Thymus histology and concomitant autoimmune diseases in J apanese patients with muscle‐specific receptor tyrosine kinase‐antibody‐positive myasthenia gravis

Background and purpose The differences in the characteristics of thymus histology, coexisting autoimmune diseases and related autoantibodies between anti‐muscle‐specific receptor tyrosine kinase ( M u SK )‐antibody ( A b)‐positive myasthenia gravis ( MG ) patients, and anti‐acetylcholine receptor ( AC h R )‐ A b‐positive MG patients are not clearly defined. Methods The types of thymus histology, coexisting autoimmune diseases and associated Abs in 83 M u SK ‐ A b‐positive patients nationwide were investigated and were compared with those in AC h R ‐ A b‐positive patients followed at our institute ( n  = 83). As for the autoantibodies associated with thymoma, titin Abs were measured. Results Thymoma was not present in any of the M u SK ‐ A b‐positive patients but presented in 21 patients (25.3%) amongst the AC h R ‐ A b‐positive patients. Titin Abs were absent in M u SK ‐ A b‐positive patients but positive in 25 (30.1%) of the AC h R ‐ A b‐positive patients. Concomitant autoimmune diseases were present in eight M u SK ‐ A b‐positive patients (9.6%) amongst whom H ashimoto's thyroiditis and rheumatoid arthritis predominated, whereas 22 AC h R ‐ A b‐positive patients (26.5%) had one or more concomitant autoimmune diseases of which G raves' disease predominated. Conclusions Differences in frequency of thymoma and thymic hyperplasia, coexisting autoimmune diseases and autoantibody positivity between M u SK ‐ A b‐positive and AC h R ‐ A b‐positive MG were indicated, suggesting that, in contrast with AC h R ‐ A b‐positive MG , thymus does not seem to be involved in the pathogenic mechanisms of M u SK ‐ A b‐positive MG .