Electrophysiology Aids Myasthenia Gravis, Related Disorders

A new report highlights the strengths and shortcomings of electrophysiological techniques in diagnosing neuromuscular junction (NMJ)-related diseases, including myasthenia gravis (MG).

The authors of the review, which was published in World Neurosurgery, say both repetitive nerve stimulation (RNS) and single fiber electromyography (SFEMG) are effective tools to detect NMJ abnormalities, but they also argue that more work should be done to study the techniques as tools to assess sarcopenia in older patients.¹

Normal functioning of the NMJ depends on the release of the neurotransmitter acetylcholine (ACh) and the correct functioning of ACh receptors, wrote the study authors. Most patients with MG have antibodies against ACh receptors, resulting in muscle weakness and fatigue, they said. Similar aberrations are at play in Lambert-Eaton myasthenic syndrome, they added.

Both RNS and SFEMG have been around for decades, the authors said. The RNS-based “Jolly test”—named for its developer, the German neurologist Friedrich Jolly—has been around for decades old. The test uses RNS to diagnose NMJ disorders.

New research shows how repetitive nerve stimulation and single fiber electromyography can help diagnose disorders such as myasthenia gravis. | Image Credit: Andrii-stockk.adobe.com

The present authors sought to take a holistic view of the utility of the 2 electrophysiological techniques, while also evaluating the extent to which they can be used to assess sarcopenia among older patients. Discussing RNS, they said the technique can be used to distinguish presynaptic and postsynaptic lesions, and is a helpful tool to assess disease severity and differentiate between different myasthenic disorders. They said low-frequency RNS helps diagnose postsynaptic lesions, while high-frequency RNS can be used to identify presynaptic terminal abnormalities.

However, they also said its sensitivity and specificity can be influenced by factors such as test temperature, limb movement, and patient age.

“Hence, caution must be exercised when using RNS for diagnostic purposes,” they said, “taking into account these potential issues and integrating them with other examination results for comprehensive analysis.”

SFEMG tends to be more precise, they said, as it directly measures the changes in action potentials of individual motor units. They said the key parameters for evaluating NMJ function include fiber density, jitter values, and pulse blocking.

“Patients with NMJ disorders often exhibit increased fiber density and jitter values, along with frequent occurrences of pulse blocking,” they said.

Yet, although significant research has been devoted to the use of the 2 techniques in diagnosing MG and related disorders, much less effort has been spent evaluating the tools in older patients with sarcopenia, they said.

The investigators noted that an estimated 10% of older people living at home and more than one-third of older individuals in nursing homes experience sarcopenia.² Sarcopenia appears to be related to both neurological and muscle factors, they said.

“As the critical nexus connecting nerves and muscle tissues, the structural integrity and proper functioning of the NMJ are essential for stable neural control and effective muscle force generation,” they wrote.

The authors said both RNS and SFEMG could be useful in evaluating elderly patients for sarcopenia. They said SFEMG can help uncover NMJ transmission failures.

“However, clinical electrophysiological studies of NMJ function in older populations, especially those with sarcopenia, remain relatively scarce and yield inconsistent results,” they wrote.

Thus, they said, future research should be devoted to better understanding the precise role of NMJ dysfunction in the development of sarcopenia.

The investigators said such research may not only yield insights into the pathogenesis of sarcopenia, but may also help identify potential therapeutic strategies.

References

1. Yang XG, Peng Z, Luo HT, Lu S. Application of electrophysiological techniques in assessing of neuromuscular junction-related disorders. World Neurosurg. Published online August 17, 2024. doi:10.1016/j.wneu.2024.08.076

2. Fielding RA, Vellas B, Evans WJ, et al. Sarcopenia: an undiagnosed condition in older adults. current consensus definition: prevalence, etiology, and consequences. International Working Group on Sarcopenia. J Am Med Dir Assoc. 2011;12(4):249-256. doi:10.1016/j.jamda.2011.01.003