Case Report: Adult-onset Still's disease presenting with massive hemorrhagic pericardial effusion without the characteristic rash.

Adult-onset Still's disease (AOSD) is a rare systemic autoinflammatory disorder without universally accepted disease-specific biomarkers, although serum ferritin and glycosylated ferritin may provide supportive diagnostic value. Therefore, diagnosis relies on a combination of clinical manifestations, laboratory abnormalities, and exclusion of alternative diagnoses, commonly using criteria such as Yamaguchi or Fautrel. Although serosal involvement has been reported, presentation with massive hemorrhagic pericardial effusion in the absence of the characteristic rash is extremely rare and may mimic tuberculosis, malignancy, or other rheumatic diseases. We report a 67-year-old Asian man who was referred to our department from a local hospital because of persistent cough, fever, and polyserositis predominantly characterized by massive hemorrhagic pericardial effusion. Laboratory evaluation revealed marked systemic inflammation, elevated ferritin levels, a positive interferon-gamma release assay, elevated adenosine deaminase in pericardial fluid, increased tumor markers, and anti-SAE1 positivity, initially raising suspicion for tuberculous pericarditis, malignancy, or autoimmune myopathy. However, comprehensive microbiological, cytological, imaging, and neuromuscular assessments found no evidence supporting these conditions, and the patient did not develop the typical evanescent rash of AOSD. After initiation of diagnostic glucocorticoid therapy, rapid defervescence and marked resolution of serous effusions were observed, supporting the diagnosis. This case highlights an atypical presentation of AOSD and emphasizes that AOSD should be considered in patients with unexplained serositis, particularly hemorrhagic pericardial effusion accompanied by systemic inflammation.