Developmental associations between motor and communication outcomes in Fragile X syndrome: Variation in the context of co-occurring autism

Fragile X syndrome (FXS), the leading heritable cause of intellectual disability, has a co-occurrence rate of autism spectrum disorder (ASD) estimated at similar to 60%. The onset and rates of motor development in FXS are slower relative to neurotypical development, and even more so in the context of co-occurring FXS + ASD. Extant evidence suggests these differences are likely to affect communication, yet this developmental process or how it varies in the context of co-occurring ASD remains unknown in FXS. We aimed to delineate developmental associations between early motor abilities and their rate of development from 9 to 60 months of age on communication outcomes in 51 children with FXS, 28 of whom had co-occurring ASD. We also aimed to identify variation in these developmental associations in the context of co-occurring ASD. Results captured within-syndrome variability in these developmental associations as a function of co-occurring ASD. Fine motor proved to be a robust predictor of receptive communication regardless of co-occurring ASD, but we identified differences between FXS with and without ASD in the association between aspects of motor development and expressive outcomes. Findings provide evidence for differential developmental processes in the context of co-occurring ASD with implications for timely developmental intervention. Lay abstract Fragile X syndrome (FXS), the leading heritable cause of intellectual disability, has a co-occurrence rate of autism spectrum disorder (ASD) estimated at similar to 60%. Children with FXS experience delayed achievement and slower development of key motor abilities, which happens to an even greater extent for children with both FXS and ASD. A multitude of studies have demonstrated that motor abilities are foundational skills related to later communication outcomes in neurotypical development, as well as in the context of ASD. However, these associations remain unexamined in FXS, or FXS + ASD. In this study, we aimed to determine the associations between early motor skills and their rate of development on communication outcomes in FXS. Furthermore, we investigated whether these associations varied in the context of co-occurring FXS + ASD. Results revealed within-FXS variation in the context of co-occurring ASD between some aspects of motor development and communication outcomes, yet within-FXS consistency between others. Findings provide evidence for variability in developmental processes and outcomes in FXS in the context of co-occurring ASD and offer implications for intervention.