Myasthenia gravis with features of the myasthenic syndrome. An investigation with electrophysiologic methods including single-fiber electromyography.

A 47-year-old woman had myasthenia gravis with only moderate weakness and a consistently poor response to anticholinesterase medication. She was investigated with routine electrophysiologic studies and single-fiber electromyography. The studies showed a limited effect of anticholinesterase medication. There was a decrement to 2 Hz stimulation of not less than 25 percent regardless of medication and administration of edrophonium. With faster rates of stimulation (10 to 20 Hz), there was a facilitation up to 190 percent. In addition, with postcontractive or post-tetanic potentiation the action potential in the abductor digiti minimi muscle was increased from the initial value of 2.5 mv or less to up to 6 mv. Single-fiber electromyographic investigations showed increased jitter and blocking with little response to drugs, as well as facilitation in a large number of motor end-plates in association with both voluntary activity and repetitive stimulation. The findings indicated a mixture of phenomena seen in myasthenia gravis and the myasthenic syndrome.