Specificity of omega-conotoxin MVIIC-binding and -blocking calcium channel antibodies in Lambert-Eaton myasthenic syndrome.

An immunoprecipitation assay was used to measure omega-conotoxin MVIIC (P/Q-type) binding and blocking calcium channel antibodies in 67 patients with Lambert-Eaton myasthenic syndrome (LEMS) and in a large control population. We first showed the presence of omega-conotoxin MVIIC-blocking antibody in LEMS patients. Binding antibodies were detected in 55 of 67 (82.1%) LEMS patients and in 2 of 296 (0.7%) controls. In contrast, blocking antibodies were positive in 14 of 67 (20.9%) LEMS patients and 8 of 171 (4.7%) controls. No LEMS patient had negative binding antibodies and positive blocking antibodies. The immunoprecipitation assay detected no antibodies against the whole P/Q-type calcium channel in either the paraneoplastic cerebellar degeneration or the amyotrophic lateral sclerosis sera. Neither the omega-conotoxin MVIIC-binding nor the -blocking calcium channel antibodies were correlated with clinical severity across the individuals, but longitudinal studies of some LEMS patients showed an inverse relation between binding antibody titre and disease severity. We concluded that the 125I-omega-conotoxin MVIIC assay for anti-P/Q-type voltage-gated calcium channel antibodies is highly specific for LEMS and that this sensitive binding antibody assay could be more valuable than the blocking antibody assay in the diagnosis of LEMS.