Specific Response Pattern to Nerve Stimulation May Help Diagnose LEMS

People with Lambert-Eaton myasthenia syndrome (LEMS) have a specific response pattern to repetitive nerve stimulation — known as RNS — that may enable physicians to easily distinguish such cases from the neuromuscular condition myasthenia gravis (MG), a study reports.

The study, “Lambert-Eaton myasthenia syndrome: specified description of a response pattern to low-frequency repetitive nerve stimulation,” was published in the Journal of Electromyography and Kinesiology.

LEMS and MG are autoimmune diseases that affect the neuromuscular junction, or NMJ — the place where motor neurons and muscle fibers communicate. Motor neurons are the nerve cells responsible for controlling voluntary movements. Both disorders are characterized by muscle weakness, reduced reflexes, and fatigue.

RNS is a diagnostic test often used to identify disorders that cause NMJ impairments. The approach uses electrical signals to repeatedly stimulate nerve cells associated with specific muscles in the body.

Weak muscle responses to each stimulus are indicative of impairments in the neuromuscular junction. However, the use of RNS to distinguish between different disorders leading up to NMJ changes is still challenging.

“In recent years, low-frequency RNS response in LEMS, which was initially thought to simply decrease, as has been observed in myasthenia gravis (MG), was suggested as a useful discriminating indicator,” the researchers wrote.

To explore this possibility, physicians in China reviewed the medical records of 18 people with LEMS and 24 with MG, who were selected from a hospital database containing patient data from 2010 to 2017.

All of the patients underwent low-frequency RNS, in which 10 electrical stimuli of 3 Hz lasting 0.2 milliseconds were applied to the abductor pollicis brevis (APB), a hand muscle that controls thumb movement.

After comparing the response patterns in the two groups, the physicians discovered that, in patients with LEMS, the APB muscle took longer (7-9 stimuli) to reach minimum response, or nadir, compared with people with MG (4-5.25 stimuli).

In the LEMS group, this minimum value of APB muscle response also was followed by a mild recovery of 1.4%, lower than in MG patients (3.5%). That difference was significant, the researchers said.

In addition, the median reduction in APB muscle response was significantly higher among those with LEMS, compared with people with MG (36.7% vs. 21%).

“We identified a peculiar response to low-frequency RNS of the APB in LEMS, with a delayed nadir followed by a mild recovery, and we suggest that this response is more precise than previous studies in distinguishing LEMS,” the investigators wrote.

“The awareness of the distinctive pattern associated with LEMS may prompt physicians to consider the possibility of LEMS and lead to further electrophysiological [electrical properties] explorations,” they concluded.

“These new indicators might improve our knowledge of LEMS,” they added.