[Seronegative rheumatoid arthritis combined with elbow pigmented villonodular synovitis: A case report].

We report a case of elbow pigmented villonodular synovitis (PVNS) combined with seronegative rheumatoid arthritis (SNRA) and review relevant literature to summarize the characteristics, treatment, and outcomes of this comorbidity. The patient was a 72-year-old female. She developed swelling and pain of the right elbow in 2002, and underwent surgical treatment. Postoperative histopathology confirmed PVNS. In 2007, she developed swelling and pain in the left elbow and underwent synovectomy, and synovectomy again yielded pathology consistent with PVNS. From 2021 onwards, she experienced recurrent bilateral knee pain and swelling. In June 2023, laboratory tests revealed markedly elevated inflammatory markers: C-reactive protein (CRP) 175.3 mg/L, erythrocyte sedimentation rate (ESR) 116 mm/h, hemoglobin 83 g/L, and platelet count 537×109/L. Rheumatoid factor, anti-cyclic citrullinated peptideantibody, anti-keratin antibody, and anti-perinuclear factor were all negative. Right knee magnetic resonance imaging (MRI) showed severe degenerative changes, suprapatellar bursitis with large effusion, and ligamentous injuries. Given her prior PVNS history and knee MRI findings, right knee replacement was performed; however, intraoperatively no nodular or mass-like synovial proliferation was observed. Postoperative synovial pathology demonstrated massive chronic inflammatory cell infiltration, small vessel proliferation, and fibrinoid necrosis-findings typical of rheumatoid arthritis rather than PVNS. After surgery, the left knee continued to experience recurring pain and swelling. In April 2024, she developed swelling and pain in both wrists and right ankle joints. Ultrasound examinations showed synovial proliferation in the wrists, knees, ankles, and metatarsophalangeal joints, with abundant blood flow signals and bone erosion. The patient was diagnosed as SNRA combined with PVNS. Treatment with tocilizumab 400 mg intravenously monthly plus methotrexate 10 mg weekly was initiated. After three months, CRP decreased to 0.73 mg/L, ESR to 6 mm/h. A literature review using Wanfang, CNKI, and PubMed databases with keywords "rheumatoid arthritis" and "pigmented villonodular synovitis" identified only four previously reported cases of RA combined with PVNS. Including the present case, there were 5 cases (4 females, 1 male), the median age of PVNS onset was 45 (32.5-58.0) years. The knee was the most commonly affected joint (4/5 cases), followed by the elbow. PVNS preceded RA in two cases (by 3 and 12 years), rheumatoid arthritis (RA) preceded PVNS in two cases (by 3 and 15 years), and one case had simultaneous onset. Four of the five patients were seropositive for rheunatoid factor/anti-cyclic citrullinated peptide antibody (RF/anti-CCP); only the index case was seronegative. All five patients underwent synovial resection; two (40%) experienced postoperative recurrence. In conclusion, the coexistence of PVNS and SNRA is extremely rare and poses diagnostic challenges due to overlapping clinical manifestations. Physicians should consider the possibility of RA when a patient with PVNS develops symmetric polyarthritis, and conversely, suspect concomitant PVNS when a patient with RA has persistent monoarticular symptoms uncontrolled by disease-modifying therapy. Synovial fluid analysis, MRI findings, and especially histopathological examination are crucial for differentiation. Early recognition and appropriate immunotherapy can significantly improve prognosis.