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

- We compared the decremental pattern during low frequency repetitive nerve stimulation in 34 patients with Lambert-Eaton myasthenic syndrome (LEMS) and 44 patients with myasthenia gravis (MG).
- In LEMS, the decrement usually became progressively greater during a train of stimuli, whereas in MG, the decrement usually became less.
- A progressive decrement during low frequency RNS should suggest the possibility of LEMS and may prompt further clinical, serological and electrodiagnostic testing.

ObjectiveWe 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).MethodsLF-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.ResultsThe 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.ConclusionWhen 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.SignificanceA 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.