From Synovial Fluid to Musculoskeletal Ultrasound: A Portrayal of Joint Involvement in Camptodactyly-Arthropathy-Coxa Vara-Pericarditis Syndrome: A Case Series

Background/Objectives: To describe the musculoskeletal ultrasound (MSUS) findings of pediatric patients with genetically confirmed CACP syndrome and to assess their clinical, laboratory, and synovial fluid characteristics. Methods: This single-center, cross-sectional study included 6 pediatric patients from 2 consanguineous families with genetically confirmed PRG4 mutations. Clinical features, laboratory results, synovial fluid analysis, and MSUS examinations of the knee joints were evaluated. Synovial effusion, hypertrophy, and power Doppler activity were assessed using the OMERACT pediatric ultrasound definitions. Results: All patients presented with early-onset camptodactyly and symmetrical large-joint arthropathy. The knees, ankles, wrists, and elbows were most frequently affected, while hip involvement was observed in four. Synovial fluid was gelatinous, viscous, clear to honey-colored, and sterile. MSUS of the longitudinal suprapatellar scan of the knee joints revealed marked grade 3 synovial effusion, circumferential synovial hypertrophy, and synovial thickening in all patients, without the power Doppler activity. Fine particulate hyperechoic texture and fibrinous bands were also noted. Conclusion: This case series highlights characteristic MSUS findings of CACP syndrome, synovial hypertrophy, and effusion without power Doppler activity, which may support earlier differentiation. Larger studies are needed to confirm the diagnostic value of MSUS and its role during follow-up.