Purpose: Single-fiber electromyography is a sensitive electrophysiological method for the determination of neuromuscular dysfunction. Normative values for stimulated single-fiber electromyography have been reported for the orbicularis oculi (OOc) with no distinction made between the upper and lower OOc. The purpose of this study is to demonstrate that the upper and lower OOc differ in how they behave during dysfunction.
Methods: Patients in whom myasthenia gravis was suspected were included in the study. Strict selection criteria for single fiber potentials were used.
Results: Seventy-six patients were entered prospectively into this study. Twenty-six patients completed the study and obtained a final diagnosis. Twenty-one patients had abnormal stimulated single-fiber electromyography results. The lower OOc was found more likely to be abnormal based on the number of abnormal single fiber potentials (27.6% and 20.7% potentials from the lower and upper OOc, respectively). Both upper and lower OOc were abnormal in 13 patients and the lower OOc was solely abnormal in seven patients. The upper OOc was solely abnormal in two patients, neither of which had a diagnosis of myasthenia gravis.
Conclusions: The lower OOc is an important muscle to examine, not only to rule out myasthenia gravis, but also to prevent the occurrence of false negative results.