Initial repetitive nerve stimulation test predicts conversion of ocular myasthenia gravis to generalized myasthenia gravis

Abstract

Background and Purpose A major concern with ocular myasthenia gravis (MG) is the potential conversion to generalized MG. This study was conducted to determine if the repetitive nerve stimulation (RNS) test could predict the conversion from ocular to generalized MG. Methods The RNS test was conducted in a consistent manner on five muscles in the face and limbs in every patient. Subjects were divided into those who remained as ocular MG (ROMG group) and those who experienced conversion to generalized MG during follow-up (GOMG group). Results Conversion to generalized MG occurred in 24 (21.4%) of 112 MG patients with ocular onset. The proportion of patients displaying abnormal decreases in responses in the trapezius, abductor digiti minimi, or flexor carpi ulnaris muscles on the RNS test was higher in the GOMG group (p<0.001, p=0.002, and p<0.001, respectively). The Cox proportional-hazards model revealed that an abnormal result on the RNS test was significantly associated with conversion to generalized MG [hazard ratio (HR)=3.13, 95% confidence interval (CI)=1.18– 8.32]. Notably, the HR was higher for abnormal results on the RNS test for the limb muscles, at 5.19 (95% CI=2.09–12.90). Conclusions An abnormal result on the RNS test, especially in the limb muscles, is an independent predictor of the conversion from ocular to generalized MG. Applying the RNS test to limb muscles could be useful for predicting the conversion to generalized MG in patients with ocular onset.