Evoked Potentials (EP) by za187cY


									       Evoked Potentials (EP)
• Sensory EP is a change in EEG resulting
  from stimulation of a sensory pathway
• Sensory EP is extracted from EEG using
  computer averaging techniques
• EEG is recorded during repetitive natural
  stimulation (eg. tap on skin or flash of light)
• Computers samples the EEG before & after
  stimulation & sample data are averaged.
      Evoked Potentials (cont)
• Sensory EP consist of multiple components
  related to various aspects of subcortical &
  cortical processing (scalp electrodes)
• Clinically useful for assessing the function
  of sensory systems or evaluating
  demyelinating diseases (eg. M.S.)
  – Destruction of myelin causes conduction
    velocity to decrease which increase latencies
Somatosensory Evoked Potential
• Repetative electrical stimulation of a
  peripheral nerve is used to elicit the SSEP
  which are recorded over the scalp & spine
• Configuration & latency of the responses
  depend on the nerve that is stimulated
         Clinical uses for EP
• Detection of lesions in multiple sclerosis
  – functional status sometimes undetected with
• Detection of other CNS disorders
  – e.g. Spinocerebellar degeneration
• Assessment & prognosis following CNS
  injury (trauma or hypoxia)
• Intraoperative monitoring
• The electrical activity within an accessible
  muscle can be recorded via insertion of a
  needle electrode into it.
• Patterns of activity at rest and during
  contraction have been characterized under
  normal and abnormal conditions
     Electromyography (EMG)
• Activity at rest
  – Normal
     • no spontaneous electrical activity except at the end-plate
       region (neuromuscular junctions)
  – Abnormal
     • fibrillation potentials & positive sharp waves are
       associated with muscle fiber irritability & are typically
       found in denervated muscle or sometimes in myopathies
       (especially in inflammation)
     • fasciculation potentials - spntaneous activation of
       individual motor units (occasionally in normal muscle),
       characteristic of neuropathic disorders with primary
       involvement of anterior horn cells (e.g. ALS)
• Activity during voluntary muscle contraction
  – Normal
     • a slight voluntary contraction of a muscle activates a
       small number of motor units
     • normal motor unit potentials have limits of duration,
       amplitude, configuration, & firing rates characteristic
       for muscle tested & # of motor units activated
  – Abnormal
     • in many myopathies-  incidence small short duration
       polyphasic motor units & increased number of motor
       units activated for a given degree of voluntary activity
     • in neuropathies- loss of motor units  # of units
Sensory nerve conduction studies
• Stimulation of nerve to measure conduction
  velocity& amplitude of action potentials in
  sensory fibers when these fibers are
  stimulated at one point and response
  recorded at another point along course of
  the nerve
• provide a means of confirming the presence
  & extent of peripheral nerve damage

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