Single fiber electromyography of the frontalis muscle: A view from the electromyography laboratory perspective

Objective: The present study reports single fiber electromyography (SFEMG) findings in patients with ocular symptoms referred to the electromyography (EMG) laboratory with a presumed diagnosis of myasthenia gravis (MG). This study also examines repetitive nerve stimulation and serologic and clinical findings. Method: We reviewed the medical records of 45 consecutive patients from January 2019 to September 2021 retrospectively. SFEMG was performed on the frontalis muscle (Front) during voluntary contraction using a disposable concentric needle electrode (CNE) (25 mm, 30 G). The number of single fiber-like action potential (SFLAP) pairs, the mean consecutive difference (MCD), the mean sorted difference (MSD), and the percentage of abnormally high individual jitters were noted. Results: SFEMG results were normal for 16 patients, abnormal for 15, and borderline for 14. The mean jitters for the normal, borderline, and abnormal SFEMG groups were 19.75±3.84 µs, 27.21±4.89 µs, and 74.4±38.74 µs, respectively. A minimum of 20 SFLAP pairs were acquired in the normal or borderline SFEMG groups, whereas an analysis of fewer than 20 SFLAP pairs sufficed for 53% of the patients in the abnormal SFEMG group (p=0.001). SFEMG revealed diagnostic high jitter in 79% of patients with MG. The acetylcholine receptor (AchR) antibody titer correlated positively with mean MCD and MSD (p<0.01) but not with the percentage of high jitter. Conclusion: CNE SFEMG of the Front is beneficial for diagnosing ocular and mild MG. The correlation of absolute AChR antibody titers with mean MCD and MSD, but not with the percentage of high jitter, is notable.