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Which patients are most likely to have a positive RNS test for myasthenia gravis?



Patients with suspected myasthenia gravis are more likely to have positive repetitive nerve stimulation (RNS) findings if they undergo testing in an inpatient setting, are seropositive, or are classified as Myasthenia Gravis Foundation of America (MGFA) Class III or higher, according to research presented at the annual meeting of the American Association of Neuromuscular and Electrodiagnostic Medicine.

Low-frequency RNS is a common test that neurologists perform to evaluate a patient for myasthenia gravis. The effects of various clinical factors on the diagnostic yield of this test are unknown, however.

Myasthenia gravis is “mostly a clinical diagnosis,” study first author Tingting Hua, a medical student at the University of Missouri in Columbia, said in an interview. “RNS is just one of the helpful diagnostic tests for it. If we can find out in what kind of populations of patients this test is more helpful, maybe that would help cut down unnecessary tests in patients for whom it’s not necessarily helpful.”

Ms. Hua and her colleagues conducted research to assess the effects of clinical, serologic, and demographic factors on the diagnostic yield of RNS. They retrospectively analyzed patients with an established diagnosis of myasthenia gravis and at least 1 year of follow-up. The variables that the investigators examined were demographic characteristics, MGFA class, RNS study results, antibody test results, thymoma status, and treatments received.

Ms. Hua and her colleagues included 65 patients in their analysis. Thirty-one patients were female. Fifty-five patients were white, eight were black, and two were categorized as “unknown.” Of this population, 32 patients (49.2%) were in MGFA Class I, 14 (21.5%) were in MGFA Class IIa, 13 (20.0%) were in MGFA Class IIb, and the remaining 6 (9.2%) were in MGFA Classes IIIa through V. Twenty-seven patients (42%) had positive RNS studies. Twenty-one patients (32%) were seropositive for myasthenia gravis antibodies.

Eleven patients underwent RNS in an inpatient setting, and 54 were tested in an outpatient setting. Acetylcholine receptor (AChR) binding antibody titer ranged from 0.12 nmol/L to 118 nmol/L. The RNS results were significantly more likely to be positive for seropositive patients, compared with seronegative patients. Patients with MGFA Class III or higher also had higher likelihood of positive RNS results, compared with patients in lower classes. Finally, the diagnostic yield was highest for patients with MGFA Class III or higher who were tested in an inpatient setting.

The study was supported by a Missouri School of Medicine Summer Research Fellowship.

SOURCE: Hua T et al. AANEM 2019, Abstract 9.

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