electromyography (EMG)

Indications

• neuromuscular disease*
  • peripheral nerve disease
    • useful in determining the site of injury in a peripheral nerve
      • can determine if neuropathy if axonal or demyelinating
    • useful in detecting spinal nerve root lesions
    • nerve root compression
      • carpal tunnel syndrome
    • presence of weakness, asymmetry, proximal signs, rapid course or atypical findings in a patient with peripheral neuropathy
  • primary muscle diseases
• spinal cord disease*
  • anterior horn cell disease
• can differentiate active (inflammatory) myopathy from chronic myopathy & acute denervation from chronic denervation

* EMG can distinguish neuropathy from other neuromuscular disorders

* increased activity with lower motor neuron lesion; normal with upper motor neuron lesion

Contraindications

• patients with bleeding tendencies
• patients unusually sensitive to recurrent infection

Principle

• the study of electrical potentials generated by the depolarization of muscle
• EMG techniques include measurement of latencies of F-responses, H-reflexes, blink reflexes & single-fiber EMG
• the usual macro EMG techniques measure the summated electrical activity of all fibers of a motor unit
• amplitude & area are increased in re-innervation & decreased in primary muscle disease

Clinical significance

• myotonic discharges consist of rhythmic discharges lasting for long periods after the external source of excitation has come to rest.
• this may be seen in association with:
  • myotonia
  • polymyositis (infrequent)
  • glycogen storage disease, type II

Procedure

Recording technique (4 phases of examination):

• insertional activity:
  • a needle electrode is placed in the muscle & electrode activity associated with its insertion is evaluated
  • normal: brief activity with insertion
  • lower motor neuron lesion: increased activity
  • upper motor neuron lesion: normal
  • myopathy: normal
  • myotonia: myotonic discharges
  • polymyositis: increased activity
• spontaneous activity
  • the muscle is evaluated at rest, with the needle stationary in a relaxed muscle
  • normal: none
  • lower motor neuron lesion:
    • fibrillation potential
    • positive sharp waves
  • upper motor neuron lesion: none
  • myopathy: none
  • myotonia: none
  • polymyositis
    • fibrillation potential
    • positive sharp waves
• motor unit potentials
  • muscle potentials evoked by isolated discharges of motor neurons are recorded with mild voluntary contraction of the muscle
  • normal: 0.5-1.0 mV, 5-10 ms
  • lower motor neuron lesion: large unit, limited recruitment
  • upper motor neuron lesion: normal
  • myopathy: small unit, early recruitment
  • myotonia: myotonic discharge
  • polymyositis: small unit, early recruitment
• recruitment & interference pattern
  • the change in electrical potential is assessed as the level of muscle contraction gradually increases & eventually reaches a maximum
  • normal: full
  • lower motor neuron lesion: reduced, fast firing pattern
  • upper motor neuron lesion: reduced, slow firing pattern
  • myopathy: full, but low amplitude
  • myotonia: full, but low amplitude
  • polymyositis: full, but low amplitude
• repetitive stimulation protocol if myasthenia gravis suspected

Interpretation

• except for end-plate noise & brief insertional activity, there are normally no electrical discharges in a relaxed muscle.
• under normal circumstances, the smallest unit of volitional contraction is a motor unit.
• fibrillation potentials & positive sharp waves are derived from single, spontaneous discharges of individual motor units.
• interference pattern:
  • with greater contraction, many motor units begin to fire rapidly
  • simultaneous activation of different units no longer allows recognition of individual motor unit potentials
  • this summated response is referred to as the interference pattern
  • it is a measure of the density or number of spikes & the amplitude of the available motor unit potentials

More general terms

• electrical stimulation

More specific terms

• anal sphincter/urethral sphincter electromyography
• needle electromyography (N-EMG)
• needle oculoelectromyography
• single-fiber electromyography (EMG)
• surface electromyography (S-EMG)

Additional terms

• EMG blink-reflex
• EMG F-response
• EMG H-reflex