Is the decremental pattern in Lambert-Eaton syndrome different from that in myasthenia gravis?

Objective: We reviewed our experience to determine if the decremental pattern during low frequency repetitive nerve stimulation (LF-RNS) distinguishes between the Lambert-Eaton myasthenic syndrome (LEMS) and myasthenia gravis (MG).

Methods: LF-RNS studies were reviewed from 34 LEMS and 44 MG patients, 4 of whom had antibodies to muscle specific kinase (MuSK). In each train we calculated the ratio between the early and the later decrement. Receiver-operator characteristic curves were calculated to determine the ratio that best distinguished between LEMS and MG.

Results: The late decrement was more often greater in LEMS and the converse was true in MG, but with some overlap in values in individual patients. A late decrement more than 102% of the early decrement discriminated between LEMS and MG in 90% of studies. The decremental pattern in MG patients with MuSK antibodies resembled that in LEMS.

Conclusion: When the decrement becomes progressively greater during low frequency RNS, the patient is more likely to have LEMS than MG, and in MG, is more likely to have MuSK antibodies.

Significance: A progressive decrement in patients otherwise felt to have MG should prompt further clinical, serological and electrodiagnostic tests. Further studies are needed to assess the decremental pattern in MuSK MG.