Camptodactyly-arthropathy-coxa vara-pericarditis (CACP) syndrome is a rare autosomal recessive disorder caused by PRG4 mutations that impair lubricin production. Resulting noninflammatory hyperplasia produces congenital or early-onset camptodactyly and noninflammatory arthropathy, affecting large joints. Because clinical features overlap with trigger finger and juvenile idiopathic arthritis (JIA), misdiagnosis is common. We describe the second genetically confirmed Brazilian case of CACP, involving two siblings. Both showed congenital trigger fingers (later reclassified as camptodactyly) and developed painless, cold swelling of large joints, initially labeled JIA. Laboratory tests showed normal inflammatory markers, and synovial fluid revealed low white cell counts. Imaging demonstrated joint effusion and synovial debris without inflammatory signs. Whole-genome sequencing identified a homozygous c.3756dup mutation in the PRG4 gene, introducing a premature stop codon and truncating lubricin. This report highlights the importance of recognizing CACP syndrome by identifying distinctive clinical, laboratory, and imaging characteristics, notably congenital camptodactyly and noninflammatory joint swelling, to prevent misdiagnosis and guide supportive management.

Camptodactyly-arthropathy-coxa vara-pericarditis syndrome is a rare autosomal recessive disorder with camptodactyly, non-inflammatory arthropathy, coxa vara, and pericarditis. Symptoms usually begin in early childhood with swelling of interphalangeal joints, wrists, and knees. Pericarditis is rare. It is caused by proteoglycan 4 gene mutations and often misdiagnosed as juvenile idiopathic arthritis due to overlapping features. A 4-year-old boy presented with 6 months of knee and wrist swelling and nocturnal pain. His parents were first-degree cousins, and a maternal uncle had rheumatoid arthritis. He had prior hip surgeries at 2.5 years. Examination showed swelling in knees, ankles, and wrists, and 30° wrist extension limitation. No systemic symptoms were noted. The erythrocyte sedimentation rate, C-reactive protein level, and antinuclear antibody test results were normal. Magnetic resonance imaging showed effusion and synovial thickening. Initially diagnosed with polyarticular juvenile idiopathic arthritis, he received methotrexate and etanercept. Due to progressive gait issues and radiographic coxa vara, diagnosis was revised. Genetic testing confirmed homozygous proteoglycan 4 (c.915delC) mutation. This case underscores an atypical camptodactyly-arthropathy-coxa vara-pericarditis presentation without camptodactyly. Persistent non-inflammatory arthropathy despite immunosuppression should raise suspicion for camptodactyly-arthropathy-coxa vara-pericarditis. Das Camptodaktylie-Arthropathie-Coxa-vara-Perikarditis (CACP)-Syndrom ist eine seltene autosomal-rezessive Erkrankung, die durch Camptodaktylie, nicht-entzündliche Arthropathie, Coxa vara und Perikarditis gekennzeichnet ist. Die Symptome beginnen typischerweise im frühen Kindesalter mit Schwellungen der Interphalangealgelenke, Handgelenke und Knie. Eine Perikarditis ist selten. Das Syndrom wird durch Mutationen im PRG4-Gen verursacht und aufgrund überlappender klinischer Merkmale häufig mit juveniler idiopathischer Arthritis (JIA) verwechselt.  Ein 4-jähriger Junge stellte sich mit einer sechsmonatigen Vorgeschichte von Schwellungen und nächtlichen Schmerzen in den Knien und Handgelenken vor. Die Eltern waren Cousin und Cousine ersten Grades, ein mütterlicher Onkel hatte rheumatoide Arthritis. Im Alter von 2,5 Jahren hatte der Patient Operationen wegen Hüftdysplasie. Die körperliche Untersuchung ergab Schwellungen in Knien, Sprunggelenken und Handgelenken sowie eine 30°-Streckungseinschränkung in beiden Handgelenken. Systemische Symptome lagen nicht vor. ESR, CRP und ANA waren unauffällig. Die MRT zeigte Gelenkergüsse und synoviale Verdickungen. Zunächst wurde eine polyartikuläre JIA diagnostiziert und mit Methotrexat und später Etanercept behandelt. Aufgrund zunehmender Gangstörungen und radiologischer Zeichen einer Coxa vara wurde die Diagnose überdacht. Eine genetische Analyse bestätigte eine homozygote PRG4-Mutation (c.915delC).  Dieser Fall verdeutlicht eine atypische Präsentation des CACP-Syndroms ohne Camptodaktylie. Eine persistierende, nicht-entzündliche Arthropathie trotz immunsuppressiver Therapie sollte den Verdacht auf ein CACP-Syndrom lenken.

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. 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. 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. 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.

We report the case of a patient with a late diagnosis of camptodactyly-arthropathy-coxa vara-pericarditis syndrome, following his first episode of constrictive pericarditis in adulthood. We also aim to review the existing literature on this rare disease.

Camptodactyly-arthropathy-coxa vara-pericarditis syndrome (CACP) is an example of a rare non-inflammatory familial arthropathy inherited in an autosomal recessive manner. The proteoglycan 4 (PRG4) gene, located on chromosome 1q25-q31, is responsible for encoding a lubricating glycoprotein found in the synovial fluid and on the surface of articular cartilage. Pathogenic mutations in the PRG4 gene have been associated with CACP disease. The present study investigated the clinical and molecular findings of the patient with CACP. Whole-genome sequencing was performed in this patient to investigate the genomic variations. The case presented in this study is a 20-year-old male who was admitted to the clinic. He had swelling in his wrists and limited mobility in his elbows as well as a family history of anaemia. The patient was initially diagnosed with juvenile idiopathic arthritis (JIA). Further genetic testing revealed a homozygous frameshift variant in the PRG4 gene (C.1290del; p.T431Lfs*481), which was classified as likely pathogenic and consistent with a diagnosis of CACP. Although this specific variant has been previously reported in the literature, this study contributes to the existing body of knowledge by emphasising the importance of comprehensive genetic analysis in differentiating CACP from other childhood rheumatic diseases such as JIA. Additionally, we discuss its location in exon 7 and potential effects on gene expression, including the possibility of nonsense-mediated decay or a truncated protein product.