Post-herpetic neuralgia occurs in 10% of patients with shingles overall but more commonly (greater than 50%) in patients over 60. It resolves in 1 month in 50% of patients and within 1 year in 80%.
Unfortunately, it may become severe and intractable. Occasional patients become suicidal. Early treatment of the rash with antivirals decrease the duration of post- herpetic neuralgia. Famciclovir and valaciclovir are better for post herpetic neuralgia than aciclovir (median time to resolution of neuralgia 55 days in patients treated with famciclovir vs 128 days for those treated with aciclovir).
The risk factors for developing PHN are
i) Advancing age (>60)
ii) Severity of prodromal and acute zoster pain
iii) Marked rash severity
iv) Rash on head/face (esp ophthalmic) and ocular involvement
v) Clinical depression (secondary to zoster)
vi) Failure to treat or delayed treatment with famciclovir or valaciclovir
vii) Mechanical allodynia (touch) and pinprick hypesthesia
Treatment of postherpetic neuralgia
i) Amitriptyline – lower doses (eg 10 mg at night) that those used to treat depression are effective and relief can be obtained within 2 weeks. However, it is essential to start treatment early. If started between 3 and 12 months 66% of patients obtain relief. Started between 13 and 24 months 41% obtain relief. If started after 2 years, only 30% of patients obtain relief.
ii) Gabapentin – start at 600 mg per day and increase to 3600 mg/day. This is effective for allodynia (pain to touch for which amitriptyline is ineffective)
iii) Pregbalin 300 to 600 mg /day
iv) Regional nerve blocks with bupivacaine/methylprednisolone eg to supraorbital nerve
v) Topical lignocaine (EMLA) is good for allodynia and itch