• Electromyography (EMG) covers methods that allow objective investigation of peripheral nerve, neuromuscular junction and muscle functioning. Useful complement to neurologic examination in hypotonic paresis and paralysis and in muscle weakness.
• "Peripheral nerve" in its physiological sense includes nerve cell bodies in the brain stem, spinal cord and ganglions and nerve roots from which the peripheral nerve originates.
• Electromyographic examination is divided into several procedures. The most reliable and widely used in a clinical setting are:
  • Detection electromyography.
  • Maximum motor nerve conduction velocity measurement.
  • Maximum sensory/mixed nerve conduction velocity measurement.
  • Repetitive motor nerve stimulation.
  • In addition, ventral roots can be specifically investigated through F-wave studies.
• In detection EMG, muscles are investigated, looking for spontaneous electrical activity.
• In motor nerve conduction velocity studies, repetitive stimulations and F-wave studies, motor nerves are electrically stimulated and resulting electrical activity in their target muscles, evaluated.
• In sensory nerve conduction studies, nerves are stimulated and recordings obtained from the same nerves.

• Denervated muscle fibers undergo many biochemical and physiological changes including:
  • Increase in sensitivity to acetylcholine.
  • Drifts in membrane potential.
• These result in spontaneous potentials observed between 5 - 10 days after a nerve injury.
• Parts of muscle fibers isolated from the end plate by a muscle lesion can survive and behave like denervated muscle cells, exhibiting spontaneous electrical activity.
• In both cases, spontaneous activities can be recorded by detection EMG.
• Disease processes may affect nerve cell body or processes, myelin sheaths or both. As a general rule, the larger a nerve fiber, the faster its conduction velocity. A larger fiber also has higher metabolic requirements and usually is more susceptible to insults. Disappearance of largest fibers results in a slight maximum conduction velocity drop and a marked diminution of the amplitude of the resulting muscle potential.
• Myelin sheath damage induces dramatic conduction slowing and conduction blockade, both identifiable through nerve conduction studies.

Uses

• Diagnostic technique to investigate neuromuscular abnormalities if neurological examination is inconclusive.
  • Gives picture of severity of lesion.
  • May localize it more or less precisely.
  • May discriminate whether nerve fibers or myelin sheaths in a nerve are most affected.

Advantages

• Highly sensitive.
• May evidence changes before they become clinically detectable.

Disadvantages

• Material expensive.
• Operator must be experienced
• Method restricted to academic and referral institutions due to above.
• Technical problems:
  • High quality grounding of the recording apparatus due to high amplifier gains.
  • Requires careful upkeep of electrodes and connecting wires.
  • Requires skillful operation.

Alternative techniques

• No alternative for functional exploration of peripheral nervous and muscular systems although lesional information is best drawn from microscopic examination of nerve and muscle biopsies.
• Serum measurement of CK enzyme and antibodies against end plates may contribute to the diagnosis of myositis Myositis ossificans /myopathy and myasthenia gravis Myasthenia gravis respectively.

Time required

Preparation

• Sedation.
• Anesthesia: 20 minutes.

Procedure

• 90-120 minutes needed for complete exploration but procedure may be tailored to individual cases and may take more/less time.

Decision taking

Criteria for choosing test

• Confirmation of, and discrimination among, peripheral nerve, nerve-muscle junction or muscle problems.
• When need refining site (local vs general, proximal vs distal etc) of nervous lesions.

Risk assessment

• No specific risk associated with the method.
• The only risk is related to the need for anesthesia. Extremely weak patients may need assisted ventilation.
• Patients with neuromuscular weaknesses ie laryngeal paralysis Larynx: paralysis and megaesophagus Megaesophagus may predispose to aspiration pneumonia Lung: aspiration pneumonia.