Post-herpetic neuralgia

Post-herpetic neuralgia (PHN) refers to the development of pain in a dermatomal distribution after a varicella zoster infection. Zoster infection reflects latent virus harbored in nerve cells, which reactivates due to a decrease in immunity, most commonly as patients age. Varicella zoster frequently causes pain, but only a subset of patients develop post herpetic neuralgia, or pain that persists beyond the initial zoster rash. PHN occurs in fewer than 10% of patients overall, but at higher rates in patients over 60, with rates ranging from 15-40%. Mortality is rare, but increases with age.

The definition of PHN is debated. Commonly, it is described as post herpetic pain that persists after 3 months. Prior to developing PHN, patients usually have acute herpetic pain (up to 1 month) and subacute pain (up to 3 months).

The nature of the pain can vary. Most commonly the discomfort follows essentially the same dermatomal distribution as the recent zoster rash, although there may be some extension. It can cause burning, as well as allodynia, or hypersensitivity to non-painful stimuli. The pain can also be associated with sensory deficits and local areas of anesthesia.

The pain is hypothesized to result from hemorrhage of the nerves that harbored the virus initially and were the site of the zoster outbreak, and subsequent fibrosis of the same nerve fibers. Biopsies of patients with PHN showed greater destruction of the central and peripheral nervous system, relative to other patients with zoster who did not develop PHN.

The most important issue related to PHN is prevention of zoster infection, as treatment of PHN is of marginal efficacy, with significant side effects and long term implications for quality of life and independence of elderly patients in particular. However, current data suggests that vaccination of patients between 50 and 59 is unlikely to be cost effective and should be reserved for older patients.

The diagnosis of PHN is clinical. Confirmatory tests can be performed at the time of an initial zoster infection, including DFA for the herpes virus. However, after the initial infection, diagnosis is based on the symptoms and recent history of herpes zoster.

A typical patient with PHN is an older patient, either male or female, with history of recent zoster infection, with a neuropathic pain syndrome in a dermatomal distribution, possibly with evidence of scarring from a recent rash. See Figure 1 and Figure 2 for a map of anterior and posterior dermatomes.

Figure 1.

Figure 2.

Key symptoms:

Neuropathic pain syndrome, including burning, anesthesia, and hypersensitivity/allodynia in distribution of prior herpes zoster infection, usually dermatomal.

Evidence of prior zoster, such as rash.

Dermatomal distribution, usually upper thoracic, cervical or trigeminal.

Patients at higher risk for development of PHN are older, with more severe pain or rash associated with the initial infection. PHN occurs in fewer than 10% of patients overall, but at higher rates in patients over 60, with rates ranging from 15-40%.

Risk factors:

Age >50

Prodromal pain

More severe initial rash

More severe initial pain

Ophthalmic involvement

Cancer, especially lymphoproliferative*

Immunocompromise (HIV, high dose steroid use)*

*These two groups are more likely to get herpes zoster, but not more likely to get PHN once they get zoster.

Radiculopathy – more radicular, more classic neuropathic pain syndrome.

Peripheral neuropathy, due to diabetes, B12 deficiency, hypothyroidism – more often peripheral neuropathy, less often on face or trunk.

Skin exam:

Scarred dermatomal rash

Neurologic exam:

Cranial nerves:

Decreased visual acuity for V1 distribution zoster

Sensory:

Allodynia in dermatomal area

Anesthesia to cold, light touch, vibration in dermatomal area, and at times beyond

Distribution usually thoracic, cervical or trigeminal

None. Diagnosis is made clinically.

None. Diagnosis is made clinically.

No radiologic imaging is necessary.

No need to check any virus antibodies or PCR.

Symptomatic treatment of PHN, with neuropathic pain medications as well as opioids.

Pain control:

Multiple agents have some efficacy in pain management, but pain control is difficult. Overall, fewer than half of PHN patients will have a 50% reduction in pain with treatment. A combination approach can be beneficial, however, the side effects can also be additive, particularly in the elderly population. Classes of medication include TCAs (amitriptyline, nortriptyline), anti-epileptics (gabapentin), opiates, both short and long acting, and topical agents. There is no role for antiviral medication after PHN has already developed.

The best evidence supports TCAs (amitriptyline, nortriptyline) and antiepileptic agents (gabapentin, pregabalin), in conjunction with topical lidocaine or capsaicin. There is only anecdotal data behind topical agents, such as lidocaine and capsaicin. The evidence for opiates is unconvincing.

Recent trials of invasive interventions, such as nerve blocks, have not been shown to be effective.

See Table I for treatment options for PHN.