Age of walking and intellectual ability in autism spectrum disorder and other neurodevelopmental disorders: a population-based study

Abstract

Background: Delayed walking is common in intellectual disability (ID) but may be less common when ID occurs with autism spectrum disorder (ASD). Previous studies examining this were limited by reliance on clinical samples and exclusion of children with severe motor deficits.

Objective: To examine in a population-based sample if age of walking is differentially related to intellectual ability in children with ASD versus other neurodevelopmental disorders (NDD).

Methods: Participants were from the nested Autism Birth Cohort Study of the Norwegian Mother, Father and Child Cohort Study (MoBa). Cox proportional hazards regression assessed if diagnosis (ASD n = 212 vs. NDD n = 354), continuous nonverbal IQ, and their interaction, were associated with continuous age of walking.

Results: The relationship between nonverbal IQ and age of walking was stronger for NDD than for ASD (Group × nonverbal IQ interaction, χ² = 13.93, p = .0002). This interaction was characterized by a 21% decrease in the likelihood of walking onset at any given time during the observation period per 10-point decrease in nonverbal IQ (hazard ratio = 0.79, 95% CI: 0.78-0.85) in the NDD group compared to 8% (hazard ratio = 0.92, 95% CI: 0.86-0.98) in the ASD group.

Conclusions: The finding that age of walking is less strongly related to low intellectual ability in children with ASD than in children without other NDDs supports the hypothesis that ID in ASD may result from heterogeneous developmental pathways. Late walking may be a useful stratification variable in etiological research focused on ASD and other NDDs.

Keywords: Intellectual disability; MoBa; epidemiology; gross motor milestones; late walking.

Figure 1.. Participant flow

Note: Of the children invited to participate in the ABC study based on selection criteria, one child did not complete the assessment and one child had Rett syndrome. Both were therefore excluded. ASD=autism spectrum disorder. NDD=neurodevelopmental disorders without ASD. IQ=intelligence quotient. MoBa=the Norwegian Mother, Father and Child Cohort Study. Of the children without any clinical diagnosis (n=454), 444 children had data on walking and IQ. ASD included ABC Protocol ASD diagnosis as of 19 Oct 2014 (Autistic Disorder, Asperger Disorder and Pervasive Developmental Disorder Not Otherwise Specified), childhood disintegrative disorder and profound disability with autism.*Rett syndrome was excluded (n=2) because of insufficient information on whether ASD diagnostic criteria were met (i.e., for either Autistic Disorder or PDD-NOS).

Figure 2.. Illustration of differential relationship between nonverbal IQ and age of walking for NDD and ASD groups

Note: Plotted values are model-estimated probabilities of walking at a given age (X-axis) and given value for nonverbal IQ (line color). IQ was not categorized for analysis or for plotting; instead, the model was used to generate the estimated probability of walking at the given IQ values.