Low-Frequency Repetitive Nerve Stimulation in Myasthenia Gravis and Lambert-Eaton Myasthenic Syndrome

Lambert-Eaton myasthenic syndrome (LEMS) and myasthenia gravis (MG) exhibit distinct decrement patterns in low-frequency repetitive nerve stimulation. While a delayed nadir in LEMS is well-documented, we identified additional unique patterns. This study systematically compared low-frequency repetitive nerve stimulation features between the two conditions using prolonged protocols.

We retrospectively analyzed 20 patients with LEMS (March 2019-August 2025) and 34 age-matched patients with MG (April 2023-August 2025). All subjects underwent low-frequency repetitive nerve stimulation (3 Hz, 5 Hz; 25 stimuli per train) on the abductor digiti minimi and trapezius muscles. Key parameters included compound muscle action potential nadir position and postnadir amplitude recovery.

As previously reported, patients with LEMS had significantly lower baseline compound muscle action potential amplitudes than patients with MG. In MG, 90.9% of tested muscles showed a compound muscle action potential nadir at the 4th or 5th response. By contrast, 65.5% of LEMS muscles exhibited a delayed nadir between the 7th and 10th responses. Notably, 24.1% of LEMS muscles displayed a nadir at waves 4 to 5 followed by substantial amplitude recovery. In this subgroup, the amplitude difference between the nadir and the 10th or 25th response showed high sensitivity and specificity in differentiating LEMS from MG. Furthermore, 10.3% of LEMS muscles exhibited direct facilitation, characterized by an amplitude increase without an initial decrement.

In addition to the previously reported delayed decrement, we identified two additional abnormal low-frequency repetitive nerve stimulation patterns in LEMS: early decrement with postnadir recovery and direct facilitation. These three LEMS-specific patterns differ from those observed in MG and may aid in the differential diagnosis of MG and LEMS.