effective, but can be associated with local irritation and an unpleasant burning sensation that may decrease with repeated applications

Systemic treatments:

• Tricyclic antidepressants, such as nortriptyline, desipramine and amitriptyline
  • Are used widely for the treatment of numerous neuropathic pain states, including PHN, have a number of presumptive mechanisms of action, including blocking of norepinephrine and serotonin reuptake and local anesthetic-like sodium channel blockade

  • A limiting factor in the clinical use of tricyclics is their side effect profile, including dry mouth, fatigue, dizziness, sedation, constipation, urinary retention, and palpitations, weight gain, etc.

• Antiepileptics, such as gabapentin and pregabalin
  • Are also used for the treatment of many types of neuropathic pain, including PHN

  • Generally they are well tolerated; the most frequently reported side effects are somnolence, dizziness, peripheral edema, fatigue, headache, and dry mouth

• Tramadol
  • Has been shown to have efficacy in diabetic peripheral neuropathy and other polyneuropathies

  • Tramadol has a side effect profile similar to that of other µ-receptor agonists, including nausea, vomiting, dizziness, itching, somnolence, etc.

• Opioid analgesics
  • The role of opioids in the management of neuropathic pain such as PHN has been intensely debated, and should be handled on a case by case basis, taking into account all considerations for appropriate patient selection and side effects

Interventional techniques such as sympathetic nerve blocks, epidural blocks, and spinal cord stimulation have been investigated as possible treatments. There is little scientific basis for recommendations about the efficacy of other treatments, such as other topical agents or complementary or alternative techniques. However psychological interventions are likely to be of value in any situation that affects the quality of life in patients with chronic pain.

Research is currently underway exploring the promise of other therapies, such as NMDA receptor antagonists, valproic acid, topical non-steroidal anti-inflammatory agents, and ketamine. Current evidence-based recommendations generally consider:

• Tricyclic antidepressants, antidepressants, and topical lidocaine 5% patch as first-line analgesic treatments for PHN
• Tramadol or opioids are recommended as second-line therapy
• Topical capsaicin and possibly valproate as third-line therapy

References:

1. Dworkin RH, Schmader KE. Essentials of Pain Medicine and Regional Anesthesia (Second Edition), 2005. Pages 386-393
2. Dworkin RH. Recent advances in reducing the burden of herpes zoster and postherpetic neuralgia. Journal of Pain. 2008 Jan;9(1):S19-S30.
3. Gnann JW. Vaccination to prevent herpes zoster in older adults. Journal of Pain. 2008;9:S31-S36.
4. Wu C, Raja S. An update on the treatment of postherpetic neuralgia. Journal of Pain. 2008 Jan;9(1):S19-S30.