Prevalence and Risk Factors for Attention-Deficit/Hyperactivity Disorder Among Children with Autism Spectrum Disorders

Background: Approximately 30% to 53% of children with an Autism Spectrum Disorder (ASD) also meet diagnostic criteria for Attention-Deficit/Hyperactivity Disorder (ADHD) (Leyfer et al., 2006; Sinzig et al., 2009).  In addition several studies have demonstrated that the presence of comorbid ADHD symptoms in children with ASD deleteriously affects adaptive behavior, executive functioning, core ASD symptoms, and externalizing behaviors in children (Yerys et al., 2009; Matsushima et al., 2008).  However, the studies reporting these findings are fraught with limitations including small sample sizes and significant variability in the measurement and clinical definition of ADHD.  Thus, a greater understanding of the prevalence and phenotype of this disorder is of paramount importance.

Objectives: To examine a) the prevalence of parent-reported ADHD symptoms in children with ASD; and b) the demographic, cognitive, psychiatric, and adaptive-behavioral correlates associated with comorbid ADHD.

Methods: Cross-sectional data from 169 children, ages 2 to 16 years (M = 6.39; SD = 3.3), were derived from a local research registry project housed in an urban outpatient pediatric clinic. Children were evaluated for ASD by trained diagnosticians using DSM-IV-TR criteria and the Autism Diagnostic Observation Schedule (ADOS; Lord et al, 2002). Children also underwent standardized cognitive testing. Upon enrollment, parents completed questionnaires such as the Child Behavior Checklist (CBCL; Achenbach and Rescorla, 2001), an established measure of childhood psychopathology. Clinical cutoff values from the DSM-oriented empirical CBCL ADHD scale were used to establish prevalence. Bivariate and multivariate linear regression analyses were conducted to examine six correlates of ADHD:  1) demographic characteristics (i.e., child age, race, parental education), 2) standardized IQ scores, 3) adaptive-behavioral functioning (Vineland Adaptive Behavior Scale II; Sparrow et al., 2005), 4) psychiatric comorbidities from the DSM-oriented scales shared by the preschool and school age versions of the CBCL (anxiety, oppositional and affective problems T scores), and 6) ASD severity using adjusted ADOS raw scores (Gotham et al., 2006).

Results: In this sample, 15% of children had clinical ADHD, and 18% had sub-clinical ADHD per parent-report. In the bivariate analyses, oppositional (r = .52), anxiety (r = .48) and affective (r = .46) problems were positively correlated with ADHD.  Results of the multivariate model demonstrated that all three of these psychiatric comorbidities were independently associated with increased ADHD symptoms (all p < .01).  Demographic characteristics, IQ, and adaptive functioning were not associated with ADHD symptoms.

Conclusions: Prevalence of parent-reported ADHD symptoms was lower in this sample compared to previously reported estimates, perhaps due to the single informant and normative based assessment utilized in this study.  Children with ADHD are likely to present with a more complex psychiatric profile characterized by the presence of comorbid oppositional, anxiety, or affective problems. These data highlight the importance of developing standardized, comprehensive, and multi-informant mental health assessment methods for children with ASD and comorbid ADHD.