Volitional single fiber electromyography of the masseter muscle; normative values and in myasthenia gravis

Abstract

This study was aimed at establishing normative values for volitional single fiber electromyography (SFEMG) 
of the masseter muscle in Egyptians and evaluating the sensitivity of this test in the diagnosis of 
generalized myasthenia gravis. Twenty two patients with myasthenia gravis (mean age of 37.72 years; 
range, 20 to 59; mean duration of illness, 4 years; range, 1 months to 16 years) where enrolled in the study 
to assess for SFEMG of the masseter muscle and of the extensor digitorum communis (EDC) muscle. 
Twenty normal individuals (mean age 34.3; range 22 to 55 years) where similarly studied to determine the 
normative values of SFEMG of the masseter muscle. The mean jitter of the masseter in the patients’ group 
was 63.4 ± 9.98 us, compared to 21.3 ± 4.9 us in the control group. Examination of the masseter muscle 
yielded 100% sensitivity in this study, compared to 90% sensitivity for EDC. We suggested a normal upper 
limit of masseter’s mean jitter of 26 microsecond/ study (mean +/- 1 SD) and 28 us / individual fiber pair (The 
95th upper percentile was 27.51 us). Volitional SFEMG of the masseter muscle is highly sensitive test to 
diagnose generalized MG that is not related to weakness of this particular muscle but correlated to the 
degree of the disease’s severity.