Trajectories of Receptive Vocabulary Development from 4 to 8 Years in Children with and without Autism Spectrum Disorder: A Population-Based Study

Background:  Language difficulties are common in children with autism spectrum disorder (ASD), occurring in around 60% of children. Language difficulties can severely impact functioning and participation and result in a number of adverse sequelae such as poor academic achievement and behavioural and emotional difficulties. To date the literature has reported most children with ASD make some progress in their language ability over time. The majority of studies have used clinical samples however, and no studies have compared language development in children with ASD to children without ASD in a large population-based sample using direct standardised measures of language.

Objectives:  The aim of this study was to describe trajectories of receptive vocabulary development in children with ASD compared to a large population-based sample of children without ASD.

Methods:  Participants were drawn from the Longitudinal Study of Australian Children (LSAC), which is a nationally representative, population-based study. There are two cohorts of children followed in LSAC using bi-yearly direct assessments and/or questionnaires. One cohort was recruited at birth (n=4983) and the other in kindergarten (n=5107). Both cohorts were combined for the current study. Of the combined cohort, 237 children were reported by parents to have received a diagnosis of ASD by 10 years. Receptive vocabulary (Adapted Peabody Picture Vocabulary Test-III) for children with ASD was compared to children without ASD at 4 years (n=188, n=7136, respectively), 6 years (n=215, n=7297) and 8 years (n=216, n=7408). Mean trajectories of receptive vocabulary development were plotted using generalised estimating equations and compared across the two groups. The proportion of children in each group who had declining, stable and improving trajectories was also compared based on one standard deviation change as the cut point for defining the 3 different trajectory types.

Results:  There was variability in individual scores for both the ASD and non-ASD groups. Mean scores at 4, 6 and 8 years were lower for children with ASD compared to those without ASD at all 3 time points (4 years ASD: 63.3 (SD 6.5), non-ASD: 64.7 (6.3); 6 years ASD: 73.2 (6.3), non-ASD: 74.1 (5.1); 8 years ASD: 77.7 (6.0), non-ASD: 78.7 (4.9)). Estimated mean receptive vocabulary scores were 0.68 units lower for the ASD than the non-ASD group across the three waves of data collection (p=0.050; 95% confidence interval 0.001-1.364). The estimated mean difference approached significance. There was no significant difference (chi²; p>0.05 for all comparisons) between the proportions of children with ASD who had stable (ASD=54%, non-ASD=54%), improving (ASD=38%, non-ASD=35%) and declining (ASD = 8%, non-ASD=11%) trajectories. Trajectory plots of receptive vocabulary showed children with ASD progressed at a similar pace as children without ASD.

Conclusions:  Receptive vocabulary trajectories were heterogeneous for children regardless of ASD diagnosis. The trajectory of receptive language development growth over time was similar across children with and without ASD, but mean scores for children with ASD were lower at each time point.