Inflammatory myositis in systemic lupus erythematosus.

Background: The coexistence of inflammatory myositis in systemic lupus erythematosus (SLE) has not been extensively studied. In this study, we describe the incidence, distinct types of inflammatory myositis, and risk factors for this finding in a cohort of pediatric and adult SLE patients.

Methods: We identified SLE patients with coexisting myositis followed between 2010 and 2019 at two pediatric hospitals and one adult hospital. Demographic, clinical, laboratory, and pathological features of myositis were collected, and descriptive statistics were applied.

Results: A total of 1718 individuals were identified as having SLE (451 pediatric and 1267 adult patients). Of these, 108 were also diagnosed with inflammatory myositis (6.3%). People of black race had a significantly higher prevalence of inflammatory myositis, as did those with childhood-onset SLE compared to adult-onset disease. In the majority of patients (68%), SLE and inflammatory myositis presented concurrently. Overlapping features of systemic sclerosis occurred in 48%, while dermatomyositis-specific rashes were present in a third. Arthralgias and inflammatory arthritis were seen in >90%. Thrombotic events and significant pregnancy-related morbidity were present in more than a third of patients. Lymphopenia, hypocomplementemia, and a positive RNP were the most common laboratory features noted. Myositis-specific antibodies (MSA) and myositis-associated antibodies (MAA) were present in >40% of patients. A review of 28 muscle biopsy reports revealed a wide array of pathological features, including nonspecific changes, dermatomyositis, polymyositis, and necrotizing auto-immune myopathy.

Conclusion: In our SLE patient population, 6.3% presented with concurrent inflammatory myositis. Dermatomyositis-specific rashes, clinical features of systemic sclerosis, arthralgias and arthritis, and cytopenias were common coexisting clinical manifestations. A high frequency of RNP, MSA, and MAA were found. People of black race and with childhood-onset disease had a higher prevalence of myositis. Our findings suggest that SLE patients of black race, with childhood-onset SLE, and who possess MSA or MAA should be routinely screened for myositis.