[Myasthenia gravis with electrophysiological features of Lambert-Eaton myasthenic syndrome: usefulness of serum autoantibodies assay and neuromuscular junction biopsy].

A 60-year-old man with a 40-year history of seronegative ocular myasthenia gravis (MG) developed generalized weakness cumulating crisis over 2 years. On admission, ophthalmoplegia, severe ptosis and marked generalized weakness were observed without autonomic symptoms and signs. The deep tendon reflexes were decreased, but were normalized after repeated muscle contractions. EMG of the abductor digiti minimi muscle showed low amplitude in compound muscle action potentials (0.6 mV), waxing phenomenon (292%) with 20 Hz repetitive nerve stimulation (RNS), waning phenomenon (34%) in 3 Hz RNS, and posttetanic facilitation (393%). Stimulated single fiber EMG showed reduced jitter with higher stimulation rates. Serum anti-P/Q-type voltage-gated calcium channel antibodies were negative with no evidence of malignancy. Sensitive assay of acetylcholine receptor antibody in serum revealed a positive titer, while conventional assay was negative. A muscle biopsy was performed and immune complex deposition was demonstrated at the endplate. A nearly complete clinical remission and normalization of electrophysiological features followed immunoadsorption and prednisolone therapy. A sensitive immunoassay of acetylcholine receptor antibodies and immunolocalization of complement at the endplate are useful diagnostic tools in cases presenting with features of myasthenia gravis and Lambert-Eaton myasthenic syndrome.