Post Herpetic Neuralgia

Stephen May

Introduction

"O’er ladies’ lips, who straight on kisses dream;Which oft the angry Mab with blisters plagues."

• Romeo and Juliet (W. Shakespeare)."Whether or not the unknown infectious material of chickenpox could under certain circumstances manifest itself instead as a zoster eruption."

• James von Bokay 1892.

Introduction

Smallpox and chickenpox different• 1904 Ernest Tyzzer

Zoster fluid could infect with varicella• 1925 Kundratitz

Weller et al in 1958 : same virus responsible for both

Hope-Simpson : zoster is due to varicella virus reactivation

Introduction

PHN is one of the most painful neuropathic conditions

Usually VZV reactivation

PHN : 1-6 mth post acute HZ

Risk increases with age

Usually a single unilateral dermatome

Thoracic and V1 most common

Pathophysiology

Acute infection : necrotising reaction in dorsal hornLarge myelinated fibres are most extensively damagedIncreased transfer at dorsal horn from smaller fibres (nociceptive info)Reduced innervation noted at affected skinChanges evident in CNS

PathophysiologyPHN – 2 types of pain

Steady, aching, boring pain

Paroxysmal lancing pain

Allodynia, hyperalgesia, hyperaesthesia often occur

Explained by 2 different mechanisms

PathophysiologySensitisation

• Acute injury -> ongoing discharge and hyperexcitability of nociceptor (peripheral sensitisation)

• Prolonged nociceptor discharge -> enhanced dorsal horn response to afferent neurones with expansion of receptive field (central sensitisation)

• Explains allodynia without marked sensory loss

Pathophysiology

Deafferentation• VZV reactivation in DRG -

>inflammation -> neural damage and oedema• Reduced endoneurial blood flow (↑P)• Neural destruction• Loss of afferent neurones -> spontaneous

activity in deafferenated central neurones• Constant pain in area of sensory loss and

minimal allodynia

Pathophysiology

Reactive sprouting of spinal terminals of AB mechanoreceptors to receptors formerly occupied by C-fibres results in hyperalgesia and allodynia

Pathophysiology

3 subtypes

1) Irritable nociceptor subtype

Damaged primary afferent nociceptors responsible for allodynia

Little deficit in sensory thresholds as sensory signalling still occurs

Pathophysiology

2) Deafferenated allodynic subtype

Primary afferents more extensively damaged

C fibres develop neuromata (spont discharge)-> dorsal horn sensitisationSelective loss of C fibres (touch preserved)Surviving AB fibres discharge normally-> light touch signalling pain

Pathophysiology

2) Deafferenated non-allodynic subtype

V. Extensive nerve damage

Skin insensitive to all modalities

Pain as a consequence of central sensitisation and neuronal reorganisation

Management : Prevention

Increasing evidence for vaccination

Acts to increase cell mediated immunity

2006 : UFDA have approved use

Amitriptyline has been suggested as prophylactic drug

Management : Acute HZ

Antivirals

Corticosteroids

Standard analgesia

? Epidural LA + steroid beneficial for pain

Possibly only short term effect and no clear evidence in reducing PHN incidence

Management : PHN

Mainstay of treatment is

Anticonvulsants

Antidepressants

Opiates

Increasing evidence for lidocaine patches as first line therapy

Management : PHN

TCA’s : NNT 2.3 for 50% pain relief30% minor SE’s, 4% more severe SENNH 22 (major), 3.7 (minor)SSRI better tolerated but less effective

GONIP trial (nortriptyline v gabapentin) : similar effects on pain and sleep, gabapentin better tolerated

Management : PHN

Only 2/3 taking carbamazepine or gabapentin for neuropathic pain get benefit

? Best for lancinating or burning pain

NNT 3.9, RR 2.5

Pregabalin may be tried where gabapentin not tolerated

? Lamotrigine – not licensed for PHN in Europe

Management : PHN

Lidocaine patch 5%

Reasonable first line choice and cost effective

NO SIGNIFICANT SYSTEMIC SE’S

Can get mild skin reaction

Shown to have central effects

Management : PHN

Opiates – partially effective, not 1st line

Capsaicin –NNT 5.3 for 0.075%• Will increase pain in irritable nociceptor

subgroup – contraindicated

TENS – to be effective : need sufficient innervation from normally conducting fast fibres

Management : PHN

Epidural LA + SteroidEvidence of effects on reducing pain

• General consensus is of little effect on PHN development? Intrathecal methyprednisoloneOccasional small scale evidence for peripheral or sympathetic blocks – no RCT

Management : Miscellaneous

Prostaglandin E2UVB in acute HZ? Myofascial component in resistant casesLA + steroid local infiltrationBotulinum toxin ALocal skin excision (irritable nociceptor subgroup)Intrathecal baclofenIv ATP

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