What is post-herpetic neuralgia?

Post-herpetic neuralgia describes chronic skin pain in an area previously affected by herpes zoster (shingles).

Herpes zoster is a localized painful blistering rash caused by reactivation of herpes varicella-zoster virus sometime after the primary generalized infection with varicella (chickenpox).

How is post-herpetic neuralgia diagnosed?

Because post-herpetic neuralgia is defined as pain after and caused by, re-activation of the herpes zoster virus, evidence of herpes zoster infection is critical for diagnosis.

Usually, the diagnosis of herpes zoster is easily made when a painful blistering rash arises that has dermatomal distribution (ie, along the pathway of a cutaneous nerve).

However, not all patients with post-herpetic neuralgia have a history of rash or acute symptoms. In these cases, a rise in antibody levels on serial blood tests may confirm the previous infection by herpes zoster virus.

Post-herpetic neuralgia is often used to describe any pain that persists after herpes zoster blisters have cleared up. Some experts prefer to reserve the term only for pain that lasts for more than 28 days or more than 120 days. ‘Acute’ and ‘sub-acute’ herpetic neuralgia may be used to describe earlier pain associated with herpes zoster infection.

What are the clinical features of post-herpetic neuralgia?

Features of post-herpetic neuralgia include:

• Dermatomal distribution: pain limited to 1–3 dermatomes and usually confined to one side of the body. Rarely, pain can be disseminated or affect both sides of the body.
• Post-herpetic neuralgia is usually a chronic neuropathic pain with a burning character, but some patients experience sharp stabbing pains.
• Pain may be constant or intermittent.
• Other symptoms and sensory changes may be present in the same dermatome/nerve distribution as the pain:
  • Itch
  • Anaesthesia (numbness)
  • Hyperaesthesia (sensitivity to touch)
  • Allodynia (pain from a light touch)
  • Other sensory change such as reaction to temperature.
• Motor dysfunction (this is rare):
  • Autonomic dysfunction (eg sweating)
  • Organ damage (eg blindness if there is ophthalmic involvement)

Chronic pain can lead to insomnia, anxiety and depression.

What are the risk factors for the development of post-herpetic neuralgia?

The main risk factors for post-herpetic neuralgia are:

• Older age
  • Post-herpetic neuralgia is rare in people younger than 40
  • About 50% of herpes zoster patients aged 70 years or older will develop post-herpetic neuralgia
• Immunosuppression
  • Blood malignancies or human immunodeficiency virus infection (HIV)
  • Treatment with systemic steroids or chemotherapy
• Severe infection
  • Pain beginning before the blisters appear
  • Severe pain in the acute phase of infection
  • Severe and prolonged blistering, ulceration and necrosis

Some other possible risk factors include:

• Being female
• Having other sensory abnormalities
• Infection of the ophthalmic nerve (leading to eye infection)

What causes post-herpetic neuralgia?

Acute herpes zoster pain is due to direct damage of peripheral nerves by the herpes zoster virus. Ongoing post-herpetic neuralgia is due to slow recovery and the involvement of the central nervous system.

Research has shown that the nerves or neurones affected by post-herpetic neuralgia are damaged. Microscopic changes include:

• Atrophy and deafferentation (loss of connections) of dorsal horns of affected dermatomes
• Pathological changes in spinal cord sensory ganglions (nerve cell bodies)
• Markedly reduced number and density of sensory nerves in affected skin

It has been suggested that these damaged nerves send fewer signals from the skin to central nervous structures, which leads to neuron hyperexcitability and the constant perception of pain. However, the exact pathways are unknown.

It is likely that acute and sub-acute pain, and sensory changes such as allodynia, arise from slightly different mechanisms.

Information Source: DermNet NZ: https://dermnetnz.org/topics/post-herpetic-neuralgia/