Objectives Myositis is an infrequent feature of SLE and may often be overlooked. We aimed to estimate the incidence of myositis in SLE, and to determine demographic and clinical factors associated with it. Methods Within our lupus cohort, we identified potential myositis cases using the SLICC Damage Index for muscle atrophy or weakness, the SLEDAI-2K item for myositis, and annually measured serum creatinine kinase. Cases were confirmed through chart review. We performed descriptive analyses of prevalent myositis cases as of January 2000. From that point onward, we studies patients without myositis to determine risk of incident myositis, using cohort analyses adjusted for demographic variables (age, sex, race/ethnicity). Results As of January 2000, there were 5 prevalent myositis cases in our SLE cohort. Among 560 SLE patients with a study visit from January 2000 onward, with no history of myositis at baseline, 5 new cases (4 females, 1 male) were identified over an average follow-up of 8.5 years (incidence 1.05 cases per 1000 person-years). There was a higher proportion of Caucasians in the non-myositis group versus myositis group, with a trend for fewer females in the myositis cases. Arthritis, Raynaud’s phenomenon, and anti-Smith antibodies were common pre-existing features, occurring in all incident myositis cases. In Cox regression analyses adjusting for age, race/ethnicity and sex, non-Caucasian patients had a markedly increased risk of developing myositis. Conclusion We found a low incidence of myositis in our SLE cohort. A cluster of variables, particularly non-Caucasian race/ethnicity, arthritis, Raynaud’s phenomenon, and anti-Smith antibodies were associated with risk of developing myositis in SLE. These variables may aid clinicians in identifying SLE patients at highest risk for this important complication.

Objectives To determine if serologic phenotypes could be identified in systemic lupus erythematosus patients developing interstitial lung disease (ILD) and/or myositis. Methods Adult SLE patients (without myositis/ILD at baseline) had annual assessments and serum sampling between 2000 and 2017. New-onset ILD was identified using the SDI pulmonary fibrosis item. New-onset myositis was identified using the SLICC Damage Index muscle atrophy/weakness item, the SLEDAI-2K item for myositis, and annual creatinine kinase testing. Chart review confirmed ILD/myositis cases and randomly sampled SLE patients from baseline formed our sub-cohort (N = 72). Cases and sub-cohort were compared regarding myositis-related biomarkers at baseline and at a randomly selected follow-up between baseline and end of observation (date of ILD/myositis diagnosis or Dec. 31, 2017). Descriptive analyses and hazards ratios (HRs) were generated for ILD/myositis incidence, focusing on baseline serology and adjusting for sex, race/ethnicity, age at SLE diagnosis, and SLE duration. Results Fourteen SLE patients developed ILD (N = 9), myositis (N = 3), and/or both (N = 2). Thirteen of those (92.9%) developing ILD/myositis had at least one biomarker at baseline, versus 47 (65.3%) SLE patients who never developed myositis/ILD. The most common biomarkers in myositis/ILD were KL-6, anti-Ro52, and anti-Ku. Baseline biomarkers tended to remain positive in follow-up. In multivariate Cox regressions, SLE patients had higher risk of developing myositis/ILD with elevated baseline KL-6 (adjusted hazard ratio 3.66; 95% confidence interval 1.01, 13.3). When updating biomarkers over time, we also saw correlations between anti-Smith and ILD/myositis. Conclusions Baseline myositis-related biomarkers were highly associated with ILD/myositis incidence. This is the first identification of biomarker phenotypes with ILD/myositis risk in SLE.