On the choice of muscle in the electrophysiological assessment of myasthenia gravis.

We performed repetitive nerve stimulation on 38 adult patients with generalised myasthenia to compare the relative diagnostic yield from deltoid and trapezius. The decrement was consistently greater in deltoid, both before and after maximum voluntary contraction. In 12 patients the test was negative in trapezius but diagnostic in deltoid, whereas the converse occurred in only one. The yield from facial muscles was comparable to that from trapezius while that from hand muscles was very low. Post-tetanic exhaustion often enhanced the decrement but the use of stimulus frequencies greater than 2-3 Hz did not. We conclude that deltoid provides the highest diagnostic yield in myasthenia when doing decremental studies.