Not Just Myocarditis: Mixed Connective Tissue Disease (MCTD) and Overlap Myositis With Anti-Ku Positivity in a Young Male With Shortness of Breath.

Mixed connective tissue disease (MCTD) is an autoimmune disorder characterized by high levels of anti-U1 ribonucleoprotein (RNP) antibodies and overlapping clinical features of autoimmune diseases, such as systemic lupus erythematosus (SLE), systemic sclerosis (SSc), and polymyositis (PM). Anti-Ku antibodies have been associated with overlap syndromes, which can present with symptoms such as Raynaud's phenomenon, arthritis, and myositis.  A 19-year-old male athlete presented with myositis, notable for cardiac involvement. Diagnostic testing revealed elevated anti-RNP and anti-Ku antibodies, and a muscle biopsy indicated scleromyositis/overlap myositis. The patient was treated with high-dose corticosteroids, intravenous immunoglobulin (IVIG), rituximab, and mycophenolate mofetil, which led to significant improvement in muscle strength and cardiac function. This case highlights the diagnostic complexity of MCTD when associated with positive anti-Ku antibodies, overlap syndromes, and cardiac involvement. Successful management emphasizes the importance of a tailored, multi-modal therapeutic approach.