Background: Adult-onset Still's disease (AOSD) is a rare systemic autoinflammatory disorder characterized by quotidian high-spiking fever, inflammatory arthralgia, and an evanescent salmon-coloured rash. Cardiac involvement is uncommon but may include pericarditis and, less frequently, myocarditis.

Objectives: To report a case of AOSD in which acute perimyocarditis was the initial clinical manifestation, and to highlight the importance of considering systemic autoinflammatory disorders in patients presenting with inflammatory myopericardial syndromes.

Methodology: Case report of an 18-year-old male, including clinical presentation, laboratory investigation, electrocardiography, echocardiography, and treatment course, followed until resolution and clinical stabilization.

Key Results/Findings: The patient presented with persistent fever, myalgias, and arthralgia. Laboratory investigation demonstrated markedly elevated C-reactive protein (CRP 419 mg/L) and increased high-sensitivity cardiac troponin T (10511 ng/L). Electrocardiography showed sinus tachycardia with T-wave inversion in the inferolateral leads, and transthoracic echocardiography revealed a small posterior pericardial effusion without ventricular dysfunction. A diagnosis of non-complicated acute perimyocarditis was established, and antiinflammatory therapy was initiated. During hospitalization, recurrent high-grade fever spikes were associated with progression of an evanescent maculopapular rash and dynamic troponin variation. Marked hyperferritinaemia (11495 µg/L) was subsequently documented. Extensive infectious and autoimmune evaluation was unremarkable, and the constellation of clinical and laboratory findings fulfilled classification criteria for AOSD. High-dose intravenous methylprednisolone led to rapid clinical and biochemical improvement. Corticosteroids were gradually tapered, and adalimumab was introduced as a steroid-sparing strategy, with no recurrence during follow-up.

Conclusion: This case highlights acute perimyocarditis as a potential initial manifestation of AOSD and underscores the importance of considering systemic autoinflammatory disorders in patients presenting with inflammatory myopericardial syndromes and persistent systemic inflammation.

Luís Cotrim completed his Integrated Master's Degree in Medicine in 2021 and holds a Postgraduate qualification in Health Data Science and Analytics, obtained in 2026. Since 2023, he has been a resident in Cardiology in Lisbon, Portugal, with particular interest in Clinical Cardiology and Heart Failure. Throughout this training path, he has delivered multiple oral communications at national and international congresses.