A Case of Adult-Onset Still's Disease With Hypocomplementemia

Adult-onset Still's disease (AOSD) causes fever, rash, pharyngalgia, and arthralgia through autoinflammation. Its complement titer has not previously received attention because this usually increases during the inflammatory process. Our female patient in her 60s was admitted to the hospital with fever, rash, arthralgia, and pharyngalgia. Her white blood cell count was 19,130/μL, hemoglobin was 11.0 g/dL, platelet count was 26.0 × 10⁴/μL, and ferritin titer was 6,175 ng/mL. Anti-nuclear antibodies and anti-neutrophil cytoplasmic antibodies were negative. The presence of infectious diseases and malignancies was excluded. She was diagnosed with hypocomplementemia at the onset of AOSD because of her low complement component 4 (C4) titer (<5.0 mg/dL). Her complement component 3 (C3) titer was 104.5 mg/dL, which was within normal limits. There was no sign of thrombotic microangiopathy (TMA) or hemophagocytosis. She was treated with high-dose corticosteroids, including pulse methylprednisolone therapy, cyclosporine, methotrexate, and intravenous immunoglobulin, but was resistant to these, and her disease repeatedly flared up. Treatment with intravenous cyclophosphamide eventually led to remission. Post-treatment, her C4 titer increased to within the normal range. Although hypocomplementemia with TMA or hemophagocytosis has been reported in AOSD patients, our patient showed no sign of either at disease onset. Hypocomplementemia of AOSD may be a sign of high disease activity and could be a predictive marker for resistance to standard therapy.