ENTRAPMENT NEUROPATHY

Introduction

• nerve in a space of limited compliance vulnerable to compression
• carpal, cubital, ulnar tunnels, deep to fibrous bands and tendinous arches
• increase in volume of a material in this space – increase pressure – compress nerve- mechanical peripheral neuropathy

Pathophysiology

• effects on structure and function
• severity depends on magnitude and duration of compression
• myelin damage – blood flow alterations - ischemic changes
• elevation of pressure in nerve - <40mmHg- sensory nerve function changes – reversible with restoration of blood flow – motor dysfunction at higher pressure and sustained elevation
• chronic compression - intraneural scarring, edema – due to prolonged ischemia
• structural changes – alterations or loss of myelin coatings of nerve fibers – in c/c high pressure compressions esp involving edges – resolve only in some time
• also inhibit axoplasmic flow – both antegrade and retrograde – diminishing the nerve function – contributing to bulging appearance of nerve proximal and distal to site of compression.

Clinical evaluation

• Sensory threshold testing
  • how well a single nerve fiber innervating a receptor or group of receptor cell is functioning
    • Vibrometry
    • Semmeasa Weinstein monofilaments
      • pressure to finger tip with filament until filament bends
      • pressure required to bend directly related to its diameter
      • filaments of successively increasing diameter to determine the sensory threshold of slowly adapting nerve fibers
    • Vibration testing
      • with 256 Hz tuning fork – to evaluate the sensory threshold of quickly adapting nerve fibers
    • Innervation density tests
      • two point and moving two- point discrimination
      • measure multiple overlapping peripheral receptor fields and the density of innervation ie. How many nerve fibers are present and correctly represented in cortex
      • compression neuropathy – nerve fibers are not lost – but not functioning well
      • nerve laceration – nerve fiber lost
      • useful in evaluating nerve laceration and recovery after repair than for evaluating compression neuropathy
• Provocative testing
  • tests compress, stretch, percuss the nerve to elicit numbness and paraesthesia in its sensory distribution
  • useful in exertional compression neuropathy
• Evaluation of muscle weakness or atrophy
  • muscles innervated by nerve is tested for bulk and strength

Electrodiagnostic studies

• only objective test of nerve
• NCV
  • sensory nerve action potential and composite motor action potential are wave form of NCV
  • measure latency and conduction velocity of sensory and motor nerve fibers
• EMG
  • activity of a single motor unit consisting of nerve cell, its fibers and the muscle group it innervates
  • positive sharp waves and fibrillation potentials – indicate recent muscle denervation
  • small highly polyphasic MUPs and decreased fibrillations – early re innervation of muscle
  • MUPs of great duration and amplitude - c/c denervation with collateral re innervation resulting form adjacent nerve sprouting

Laboratory investigations

• r/o hypothyroidism with myxedema, obesity, cervical radiculopathy, DM, alcoholism, exposure to neurotoxic chemicals

Principles of treatment

• splinting, medications, physiotherapy, corticosteroid injections, correction of metabolic abnormalities
• splinting at night – avoid positions that are harmful to nerve function
• NSAIDS, diuretics
• steroid injections after failure of non operative treatment
• indications of surgery
  • failure of non surgical management
  • acute rapidly progressive involvement
  • severe cases
  • symptom recurrence
• procedures
  • decompression
  • nerve transposition or flap coverage – if nerve bed is scarified

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