Electro lecture 6

• Peripheral Nerves Injuries


  Injuries can affect the myelin, the axons, or both
• Rarely is the myelin involved without at least some involvement of the axon (and vice versa)
• Electrodiagnostic testing


  Can determine what type of nerve injury exists, how severe the injury is, and where the injury is located
• Classification of nerve injuries

  Useful for understanding their pathological basis, making decisions on management, and predicting the prognosis for recovery
• Seddon's classification of nerve injuries

  • Neurapraxia
  • Axonotmesis
  • Neurotmesis
• Neurapraxia


  Conduction block - only the myelin is affected
• Axonotmesis


  Anatomical disruption of the axons and their myelin sheaths, but the connective tissue layers of the nerve including the endoneurial tubes remain intact
• Neurotmesis


  Complete nerve injury involving the myelin, axon, and all supporting structures
• Clinical examination and neurophysiology assessment may be the same for axonotmesis and neurotmesis, yet there is a clear difference in prognosis and management
• Sunderland's 5 degrees of nerve injury

  Based on increasing anatomical disruption of the nerve trunk and distinguishes injuries where the nerve trunk is in continuity but there is damage to endoneurium and perineurium
• Strength-Duration (S-D) Curve


  Relationship between the strength of a stimulus and its duration for producing minimal excitation
• Constant-current stimulator


  Current output set by the therapist will remain the same, voltage output would change with change in tissue impedance
• Constant-voltage stimulator


  Voltage output set by the therapist will remain the same, there will be change in the current with change in tissue impedance
• Equipment required for S-D Curve

  • Low frequency generator
  • Moist saline pad
  • Electrodes
  • Leads
  • Bandage
  • Plastic protectors
• 1. D Curve test application

  1. Apply longest stimulation first and increase until minimal contraction

  2. Shorten impulse and increase intensity as required

  3. Use minimal contraction to detect changes
• Normal S-D Curve


  Impulses of longer duration produce response with same strength, shorter durations require increased strength
• Complete denervation S-D Curve

  Steep rise and shift to the right, no response to very short impulses
• Partial denervation S-D Curve

  Kink where innervated and denervated fibers respond differently, indicates proportion of denervation
• Rheobase


  Lowest intensity with maximum pulse duration (100ms+) that just stimulates muscles or nerves
• Chronaxie


  Minimum time for which a current of intensity twice rheobase will cause contraction
• Utilization time


  Time taken by a stimulus of rheobasic strength to excite the nerve and produce a minimal muscle contraction
• Factors affecting Chronaxie


  • Texture of skin
  • Ischemia
  • Edema
  • Fatigue
  • Position of stimulating electrode
  • Denervation
  • Partial denervation
  • Re-innervation
  • Nerve root lesion
  • Peripheral neuropathy
  • Myopathy
• Factors affecting Rheobase


  • Resistance of skin and subcutaneous tissue
  • Edema and inflammation
  • Ischemia and underlying pain
  • Temperature variation
  • Position of electrode
  • Amount of subcutaneous tissue
  • Degeneration
  • Denervation
  • Partial denervation
  • Re-innervation