What is the role of single-fiber EMG in the workup of diplopia?

Updated: May 21, 2019
  • Author: Jitander Dudee, MD, MA(Cantab), FACS, FRCOphth; Chief Editor: Andrew G Lee, MD  more...
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Answer

Single-fiber electromyography (EMG) can be used to detect myasthenia gravis in seronegative cases with isolated ocular findings. Single-fiber EMG is a highly sensitive test for detecting defects in the neuromuscular junction. An electrode measures the potentials of different muscle fibers that are innervated by the same nerve. In a healthy individual, the action potential will excite the muscle fibers at the same time. If there is pathology, there will be variability in the time of excitation of the muscle fibers. Jitter represents the difference in time between the depolarization of the two muscle fibers and is increased in those with myasthenia gravis. The threshold for an abnormal test result is a jitter value greater than the upper limit of the normal value for the specific muscle or greater than 10% of muscle pairs tested with increased jitter. [11]


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