Isolated Myositis Unveiling Systemic Lupus Erythematosus: A Rare Initial Presentation.

Systemic lupus erythematosus (SLE) is a heterogeneous autoimmune disease with a broad range of clinical manifestations. While musculoskeletal involvement is common in SLE, myositis as the initial and isolated presentation is rare and can lead to diagnostic delays. We report the case of a 19-year-old female who presented with progressive, symmetrical proximal muscle weakness over several weeks, without accompanying joint, cutaneous, or systemic symptoms. Laboratory workup revealed elevated serum creatine kinase and positive antinuclear antibodies. Electromyography findings were suggestive of inflammatory myopathy, and muscle biopsy confirmed autoimmune myositis. Further autoimmune testing confirmed the presence of anti-dsDNA and anti-Sm antibodies. Based on the 2019 EULAR/ACR classification criteria, a diagnosis of SLE with predominant myositis was established. The patient showed significant clinical improvement following the initiation of corticosteroids and hydroxychloroquine. This case underscores the importance of considering SLE in the differential diagnosis of inflammatory myopathies, especially in young females presenting with isolated proximal muscle weakness. Early recognition and treatment of lupus-associated myositis can prevent disease progression and improve outcomes.