Repetitive Nerve Stimulation Test Could Be Useful to Diagnose LEMS, Myasthenic Crisis, Study Shows

A repetitive nerve stimulation test can help diagnose and differentiate between Lambert-Eaton myasthenic syndrome (LEMS) and myasthenia gravis (MG) patients undergoing a myasthenic crisis, a study shows.

The study, “Repetitive nerve stimulation test in myasthenic crisis,” was published in the journal Muscle & Nerve.  

LEMS and MG are both autoimmune disorders of the neuromuscular junction, the site where specialized nerve cells called motor neurons communicate with muscle fibers.

The first symptom of MG is weakness in eye muscles and sometimes double vision, but patients may also have difficulties swallowing and talking. In fact, an MG crisis occurs when the respiratory muscles get too weak to move enough air in and out of the lungs, and the patient is unable to breathe independently, requiring the assistance of a ventilator.

MG and LEMS are characterized by different autoantibodies against different targets — P/Q type of voltage-gated calcium channels (VGCC) in LEMS and acetylcholine receptors (AChRs) in myasthenia gravis. Blood tests for these antibodies are currently the way to diagnose these diseases.

However, testing for levels of antibodies against voltage-gated calcium channels may not be highly specific for LEMS, according to a previous study.

Moreover, due to the potential severity of an MG crisis, accurate ways to diagnose and distinguish it from other similar disorders is crucial.

The repetitive nerve stimulation (RNS) test uses electrodes to send small electrical pulses to measure the nerve’s ability to send these signals to the muscle tissue. The electrodes are usually placed on a patient’s skin over the muscles the clinicians need to test.

If the signal transmission worsens with muscle fatigue, the patient is diagnosed with MG.

Despite being considered an accurate and fast diagnostic tool for MG, reports on the use of RNS as a diagnostic tool for an MG crisis are scarce.

A group of researchers from the University of Alabama reviewed the results of the RNS test in a group of patients who had experienced a myasthenic crisis. Specifically, they reviewed the medical records of 26 patients who developed an MG crisis between 1997 and 2017.

The RNS test was performed in four muscles in 13 patients, in three muscles in six patients, and in two muscles in seven patients.

Results showed that in more than half of the patients (18 patients), an MG crisis is the first manifestation of MG as shown by abnormal RNS test outcomes.

Moreover, researchers found that the RNS test identified three different patterns of abnormal results — 22 patients had the “classic MG pattern,” one acute MG patient had a LEMS pattern, and another MG patient had a cholinergic crisis pattern, a condition of characterized by the accumulation of the neurotransmitter acetylcholine in the neuromuscular junctions.

These results suggest that “the RNS test can serve as a rapid and sensitive diagnostic tool for MG in a majority of patients,” the study concluded.