An Examination of the Prevalence of Attention Deficit/Hyperactivity Disorder In a Sample of Individuals with An Autism Spectrum Disorder

Background: A likely change in DSM-V will be the ability to diagnose - Attention Deficit/Hyperactivity Disorder (ADHD) in children who present with a Pervasive Developmental Disorder (American Psychiatric Association, 2010). However, reliable prevalence rates of co-morbidity are lacking, with estimates ranging from 28% to 78% in ASD populations (Ronald, Edelson, Asherson, & Saudino, 2009). In addition, little is known regarding the mutual impact of ADHD and ASD on clinical symptoms and function.   

Objectives: The goal of this study is to assess the prevalence of ADHD in a sample of individuals with an ASD, as well as the characteristics of these Individuals. Our specific aims are:

1. To replicate previous findings documenting the existence of comorbid ASD-ADHD in a population of individuals with an ASD.
2. To describe the relationship between comorbid ADHD and ASD severity and/or IQ. Specifically, we hypothesize that, among individuals with ASD:

a)      Those with comorbid ADHD have lower IQ, compared to those without ADHD.

b)      Those with comorbid ADHD have more severe ASD (as determined by the Calibrated Severity Score; Gotham, Pickles, & Lord, 2009) than those without ADHD.

c)      Those with both higher ASD severity and lower IQ are more likely to have comorbid ADHD.

Methods :  We propose to test these hypotheses in a sample of children with an ASD who participated in a study through the Simons Simplex Collection (SSC). ASD diagnosis was established using the Autism Diagnostic Interview – Revised and the Autism Diagnostic Observation Schedule. Cognitive, adaptive and behavioral testing was administered, including the Child Behavior Checklist (CBCL) and the Teacher Report Form (TRF). The comorbidity rate will be established by counting the number of individuals in the clinical range on the ADD/ADHD subscale of both the CBCL and TRF. The CSS will be used to calculate symptom severity.  Individuals will be categorized into Clinical, Sub-Clinical, and Non-Clinical ADHD groups; differences between groups will be calculated for ASD severity and IQ. 

Results: In preliminary analysis, using a local sample of 309 individuals with an ASD at Children’s Hospital Boston, we found that 29 (9.4%) fell in the ADHD clinical range per the CBCL (TRFs were not available for the majority of this sample). There was a trend for individuals in the Clinical group to have lower IQ (p= 0.29) and higher severity (p= 0.25), although differences were not significant. 

Conclusions:   Prevalence rates in our sample suggest that ASD-ADHD comorbidity may not affect as many individuals as found in previous research. This variability may be due to inconsistencies in ADHD classifications. Thus it is critical that stringent criteria be set for assigning ADHD status, which we will do using TRFs in the SSC sample. In addition, we found differences in IQ and severity that showed a trend in the predicted direction. Even though these did not reach significance, we expect that they will in the full SSC sample, in which ADHD classification can be confirmed with TRFs. These data will be analyzed in January 2011.