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Different Developmental Trajectories of ASD: Slow and Rapid Onset of Symptoms in Toddlers from the General Population
The heterogeneity of ASD is widely documented, however there is little documentation regarding variability in onset of ASD symptoms. This question is particularly difficult to answer as it requires the prospective study of ASD. A recent study by Ozonoff et al. (2015) examined the onset of ASD in a baby-sibling population, and found over half the sample that eventually received an ASD diagnosis at age three, did not initially meet criteria for ASD between 1-2 years.
Objectives:
This study aimed to analyze differential developmental trajectories of toddlers with ASD, and specifically compare toddlers initially identified as ASD that retained a diagnosis (Rapid-Onset) versus children initially identified as non-ASD who were identified as ASD at a later evaluation (Slow-Onset) in the general population.
Methods:
ASD and contrast toddlers were primarily identified using the One-Year Well-Baby Check-Up Approach (Pierce et al., 2011). Children were evaluated using the Mullen, Vineland, ADOS, and the Geometric Preference Test, a preferential looking eye-tracking test designed to quantify a toddlers’ preference for geometric or social stimuli (Pierce et al. 2011; 2015). Toddlers were included if initially evaluated ≤24 months (M=17.1) and diagnosis was confirmed at three. Children were reassessed every 9-12 months (M= 3.14 evaluations). Seventy-four language delayed (LD) and 132 typically developing (TD) toddlers were included for comparison. Growth curve analysis was used to examine longitudinal trajectories. Visual preference patterns based on eye-tracking were also compared at intake.
Results:
Within our sample 86 toddlers were considered Rapid-Onset ASD (RO-ASD; i.e., initially identified as ASD and retained that diagnosis over time) and 41 toddlers were identified as Slow-Onset ASD (SO-ASD; i.e., were initially identified as non-spectrum or having a delay such as language delay, but were identified as ASD at a subsequent evaluation). See figure 1 for trajectories of ADOS scores. An ANOVA showed a significant difference in looking times for geometric stimuli across groups (F(3, 127)=11.406, p < .001). The RO-ASD group showed the highest preference for geometric images (M=42.1 sec), followed by SO-ASD (M=30.3), LD (M=24.9), and TD group (M=18.2; see figure 2). Eleven RO-ASD and 2 SO-ASD cases looked at geometric images at very elevated levels (≥69%) which has been demonstrated to be highly associated with diagnosis of ASD (Pierce et al., 2011).
Conclusions:
Our study showed strikingly different symptom onset patterns within ASD toddlers from the general population. For language and ADOS scores, the RO-ASD group initially showed more impairment than the SO-ASD group, but both groups showed similar performance at age three. Importantly, the SO-ASD group was not initially identified as ASD based on their behavioral presentation, and instead were often identified as having some other delay (i.e. LD). Interestingly, a small subset of SO-ASD children showed elevated preference for geometric images at intake. This finding highlights the need for robust biological markers of autism as they may be able to identify children before behavioral symptoms become clear. These results also emphasize the necessity of repeated screening and follow-up for children showing any delay, as ASD symptoms emerge differently across children.