Aim: To compare the diagnostic yield of single fiber electromyography (SFEMG) and repetitive nerve stimulation (RNS) in consecutive patients with myasthenia gravis (MG).
Methods: Consecutive 33 patients with MG diagnosed on the basis of clinical features, positive neostigmine test and/or acetylcholine receptor antibody assay were categorized into stage 1 (3), 2A (10), 2B (12) and 2C (8 patients). Low rate repetitive nerve stimulation (3Hz) was performed in distal muscles (abductor digiti minimi, anconeus, flexor carpi ulnaris, tibialis anterior) and proximal muscles (deltoid, serratus anterior, trapezius and nasalis). Decrement exceeding 10% was considered abnormal. Single fiber EMG was performed in extensor digitorum communis (EDC), recording 20 potential pairs. The abnormality was defined as mean jitter exceeding 40 micros or 10% of potential pairs having block or jitter exceeding 54 micros. The abnormality in RNS and SFEMG was compared and correlated with severity of MG.
Results: RNS study was carried out in 33 and SFEMG in 30 patients. In 2 patients SFEMG was not possible due to lack of cooperation and in one due to severe weakness. The RNS study was normal in 6 (2 in stage 1, 3 in 2A and 1 in 2B) patients and in all of them SFEMG was abnormal. The abnormality in RNS and SFEMG correlated with severity of MG.
Conclusion: SFEMG is indicated in the patients with MG in whom RNS test is negative.