Developmental Trajectories of ADHD Symptoms in Infants at Risk for ASD

Background:  Comorbidity rates between ASD and attention-deficit/hyperactivity disorder (ADHD) are as high as 40-70% (Antshel et al., 2016), making it important to better understand the presentation of ADHD symptoms in ASD. This is especially critical since the presence of ADHD symptoms in young children can delay a diagnosis of ASD by several years (Miodovnik et al., 2015). Very little research has focused on the early development of ADHD symptoms in infants developing ASD, despite the fact that a better understanding of the onset and early course of ADHD symptoms among infants at risk for ASD could improve early diagnostic and classification efforts and have implications for early intervention.

Objectives: We examined when differences in ADHD-related behaviors emerged among participants with either ASD or typical development (TD) outcomes.

Methods:  Participants were drawn from a larger longitudinal study of infants at high- and low-risk for ASD (infant siblings); a subset of those with 36-month outcomes of either ASD (n = 14) or TD (n = 36) was included in the present investigation. We conducted second-by-second behavioral coding of videos from the 12-, 18-, 24-, and 36-month visits, examining four coded behaviors (Table 1) related to ADHD symptomatology (inattention, hyperactivity, out-of-seat behavior, and grabbing). Behaviors were coded from 5-minute video segments of structured testing. Group differences in ADHD-related behaviors from 12 to 36 months were examined via a Generalized Estimating Equations approach, using a negative binomial distribution with log link for each coded behavior.

Results:  Estimated marginal means of each coded behavior are displayed in Figure 1. The interaction between inattention, visit, and outcome was significant (Wald χ² = 13.71, df = 3, p = .003). The ASD group significantly increased in the frequency of inattentive behavior over time, and significantly differed from the TD group at 36 months of age; there was also a trend toward more frequent bouts of inattention in the ASD group at 24 months. The interaction between out-of-seat behavior, visit, and outcome was also significant (Wald χ² = 10.44, df = 3, p = .015). The ASD group decreased over time and engaged in significantly fewer bouts of out-of-seat behavior at 36 months of age compared to the TD group. The interaction between hyperactive behavior, visit, and outcome was not significant (Wald χ² = 4.28, df = 3, p = .233), nor was the 3-way interaction for grabbing behavior (Wald χ² = .95, df = 3, p = .813).

Conclusions:  Infants developing ASD showed more frequent bouts of inattention than infants developing typically by 36 months of age, as well as less frequent bouts of out-of-seat behavior by 36 months. Our findings contribute to the growing literature implicating early attentional processes in ASD (reviewed in Johnson et al., 2014) and is consistent with prior research documenting elevated rates of ADHD-related inattention symptoms in older children with ASD (Leyfer et al., 2006). Coding of the full sample is ongoing and may help clarify the unexpected finding of minimal group differences in hyperactive-impulsive behaviors.