Evoked Potentials (EP)

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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.

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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

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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

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Clinical uses for EP

• Detection of lesions in multiple sclerosis
• functional status sometimes undetected with MRI
• Detection of other CNS disorders
• e.g. Spinocerebellar degeneration
• Assessment prognosis following CNS injury
  (trauma or hypoxia)
• Intraoperative monitoring

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Electromyography

• 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

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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)

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EMG

• 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
  activated

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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