Repetitive facial nerve stimulation in myasthenia gravis 1min after muscle activation is inferior to testing a second muscle at rest

Objectives: To explore the increased diagnostic yield of repetitive nerve stimulation (RNS) following activation for myasthenia gravis (MG) diagnosis, and compare with testing an additional muscle at rest only.

Methods: We performed a retrospective chart review of patients diagnosed with MG attending the neuromuscular clinic from 2013 to 2015 and extracted data on electrophysiological studies, including frequency of decrement of 10% or above, with facial RNS at rest, and following activation.

Results: The total cohort included 102 patients with MG, 65 with generalized, and 37 with ocular MG. Facial RNS sensitivities for diagnosing MG were 32-46% and 14-19% respectively. The increase in RNS sensitivity following muscle activation was 6-8% for frontalis muscle recordings, and 0-2% for nasalis muscle recordings. Recording from both muscles at rest only, increased the sensitivity by 9-15%.

Conclusion: RNS is a valid method for confirming a clinical diagnosis of MG. The increased diagnostic yield of RNS following activation is controversial. Our study shows that the increased diagnostic yield of facial RNS following activation is modest, and is less than performing facial RNS in a second muscle.

Significance: Performing facial RNS in an additional muscle at rest appears to be more sensitive than facial RNS in a single muscle after activation.