Adult-Onset Still’s Disease Revealed by Myopericarditis: A Case Report from Parakou, Benin

Introduction: Adult-onset Still’s disease (AOSD) is a rare systemic inflammatory disorder of unknown etiology, characterized by marked clinical polymorphism and the absence of a specific diagnostic biomarker. Cardiac involvement, particularly myopericarditis, is uncommon and may represent a diagnostic challenge. Case Presentation: We report the case of a 25-year-old male with no significant past medical history, admitted for febrile myopericarditis. The initial clinical presentation included high-grade fever (40.5°C), retrosternal chest pain, marked neutrophilic leukocytosis (25.3 × 10⁹/L with 88.9% neutrophils), elevated inflammatory markers, and increased cardiac troponin I levels. Transthoracic echocardiography revealed a moderate circumferential pericardial effusion with preserved left ventricular systolic function (ejection fraction 60%). During hospitalization, the patient developed diffuse disabling arthralgia, odynophagia with an erythematous pharynx, and a transient evanescent rash. The diagnosis of AOSD was established according to the Yamaguchi classification criteria. High-dose corticosteroid therapy (prednisone 1 mg/kg/day) was initiated, leading to rapid clinical improvement, with complete resolution of fever and arthralgia within two weeks. Conclusion: This case highlights an atypical presentation of AOSD revealed by myopericarditis and underscores the diagnostic challenges encountered in resource-limited settings. Early recognition and prompt corticosteroid therapy are associated with favorable outcomes.