Anti-MDA5 Dermatomyositis Presenting With Lupus-Like Mucocutaneous Features and Inflammatory Arthritis: A Case Report.

Understanding diagnosis and management of specific autoimmune diseases is based on existing classifications or diagnostic criteria. However, patients frequently present with broad, multisystem features that mimic multiple autoimmune diseases. Recognizing clinical mimicry is crucial because overclassifying these features as distinct coexisting diseases often presents a diagnostic challenge. We report the case of a 36-year-old man who presented with progressive weakness. The patient initially complained of facial rash and skin lesions, which then developed into tetraparesis, arthralgia, and difficulty swallowing. Physical examination revealed various skin and musculoskeletal manifestations, including erythematous plaques on the face without involving the nasolabial folds, discoid rash, heliotrope rash, Gottron's sign, and multiple ulcers. Neurological examination showed decreased muscle strength and motor lesions. Laboratory results showed an increase in inflammatory markers, positive antinuclear antibodies (ANAs) as well as positive myositis-specific antibodies for melanoma differentiation-associated protein (MDA)-5. Electromyography (EMG) examination showed findings consistent with myogenic lesions. Chest CT scan showed interstitial lung disease (ILD), and hand radiography showed juxta-articular wrist osteopenia consistent with early signs of inflammatory arthritis. Based on these findings, the patient was diagnosed with MDA-5 dermatomyositis (DM) with lupus-like features and inflammatory arthritis, representing a high-risk DM phenotype that clinically mimics a triple overlap syndrome, rather than three distinct confirmed diseases. Initial treatment involved the administration of high-dose methylprednisolone followed by a gradual dose reduction, hydroxychloroquine, and mycophenolate sodium after the secondary skin infection resolved. The patient received multidisciplinary care involving various specialties. After treatment, the patient's motor weakness improved. Distinguishing severe phenotypes of connective tissue diseases (CTDs) from true overlapping syndromes is a diagnostic challenge due to overlapping symptoms and laboratory findings. Accurate diagnosis requires a comprehensive clinical and serological evaluation, as well as a deep understanding of the classification criteria for each disease to avoid overclassification. Early diagnosis and aggressive treatment targeting the primary driving pathology can improve the prognosis and quality of life of patients.