Early Phenotype and Developmental Trajectories of Children with ASD with and without Comorbid ADHD

Background:  

A growing number of researchers are reporting symptoms of Attention Deficit/Hyperactivity Disorder (ADHD) in a substantial proportion of children with ASD (e.g., Mayes, Calhoun, Mayes, & Molitoris, 2012).  There is preliminary evidence that when ADHD is comorbid with ASD, the risk for a more severe behavioral phenotype increases (Holtmann et al., 2007; Rao & Landa, in press; Yerys et al, 2007).  A major need in the field of autism research is to better understand whether early phenotypic characteristics and trajectories of children with ASD who develop comorbid ADHD differ from those without ADHD comorbidity.

Objectives:  

To determine whether early developmental differences exist in cognitive functioning, language acquisition, ASD symptom severity in children with ASD who were identified at school-age as having comorbid ADHD or no ADHD comorbidity.

Methods:  

Participants were 44 children (ages 4-8 years) with ASD participating in a prospective study of child development.  Data were collected at ages:  24 months (T1); 36 months (T2); and 4 to 8 years (T3).  At T3, parent ratings on the BASC-2 were used to classify children as ASD-Only (n = 29) or ASD+comorbid ADHD (n = 15).  The Mullen Scales of Early Learning (MSEL) age equivalency scores were used at T1 and T2 to assess nonverbal cognitive (Visual Reception [VR] scale) and language functioning (Expressive Language [EL] and Receptive Language [RL] scales). Age equivalents from the Peabody Picture Vocabulary Test-P3, and the Oral Vocabulary subtest of the Test of Oral Language Development-P3 were measures of receptive and expressive language at T3.  Severity of ASD symptoms was measured with the Autism Diagnostic Observation Scale (ADOS) at Ts1-3.  

Results:  

Independent samples t-tests revealed no significant between-group differences in gender, ethnicity, recruitment source, or age of assessment at T3 (Table 1). One-way ANOVA revealed that the ASD+ADHD group had significantly lower nonverbal IQ than the ASD Only group at T1 (F = 4.78, p = .04) and T2 (F = 9.87, p = .003). Similarly, expressive language at T1 (F = 4.65, p = .04) and T2 (F = 6.11, p = .02), and receptive language at T2 (F = 9.12, p = .004) and T3 (F = 4.71, p = .04) were more impaired in the ASD+ADHD group than in the ASD-Only group.  There were no significant between-group differences in severity of ASD symptoms at any time of assessment (see Figure 1).

Conclusions:  

Findings indicate that, by the toddler years, children with ASD who later exhibit clinically significant ADHD symptoms present with more severe cognitive and language phenotypes than those without comorbid ADHD symptoms.  Future studies replicating and expanding upon these findings are needed so that early interventions targeting this specific behavioral phenotype may be developed.