Relative fatigability of muscles in response to repetitive nerve stimulation in myasthenia gravis

Repetitive nerve stimulation (RNS) plays an important role in the diagnosis of myasthenia gravis (MG). Technically, this test can be more easily performed on distal muscles than proximal muscles. On the other hand, proximal muscles have been shown to be more easily fatigued or more sensitive than the distal ones.

Evaluate the relative fatigability of different muscles in response to RNS in ocular and generalized MG patients.

Two hundred patients with 44 ocular MG and 156 generalized MG were studied The relative fatigability of each muscle was calculated as percentage of cases with abnormal response. A decrement of 10% or more of the amplitude of the compound muscle action potentials was considered abnormal.

The cases with abnormal response of adductor digiti minimi, anconeus, trapezius, nasalis and orbicularis oculi were 17%, 50%, 55%, 62%, 66% in ocular MG and 53%, 77%, 79%, 85%, 79% in generalized MG respectively. These results showed that in generalized MG the response in all the muscles was more frequently abnormal and was fatigable in a more comparable degree, whereas in ocular MG, the facial muscles were more often abnormal than the limb muscles.

The abnormal response because of fatigability was more widespread in generalized MG whereas facial muscles were relatively more sensitive in ocular MG. This finding may be useful for the selection of the muscles to be tested in RNS especially in ocular MG.