Procedure

Electromyogram

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Electromyogram, Electromyography, EMG

  • Indications
  1. Muscle disease
  2. Neuromuscular junction disease
  3. Nerve or nerve root disease with denervation
  • Background
  1. Electrical test to measure muscle physiology
  • Myopathy Findings
  1. Nonspecific for Myopathy type
  2. Motor unit potential abnormalities
    1. Low amplitude
    2. Short duration
    3. Complex, polyphasic
  3. Increased Insertional activity
    1. Ectopic high frequency discharges
  4. Fibrillation potentials present in some myopathies
    1. Polymyositis
    2. Muscular Dystrophy
  • Myotonia findings
  1. Waxing and waning hyperexcitability
  2. Repetitive discharges
    1. Electromyogram machine creates "dive bomber" sound
  • Neuromuscular junction disorder findings
  1. General
    1. Increased variation in muscle response time (latency)
  2. Presynaptic disorders
    1. Examples
      1. Botulism
      2. Eaton -Lambert Syndrome
    2. Findings
      1. Enhanced response to repetitive nerve stimulation
      2. Normal amplitude miniature end-plate potentials
  3. Postsynaptic disorders
    1. Examples
      1. Myasthenia Gravis
    2. Findings
      1. Decreased response to repetitive nerve stimulation
      2. Subnormal amplitude miniature end-plate potentials
  • Denervation Findings
  1. Increased polyphasic action potentials
  2. Ectopic high frequency discharges
  3. Fibrillations
    1. Onset 3 to 4 weeks after nerve injury
  4. Fasciculations
  5. Positive sharp waves
  6. F Response prolonged (proximal Motor Nerve reflex arc)
    1. Guillain-Barre Syndrome
  7. Hoffman Reflex (Stretch reflex arc - sensory and motor)
    1. Sensory Nerve root lesion
    2. Motor Nerve root lesion