Stimulated Single-Fiber Electromyography an Accurate Diagnostic Tool in LEMS, Case Report Suggests

Using stimulated single-fiber electromyography is associated with an early and accurate diagnosis of Lambert-Eaton myasthenic syndrome, according to a case report.

The study, “Stimulated single-fiber electromyography (sSFEMG) in Lambert-Eaton syndrome,” was published in the journal Clinical Neurophysiology Practice.

Lambert-Eaton myasthenic syndrome (LEMS) is an autoimmune disease that results in impaired signaling between nerve and muscle cells (neuromuscular transmission) by affecting the calcium channels known as presynaptic voltage-gated calcium channels (VGCCs).

A diagnosis of LEMS is confirmed by detecting circulating antibodies against VGCCs, as well as neurophysiological studies.

Electromyography (EMG) is a diagnostic test used to assess the health of muscles and the nerve cells that control them.

Single-fiber electromyography (SFEMG) is used to study the neuromuscular junction. One of the findings that confirms a LEMS diagnosis is an increased neuromuscular jitter (an indicator of irregularity of neuromuscular transmission) in SFEMG.

There are two types of SFEMG — stimulated and voluntary. Stimulated SFEMG (sSFEMG) has several advantages over voluntary SFEMG (vSFEMG). Namely, it allows the study of the neuromuscular junction without voluntary activation, or movement of voluntary muscles; it controls the rate of stimulation; it requires only limited patient cooperation; and it is more comfortable.

In this report, physicians discuss the case of a 60-year-old woman who had muscle weakness, waddling gait (a form of gait abnormality due to muscle weakness), and bilateral ptosis (drooping of the eyelid).

Physicians performed a needle electromyography, the results of which suggested a diagnosis of noninflammatory myopathy (disease of muscle fibers). The patient was then referred to neurologists for further diagnostic tests.

A more detailed medical history revealed that the patient had signs suggestive of an autonomic dysfunction (a disease associated with the impairment of the autonomic nervous system — the part of the nervous system responsible for control of bodily functions.

Neurologic examination revealed that the patient’s deep tendon reflexes were absent at rest but showed the post-exercise facilitation phenomenon, where there is a short-term return of normal tendon reflexes and muscle strength after periods of exercise. This suggested that the patient had myasthenic syndrome, a disorder of the neuromuscular junction.

Physicians then performed both vSFEMG and sSFEMG in the right extensor digitorum communis muscle — which helps with the movement of the phalanges, wrist, and elbow — using a concentric needle EMG.

vSFEMG showed a nonspecific increased duration of the neuromuscular jitter, which is also typically observed in patients with myasthenia gravis — another neuromuscular junction disease.

In contrast, sSFEMG showed the typical rate-dependent reduction of the neuromuscular jitter and blocking, which is a finding specific to LEMS.

According to the authors, “sSFEMG represents, therefore, a very helpful approach in the differential diagnosis of LEMS thanks to the peculiar rate-dependence of neuromuscular jitter that is not observed in myopathies.”

A diagnosis of LEMS was confirmed by demonstration of circulating VGCC antibodies.

This study highlights the importance of correlating patterns observed on an EMG with other clinical characteristics and neurophysiological characteristics to differentiate between myasthenic and myopathic syndrome.

“sSFEMG findings in LEMS underlie the stimulation rate-dependency of both neuromuscular jitter and blocks, reflect the basic pathophysiology [disease development] of this neuromuscular syndrome, thus making this technique very specific and, therefore, clinically useful for an early and accurate diagnosis,” the authors concluded.