Antibodies to skeletal muscle in myasthenia gravis. Part 2. Prevalence in non-thymoma patients.

Anti-skeletal muscle antibodies (AMA) were measured in 340 non-thymoma patients with myasthenia gravis (MG) by both an ELISA with citric acid muscle extract and by immunofluorescence assay. Three a-AChR negative but AMA-positive patients were found. The occurrence of AMA was related to the age at onset in patients with an onset of MG beyond 40 years and with duration of disease in patients with an earlier onset of MG. This stresses the heterogeneity of late onset and early onset MG. Serial measurements in individual patients revealed no evidence for a thymic role in the generation of AMA. There was no difference in specificity of AMA in early and late onset patients. The presence of AMA in CT-negative patients is not necessarily suggestive of a radiological shortcoming in the detection of thymoma since in 16 AMA-positive patients from our series no thymoma was found at operation or obduction.