P58 An unusual case of a swollen knee

• Published in: Rheumatology advances in practice 2024-11, Vol.8 (Supplement_1)
• Main Authors: Mackay, Madeleine, Pai, Nandita, Deepak, Samundeeswari, Warrier, Kishore
• Format: Article
• Language: English
• ISSN: 2514-1775; 2514-1775
• DOI: 10.1093/rap/rkae117.089

Abstract Introduction This is a rare presentation of a rapidly progressive monoarthritis in a child with a complex medical background. She has short stature with growth hormone deficiency, on growth hormone replacement; microcephaly, developmental delay, anaemia, multiple allergies and previous febrile seizures. Parents are consanguineous. Genetic testing observed several regions of homozygosity (consistent with consanguinity), but Trio exome sequence analysis did not identify a genetic cause for her problems. It outlines the importance of careful investigation, revisiting diagnosis when there is not the expected response to treatment, and how medical background may influence the differential diagnosis to consider. Case description The patient presented at age 7 years to her local A&E with a 5-day history of left knee swelling, no pain and weight bearing with a normal gait. She was systemically well. She had a history of viral URTI one week prior. She was diagnosed with likely reactive arthritis and was followed up in orthopaedic clinic 3 weeks later. Her swelling persisted and she was referred to Rheumatology. On review in the Paediatric Rheumatology Clinic 3 months later, she had a large effusion in the left knee, the knee was warm but had good range of movement with minimal restriction at the end of extension and no pain. There was no other joint involvement, or systemic symptoms. An urgent MRI scan and joint aspiration were arranged. Investigations showed low Hb (108 g/L), high plt (537 × 109/L), high WCC (15.2 × 109/L) but normal inflammatory markers (CRP 1.2 mg/L, ESR 14 mm/hr). Her ANA was weakly positive (400). RF, Lyme serology, and QuantiFERON negative. Synovial fluid was negative for microscopy, culture, acid fast bacilli and TB culture. MRI of her left knee showed a very large joint effusion with significant thickened and enhancing synovium; erosive osseous lesions at the lateral femoral condyle and medial tibial plateau. Appearances were unusual. Differentials included an atypical infection such as TB as well as juvenile idiopathic arthritis (JIA). An urgent referral was sent to orthopaedics for synovial biopsy to rule out atypical infection. This showed a hyperplastic thickened synovium suggestive of chronic inflammation with calcification, no evidence of infection or PVNS. She was treated as presumed JIA. She had minimal response to an intra-articular steroid injection to the right knee under and was started on methotrexate. Discussion The p