RS3PE as a Paraneoplastic Manifestation of Acute Myeloid Leukaemia With IDH1 and KMT2A Mutations: A Case Report.

Remitting seronegative symmetrical synovitis with pitting oedema (RS3PE) is an inflammatory syndrome of older adults that has been increasingly associated with underlying malignancy. A 67-year-old woman presented with abrupt-onset symmetrical polyarthritis and marked bilateral pitting oedema. Laboratory investigations showed elevated inflammatory markers and normocytic anaemia, with negative rheumatoid serology. She achieved complete remission with low-dose corticosteroids. Months after steroid withdrawal, inflammatory arthritis recurred alongside worsening anaemia (haemoglobin 7.6 g/dL), leukopenia (white blood cell count 2.5 × 10⁹/L; absolute neutrophil count 0.38 × 10⁹/L), platelet count of 150 × 10⁹/L, and circulating blast cells. Bone marrow biopsy confirmed acute myeloid leukaemia (AML) with 33%-40% blasts. Molecular analysis identified IDH1 and KMT2A mutations. Treatment with azacitidine-venetoclax resulted in complete remission of both haematological disease and rheumatologic manifestations without corticosteroids. RS3PE may represent an early paraneoplastic manifestation of AML. Recurrence of symptoms after steroid withdrawal should prompt evaluation for malignancy. Molecular characterisation may provide additional insight into disease mechanisms and treatment response.