Afterdischarges in motor nerve stimulation studies help distinguish slow channel congenital myasthenic syndrome (SCCMS) from acquired myasthenia gravis (MG) We present a patient with fatigable weakness in whom afterdischarges were not initially apparent. After she failed to respond to treatment for MG, afterdischarges were demonstrated in some, but not all, of her muscles. Genetic testing confirmed SCCMS. This case indicates that electrophysiological distinction between SCCMS and MG may require motor nerve stimulation studies in multiple muscles.