Do Developmental Profiles of Toddlers with ASD Differ Based on Sibling Status Vs. Clinical Referral?

Background: Great strides have been made in diagnosing children with ASD at very young ages, with good diagnostic stability evidenced by 18-24 months of age in clinically-referred samples. However, the developmental and diagnostic profiles of younger siblings of children with ASD (i.e., those with a genetic propensity for the disorder) appear to be more complex in that siblings who develop ASD tend to be higher functioning and have more subtle autism symptomatology, making it challenging to provide accurate diagnoses upon detection of risk.

Objectives: This study directly compares the developmental profiles of 18-24 month-old children with ASD based on whether they were clinically-referred for an evaluation vs. evaluated due to sibling status. Profiles of autism symptomatology, developmental, play, and adaptive skills are analyzed between the groups.

Methods: Participants included 76 children with ASD, 19 of which participated in a research study on infant siblings of children with ASD (i.e., Siblings) and 57 of which participated in an early detection study and did not have a sibling with ASD (i.e., Clinically-referred). Preliminary diagnoses for the Siblings were provided at 18-months, with confirmatory diagnoses of ASD made at 36-months. Preliminary diagnoses for the Clinically-referred children were made during their initial visit, prior to the age of 24 months, and confirmatory diagnoses of ASD were made during a follow-up visit at 36 months. The mean age of the Siblings was 18.74 months (SD=1.49) and 84% were male. The mean age of the Clinically-referred sample was 21.34 months (SD=3.28) and 88% were male.

Results: A significant difference in age was observed between the Siblings and Clinically-referred samples, with the latter being slightly older (t=4.6; p<.001). Despite being younger, Siblings had significantly higher Visual Reception [F(1,72)=12.38; p<.01], Fine Motor [F(1,72)=8.49; p<.01], Receptive [F(1,72)=9.15; p<.01] and Expressive Language [F(1,72)=8.90; p<.01] scores on the Mullen Scales of Early Learning, with all scores falling within the average range, whereas mean Mullen scores for Clinically-referred children fell 1-1.5 standard deviations below the mean. Clinically-referred children evidenced significantly more autism symptomatology on the ADOS, Toddler Module [F(1,72)=12.86; p<.01], with a mean Social/Affect Algorithm score of 16.46 (SD=4.49) compared to 11.76 (SD=5.50) for the Siblings. No differences were observed in restricted/repetitive behaviors as measured on the ADOS-T, or within any domains of adaptive behavior, as measured by the Vineland Adaptive Behavior Scales.

Conclusions: Results confirm preliminary findings suggesting that siblings of children with ASD who themselves develop the disorder have different developmental profiles than children with ASD who are clinically-referred for evaluation due to concerns. Siblings tend to be higher functioning but still exhibit significant levels of autism symptomatology, albeit not to the severity of clinically-referred children. Nonetheless, their symptoms are detected earlier, likely due to close monitoring for risk. Although differences are likely referral-based, these data raise the question of differing neuroendophenotypes between children with and without genetic liability for ASD. They also highlight the complexities that arise in, and can certainly impact, the assessment and diagnosis of two varying cohorts of children, both at significant risk for ASD.