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

## TL;DR

This paper reports a rare genetic disorder in two Brazilian siblings, highlighting how it can be misdiagnosed as arthritis due to similar symptoms.

## Contribution

The study presents the second genetically confirmed case of CACP syndrome in Brazil, emphasizing diagnostic challenges and clinical features.

## Key findings

- CACP syndrome was confirmed through whole-genome sequencing, revealing a PRG4 gene mutation.
- Clinical features like congenital camptodactyly and noninflammatory joint swelling were key to diagnosis.
- Misdiagnosis as juvenile idiopathic arthritis was common due to overlapping symptoms.

## Abstract

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.