Conference Coverage

Congenital myasthenic syndrome diagnosed best with repetitive stimulation and jitter analysis



The use of both repetitive stimulation (RS) and jitter analysis are important for accurate diagnosis of congenital myasthenic syndrome (CMS) suggests newly presented research.

“In case RS is negative, SFEMG [single fiber electromyography] alone is not very specific and cannot distinguish CMS from mitochondrial myopathies, even in the presence of impulse blocking,” Vitor Marques Caldas, MD, a neurologist at the Syrian Libanes Hospital in Brasilia, Brazil, and a PhD student at the University of São Paulo, told attendees at the annual meeting of the American Association for Neuromuscular and Electrodiagnostic Medicine. “An isolated SFEMG test can lead to a misdiagnosis of myasthenia syndrome if not interpreted in the right clinical context.”

The researchers sought to understand the relative sensitivity and specificity of low-frequency RS versus jitter analysis using disposable concentric needle electrodes (CNE).

The study involved 69 patients, of whom 19 had mitochondrial myopathy, 18 had congenital myopathy, 18 had CMS, and 14 were asymptomatic controls. The control group all tested normal with both RS and jitter analysis.

The 18 participants with CMS, average age 24 years, received low-frequency RS in at least six different muscles: two distal muscles (abductor digiti minimi and tibialis anterior), two proximal muscles (deltoid and trapezius) and two facial muscles (nasalis and orbicularis oculi). They also underwent jitter analysis of their orbicularis oculi muscle under voluntary activation using CNE.

These patients had heterogeneous genetic profiles: 11 had the CHRNE gene mutation, 2 had the RAPSN gene mutation, 2 had the COLQ gene mutation, 2 had the DOK-7 gene mutation, and 1 had the COL13A1 mutation.

All but two patients with congenital CMS tested positive (88.9%) with RS: one female with CHRNE mutation and one male with RAPSN mutation. Using mean jitter, all but one patient tested positive (94.4%): a female with DOK-7 mutation who had tested abnormal on RS.

All patients with CMS tested positive with at least one of the two tests, but only 83.3% tested positive with both tests, resulting in a sensitivity of 83.3%, a specificity of 100%, and overall accuracy of 95.6% using both tests.

Among the 19 patients with mitochondrial myopathy, 5 had abnormal jitter analysis.

When the researchers looked only at participants with abnormal jitter analysis but normal RS, two of these were patients with CMS, but another seven had congenital or mitochondrial myopathies. Using abnormal jitter alone therefore resulted in a sensitivity of 100% but a specificity of only 86%, for overall 86.5% accuracy.

“It’s important to notice that if you have an abnormal jitter, we have to look at the clinical symptoms of the patients,” Dr. Marques Caldas said in an interview. “Jitter abnormalities are not enough to distinguish between myasthenic disorder and a myopathic disorder.”

The research used no external funding, and Dr. Marques Caldas had no disclosures.

SOURCE: Caldas VM et al. AANEM 2019. Unnumbered Abstract: Sensitivity of neurophysiologic tests regarding the neuromuscular junction in patients with congenital myasthenic syndromes.

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