Severe Late-Onset Muscular Dystrophy Mimicking Inflammatory Myopathy in an 18-Year-Old Woman: A Case Report

Abstract

Background: Muscular dystrophies (MDs) are a heterogeneous group of inherited disorders characterized by progressive muscle degeneration and weakness. Although most cases are diagnosed in childhood, late-onset forms may present during adolescence or adulthood and mimic inflammatory myopathies, leading to diagnostic delays and inappropriate treatment. Early recognition is essential to avoid unnecessary immunosuppressive therapy and to enable appropriate genetic counseling. Case Presentation: We report the case of an 18-year-old woman, born to a first-degree consanguineous marriage, who presented with a two-year history of progressive proximal muscle weakness predominantly affecting the lower limbs. Laboratory investigations revealed markedly elevated serum creatine phosphokinase (CPK) levels (7982 IU/L), and electromyography demonstrated a myogenic pattern. The patient was initially treated with high-dose corticosteroids followed by methotrexate, without clinical improvement. A muscle biopsy of the quadriceps revealed marked fiber size variation, internal nuclei, endomysial fibrosis, minimal inflammatory infiltrate, and reduced sarcolemmal protein expression, confirming a diagnosis of muscular dystrophy. Genetic testing was recommended. The patient was managed with supportive therapy including physiotherapy. Conclusion: This case highlights the diagnostic challenge of distinguishing late-onset muscular dystrophy from inflammatory myopathies. Key indicators include treatment resistance, consanguinity, and characteristic histopathological findings. Early and accurate diagnosis is crucial to prevent inappropriate therapy and to provide genetic counseling.