Proximal Muscles Show Earlier RNS Response in MG, ALS

Patients with myasthenia gravis (MG) and amyotrophic lateral sclerosis (ALS) began showing a gradually decreased response to repetitive nerve stimulation (RNS) in the deltoid muscle at the second or third response, which was earlier than stabilization completion in the abductor pollicis brevis (APB) muscle, according to a study in Clinical Neurophysiology Practice.¹

“The present study documented a previously unreported phenomenon that the decremental response in RNS is completed, ie, reaches a plateau, more rapidly in a proximal muscle (deltoid) than in a distal muscle (APB),” the authors wrote. “The earlier completion of the decremental response in the deltoid muscle documented in the present study may well be related to such a lower safety factor in proximal muscles, although the precise mechanism is yet to be clarified.”

The term safety factor denotes the ability of neuromuscular transmission to function effectively under a range of physiological conditions and stresses, according to previous research.²

The retrospective study evaluated 11 patients with MG and 11 patients with ALS in Japan who underwent RNS treatment between 2008 and 2017. The patients with ALS patients included 7 men and 4 women aged 43 to 83 years. The patients who had MG had a generalized form of the condition and consisted of 4 men and 7 women aged 35 to 82 years. The anti–acetycholine receptor antibody waspresent in 10 of the patients with MG, and the 11th patient was positive for the anti–muscle-specific kinase antibody.

Patients included in this study had a compound muscle action potential amplitue that had decreased more than 10%. | Image Credit: syahrir-stock.adobe.com

Patients selected for the study had more than a 10% decrease of the compound muscle action potential amplitude (CMAPamp) from the first to the fourth response for both the deltoid and the APB muscles.

For patients in both cohorts, the second and third responses were significantly higher in the deltoid muscle than in the APB muscle. Initial CMAPamp was significantly larger in MG compared with ALS for both the APB and deltoid muscles. The decremental percentage was not significantly different between MG and ALS for the APB muscle. It was significantly larger, however, in MG compared with ALS for the deltoid muscle.

For patients who have ALS, overall impairment as measured by the normalized CMAPamp was not significantly different between the APB and deltoid muscles. Yet, the decremental percentage following RNS was significantly higher in the latter.

In patients who have MG patients, the normalized CMAPamp was more significant in the APB vs the deltoid muscle.

The study disproves the conventional belief that the decremental response in RNS stabilizes at the fourth or fifth response, the authors stated. Their findings signify that in cases of unexpected early completion of the decrement such as at the second response in RNS, the reason may be rapid completion in a proximal muscle and not due to a technical error.

The results were found to be independent of disorder—MG or ALS—or initial CMAP amplitude.

Examinations were done at rest and conducted using an ordinary electromyography machine. Patients received stimulations at a frequency of 3 Hz 10 times in the APB muscle and usually 6 times in the deltoid muscle. Stimulations of the deltoid muscle were capped at 6 to minimize discomfort.

“It is well known that proximal muscles are more severely affected in MG, both clinically and electrophysiologically. This may be due to a lower safety factor in these muscles. Even in ALS, which is usually believed to affect distal muscles preferentially, the decremental response was greater in proximal muscles in our previous study. The present study further clarified the latter point,” the authors wrote.

References

1. Ueta Y, Kanbayashi T, Miyaji Y, et al. The speed of completion of the decremental responses on repetitive nerve stimulation. Clin Neurophysiol Pract. 2024;9:211-216. doi:10.1016/j.cnp.2024.06.003

2. Wood SJ, Clarke RS. Safety factor at the neuromuscular junction. Prog Neurobiol. 2024;64(4):393-429. doi:10.1016/s0301-0082(00)00055-1