Patients affected by one autoimmune disorder have a higher risk of developing a second one, and the prevalence is higher in females than in males. MG patients have an increased risk of other autoimmune disorders compared to the rest of the population without MG. Autoantibodies that are characteristic for autoimmune disorders can be found in MG patients without any of the clinical symptom . Furthermore the prevalence of manifest autoimmune disorders in patients with MG has been reported in the ranges from 8.7 to 25% in the literature [4, 9,10,11]. In a systematic review, they stated that autoimmune thyroid disease was the most frequent autoimmune disorder, occurring in 10% of MG patients . Other common autoimmune associates with MG are systemic lupus erythematosus (SLE), RA, dermatomyositis, polymyositis, and Addison’s disease . MuSK antibodies are present in 10–70% of all MG patients without AChR antibodies. Only few studies have reported the associated autoimmune disorders in MuSK-MG. An association between MuSK-MG and SLE or relapsing-remitting multiple sclerosis (MS) has been suggested in previous reports [1, 12, 13].
Combined RA and MG occurrence has been calculated as 4% in a previous study with 75 MG patients . From the literature review only one RA patient who is treated with penicillamine has been reported to have MuSK antibody positivity besides the AchR antibodies . Our patient had not received any treatment with penicillamine which may become a triggering factor for her MG . All the cases which have been reported in the literature have RA and AchR antibody-positive MG except the penicillamine-related case [4, 7, 15]. To our knowledge, our case is the first reported case who has RA and anti-MuSK antibody-positive MG. If we concentrate also on the disease progression of co-occurred autoimmune disorder, it is reported that MG presentation was generalized in all the patients who have RA like our patient. Additionally, the manifestations of RA were also classically less severe .
The pathogenesis for the co-occurrence of different varieties of autoimmune disorders is unclear; however, genetic, infectious, and immunological factors have been implicated, and abnormalities in both humoral and cell-mediated immunity have been described. Genetic studies on the susceptibility genes in autoimmune disorders reveal that is the most strong relationship at the human leukocyte antigen (HLA) locus [1, 4]. Particular role of the HLA-B8-DR3 and HLA DR14-DQ5 had been suggested in, respectively, MS and pemfigus association with MuSK-MG [5, 16,17,18]. CTLA4 gene polymorphisms are also reported as associated with MG and other autoimmune diseases such as type 1 diabetes mellitus, autoimmune thyroid disease, SLE, RA, and celiac disease .