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

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

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

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

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