From Misdiagnosis to Genetic Confirmation: A Brazilian Familial Report of Camptodactyly–Arthropathy–Coxa Vara–Pericarditis Syndrome—A Case‐Based Review

Background 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. Case Presentation 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. Conclusion 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.