Serologic profile of myasthenia gravis and distinction from the Lambert- Eaton myasthenic syndrome

Autoimmune serologic testing is indicated in the initial investigation of a patient believed to have myasthenia gravis (MG) or the Lambert-Eaton myasthenic syndrome (LES). Serologic confirmation of mild, ocular, or recent- onset MG is ensured by a panel of antibody tests more than by any single test, and by testing before commencing immunosuppressant therapy. A positive result for the acetylcholine receptor (AChR)-binding antibody test is characteristic but not diagnostic of MG. AChR-binding or striational autoantibodies are found in 13% of patients with LES, a disorder that tends to be mistaken for MG when the presenting symptoms include ocular or bulbar muscle weakness. Seropositivity for P/Q-type calcium channel-binding antibodies distinguishes these disorders (LES 95%; MG ≤5%). The AChR- blocking antibody assay is not recommended as a screening test for MG, but if the assay for AChR-modulating antibodies is positive, a positive result for AChR-blocking antibodies provides an additional quantitative baseline for future comparisons in monitoring an individual patient's immune response in relation to the clinical course and treatment. Seropositivity for striational antibodies is most predictive of thymoma when coupled with an AChR modulating antibody value of >90% AChR loss. Results of all serologic tests must be interpreted by a physician in the context of a patient's history, physical and electromyographic findings, response to Tensilon, chest imaging, and other serological findings (e.g., striational antibodies, thyroid and gastric autoantibodies, and P/Q-type calcium channel-binding antibodies).