ENTRAPMENT NEUROPATHY - DNB Orthopaedics MS Orthopedics MRCS Exam GUIDE - Orthodnb.com

DNB Orthopaedics  MS Orthopedics  MRCS Exam GUIDE - Orthodnb.com

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Saturday, 9 June 2018

ENTRAPMENT NEUROPATHY

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
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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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