Dual Autoimmunity: A Case Report of the Sequential Development of Systemic Lupus Erythematosus in a Patient With Anti-MDA5 Dermatomyositis.

Dermatomyositis (DM) and systemic lupus erythematosus (SLE) are chronic rheumatic diseases that can affect multiple organ systems. Both conditions share several similarities, including pathogenic mechanisms, clinical manifestations, and pharmacological treatments. However, the coexistence of DM and SLE is rarely encountered in clinical practice. Here, we present the case of a 42-year-old woman who developed DM, characterized by proximal muscle weakness in the upper and lower extremities, dysphagia, heliotrope rash, periungual erythema, erythematous skin lesions on the neck and arms, elevated serum aldolase and creatine phosphokinase (CPK) levels, and positive anti-melanoma differentiation-associated gene 5 (MDA5) antibodies. A skin biopsy confirmed the diagnosis of dermatomyositis. She was initially treated with high-dose corticosteroids and mycophenolic acid, resulting in early improvement. However, three months after the onset of DM, she presented with persistent DM manifestations and the development of new-onset pancytopenia, arthritis, discoid lesions, positive antinuclear antibodies, and C3 hypocomplementemia, consistent with SLE. Rituximab and tacrolimus were added to her regimen of glucocorticoids and mycophenolic acid, and she responded well to therapy, with resolution of all clinical manifestations by four months after starting rituximab and tacrolimus. At the 18-month follow-up, she remained in complete clinical remission from both DM and SLE. This case underscores the complexity of autoimmune diseases. Although the coexistence of DM and SLE is uncommon, healthcare providers should maintain a high index of suspicion in patients presenting with atypical symptoms or overlapping features. It also emphasizes the challenge of managing multiple autoimmune conditions concurrently.