Relative Fatigability of Muscles in Response to Repetitive Nerve Stimulation in Myasthenia Gravis
Rawiphan Witoonpanich MD, FRCP*, Samai Barakul *, Charungthai Dejthevaporn MD, PhD, FRCP(T)*
Affiliation : * Division of Neurology, Department of Medicine, Faculty of Medicine, Ramathibodi Hospital, Mahidol University
Background : 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.
Objective : Evaluate the relative fatigability of different muscles in response to RNS in ocular and generalized
MG patients.
Materials and Methods : 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 decre-
ment of 10% or more of the amplitude of the compound muscle action potentials was considered abnormal.
Result: The cases with abnormal response of adductor digiti minimi, anconeus, trapezius, nasalis and orbicu-
laris 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.
Conclusion : 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.
Keywords : Myasthenia gravis, Repetitive nerve stimulation, Fatigability
