Macrophage Activation Syndrome in MDA5-Positive Dermatomyositis: A Diagnostic and Therapeutic Odyssey in a Young Adult

Introduction Macrophage activation syndrome (MAS) is a rare, potentially fatal hyperinflammatory complication of autoimmune disease, characterized by fever, cytopenias, hyperferritinemia, and multiorgan involvement (1). In patients with inflammatory myopathies, MAS may mimic infection, pancreatitis, or medication reactions, posing diagnostic and management challenges (2). Case Description A 21-year-old male with atopic dermatitis and MDA5-positive dermatomyositis presented initially in October 2023 with fever, fatigue, lymphadenopathy, and rash; EBV serology suggested infectious mononucleosis. Over subsequent months, he developed daily fevers, unintentional weight loss, proximal muscle weakness, inflammatory arthritis, and evolving rash. Laboratory evaluation revealed elevated creatine kinase (CK) (∼300s), ferritin 4,000–5,000 ng/mL, pancytopenia, elevated liver function tests (LFTs), and inflammatory markers. Imaging ruled out pancreatitis despite abdominal symptoms. Extensive workup included skin, liver, and bone marrow biopsies; autoimmune and infectious panels were otherwise unrevealing. MAS was suspected in February 2024 after worsening cytopenias and rising ferritin. The patient was admitted and treated with pulse IV methylprednisolone and intravenous immunoglobulin (IVIG), resulting in stabilization. Rituximab was attempted in April 2024 but discontinued due to an infusion reaction. Maintenance therapy included mycophenolic acid, hydroxychloroquine, and IVIG, with a gradual steroid taper. Long-term follow-up (through 2025) showed sustained remission, resolution of rash and systemic symptoms, and no evidence of active myositis or MAS flares. Discussion MAS complicating MDA5-positive dermatomyositis is rare, life-threatening, and diagnostically challenging (2, 3). High clinical suspicion is warranted in patients with fever, hyperferritinemia, cytopenias, and multisystem involvement. Prompt high-dose corticosteroids and IVIG can induce remission, while biologics may be required in refractory cases. Multidisciplinary care and longitudinal monitoring (labs, IVIG cycles, and pulmonary evaluation) optimize outcomes. Conclusions MAS should be considered in young adults with inflammatory myopathies presenting with persistent fever, cytopenias, and hyperferritinemia. Early recognition, aggressive immunosuppression, and coordinated care are essential to achieve remission and prevent long-term complications.