Reliability and validity of the FSHD-composite outcome measure in childhood facioscapulohumeral dystrophy

•There is a paucity of research and development in outcome measures to evaluate function in childhood FSHD.•Estimated intra-rater test re-test reliability for both adult and pediatric versions of the FSHD-COM is excellent.•Estimated correlations of both versions of the FSHD-COM with other measures were moderate to very strong.•The FSHD-COM and FSHD-COM Peds have potential to measure physical function across the lifespan in FSHD. This study aims to investigate intra-rater reliability and construct validity of the Facioscapulohumeral Dystrophy Composite Outcome Measure (FSHD-COM), in childhood FSHD. Participants included eighteen children with FSHD, and matched healthy controls. Reliability data were collected from 15 participants with FSHD over two testing sessions. Validity data were collected from all participants. Participants with FSHD completed; the FSHD-COM (and modified pediatric version), Motor Function Measure-32 (MFM-32), FSHD Severity Scales, Performance of the Upper Limb 2.0, Pediatric Quality of Life™ Neuromuscular Module and pediatric FSHD Health-Index Questionnaire. Both versions of the FSHD-COM showed excellent intra-rater reliability (ICC1,2 > 0.99, lower 95%CI > 0.98) with a Minimal Detectable Change (MDC95%) of ≤14.5%. The FSHD-COM had robust and widespread correlations with other related outcome measures. The FSHD-COM versions and 6 min walk test effectively discriminated between children with and without FSHD; the MFM-32 and 10 m walk/run test did not. Ceiling effects were not observed on either version of the FSHD-COM. Reliability and validity findings in this childhood FSHD study concord with estimates in adults. Both versions of the FSHD-COM were effective in discriminating disease in children with mild FSHD symptoms. The FSHD-COM has the potential to be a useful measure of function across the life span.