Behavior Profiles of Children with Attention Deficit Hyperactivity Disorder Behaviors and Children with Autism Spectrum Disorder on the Parent PDD Behavior Inventory

Background:  Substantial overlap exists between Autism Spectrum Disorder (ASD) and Attention Deficit Hyperactivity Disorder (ADHD). Etiologically, similarity exists in genetic liability for the two disorders.  Phenotypically, comorbidity exists between ASD and symptoms of ADHD with rates varying between 28% and 92% and both groups have similar problems with social cognition.  Where there is comorbidity between ASD and ADHD, studies suggest that a more severe phenotype exists: increased oppositional and anxiety behaviors, decreased IQ, and adaptive skills and worse response to intervention. 

In gathering information on the PDDBI prior to clinical observation, we came across highly unusual profiles indicating very severe behavior problems along with relatively intact communication skills leading us to expect to see a high-functioning child with ASD.  To our surprise, both clinical observations and assessment with the ADOS-G indicated the child did not have ASD.   Instead diagnoses of ADHD, and/or anxiety/mood disorders were provided. 

Objectives:  The purpose of this study was to investigate this phenomenon by examining similarities and differences in parent PDDBI ratings of children with ASD and children with ADHD behaviors.

Methods:  The sample consisted of 139 children (2.4 to 13.8 years) referred for differential diagnosis.  Parent ratings of children with ASD (with and without ADHD behaviors) were compared with parent ratings of children with ADHD behaviors who did not have ASD (the ADHD group) using the PDD Behavior Inventory (PDDBI), a rating instrument that is age-standardized on children with ASD and which assess both maladaptive behaviors and adaptive skills.  Analyses included MANOVAs and classification trees (CART analysis).

Results:  The ADHD group’s maladaptive behaviors were rated as more severe than the ASD group, especially on those domains assessing fears and aggression.  Further, the ADHD group was rated as having greater severity of fears and aggression when compared with those in the ASD group who also had comorbid ADHD behaviors and better expressive language ability than the ASD group (especially when compared with those in the ASD group with comorbid ADHD behaviors).  Using four domains from the PDDBI (FEARS, EXPRESS, AROUSE and RITUAL), CART classified 94% of the ASD group and 87% of the ADHD group correctly.   

Conclusions:  These data extend findings from previous studies and suggest that a unique ADHD profile may exist for children with ADHD on the PDDBI.  It is hypothesized that the increased severity of fears and aggression in the ADHD group reflects their increased ability to communicate these problems to others. Children with both ASD and ADHD behaviors were more severely affected than their ASD cohorts whose parents did not report ADHD behaviors.  As a group, they were the ones whose social communication scores and scores for classic ASD problems were similar to the standardization sample while those ASD children who did not have ADHD traits presented with a more mild phenotype.  It is predicted that some of these children will likely have a different genetic background than the ASD-ADHD group based on our research showing that an X-linked genotype linked to serotonin metabolism affects the severity of parent PDDBI scores for males with autism.