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.
Keywords: Systemic lupus erythematosus; cohort; myositis.