Direct Quantitative Measurement of Motor Coordination in Sibling Pairs Discordant for Autism: New Evidence for Motor Impairment as a Core Component of Autistic Syndromes

Background: Studies of unaffected family members of children with Autism Spectrum Disorders (ASD) have revealed an aggregation of sub clinical autistic social impairment among these individuals suggesting the possibility that such impairment constitutes an autism endophenotype.  Since previous studies have found children with autism to have a high incidence of motor coordination abnormalities, we conducted an initial exploration of how such abnormalities are distributed in ASD-affected families.
Objectives: This study examined the distribution of motor impairment in siblings concordant and discordant for autism.
Methods: Sibling pairs discordant (N = 33) and concordant (N = 50) for ASD were assessed using the Bruininks Oseretsky Test of Motor Performance, 2^nd Edition (BOT-2), a standardized, comprehensive, direct observational measure of motor coordination.  In addition to a total motor composite score, the BOT-2 generates four gender-specific motor area composite scores, scale scores, standard scores and percentile ranks for fine manual control, manual coordination, body coordination, and strength and agility, and is capable of detecting subtle differences between clinical and nonclinical subjects across a wide range of motor abilities.

Results: As predicted, mean scores for motor skills among ASD-affected children were significantly impaired across all domains (effect size = 1.5 to 1.8), however the scores for unaffected siblings in this primarily simplex autism sample were within the normal range. ASD-affected children with phrase speech had significantly better motor skills than those who were non-verbal (t = 3.22; df = 60; p = .002).  Total motor coordination scores of at least 1 SD below the general population mean were seen in 84% of the affected group and scores of at least 2 SDs were observed in 46%. Of particular note is that among the ASD-affected children, motor impairment scores were continuously (not dichotomously) distributed, and moderately correlated with degree of social impairment for the affected children (N = 79; r = .366; p < .05).

Conclusions: The current observations—that  motor coordination exhibits a pathologically-shifted, continuous distribution in ASD, that the degree of motor impairment is correlated with the degree of social impairment in ASD, but absent among unaffected siblings (in simplex families)—suggest that motor coordination abnormalities, as measured by the BOT-2 constitute core features of the autistic phenotype.  Further studies of quantitative impairments of motor coordination among unaffected children in multiplex families (for which the sample size in this study was too small to draw conclusions) are warranted to explore possible endophenotypic characteristics of motor impairments in ASD-affected families.  Moreover, pursuit of the genetic and neurobiologic underpinnings of motor coordination abnormalities in ASD may provide critical clues to the pathogenesis of autistic syndromes.