The present study investigated the incidence of attention deficit hyperactivity disorder in a sample of 86 adults with severe to profound mental retardation. Participants were evaluated by supervisory staff using the diagnostic criteria from the Diagnostic and Statistical Manual of Mental Disorders (American Psychiatric Association, 1994) and Conners’ (1990) Hyperactivity Index. Using the most conservative measure, 15% of the sample met the diagnostic criteria. This measure was not influenced by the subjects’ chronological age, adaptive age, IQ, or gender. Implications of this finding for continued research and practice were discussed.
The prevalence of attention deficit hyperactivity disorder (ADHD) in school-aged children with average intelligence is estimated at 3 to 5%, with a 4:1 male-to-female ratio (American Psychiatric Association, 1994). Using a sample (N = 245) of 6- to 18-year-old children with mental retardation, Epstein, Cullinan, and Gadow (1986) reported 19.7% of the boys and 15.7% of the girls were hyperactive, based on a cutoff score of 15 on the abbreviated version of the Conners Rating Scale (Conners, 1973).
Historically, ADHD was considered a problem of childhood and presumed to dissipate during adolescence. However, more recent evidence indicates that this disorder persists into adolescence and adult life (Wender, 1987). The prevalence of ADHD in adults is not known (Shaffer, 1994), but is considered to affect up to 3% of the adult population (Feifel, 1996). The prevalence of ADHD in adults with mental retardation has been only indirectly studied. In a recent study by Cherry, Matson, and Paclawskyj (1997), 52.4% of younger adults and 60.7% of older adults with severe and profound mental retardation met the diagnostic criterion for Impulse Control and Miscellaneous Problems. Impulsivity is considered one of the hallmark characteristics of ADHD (Barkley, 1990).
Given that the prevalence of ADHD is considered higher in children with mental retardation than children with normal intelligence (Handen, McAuliffe, Janosky, Feldman, & Breaux, 1994; Pearson, Yaffee, Loveland, & Lewis, 1996), coupled with a growing consensus that ADHD continues into adulthood (Barkley, 1990), it seems logical that this disorder would be present in adults with mental retardation, possibly to a greater extent than adults with normal intelligence. The purpose of the present study was to investigate the incidence of ADHD in a sample of adults with mental retardation.
Subjects and procedures
The subjects were selected from a Midwestern residential facility serving 182 adults with mental retardation. The subject selection criteria for this study included being ambulatory, a diagnosis of severe to profound mental retardation, and the absence of psychotropic medications. Eighty-six subjects (61 males, 25 females) met these criteria with an average age of 41.4 years (SD = 12.4; range = 21 to 72), an average IQ of 22.2 (SD = 12.89; range = 12 to 49), and an average adaptive age of 3.3 years (SD = 1.91; range = 0.5 to 8.2).
Two instruments were used. The first instrument included the diagnostic criteria for ADHD from the Diagnostic and Statistical Manual of Mental Disorders, Fourth Edition (DSM-IV; American Psychiatric Association, 1994) and were represented by two subscales. Included were nine items representing an Inattention Scale and nine additional items representing a Hyperactivity and Impulsivity Scale. Each item is rated for the presence (scored 1) or absence (scored 0) of each criterion, resulting in scale scores ranging from 0 to 9 for inattention and ranging from 0 to 9 for hyperactivity and impulsivity, respectively. The second instrument was the Hyperactivity Index from the Conners’ Rating Scales (Conners, 1990), which includes 10 items that are rated on a four point scale (0 = not at all, 1 = just a little, 2 = pretty much, and 3 = very much). Hyperactivity Index scores could range from 0 to 30.
Ten supervisory staff members, responsible for ensuring the quality of the subjects’ daily programs and monitoring their effectiveness, were selected to rate subjects using the two instruments. All staff had a minimum of a bachelor’s degree, at least 1 year of experience working with individuals with mental retardation, and at least 6 months of experience working with the subjects they were assigned to rate. Staff were given instructions regarding administration of the study’s measures. Each rater was given a list of subjects who they were familiar with and instructed to rate the subjects relative to other adults with mental retardation at similar developmental levels. Raters completed the scales while in the presence of the subject being rated.
Coefficient alpha was computed for each study instrument as a measure of internal reliability: DSM-IV’s Inattention Scale = .87, DSM-IV’s Hyperactivity and Impulsivity Scale = .85, and Conner’s Hyperactivity Index = .92. For purposes of data analyses, subject variables were categorized to produce cell sizes with sufficient subject numbers for data analyses. Subject categories included: age, younger = 38 years or less, older = 39 years or more; IQ, lower = 20 or less, higher = 21 or more; adaptive age, lower = 2.75 years or less, higher = 2.80 years or more; and gender, males, females.
A multiple analysis of variance (MANOVA) was computed with age and gender of the subjects as the independent variables and total scores on the Inattention Scale, Hyperactivity and Impulsivity Scale, and Hyperactivity Index as the dependent measures. Adjusted mean scores for the dependent measures by the independent variables are shown in Table 1. No significant age, gender, or interaction effects were found. A second MANOVA was computed with adaptive age and IQ of the subjects as the independent variables and total scores on the Inattention Scale, Hyperactivity and Impulsivity Scale, and Hyperactivity Index as the dependent measures. Adjusted mean scores for the dependent measures by the independent variables are shown in Table 1. A significant effect was found for IQ [F(3, 80) = 2.90, p = .04]. Univariate F-tests indicated that subjects with lower IQS had significantly higher scores on the Inattention Scale (M = 5.93) than subjects with higher IQS (M = 3.80) [F(1, 82) = 7.39, p = 0.008]. There were no significant adaptive age or interaction effects.
