Postherpetic Neuralgia

Postherpetic neuralgia is neuropathic pain persisting for 3 months or more after healing of the herpes zoster rash in the affected dermatome. It is the commonest complication of shingles.

• Risk factors: age over 50 years, severe acute pain, severe rash, ophthalmic division involvement, immunosuppression, diabetes, delayed antiviral therapy.
• Mechanism is nerve and ganglion injury with peripheral and central sensitisation.

Burning, stabbing or shooting pain with allodynia (pain from light touch) and hyperalgesia in the dermatome, which may be thoracic, trigeminal or cervical.

• Sensory loss, scarring and pigment change over the area; sleep disturbance, depression and anxiety.
• Differential diagnosis: acute zoster pain, trigeminal neuralgia, radiculopathy, diabetic neuropathy, post surgical neuropathic pain, angina or pleurisy (thoracic), cluster headache, musculoskeletal pain, spinal tumour.
• Complications: chronic disabling pain, insomnia, depression, social withdrawal, weight loss, reduced mobility and self care, suicidal thoughts.

Clinical, from history of prior zoster in the same distribution with persistent pain.

• Sensory examination: mapping of allodynia and loss; pain scale and neuropathic pain screening.
• Glucose, vitamin B12, renal and liver function before drug therapy; HIV test and CD4 in recurrent disease.
• Imaging only when there are atypical features (motor weakness, no rash history).

Combine topical and systemic neuropathic agents with psychological support.

• Supportive: explain the course (improves over 6 to 12 months in most), sleep hygiene, loose clothing, cool compresses, TENS, cognitive behavioural therapy, treat depression, early specialist pain clinic review when pain is severe.
• Pharmacological: first line: gabapentin 300 mg at night, increasing by 300 mg every 1 to 3 days to 1800 to 3600 mg daily in three divided doses; or pregabalin 75 mg at night increasing to 150 mg twice daily (maximum 300 mg twice daily); or amitriptyline 10 mg at night increasing by 10 mg weekly to 75 mg (nortriptyline 10 to 25 mg at night for older adults). Topical: lidocaine 5 percent plaster, up to three plasters applied for 12 hours on and 12 hours off to intact skin; capsaicin 0.075 percent cream four times daily or capsaicin 8 percent patch for 30 to 60 minutes every 3 months. Second line: duloxetine 30 mg daily increasing to 60 mg daily; tramadol 50 mg twice daily increasing to 100 mg four times daily; morphine or oxycodone for severe refractory pain under specialist care. Reduce doses in renal impairment and older adults.
• Procedures: intercostal or epidural nerve block with local anaesthetic and steroid; spinal cord stimulation or intrathecal therapy for refractory pain.
• Special groups: pregnancy, avoid gabapentinoids and tricyclics when possible and use topical lidocaine; renal impairment, reduce gabapentin and pregabalin doses (creatinine clearance 30 to 59 mL/min, gabapentin 400 to 1400 mg daily); elderly, start low (gabapentin 100 mg at night, amitriptyline 10 mg), watch for falls and confusion; HIV, review interactions.
• Red flags: new motor weakness, rapidly progressive pain, suicidal thoughts, signs of spinal cord compression, severe opioid adverse effects.
• Follow-up: review at 2 to 4 weeks for response and adverse effects; target 30 percent pain reduction; continue 3 to 6 months then taper. Prevention: recombinant zoster vaccine, 2 doses 2 to 6 months apart, for adults aged 50 years and over and for immunocompromised adults.