Electrophysiological differences in seropositive and seronegative Lambert-Eaton myasthenic syndrome

In order to determine whether there is any difference between voltage-gated calcium-channel antibody (VGCC-Ab)-positive and -negative groups in Lambert-Eaton myasthenic syndrome (LEMS), we compared the clinical and electrophysiological features between 13 patients with VGCC-Ab and 6 VGCC-Ab-negative patients. No obvious difference was observed in the various clinical features or findings on single-fiber electromyography between seropositive and seronegative cases. In seropositive cases, the compound muscle action potential (CMAP) amplitude was lower but the increment on post-exercise facilitation (PEF) and high-rate stimulation (HRS) was significantly higher than in the seronegative group, indicating that the repetitive nerve stimulation (RNS) test in the seropositive group is more typical of LEMS and more severe. A 100% increment as the diagnostic criterion in the routine RNS test was satisfied in all seropositive cases but in only three seronegative cases, whereas a 60% increment as the diagnostic criterion was found in all seronegative cases. The classic triad (low CMAP amplitude, decrement at low rate of stimulation, and increment at PEF or HRS) of RNS is rare, adding to the difficulty in diagnosing LEMS in the seronegative group, and making a 60% increment criterion more critical for the diagnosis of this disorder.