Combined type versus ADHD predominantly hyperactive-impulsive type: is there a difference in functional impairment?

The purpose of this study was to evaluate whether preschool children with attention-deficit/hyperactivity disorder predominantly hyperactive-impulsive type (ADHD-HI) and ADHD combined type (ADHD-C) have different levels of functional impairment in four domains: externalizing (oppositional and disruptive) behaviors, internalizing (anxious) behaviors, social skills, and preacademic functioning.

The subjects were 102 children 3 to 5 years of age, meeting DSM-IV criteria for ADHD. Children with ADHD-C versus ADHD-HI were compared across at least two measures for each of the four functional domains. Oppositional and anxious behaviors were assessed on the Conners Parent and Teacher Rating Scales. In addition, off-task and disruptive behaviors were assessed by direct observation in the preschool setting. Social skills were assessed on the parent and teacher versions of the Social Skills Rating System and preacademic skills were assessed on the letter word identification, passage comprehension, and applied problems subtests of the Woodcock-Johnson III Tests of Achievement and the initial sound fluency subtest of the Dynamic Indicators of Basic Early Literacy Skills 5th Edition.

There were no significant differences between the groups on rating scale T scores for parent-reported oppositional symptoms (ADHD-C vs ADHD-HI; 66.7 +/- 13.5 vs 65.7 +/- 11.7; p = .73); parent-reported anxious symptoms (53.5 +/- 11.1 vs 53.2 +/- 9.7; p = .90); teacher-reported oppositional symptoms (70.9 +/- 15.6 vs 75.5 +/- 14.7; p = .17); or teacher reported anxious symptoms (59.2 +/- 11.6 vs 58.5 +/- 12.2; p = .77). No statistically significant differences were found between the groups when examining off-task and/or disruptive behavior during structured and free play observations at school. No significant differences between the subtypes were found for social skills or preacademic functioning.

Across the four areas of functioning assessed in this study, preschool children with ADHD-HI and those with ADHD-C demonstrated similar levels of functioning. This study, in combination with data from longitudinal studies demonstrating that most children with ADHD-HI are later diagnosed with ADHD-C, suggests that ADHD-HI may represent an earlier form of ADHD-C as opposed to a distinct subtype.