What is the role of electrodiagnostic studies in the evaluation of myasthenia gravis (MG)?

Updated: Aug 27, 2018
  • Author: Abbas Jowkar, MD; Chief Editor: Nicholas Lorenzo, MD, MHA, CPE  more...
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Answer

Routine motor and sensory nerve conduction studies (NCS) must not be omitted before embarking on electrodiagnostic studies that demonstrate a defect of neuromuscular transmission. Routine NCS is done to ensure the integrity of any nerve that subsequently will be used in RNS. A decrement in RNS can be seen in other conditions (neuropathies, motor neuron disease, inflammatory myopathies) and myotonic disorders. At least one motor and sensory conduction study should be performed in an upper and lower extremity. Compound mucle action potentials (CMAP) amplitudes should be normal with only 3% to 15% showing CMAPs that are diffusely low. If CMAP amplitudes are low or borderline, repeat distal stimulation after 10 seconds of exercise to exclude a presynaptic NMJ transmission disorder such as LEMS. The routine needle EMG of distal and proximal muscles, especially weak muscles may reveal unstable or small amplitude, short duration, polyphasic motor unit potentials, with or without early recruitment. Unstable motor units represent muscle fibers that are blocked or come into action potential at varying intervals, resulting in MUAPs that change in configuration from impulse to impulse. Fibrillation potentials and other abnormal spontaneous activity are not seen in NMJ disorder, except in the case of botulism.


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