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

Updated: Mar 13, 2019
  • Author: Pradeep C Bollu, MD; Chief Editor: Nicholas Lorenzo, MD, MHA, CPE  more...
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SFEMG demonstrates increased jitter in virtually all patients with myasthenia gravis (MG). SFEMG is most valuable clinically in the patient with suspected MG in whom results of other tests of neuromuscular transmission and acetylcholine receptor (AChR) antibody measurements are normal. [1, 3] When abnormal neuromuscular transmission has been demonstrated by RNS, the finding of abnormal jitter does not add to the diagnosis, although baseline jitter values may be useful for comparison with subsequent studies (see Assessment of Neuromuscular Transmission).

Although no one muscle is more abnormal in every patient with MG, the extensor digitorum communis (EDC) is abnormal in most.

If the symptoms or weakness is limited to the ocular muscles, the orbicularis oculi or frontalis may be examined first. In most cases, the EDC is examined first.

If the EDC is examined first and is normal, the frontalis or orbicularis oculi muscles usually are examined next. If the first of these facial muscles is normal, the other should be examined before excluding the diagnosis of MG.

If any limb muscle is weak and jitter is normal in all other muscles tested, a weak limb muscle also should be examined. If jitter is normal in a muscle with definite weakness, the weakness is not due to MG.

Jitter is less abnormal in patients with ocular myasthenia than in those with generalized disease. Jitter is increased in a limb muscle in more than half the patients with ocular myasthenia, demonstrating that the physiologic abnormality is more widespread than the clinical findings would suggest.

In patients with antibodies to muscle-specific tyrosine kinase (MuSK), the electrodiagnostic abnormalities are more focal and single-fiber studies should be performed in clinically weak muscles. [17]

Jitter usually is increased even when patients are taking cholinesterase inhibitors. In patients with mild or purely ocular weakness, however, jitter may be normal unless these medications have been discontinued for at least 24 hours. Comparisons among studies are valid only if they were made at the same time after the same dose of cholinesterase inhibitors. Changes in jitter measurements correlate with changes in disease severity in most patients. [18]  Jitter also is increased in diseases of nerve and muscle; these must be excluded by other electrophysiologic and clinical examinations before concluding that the patient has MG. Increasing temperature can increase the jitter in patients with MG. Neuromuscular jitter can be increased in the setting of Lambert–Eaton myasthenic syndrome (LEMS) and this finding alone cannot be used to differentiate it from MG. 

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