ELECTROMYOGRAPHY

ELECTROMYOGRAPHY (EMG)

• graphic recording of electrical activity of a muscle at rest and action
• using electromyograph and recorded to electromyogram
• detect the electrical potential generated by muscle cell when these cells contract and at rest

Normal muscle

• at rest shows no electrical activity – voluntary contracture – action potential develops in motor unit
• single motor unit potential – weak contraction
• strong contraction – number of motor units fire simultaneously – superimposed to give – interference pattern

Denervated muscle

• spontaneous electrical activity at rest – denervated potential
• after 15-20 days of muscle denervation

Types of EMG needles used

• concentric
• monopolar
• single fiber
• macro electrode

Three types of activity recorded in EMG

1. Insertional activity

• brief action potential – only few seconds – stops immediately when needle movement stops
• decreased in fibrosis or fat tissue replacement
• prolonged in early denervation (irritability), myotonic disorders

2. Spontaneous activity

• monophasic (end plate noise) or biphasic ( end plate spikes)
• when needle near a motor end plate
• very short duration

3. Voluntary activity

• action potentials from Type I muscle fibers followed by strong ones from Type II fibers
• interference pattern
• prolonged in myopathy
• shortened in neuropathy

Indications

• peripheral neuropathy evaluation
• identifying predominant pathophysiology : axonal/ demylinating, sensory/ motor, acute/sub acute/chronic
• localizing level of lesion ( myelopathic, neuropathic, myopathic)
• objective and qualitative measure of nerve function

Normal results

• muscles at rest – electrically inactive
• after the electrical activity of needle insertion – no abnormal spontaneous activity detected
• voluntary contraction – action potential begin to develop
• strength of contraction increases – more and more fibers produce action potentials
• fully contracted – disorderly group of action potentials of varying rates and amplitudes

Neuropraxia

• normal insertional activity – silent rest activity – no biphasic or triphasic potentials – not interference

Axonotmesis / Neuronotmesis

• increased insertional activity – fibrillation and positive sharp waves in rest – no biphasic, triphasic, interference potentials

Demyelinating neuropathy

• normal insertional and silent rest activity – no bi and triphasic – incomplete interference potential

Contraindications

• abnormal clotting factors, anticoagulant therapy
• extreme swelling
• dermatitis, blood transmittable disorders
• recent MI, pacemakers

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