Rheumatic Disease Risk Increased in Myasthenia Gravis After Thymectomy

Myasthenia gravis was associated with incident autoimmune rheumatic diseases, with a particularly increased risk for rheumatoid arthritis (RA), primary Sjogren syndrome (pSS) and systemic lupus erythematosus (SLE) in those with myasthenia gravis who underwent thymectomy, according to a report published in Rheumatology.

Investigators were interested in exploring the relationship between myasthenia gravis and autoimmune rheumatic diseases because of prior evidence that suggested the conditions share common pathogenic mechanisms. Etiologic similarities could offer researchers and clinicians important clues about the nature of these associated disorders and their interactions with myasthenia gravis.

This population-based, nationwide cohort study used insurance claims between 2001 and 2012 provided by the National Health Research Institute of Taiwan to identify patients with newly diagnosed myasthenia gravis. Data from participants were compared with data from age- and sex-matched controls from the general population. The primary outcome was the incidence rate of rheumatic disease, with Cox regression used to estimate hazard ratios (HR) that were adjusted for sex, age, and comorbidities and stratified according to age, sex, and treatment.

Data from a total of 6478 patients with myasthenia gravis (58.03% women; mean age, 50.55 years) were compared with data from 25,912 participants without myasthenia gravis (58.03% women; mean age, 50.52 years). The risk for rheumatic disease was 6.25 times higher in the myasthenia gravis cohort vs the control cohort, with an adjusted HR (aHR) for total rheumatic diseases of 6.34 (95% CI, 4.54-9.12; P <.001). In addition, the entire myasthenia gravis cohort exhibited a significantly greater chance of being diagnosed with pSS (aHR, 15.84; 95% CI, 8.39-23.91; P <.001), SLE (aHR, 11.32; 95% CI, 5.04-25.42; P <.001) or other types of rheumatic diseases (aHR, 4.07; 95% CI, 1.31-12.62; P =.015).

Among those in the myasthenia gravis group, those who underwent thymectomy had increased risks for RA (aHR, 4.41; 95% CI 1.32-14.73; P =.016), pSS (aHR, 15.06; 95% CI 5.27-43.01; P <.001), and SLE (aHR, 23.68; 95% CI 8.52-65.84; P <.001), as well as an increase in total rheumatic disease risk (aHR, 10.48; 95% CI, 5.96-18.45; P <.001). In contrast to thymectomy, plasmapheresis did not significantly affect the odds of rheumatic disease development and diagnosis. Overall, total rheumatic disease and subgroup rheumatic disease incidence rates were significantly higher in the myasthenia gravis group vs the non-myasthenia gravis group (log-rank P <.001).

Study strengths included a large sample size, large validation cohort, and long-term monitoring for and identification of concurrent autoimmune disorders.

Study limitations included possible misclassification bias, inability to analyze correlations between disease activity and severity of myasthenia gravis and rheumatic diseases, and lack of important laboratory and clinical information such as myasthenia gravis subtype.

“The clinical identification of the subgroup of patients with [myasthenia gravis] at an increased risk of associated autoimmune disease would be of considerable interest,” concluded the authors, discussing the clinical and therapeutic relevance of their findings. They recommended that future research continue to explore the underlying pathogenesis of these conditions in an effort to expand therapeutic options.

Reference

Chang CC, Lin TM, Chang YS, et al. Thymectomy in patients with myasthenia gravis increases the risk of autoimmune rheumatic diseases: a nationwide cohort study [published online September 4, 2018]. Rheumatology (Oxford). doi:10.1093/rheumatology/key236