What is the role of single-fiber electromyography (SFEMG) in the evaluation of myasthenia gravis (MG)?

Updated: Aug 27, 2018
  • Author: Abbas A Jowkar, MBBS; Chief Editor: Nicholas Lorenzo, MD, CPE, MHCM, FAAPL  more...
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SFEMG provides the most sensitive measure of myasthenia gravis. A normal SFEMG of a clinically weak muscle effectively rules out the diagnosis of MG.

A concentric needle electrode and other monopolar and bipolar needle electrodes can record single motor unit potentials, but they cannot discriminate individual muscle fibers within the motor unit. The single-fiber needle used in SFEMG, which has a small recording surface, allows recording from individual muscle fibers.

SFEMG is capable of determining jitter (ie, variability of the time interval between the action potentials of 2 single adjacent muscle fibers in the same motor unit) and fiber density (ie, number of single-fiber action potentials within recording radius of the needle). Increased jitter (with or without impulse blocking) and normal fiber density are suggestive of a neuromuscular fiber transmission defect (see the image below).

Single-fiber electromyography showing so-called ji Single-fiber electromyography showing so-called jitter phenomenon (second action potential wave group).

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