Multicentric Reticulohistiocytosis mimicking seronegative rheumatoid arthritis: a case report of destructive erosive arthritis with partial response to Upadacitinib

Multicentric reticulohistiocytosis (MRH) is a rare non-Langerhans cell histiocytosis that can mimic seronegative rheumatoid arthritis through destructive inflammatory arthritis and papulonodular skin lesions. We report a 44-year-old woman with progressive polyarthritis, purplish papulonodular lesions and destructive erosive disease of the hands and feet. Rheumatoid factor, anti-cyclic citrullinated peptide antibody and antinuclear antibody were negative. Radiographs showed sharply demarcated erosions with prominent distal interphalangeal involvement, and skin biopsy confirmed MRH. Available malignancy screening, including oncology review, mammography, cervical screening and blood tests, was reassuring, although planned PET-CT was not completed due to loss to follow-up. Methotrexate and corticosteroids were not tolerated, certolizumab produced partial response, and upadacitinib led to modest cutaneous improvement but persistent active synovitis at four months. This case highlights MRH as a diagnostic mimic of seronegative rheumatoid arthritis and illustrates discordant treatment response to JAK inhibition.