Predictive Value of Single-Fiber Electromyography for Ocular Myasthenia Gravis in a Low Prevalence Cohort

Single-fiber electromyography (SFEMG) is a sensitive test to diagnose ocular myasthenia gravis (OMG) when the clinical suspicion is high but the predictive value of the test is also dependent on the prevalence of the condition in the referral population. In a low prevalence population, false positive results can be as common as true positive results. Most of the patients referred for SFEMG at our center do not have symptoms or signs that are strongly suggestive for OMG.

A retrospective chart review was performed of all patients referred to our laboratory for possible OMG for a 1-year period and analyzed the long-term accuracy of SFEMG in a cohort of 33 patients.

Eleven of these (33%) were eventually diagnosed with a disease of neuromuscular transmission and SFEMG was abnormal in all of these (100% sensitivity). With the requirement that 3 jitter pairs be abnormal or that mean jitter was elevated, SFEMG was normal in the remaining 22 patients (100% specific).

The results confirm that SFEMG is an excellent confirmatory test when clinical suspicion for OMG is high and also demonstrate that when strict criteria are applied, the test accurately excludes OMG in patients with ocular complaints that are not truly suggestive of OMG.