RNS Not a Meaningful Prognostic Tool in Myasthenia Gravis

Repetitive nerve stimulation (RNS) is a standard tool in the diagnosis of myasthenia gravis (MG), but a new study has found that the test does not appear to be useful in predicting long-term patient outcomes.

MG is an autoimmune disease that affects the neuromuscular junction. In most cases, patients have autoantibodies that target the muscle-type acetylcholine receptor (AChR) at the postsynaptic membrane. Other patients have autoantibodies that target other antigens, and some patients have no detectable autoantibodies.

What unites patients with MG are symptoms of increased fatigability, along with pronounced weakness of ocular, bulbar, and/or limb muscles, noted the study authors in Scientific Reports.

“The disease course is highly variable with long periods of minimal manifestation in some patients and frequent exacerbations of muscle weakness in others,” they wrote, adding that treatment options are available for patients with severe symptoms, and early diagnosis often leads to better long-term outcomes.

One of the methods used to diagnose the condition is RNS, although the authors said the results of such testing can vary based on disease severity and the particular muscles affected. The test measures compound muscle action potentials (CMAP); decreases in CMAP are linked with clinical muscle fatigability. One recent study suggested that RNS might be a useful tool to predict the shift from purely ocular MG to generalized MG. Based on that report, the study inestigators wondered whether RNS test results might also be predictive of a patient’s long-term prognosis.

To find out, the authors looked at 17 years of data (2000-2016) from patients treated for MG at their university medical center. Patients were included in the study if they were observed for at least 1 year and had at least 2 neurological examinations. Ninety-four patients met inclusion criteria, most of whom (88.3%) tested positive for AChR autoantibodies.

The data showed RNS had a higher sensitivity in patients with generalized disease (71.6%) than those with ocular-only MG (38.5%). The test was also more sensitive in patients with AChR autoantibodies. However, CMAP decrement was not significantly associated with AchR autoantibody titers, and it was also not associated with clinical symptom severity, the investigators said.

“We found a correlation between CMAP decrement and the worst recorded clinical status on a group level, but according to our data, [it was] not strong enough on an individual level to be clinically useful,” they found.

In short, they said they found nothing that would support the use of RNS as a predictive tool, although they said it might be possible to design a prospective study that would better be able to identify the potential usefulness of RNS as a prognostic tool.

In the meantime, they said RNS continues to have an important role in MG diagnosis, even though that role may be diminishing with the development of newer diagnostic tools.

“In the era of highly sensitive assays for the detection of specific MG autoantibodies, however, the clinical use of RNS might become limited to seronegative MG cases to confirm diagnosis, where a sufficient number of muscles need to be tested to maximize sensitivity,” they said.

Reference

Tomschik M, Renaud E, Jäger F, et al. The diagnostic and prognostic utility of repetitive nerve stimulation in patients with myasthenia gravis. Sci Rep. Published online February 20, 2023. doi:10.1038/s41598-023-30154-5