Distinguishing Features of the Repetitive Nerve Stimulation Test Between Lambert-Eaton Myasthenic Syndrome and Myasthenia Gravis, 50-Year Reappraisal.

OBJECTIVE

To reappraise the distinguishing features of the repetitive nerve stimulation (RNS) tests in the abductor digiti quinti muscle between myasthenia gravis (MG) and Lambert-Eaton myasthenic syndrome (LEMS) 50 years after the 1965's Lambert seminal paper.

METHODS

The various parameters of the RNS test were compared between 34 patients with LEMS and 140 patients with MG to assess their diagnostic sensitivity.

RESULTS

RNS test was abnormal in all (100%) patients with LEMS and 76 (54%) patients with MG. The diagnostic hallmark of LEMS, ≥60% increment at high-rate stimulation or postexercise facilitation, was present in 33 (97%) of patients. The diagnostic hallmark of MG, decrement at low-rate stimulation (LRS), was present in 65 (46%) of patients with MG. The most prominent difference in the various parameters of RNS tests was noted between LEMS versus MG and normal controls.

CONCLUSIONS

Distinguishing features of the RNS test in LEMS and MG are confirmed in this direct comparison study: the low compound muscle action potential amplitude, decrement at LRS, and increment at high-rate stimulation or postexercise facilitation for LEMS and normal compound muscle action potential amplitude and decrement at LRS for MG.