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Symptoms of ADHD and their correlates in children with intellectual disabilities
Research in Developmental Disabilities 26 (2005) 456–468
Symptoms of ADHD and their correlates in children with intellectual disabilities Richard P. Hastings*, Alexandra Beck, Dave Daley, Christopher Hill School of Psychology, University of Wales Bangor, Bangor, Gwynedd, Wales LL57 2AS, UK Received 10 June 2004; received in revised form 28 September 2004; accepted 15 October 2004
Abstract Existing research suggests that children with intellectual disabilities are at increased risk for ADHD, and that the symptoms of the disorder might successfully be treated with stimulant drugs. However, there has been little exploration of ADHD symptoms and their correlates in children with intellectual disabilities. Analyses of three samples of children with intellectual disabilities are presented (total N = 338). Correlational analyses showed that younger children, and those with a diagnosis of Autism were rated as having more ADHD/hyperactivity symptoms. There was little evidence of a sex difference, and no strong associations with domains of adaptive behavior (socialization, communication, and daily living skills). However, there was a small but significant negative association between mental age and ratings of symptoms. Finally, an increased prevalence of ADHD/hyperactivity symptoms was confirmed in the children with intellectual disabilities compared to their siblings. This effect remained after controlling for chronological and mental age differences between the siblings. These findings support those from previous research and suggest that ADHD/ Hyperkinesis may be a valid psychiatric diagnosis for children with intellectual disabilities. However, a great deal more research is needed to explore the phenomenology of ADHD in intellectual disability and to develop an evidence base for psychosocial intervention. # 2004 Elsevier Ltd. All rights reserved. Keywords: Intellectual disability; Children; Attention Deficit Hyperactivity Disorder; Hyperkinesis
* Corresponding author. Tel.: +44 1248 388 214. E-mail address: [email protected] (R.P. Hastings). 0891-4222/$ – see front matter # 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.ridd.2004.10.003
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1. Introduction Attention Deficit/Hyperactivity Disorder (AD/HD: American Psychiatric Association, 1994) is a relatively common, debilitating condition of childhood, characterised by severe, persistent, and early appearing symptoms of inattention, overactivity and impulsiveness (Swanson et al., 1998). The disorder affects between 2% and 5% of children of school age (Scahill et al., 1999). ADHD frequently co-occurs with other child mental health problems, such as anti-social conduct disorder, and anxiety. It is associated with marked social and educational impairment in the school years, and is a major risk factor for the development of problems throughout adolescence and adulthood, including conduct disorder, personality disorder, and educational failure (Taylor, Chadwick, Heptinstall, & Danckaerts, 1996). AD/HD is closely related to Hyperkinesis, which is a more severe form of ADHD based on alternative rating criteria (ICD-10). While the symptoms of the two disorders are very similar, requirements relating to pervasiveness and diagnostic profile vary somewhat between the ICD and DSM taxonomies (Swanson et al., 1998). For example, according to DSM-IV criteria, it is possible to have ‘‘ADHD’’ without being inattentive. However, inattentiveness is a necessary requirement for a hyperkinetic disorder according to ICD-10 criteria (Taylor, 1998). Symptoms of ADHD have been found in several studies of children with intellectual disabilities at levels leading to the diagnosis of the disorder at apparently higher rates than those found in populations without intellectual disabilities (Dekker & Koot, 2003; Strømme & Diseth, 2000). For example, Dekker and Koot (2003) recruited a sample representative of children in Dutch special schools and conducted DSM-IV diagnostic assessment using a standardized clinical interview. They found a prevalence rate of 14.8% for any ADHD disorder, and a high degree of co-morbidity (44%) with Oppositional Defiant Disorder. There have also been a number of studies that incorporated direct comparisons of ADHD symptoms between children with and without intellectual disabilities. Large population-based studies using rating questionnaires have shown that ADHD/hyperactivity symptoms occur at significantly higher levels in children with intellectual disabilities (Linna, Piha, Kumpulainen, Tamminen, & Almqvist, 1999; Rutter, Graham, & Yule, 1970). Similar results have been found in smaller group comparisons of children with and without intellectual disabilities (Epstein, Cullinan, & Gadow, 1986; Fee, Matson, Moore, & Benavidez, 1993). One difficulty with research using behavior rating scales is that it can be difficult to map symptom profiles onto the criteria used in psychiatric diagnostic symptoms. In this respect, Emerson’s (2003) research is perhaps one of the most informative designs for comparing psychiatric problems in children with and without intellectual disabilities. Emerson (2003) conducted a secondary analysis of a representative population-based survey of mental health in over 10,000 children and adolescents in the UK (Meltzer, Gatward, Goodman, & Ford, 2000). In the absence of any explicit data on intellectual disability, Emerson used a working definition of intellectual disability (e.g., presence of ‘‘learning difficulties’’ (UK Education Department terminology for mental retardation), concerns about early language development, attending special schools for children with
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intellectual disabilities). Emerson compared psychiatric diagnoses in this intellectual disability group against the remainder of the sample presumed not to have an intellectual disability. A clinical interview was used to establish ICD-10 diagnoses, including Hyperkinesis. Emerson found that 8.7% of children in the intellectual disability group were diagnosed with Hyperkinesis compared with 0.9% of the remainder of the sample, a difference that was statistically significant (Odds ratio = 10.0; 95% CI = 6.2, 16.0). These results suggest that the presence of an intellectual disability is associated with a 10-fold increase in the risk for Hyperkinesis. Although the data are not presented, Emerson (2003) states that similar results across all disorders were found when DSM rather than ICD diagnoses were made from the clinical interviews. Despite the data reviewed above, there have been few studies focusing specifically on ADHD in children with intellectual disabilities. There is a relatively large literature on the use of stimulant medication as a pharmacological management strategy for children with intellectual disabilities and ADHD (e.g., Aman, Buican, & Arnold, 2003; Aman, Kern, McGhee, & Arnold, 1993a, 1993b; Beale & McDowell, 1994; Handen et al., 1992; Handen, Feldman, Lurier, & Murray, 1999; Handen, McAuliffe, & Caro-Martinez, 1996; Handen, McAuliffe, Janosky, Feldman, & Breaux, 1995; Johnson, Handen, Lubetsky, & Sacco, 1994; Pearson, Santos, et al., 1996a, 2003; Varley & Trupin, 1982). This research shows improvements in parent- and teacher-rated behavior, and observed behavior in the classroom. There are also data from follow-up studies of drug interventions suggesting that ADHD symptoms in children with intellectual disabilities are persistent over extended periods of time (e.g., Aman, Armstrong, Buican, & Sillick, 2002; Handen, Janosky, & McAuliffe, 1997). However, few studies have been concerned with the presentation of ADHD in children with intellectual disabilities and with the correlates of ADHD symptoms. The presentation of ADHD in children with intellectual disabilities is a crucial issue. Diagnostic systems emphasize that any symptoms of inattention, overactivity, and impulsivity should be over and above those expected for the child’s age and level of disability. Research within the developmental tradition has suggested no strong evidence for attention deficits in children with intellectual disabilities once children have been matched for mental age (Burack, Evans, Klaiman, & Iarocci, 2001), although children with specific syndromes, including Fragile X may have deficits in attention (e.g., Munir, Cornish, & Wilding, 2000; Turk, 1998). On the basis of this research on attention, and guidance from diagnostic systems, one might conclude that the apparent increased risk for ADHD in children with intellectual disabilities is due simply to their developmental delay. However, there are at least two problems with such a conclusion. First, ADHD is defined using more than symptoms of inattention and research on the impact of developmental delay on the differential prevalence of impulsiveness and overactivity is not available. Second, one would predict that ADHD symptoms should be correlated with the severity of intellectual disability (more pronounced with more developmental delay) but data suggest that this may not be the case (Dekker & Koot, 2003; Pearson & Aman, 1994). A further difficulty with the diagnosis of ADHD is the validity of classification systems for people with intellectual disabilities (Sturmey, 1993, 1995). The basic question is whether the disorder of ADHD can be identified in children with intellectual disabilities. Three research strategies have been employed that can help to answer this question. First,
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factor analysis of items reflecting a range of psychological symptoms in both adults and children with intellectual disabilities has identified hyperactivity sub-scales for several behavior rating questionnaires (Aman, Tasse´ , Rojahn, & Hammer, 1996; Freund & Reiss, 1991; Reiss & Valenti-Hein, 1994). Second, studies comparing the psychological characteristics of children with ADHD with and without intellectual disabilities suggest that they are similar (Fee, Matson, & Benavidez, 1994). Third, studies comparing children with intellectual disabilities with and without high levels of ADHD symptoms show differences in terms of activity behaviors and attention problems (Handen, McAuliffe, Janosky, Feldman, & Breaux, 1994, 1998; Melnyk & Das, 1992; Pearson, Yaffee, Loveland, & Lewis, 1996). The preceding review suggests that children with intellectual disabilities may be at increased risk for ADHD, and there is evidence that the disorder may be a valid diagnosis in the context of intellectual disability. Given the potential clinical implications of this research, most notably the use of stimulant medication as an intervention, more studies of ADHD symptoms in children with intellectual disabilities are needed. The purpose of the present paper is to explore a number of questions within three different data sets that included measures of ADHD symptoms: 1. Correlates of ADHD symptoms in children with intellectual disabilities—on the basis of previous research, we expected to find more symptoms in boys, declining symptoms with chronological age, and increased symptoms in children with Autism. 2. The relationship between ability and ADHD symptoms—on the basis of previous research, we expected to find no evidence or only weak evidence for associations between measures of ability and ADHD symptoms. 3. Increased risk of ADHD symptoms in children with intellectual disabilities—here, we used a within-family design (ADHD symptoms in children with intellectual disabilities and their siblings in the same families) and expected to replicate previous research findings. 2. Method 2.1. Participants Sample 1 consisted of 123 children between four and 18 years of age (mean = 10.89, S.D. = 4.21) administratively defined as having severe intellectual disability (i.e., they attended schools designated for such children, but no formal IQ data were available) (Cormack, Brown, & Hastings, 2000). Within this sample, there were 74 males and 49 females but fewer than 50% of children with an identified aetiology for their intellectual disability, including Down syndrome (34 children), Autism (11), Cerebral Palsy (9), and Tuberous Sclerosis (4). Data on these children were provided by their primary parental caregiver (in most cases, their mother). Sample 2 (Beck, Hastings, Daley, & Stevenson, 2004) consisted of 75 children with intellectual disabilities. The children were 48 boys and 26 girls (gender not recorded for one child) with a mean age of 9.75 years (S.D. = 4.04 years, range = 3–19 years). Twenty-three children had a diagnosis of Autism and 26 had a diagnosis of
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Down syndrome. The remaining children either had no diagnosis in addition to their intellectual disability or a diagnosis relevant to no other child in the sample. Based on the Vineland Adaptive Behavior Scale (VABS: Sparrow, Balla, & Cicchetti, 1984), 31% of the children had adaptive skills indicative of severe to profound intellectual disability and the remainder of mild or moderate intellectual disability. All data in this sample were provided by the child’s mother. The mothers also provided data on ADHD symptoms for a sibling without intellectual disabilities who was closest in age to their child with intellectual disabilities. These siblings were 35 brothers and 39 sisters (no gender recorded for one sibling) and were on average 10.34 (S.D. = 4.35) years of age. Sample 3 were from a study of parental adaptation to children with intellectual disabilities (Hastings, Beck, & Hill, in press). In this research, mothers provided data on 140 children with intellectual disabilities. The children were 10.43 years of age on average (S.D. = 4.03, range 4–18 years), 92 were male and 48 were female. Their diagnostic labels, as reported by parents, were: Autism (55 children), Down syndrome (24), Cerebral Palsy (12), and mixed aetiology intellectual disability (49). According to the VABS, 54% of the children had adaptive behavior levels indicative of severe to profound intellectual disability and the remainder of mild or moderate intellectual disability. 2.2. Measures Within each sample, mothers were asked to report on their child’s chronological age, sex, and any clinical diagnosis of Autism. For Samples 2 and 3, the severity of the child’s disability was measured using the Vineland Adaptive Behavior Scale—Survey Form (VABS: Sparrow et al., 1984). This semi-structured interview measure contains 297 items that provide an assessment of adaptive behavior across four domains: socialization, communication, daily living skills, and motor skills (only for children to a developmental age of 5 years). Not all 297 questions are used: rather, enough specific questions must be asked to allow the interviewer to make an accurate rating of adaptive behavior. Overall mental age derived from the VABS was used in the present analyses as an index of the child’s adaptive skills, along with age-standardized scores within the three core domains scored for every child in the sample (socialization, communication, daily living). For Sample 1, the only measure of intellectual disability available was a clinician’s rating of language impairment extracted from medical records. Each child was examined by a pediatrician at least once in every 12 month period. At these interviews, clinicians’ ratings of the child’s language impairment on a scale ranging from 0 (no problem) to 4 (profound), were reviewed and up-dated. 2.2.1. Measures of ADHD/hyperactivity For Sample 1, hyperactivity behaviors were measured using the parent report version of the Developmental Behavior Checklist (DBC: Einfeld & Tonge, 2002). This rating questionnaire contains 96 individual items that were derived from case files of children and adolescents with intellectual disabilities. It is one of a very small number of behavior rating questionnaires developed on and standardized for populations of children and adolescents with intellectual disabilities, and it has excellent psychometric properties (Dekker, Nunn,
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& Koot, 2002; Einfeld & Tonge, 1995; Hastings, Brown, Mount, & Cormack, 2001). The hyperactivity sub-scale contains six items from the DBC with face validity for the disorder (over-excited, poor attention span, impatient, impulsive, noisy or boisterous, and overactive). A total score for these items showed good internal consistency, differentiated children with and without a DSM diagnosis of ADHD, and was correlated significantly with the Conner’s Parent Rating Scale (Einfeld & Tonge, 2002). In the present sample, the DBC hyperactivity scale also had a good level of internal consistency (Cronbach’s alpha = .80). For Sample 2, hyperactivity behaviors for the children with and without intellectual disabilities in the family were measured using the hyperactivity sub-scale of the Strengths and Difficulties Questionnaire (SDQ: Goodman, 1997). This is a brief 25 item behavioral screening questionnaire that can be completed in about 5 min by parents. The SDQ has five sub-scales assessing pro-social behavior (e.g., ‘‘is your child kind to younger children’’), conduct disorder (e.g., ‘‘often has temper tantrums’’), emotional symptoms (e.g., ‘‘many worries often seems worried’’), hyperactivity (e.g., ‘‘easily distracted’’), and peer relationships (e.g., ‘‘has at least one good friend’’). The SDQ is a well-validated instrument and has been proven to be as effective as both the Child Behavior Checklist (Achenbach, 1991) and the Rutter Scales (Elander & Rutter, 1996) in identifying clinically significant levels of behavioral disturbance in children (Goodman, 1997; Goodman & Scott, 1999). Research with children with intellectual disabilities suggests that good levels of reliability are maintained (Beck, Daley, Hastings, & Stevenson, 2004). In the present sample, we also checked the internal consistency of the hyperactivity scale for the children with intellectual disabilities and found this to be reasonable (five items: Cronbach’s alpha = .69). For Sample 3, mothers’ ratings of ADHD symptoms were available from the Attention Deficit sub-scale of the Reiss Scales for Children’s Dual Diagnosis (Reiss & Valenti-Hein, 1990, 1994). This measure was designed specifically to assess psychopathology in children with intellectual disabilities. The Attention Deficit scale contains five items with both face validity and content validity (via factor analysis) for ADHD/hyperactivity (distracted, overactive, uncompleted activities, disobedient, runs away). In the present sample, the internal consistency of this scale was good (Cronbach’s alpha = .76) and slightly higher than that reported in previous research (Reiss & ValentiHein, 1994). 2.3. Procedure The procedure varied for the three samples. For Sample 1, the DBC survey was sent home to parents of all children attending schools for children with ‘‘severe learning difficulties’’ in a small geographical area. These data were matched with information from clinical files (language rating, and diagnostic category only were used in the present analysis). For Samples 2 and 3, invitations to participate in the research were distributed via parent organisations and by schools for children with special needs. Those parents who returned consent forms were then mailed questionnaires, including the behavior rating scales, and were interviewed over the telephone to complete the VABS.
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3. Results 3.1. Associations with gender, chronological age, and Autism Correlations between the chronological age of the child with intellectual disability and ADHD/hyperactivity symptoms were statistically significant across all three samples (Sample 1: r = .34, p < .001; Sample 2: r = .35, p < .01; Sample 3: r = .24, p < .01). In all cases, there is a relatively small correlation with age but the pattern of reducing symptoms with age is replicated. Associations with the gender and Autism diagnostic status of the child with intellectual disability were explored using independent samples ttests. The results of these analyses are summarised in Table 1. There was a gender difference for only one analysis (Sample 1) with boys rated as having more hyperactivity symptoms than girls, and an effect of Autism diagnosis for two analyses (Samples 2 and 3). In all cases, children with a diagnosis of Autism were rated as having more symptoms of ADHD/hyperactivity and this difference was statistically significant for two of the three samples. 3.2. Associations with adaptive behavior and mental age The correlation between language delay and hyperactivity behaviors in Sample 1 was not found to be significant (r = .09). For Samples 2 and 3, correlations between VABS scores and ADHD/hyperactivity ratings were calculated and are displayed in Table 2. These analyses show a consistent pattern of no significant associations between agestandardized scores on adaptive behavior domains and ADHD/hyperactivity symptoms. However, overall mental age for the children with intellectual disabilities was significantly negatively associated with ADHD/hyperactivity symptoms. 3.3. Hyperactivity in children with intellectual disabilities and their siblings Sixty per cent of the children with intellectual disabilities in Sample 2 scored above the clinical cut-off (i.e., within the ‘‘abnormal’’ range) on the SDQ hyperactivity scale, whereas this was the case for only 2.7% of their siblings. A related samples t-test revealed that children with intellectual disabilities (mean = 7.11, S.D. = 2.31) were rated as having
Table 1 Mean scores on ADHD/hyperactivity scales by gender and Autism diagnosis Samples
Scale
All samples
Boys
Girls
t
Autism
Not Autism
1
DBC hyperactivity
2
SDQ hyperactivity Reiss Attention Deficit
6.38 (3.28) 7.14 (2.40) 3.11 (2.63)
4.67 (3.00) 7.12 (2.20) 2.38 (2.18)
2.92 (p < .01) .05
3
5.70 (3.27) 7.11 (2.31) 2.86 (2.50)
6.55 (3.53) 8.09 (2.02) 4.02 (2.69)
5.62 (3.25) 6.67 (2.32) 2.11 (2.06)
S.D. in parentheses.
1.76
t .90 2.53 (p < .05) 4.49 (p < .001)
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Table 2 Associations between ADHD/hyperactivity ratings and adaptive behavior VABS score
Research samples 2 (Hyperactivity)
Mental age Communication Socialization Daily living skills *
.243* .141 .057 .127
3 (Attention Deficit) .203* .074 .024 .131
p < .05.
more hyperactivity symptoms than their siblings in the same family (mean = 2.09, S.D. = 2.01) (t(74) = 15.34, p < .001). Given the impact of chronological and mental age on hyperactivity ratings for the children with intellectual disabilities, this difference may have been due to chronological or mental age differences between the siblings. To explore these possibilities, hyperactivity scores were predicted for the children with intellectual disabilities using regression equations and then compared to their siblings’ measured scores. For chronological age, a regression equation was derived from the data on children with intellectual disabilities [SDQ hyperactivity score = (CA .198) + 9.068)]. The chronological age of the sibling was entered into the equation to calculate a predicted hyperactivity score for the child with intellectual disabilities (i.e., as if the children were of the same age). These predicted scores were compared with the measured scores of the siblings using a related samples t-test. The children with intellectual disabilities still had significantly higher scores than their siblings (t(74) = 19.89, p < .001). A similar procedure was repeated for mental age. No data on sibling mental age was available, so the assumption was made that their chronological and mental ages were equivalent. Thus, the chronological age of the sibling was entered into the regression equation for mental age as a predictor of hyperactivity in the children with intellectual disabilities [hyperactivity = (MA .217) + 8.019]. Thus, a comparison could be made as if the children were of the same mental age. Again, a related samples t-test comparing siblings’ measured scores with predicted scores for the children with intellectual disabilities still showed significantly higher scores in the children with intellectual disabilities after controlling for mental age (t(74) = 14.65, p < .001).
4. Discussion The present analyses have led to consistent results using different samples and different measures of ADHD/hyperactivity. As with children without disabilities, chronological age was negatively correlated with mothers’ reports of symptoms of ADHD/hyperactivity for children with intellectual disabilities. However, in contrast to studies of children without disabilities (Gaub & Carlson, 1997), there was little evidence that the child’s gender affected mothers’ ratings of their symptoms. Finally, consistent with previous studies of children with disabilities, a diagnosis of Autism was found to be associated with more ADHD/hyperactivity symptoms. In terms of overall level of functioning, the results were
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also consistent across the samples explored in this research. First, we found that functioning across the domains of the VABS was not associated with the scores on ADHD/ hyperactivity scales. Second, there were small but significant negative associations between symptom ratings and overall mental age. Our final analyses confirmed that children with intellectual disabilities are at increased risk for ADHD/hyperactivity and that this seems to be independent of the effects of chronological and mental age. Focusing on the main significant findings first, there are a number of implications of the present research for future studies and for clinical practice. First, we did find that overall mental age was associated with symptoms of ADHD/hyperactivity in children with intellectual disabilities. The lack of a relationship in previous research studies may be due to the fact that only crude comparisons between levels of intellectual disability have been used (e.g., mild versus severe intellectual disability). Much more research is needed to explore the impact of mental age on ADHD/hyperactivity symptoms, with a particular need to differentiate types of symptoms (e.g., inattention, impulsiveness, and overactivity). The data presented here suggest that it is important to consider mental age when diagnosing ADHD in children with intellectual disabilities. However, our data also suggest that ADHD should not be rejected as a diagnosis simply because a child has an intellectual disability. Rather, children with intellectual disabilities do seem to have an increased risk for ADHD/hyperactivity behaviors over and above that associated with their developmental delays. Our second clear finding was that a reported clinical diagnosis of Autism was associated with the increased presence of ADHD/hyperactivity behaviors. There has been research interest in the pharmacological treatment of such symptoms in children with pervasive developmental disorders (Aman & Langworthy, 2000) but little research concerned with explaining their apparently increased risk for ADHD. The DSM diagnostic criteria suggest that ADHD/hyperactivity symptoms in Autism may be better explained by the disorder itself than through the application of the ADHD label. However, more basic research is needed to understand the underlying disorder. In particular, we could find no studies comparing neuropsychological performance on inhibition and attention tasks for children with Autism who meet with diagnostic criteria for ADHD and those who do not. Such research would more clearly help to delineate the nature of the disorder of ADHD in Autism than reliance on ratings of behavioral symptoms alone. Our third main finding was based on a within-family design that confirmed an increased risk for ADHD/hyperactivity symptoms in children with intellectual disability. This design had the advantage of controlling for the overall family environment (the children and their siblings lived in the same family home) and the rater (both children were rated by the same mother). However, there may also be disadvantages with such a design. For example, we might speculate that mothers will tend to over-estimate the behavior problems of their children with intellectual disabilities (perhaps because these behaviors are stressful and/or because they wish to attract support services for their child) and under-estimate those of their children without disabilities (perhaps because of a reluctance to suggest that the sibling is negatively affected by having a brother or sister with a disability). A further strength was that we were able to conduct exploratory analyses to control for the effects of both chronological and mental age when comparing the two groups of children. This is a methodological approach that has not been adopted in previous studies comparing the
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presence of ADHD/hyperactivity in children with and without intellectual disabilities. Future research should seek to employ more careful matching to understand the limits of the apparent increased risk for children with intellectual disabilities. A limitation of the present research is that a different measure was used in each analysis. Although each individual scale seems to perform well in terms of internal consistency reliability, there are few data to support their diagnostic utility specifically with children with intellectual disabilities. Most significant is probably the question of whether these scales capture the nature of ADHD/hyperactivity in children with intellectual disabilities. Until further research confirms whether ADHD presents in a similar manner in children with and without intellectual disabilities, measurement issues cannot be advanced. At present, any clinician considering a diagnosis of ADHD or Hyperkinesis in a child with intellectual disability should probably collect multiple sources of data (several rating scales, observations), consider carefully the impact of the presence of Autism, consider the potential impact of low mental age, and ideally include a test of performance on some experimental tests found to be reliable in other populations. Tests that have been shown to differentiate children with and without ADHD include the Stop-Signal Task and the Choice-Delay Task (Solanto et al., 2001). Many questions are left unanswered in relation to ADHD/hyperactivity in individuals with intellectual disabilities. The first substantial problems to be solved are the measurement, diagnosis, and understanding of the phenomenology of the disorder in children with intellectual disability. There are then questions about the treatment of the disorder. Assuming that new measurement is used in future pharmacological studies to confirm existing positive results, there is likely to be an important role for drug treatment. However, a coherent effort should also be directed at psycho-social interventions. Recent data on parent behavior management interventions for pre-school children with ADHD suggest that such interventions can lead to clinically and statistically significant outcomes (Bor, Sanders, & Markie-Dadds, 2002; Sonuga-Barke, Daley, Thompson, Laver-Bradbury, & Weeks, 2001). Similar intervention methods might be successful with parents of school age children with intellectual disabilities, given the children’s developmental delays. Nonpharmacological interventions may be attractive especially for children with intellectual disabilities who do not benefit from drug interventions, require high doses to have an effect, or who are at risk for more negative side effects from stimulant medication. A final set of questions relate to the presence of high levels of hyperactivity symptoms in many populations of adults with intellectual disabilities (Fox & Wade, 1998). ADHD is increasingly recognized as a disorder across the lifespan (Faraone, Biederman, Feighner, & Monuteaux, 2000; Young, Toone, & Tyson, 2003) and it is important that developments in research, theory, and treatment for adults consider the particular needs of people with intellectual disabilities.
Acknowledgement Preparation of this manuscript was supported in part by the Health Foundation (grant number 1583/1475). Any opinions expressed herein are those of the authors and are not necessarily endorsed by the Health Foundation.
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Faraone, S. V., Biederman, J., Feighner, J. A., & Monuteaux, M. C. (2000). Assessing symptoms of Attention Deficit Hyperactivity Disorder in children and adults: Which is more valid? Journal of Consulting and Clinical Psychology, 68, 830–842. Fee, V. E., Matson, J. L., & Benavidez, D. A. (1994). Attention Deficit Hyperactivity Disorder among mentally retarded children. Research in Developmental Disabilities, 15, 67–79. Fee, V. E., Matson, J. L., Moore, L. A., & Benavidez, D. A. (1993). The differential validity of hyperactivity/ attention deficits and conduct problems among mentally retarded children. Journal of Abnormal Child Psychology, 21, 1–11. Fox, R. A., & Wade, E. J. (1998). Attention Deficit Hyperactivity Disorder among adults with severe and profound mental retardation. Research in Developmental Disabilities, 19, 275–280. Freund, L. S., & Reiss, A. L. (1991). Rating problem behaviors in outpatients with mental retardation: Use of the Aberrant Behavior Checklist. Research in Developmental Disabilities, 12, 435–451. Gaub, M., & Carlson, C. L. (1997). Gender differences in ADHD: A meta-analysis and critical review. Journal of the American Academy of Child and Adolescent Psychiatry, 36, 1036–1045. Goodman, R. (1997). The Strengths and Difficulties Questionnaire: A research note. Journal of Child Psychology and Psychiatry, 38, 581–586. Goodman, R., & Scott, S. (1999). Comparing the Strengths and Difficulties Questionnaire and the Child Behavior Checklist: Is small beautiful? Journal of Abnormal Child Psychology, 27, 17–24. Handen, B. L., Breaux, A. M., Janosky, J., McAuliffe, S., Feldman, H., & Gosling, A. (1992). Effects and noneffects on methylphenidate in children with mental retardation and ADHD. Journal of the American Academy of Child and Adolescent Psychiatry, 31, 455–461. Handen, B. L., Feldman, H. M., Lurier, A., & Murray, P. J. H. (1999). Efficacy of methylphenidate among preschool children with developmental disabilities and ADHD. Journal of the American Academy of Child and Adolescent Psychiatry, 38, 805–812. Handen, B. L., Janosky, J., & McAuliffe, S. (1997). Long term follow-up of children with mental retardation/ borderline intellectual functioning and ADHD. Journal of Abnormal Child Psychology, 25, 287–295. Handen, B. L., McAuliffe, S., & Caro-Martinez, L. (1996). Stimulant medication effects on learning in children with mental retardation and ADHD. Journal of Developmental and Physical Disabilities, 8, 335–346. Handen, B. L., McAuliffe, S., Janosky, J., Feldman, H., & Breaux, A. M. (1994). Classroom behavior and children with mental retardation: Comparison of children with and without ADHD. Journal of Abnormal Child Psychology, 22, 267–280. Handen, B. L., McAuliffe, S., Janosky, J., Feldman, H., & Breaux, A. M. (1995). Methylphenidate in children with mental retardation and ADHD: Effects on independent play and academic functioning. Journal of Developmental and Physical Disabilities, 7, 91–103. Handen, B. L., McAuliffe, S., Janosky, J., Feldman, H., & Breaux, A. M. (1998). A playroom observation procedure to assess children with mental retardation and ADHD. Journal of Abnormal Child Psychology, 26, 269–277. Hastings, R. P., Beck, A., & Hill, C. (in press). Positive contributions made by children with a learning disability in the family: Mothers’ and Fathers’ perceptions. Journal of Learning Disabilities. Hastings, R. P., Brown, T., Mount, R. H., & Cormack, K. F. M. (2001). Exploration of psychometric properties of the Developmental Behavior Checklist. Journal of Autism and Developmental Disorders, 31, 423–431. Johnson, C. R., Handen, B. L., Lubetsky, M. J., & Sacco, K. A. (1994). Efficacy of methylphenidate and behavioral intervention on classroom behavior in children with ADHD and mental retardation. Behavior Modification, 18, 470–487. Linna, S. L., Piha, J., Kumpulainen, K., Tamminen, T., & Almqvist, F. (1999). Psychiatric symptoms in children with intellectual disability. European Child and Adolescent Psychiatry, 8, 77–82. Melnyk, L., & Das, J. P. (1992). Measurement of attention deficit: Correspondence between rating scales and tests of sustained and selective attention. American Journal on Mental Retardation, 96, 599–606. Meltzer, H., Gatward, R., Goodman, R., & Ford, F. (2000). Mental health of children and adolescents in Great Britain. London: HMSO. Munir, F., Cornish, K. M., & Wilding, J. (2000). A neuropsychological profile of attention deficits in young males with Fragile X syndrome. Neuropsychologia, 38, 1261–1270.
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Pearson, D. A., & Aman, M. G. (1994). Ratings of hyperactivity and developmental indices: Should clinicians correct for developmental level? Journal of Autism and Developmental Disorders, 24, 395–411. Pearson, D. A., Santos, C. W., Roache, J. D., Casat, C. D., Loveland, K. A., Lachar, D., et al. (2003). Treatment effects of methylphenidate on behavioral adjustment in children with mental retardation and ADHD. Journal of the American Academy of Child and Adolescent Psychiatry, 42, 209–216. Pearson, D. A., Santos, C. W., Roache, J. D., Loveland, K. A., Casat, C. D., Farwell, E. C., et al. (1996). Effects of methylphenidate on behavioral adjustment in children with mental retardation and ADHD: Preliminary findings from a study in progress. Journal of Developmental and Physical Disabilities, 8, 313–333. Pearson, D. A., Yaffee, L. S., Loveland, K. A., & Lewis, K. R. (1996). Comparison of sustained and selective attention in children who have mental retardation with and without Attention Deficit Hyperactivity Disorder. American Journal on Mental Retardation, 100, 592–607. Reiss, S., & Valenti-Hein, D. (1990). Reiss Scales for Children’s Dual Diagnosis: Test manual (2nd ed.). Worthington, OH: IDS Publishing. Reiss, S., & Valenti-Hein, D. (1994). Development of a psychopathology rating scale for children with mental retardation. Journal of Consulting and Clinical Psychology, 62, 29–33. Rutter, M., Graham, P., & Yule, W. (1970). A neuropsychiatric study in childhood. London: SIMP/Heinemann. Scahill, L., Schwab-Stone, M., Merikangas, K. R., Leckman, J. F., Zhang, H., & Kasl, S. (1999). Psychosocial and clinical correlates of AD/HD in a community sample of school-aged children. Journal of the American Academy of Child and Adolescent Psychiatry, 38, 976–984. Solanto, M. V., Abikoff, H., Sonuga-Barke, E. J. S., Schachar, R., Logan, G. D., Wigal, T., et al. (2001). The ecological validity of delay aversion and response inhibition as measures of impulsivity in AD/HD: A supplement to the NIMH multimodal treatment study of AD/HD. Journal of Abnormal Child Psychology, 29, 215–228. Sonuga-Barke, E. J. S., Daley, D., Thompson, M., Laver-Bradbury, C., & Weeks, A. (2001). Parent-based therapies for preschool attention-deficit/hyperactivity disorder: A randomized, controlled trial with a community sample. Journal of the American Academy of Child and Adolescent Psychiatry, 40, 402–408. Sparrow, S. S., Balla, D. A., & Cicchetti, D. V. (1984). The Vineland Adaptive Behavior Scale (Interview edition survey form). Circle Time, MN: American Guidance Service. Strømme, P., & Diseth, T. H. (2000). Prevalence of psychiatric diagnoses in children with mental retardation: Data from a population-based study. Developmental Medicine and Child Neurology, 42, 266–270. Sturmey, P. (1993). The use of DSM and ICD diagnostic criteria in people with mental retardation: A review of empirical studies. Journal of Nervous and Mental Disease, 181, 38–41. Sturmey, P. (1995). DSM-III-R and persons with dual diagnosis: Conceptual issues and strategies for future research. Journal of Intellectual Disability Research, 39, 357–364. Swanson, J., Sergeant, J. A., Taylor, E., Sonuga-Barke, E. J. S., Jensen, P. S., & Cantwell, D. P. (1998). Attention Deficit/Hyperactivity Disorder and Hyperkinetic Disorder. Lancet, 351, 429–433. Taylor, E. (1998). Clinical foundations of hyperactivity research. Behaviour Brain Research, 94, 11–24. Taylor, E., Chadwick, O., Heptinstall, E., & Danckaerts, M. (1996). Hyperactivity and conduct problems as risk factors for adolescent development. Journal of the American Academy of Child and Adolescent Psychiatry, 35, 1213–1226. Turk, J. (1998). Fragile X syndrome and attentional deficits. Journal of Applied Research in Intellectual Disabilities, 11, 175–191. Varley, C. K., & Trupin, E. W. (1982). Double-blind administration of methylphenidate to mentally retarded children with Attention Deficit Disorder: A preliminary study. American Journal of Mental Deficiency, 86, 560–566. Young, Y., Toone, B., & Tyson, C. (2003). Comorbidity and psychosocial profile of adults with Attention Deficit/ Hyperactivity Disorder. Personality and Individual Differences, 35, 743–755.
Symptoms of ADHD and their correlates in children with intellectual disabilities Richard P. Hastings*, Alexandra Beck, Dave Daley, Christopher Hill School of Psychology, University of Wales Bangor, Bangor, Gwynedd, Wales LL57 2AS, UK Received 10 June 2004; received in revised form 28 September 2004; accepted 15 October 2004
Abstract Existing research suggests that children with intellectual disabilities are at increased risk for ADHD, and that the symptoms of the disorder might successfully be treated with stimulant drugs. However, there has been little exploration of ADHD symptoms and their correlates in children with intellectual disabilities. Analyses of three samples of children with intellectual disabilities are presented (total N = 338). Correlational analyses showed that younger children, and those with a diagnosis of Autism were rated as having more ADHD/hyperactivity symptoms. There was little evidence of a sex difference, and no strong associations with domains of adaptive behavior (socialization, communication, and daily living skills). However, there was a small but significant negative association between mental age and ratings of symptoms. Finally, an increased prevalence of ADHD/hyperactivity symptoms was confirmed in the children with intellectual disabilities compared to their siblings. This effect remained after controlling for chronological and mental age differences between the siblings. These findings support those from previous research and suggest that ADHD/ Hyperkinesis may be a valid psychiatric diagnosis for children with intellectual disabilities. However, a great deal more research is needed to explore the phenomenology of ADHD in intellectual disability and to develop an evidence base for psychosocial intervention. # 2004 Elsevier Ltd. All rights reserved. Keywords: Intellectual disability; Children; Attention Deficit Hyperactivity Disorder; Hyperkinesis
* Corresponding author. Tel.: +44 1248 388 214. E-mail address: [email protected] (R.P. Hastings). 0891-4222/$ – see front matter # 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.ridd.2004.10.003
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1. Introduction Attention Deficit/Hyperactivity Disorder (AD/HD: American Psychiatric Association, 1994) is a relatively common, debilitating condition of childhood, characterised by severe, persistent, and early appearing symptoms of inattention, overactivity and impulsiveness (Swanson et al., 1998). The disorder affects between 2% and 5% of children of school age (Scahill et al., 1999). ADHD frequently co-occurs with other child mental health problems, such as anti-social conduct disorder, and anxiety. It is associated with marked social and educational impairment in the school years, and is a major risk factor for the development of problems throughout adolescence and adulthood, including conduct disorder, personality disorder, and educational failure (Taylor, Chadwick, Heptinstall, & Danckaerts, 1996). AD/HD is closely related to Hyperkinesis, which is a more severe form of ADHD based on alternative rating criteria (ICD-10). While the symptoms of the two disorders are very similar, requirements relating to pervasiveness and diagnostic profile vary somewhat between the ICD and DSM taxonomies (Swanson et al., 1998). For example, according to DSM-IV criteria, it is possible to have ‘‘ADHD’’ without being inattentive. However, inattentiveness is a necessary requirement for a hyperkinetic disorder according to ICD-10 criteria (Taylor, 1998). Symptoms of ADHD have been found in several studies of children with intellectual disabilities at levels leading to the diagnosis of the disorder at apparently higher rates than those found in populations without intellectual disabilities (Dekker & Koot, 2003; Strømme & Diseth, 2000). For example, Dekker and Koot (2003) recruited a sample representative of children in Dutch special schools and conducted DSM-IV diagnostic assessment using a standardized clinical interview. They found a prevalence rate of 14.8% for any ADHD disorder, and a high degree of co-morbidity (44%) with Oppositional Defiant Disorder. There have also been a number of studies that incorporated direct comparisons of ADHD symptoms between children with and without intellectual disabilities. Large population-based studies using rating questionnaires have shown that ADHD/hyperactivity symptoms occur at significantly higher levels in children with intellectual disabilities (Linna, Piha, Kumpulainen, Tamminen, & Almqvist, 1999; Rutter, Graham, & Yule, 1970). Similar results have been found in smaller group comparisons of children with and without intellectual disabilities (Epstein, Cullinan, & Gadow, 1986; Fee, Matson, Moore, & Benavidez, 1993). One difficulty with research using behavior rating scales is that it can be difficult to map symptom profiles onto the criteria used in psychiatric diagnostic symptoms. In this respect, Emerson’s (2003) research is perhaps one of the most informative designs for comparing psychiatric problems in children with and without intellectual disabilities. Emerson (2003) conducted a secondary analysis of a representative population-based survey of mental health in over 10,000 children and adolescents in the UK (Meltzer, Gatward, Goodman, & Ford, 2000). In the absence of any explicit data on intellectual disability, Emerson used a working definition of intellectual disability (e.g., presence of ‘‘learning difficulties’’ (UK Education Department terminology for mental retardation), concerns about early language development, attending special schools for children with
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intellectual disabilities). Emerson compared psychiatric diagnoses in this intellectual disability group against the remainder of the sample presumed not to have an intellectual disability. A clinical interview was used to establish ICD-10 diagnoses, including Hyperkinesis. Emerson found that 8.7% of children in the intellectual disability group were diagnosed with Hyperkinesis compared with 0.9% of the remainder of the sample, a difference that was statistically significant (Odds ratio = 10.0; 95% CI = 6.2, 16.0). These results suggest that the presence of an intellectual disability is associated with a 10-fold increase in the risk for Hyperkinesis. Although the data are not presented, Emerson (2003) states that similar results across all disorders were found when DSM rather than ICD diagnoses were made from the clinical interviews. Despite the data reviewed above, there have been few studies focusing specifically on ADHD in children with intellectual disabilities. There is a relatively large literature on the use of stimulant medication as a pharmacological management strategy for children with intellectual disabilities and ADHD (e.g., Aman, Buican, & Arnold, 2003; Aman, Kern, McGhee, & Arnold, 1993a, 1993b; Beale & McDowell, 1994; Handen et al., 1992; Handen, Feldman, Lurier, & Murray, 1999; Handen, McAuliffe, & Caro-Martinez, 1996; Handen, McAuliffe, Janosky, Feldman, & Breaux, 1995; Johnson, Handen, Lubetsky, & Sacco, 1994; Pearson, Santos, et al., 1996a, 2003; Varley & Trupin, 1982). This research shows improvements in parent- and teacher-rated behavior, and observed behavior in the classroom. There are also data from follow-up studies of drug interventions suggesting that ADHD symptoms in children with intellectual disabilities are persistent over extended periods of time (e.g., Aman, Armstrong, Buican, & Sillick, 2002; Handen, Janosky, & McAuliffe, 1997). However, few studies have been concerned with the presentation of ADHD in children with intellectual disabilities and with the correlates of ADHD symptoms. The presentation of ADHD in children with intellectual disabilities is a crucial issue. Diagnostic systems emphasize that any symptoms of inattention, overactivity, and impulsivity should be over and above those expected for the child’s age and level of disability. Research within the developmental tradition has suggested no strong evidence for attention deficits in children with intellectual disabilities once children have been matched for mental age (Burack, Evans, Klaiman, & Iarocci, 2001), although children with specific syndromes, including Fragile X may have deficits in attention (e.g., Munir, Cornish, & Wilding, 2000; Turk, 1998). On the basis of this research on attention, and guidance from diagnostic systems, one might conclude that the apparent increased risk for ADHD in children with intellectual disabilities is due simply to their developmental delay. However, there are at least two problems with such a conclusion. First, ADHD is defined using more than symptoms of inattention and research on the impact of developmental delay on the differential prevalence of impulsiveness and overactivity is not available. Second, one would predict that ADHD symptoms should be correlated with the severity of intellectual disability (more pronounced with more developmental delay) but data suggest that this may not be the case (Dekker & Koot, 2003; Pearson & Aman, 1994). A further difficulty with the diagnosis of ADHD is the validity of classification systems for people with intellectual disabilities (Sturmey, 1993, 1995). The basic question is whether the disorder of ADHD can be identified in children with intellectual disabilities. Three research strategies have been employed that can help to answer this question. First,
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factor analysis of items reflecting a range of psychological symptoms in both adults and children with intellectual disabilities has identified hyperactivity sub-scales for several behavior rating questionnaires (Aman, Tasse´ , Rojahn, & Hammer, 1996; Freund & Reiss, 1991; Reiss & Valenti-Hein, 1994). Second, studies comparing the psychological characteristics of children with ADHD with and without intellectual disabilities suggest that they are similar (Fee, Matson, & Benavidez, 1994). Third, studies comparing children with intellectual disabilities with and without high levels of ADHD symptoms show differences in terms of activity behaviors and attention problems (Handen, McAuliffe, Janosky, Feldman, & Breaux, 1994, 1998; Melnyk & Das, 1992; Pearson, Yaffee, Loveland, & Lewis, 1996). The preceding review suggests that children with intellectual disabilities may be at increased risk for ADHD, and there is evidence that the disorder may be a valid diagnosis in the context of intellectual disability. Given the potential clinical implications of this research, most notably the use of stimulant medication as an intervention, more studies of ADHD symptoms in children with intellectual disabilities are needed. The purpose of the present paper is to explore a number of questions within three different data sets that included measures of ADHD symptoms: 1. Correlates of ADHD symptoms in children with intellectual disabilities—on the basis of previous research, we expected to find more symptoms in boys, declining symptoms with chronological age, and increased symptoms in children with Autism. 2. The relationship between ability and ADHD symptoms—on the basis of previous research, we expected to find no evidence or only weak evidence for associations between measures of ability and ADHD symptoms. 3. Increased risk of ADHD symptoms in children with intellectual disabilities—here, we used a within-family design (ADHD symptoms in children with intellectual disabilities and their siblings in the same families) and expected to replicate previous research findings. 2. Method 2.1. Participants Sample 1 consisted of 123 children between four and 18 years of age (mean = 10.89, S.D. = 4.21) administratively defined as having severe intellectual disability (i.e., they attended schools designated for such children, but no formal IQ data were available) (Cormack, Brown, & Hastings, 2000). Within this sample, there were 74 males and 49 females but fewer than 50% of children with an identified aetiology for their intellectual disability, including Down syndrome (34 children), Autism (11), Cerebral Palsy (9), and Tuberous Sclerosis (4). Data on these children were provided by their primary parental caregiver (in most cases, their mother). Sample 2 (Beck, Hastings, Daley, & Stevenson, 2004) consisted of 75 children with intellectual disabilities. The children were 48 boys and 26 girls (gender not recorded for one child) with a mean age of 9.75 years (S.D. = 4.04 years, range = 3–19 years). Twenty-three children had a diagnosis of Autism and 26 had a diagnosis of
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Down syndrome. The remaining children either had no diagnosis in addition to their intellectual disability or a diagnosis relevant to no other child in the sample. Based on the Vineland Adaptive Behavior Scale (VABS: Sparrow, Balla, & Cicchetti, 1984), 31% of the children had adaptive skills indicative of severe to profound intellectual disability and the remainder of mild or moderate intellectual disability. All data in this sample were provided by the child’s mother. The mothers also provided data on ADHD symptoms for a sibling without intellectual disabilities who was closest in age to their child with intellectual disabilities. These siblings were 35 brothers and 39 sisters (no gender recorded for one sibling) and were on average 10.34 (S.D. = 4.35) years of age. Sample 3 were from a study of parental adaptation to children with intellectual disabilities (Hastings, Beck, & Hill, in press). In this research, mothers provided data on 140 children with intellectual disabilities. The children were 10.43 years of age on average (S.D. = 4.03, range 4–18 years), 92 were male and 48 were female. Their diagnostic labels, as reported by parents, were: Autism (55 children), Down syndrome (24), Cerebral Palsy (12), and mixed aetiology intellectual disability (49). According to the VABS, 54% of the children had adaptive behavior levels indicative of severe to profound intellectual disability and the remainder of mild or moderate intellectual disability. 2.2. Measures Within each sample, mothers were asked to report on their child’s chronological age, sex, and any clinical diagnosis of Autism. For Samples 2 and 3, the severity of the child’s disability was measured using the Vineland Adaptive Behavior Scale—Survey Form (VABS: Sparrow et al., 1984). This semi-structured interview measure contains 297 items that provide an assessment of adaptive behavior across four domains: socialization, communication, daily living skills, and motor skills (only for children to a developmental age of 5 years). Not all 297 questions are used: rather, enough specific questions must be asked to allow the interviewer to make an accurate rating of adaptive behavior. Overall mental age derived from the VABS was used in the present analyses as an index of the child’s adaptive skills, along with age-standardized scores within the three core domains scored for every child in the sample (socialization, communication, daily living). For Sample 1, the only measure of intellectual disability available was a clinician’s rating of language impairment extracted from medical records. Each child was examined by a pediatrician at least once in every 12 month period. At these interviews, clinicians’ ratings of the child’s language impairment on a scale ranging from 0 (no problem) to 4 (profound), were reviewed and up-dated. 2.2.1. Measures of ADHD/hyperactivity For Sample 1, hyperactivity behaviors were measured using the parent report version of the Developmental Behavior Checklist (DBC: Einfeld & Tonge, 2002). This rating questionnaire contains 96 individual items that were derived from case files of children and adolescents with intellectual disabilities. It is one of a very small number of behavior rating questionnaires developed on and standardized for populations of children and adolescents with intellectual disabilities, and it has excellent psychometric properties (Dekker, Nunn,
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& Koot, 2002; Einfeld & Tonge, 1995; Hastings, Brown, Mount, & Cormack, 2001). The hyperactivity sub-scale contains six items from the DBC with face validity for the disorder (over-excited, poor attention span, impatient, impulsive, noisy or boisterous, and overactive). A total score for these items showed good internal consistency, differentiated children with and without a DSM diagnosis of ADHD, and was correlated significantly with the Conner’s Parent Rating Scale (Einfeld & Tonge, 2002). In the present sample, the DBC hyperactivity scale also had a good level of internal consistency (Cronbach’s alpha = .80). For Sample 2, hyperactivity behaviors for the children with and without intellectual disabilities in the family were measured using the hyperactivity sub-scale of the Strengths and Difficulties Questionnaire (SDQ: Goodman, 1997). This is a brief 25 item behavioral screening questionnaire that can be completed in about 5 min by parents. The SDQ has five sub-scales assessing pro-social behavior (e.g., ‘‘is your child kind to younger children’’), conduct disorder (e.g., ‘‘often has temper tantrums’’), emotional symptoms (e.g., ‘‘many worries often seems worried’’), hyperactivity (e.g., ‘‘easily distracted’’), and peer relationships (e.g., ‘‘has at least one good friend’’). The SDQ is a well-validated instrument and has been proven to be as effective as both the Child Behavior Checklist (Achenbach, 1991) and the Rutter Scales (Elander & Rutter, 1996) in identifying clinically significant levels of behavioral disturbance in children (Goodman, 1997; Goodman & Scott, 1999). Research with children with intellectual disabilities suggests that good levels of reliability are maintained (Beck, Daley, Hastings, & Stevenson, 2004). In the present sample, we also checked the internal consistency of the hyperactivity scale for the children with intellectual disabilities and found this to be reasonable (five items: Cronbach’s alpha = .69). For Sample 3, mothers’ ratings of ADHD symptoms were available from the Attention Deficit sub-scale of the Reiss Scales for Children’s Dual Diagnosis (Reiss & Valenti-Hein, 1990, 1994). This measure was designed specifically to assess psychopathology in children with intellectual disabilities. The Attention Deficit scale contains five items with both face validity and content validity (via factor analysis) for ADHD/hyperactivity (distracted, overactive, uncompleted activities, disobedient, runs away). In the present sample, the internal consistency of this scale was good (Cronbach’s alpha = .76) and slightly higher than that reported in previous research (Reiss & ValentiHein, 1994). 2.3. Procedure The procedure varied for the three samples. For Sample 1, the DBC survey was sent home to parents of all children attending schools for children with ‘‘severe learning difficulties’’ in a small geographical area. These data were matched with information from clinical files (language rating, and diagnostic category only were used in the present analysis). For Samples 2 and 3, invitations to participate in the research were distributed via parent organisations and by schools for children with special needs. Those parents who returned consent forms were then mailed questionnaires, including the behavior rating scales, and were interviewed over the telephone to complete the VABS.
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3. Results 3.1. Associations with gender, chronological age, and Autism Correlations between the chronological age of the child with intellectual disability and ADHD/hyperactivity symptoms were statistically significant across all three samples (Sample 1: r = .34, p < .001; Sample 2: r = .35, p < .01; Sample 3: r = .24, p < .01). In all cases, there is a relatively small correlation with age but the pattern of reducing symptoms with age is replicated. Associations with the gender and Autism diagnostic status of the child with intellectual disability were explored using independent samples ttests. The results of these analyses are summarised in Table 1. There was a gender difference for only one analysis (Sample 1) with boys rated as having more hyperactivity symptoms than girls, and an effect of Autism diagnosis for two analyses (Samples 2 and 3). In all cases, children with a diagnosis of Autism were rated as having more symptoms of ADHD/hyperactivity and this difference was statistically significant for two of the three samples. 3.2. Associations with adaptive behavior and mental age The correlation between language delay and hyperactivity behaviors in Sample 1 was not found to be significant (r = .09). For Samples 2 and 3, correlations between VABS scores and ADHD/hyperactivity ratings were calculated and are displayed in Table 2. These analyses show a consistent pattern of no significant associations between agestandardized scores on adaptive behavior domains and ADHD/hyperactivity symptoms. However, overall mental age for the children with intellectual disabilities was significantly negatively associated with ADHD/hyperactivity symptoms. 3.3. Hyperactivity in children with intellectual disabilities and their siblings Sixty per cent of the children with intellectual disabilities in Sample 2 scored above the clinical cut-off (i.e., within the ‘‘abnormal’’ range) on the SDQ hyperactivity scale, whereas this was the case for only 2.7% of their siblings. A related samples t-test revealed that children with intellectual disabilities (mean = 7.11, S.D. = 2.31) were rated as having
Table 1 Mean scores on ADHD/hyperactivity scales by gender and Autism diagnosis Samples
Scale
All samples
Boys
Girls
t
Autism
Not Autism
1
DBC hyperactivity
2
SDQ hyperactivity Reiss Attention Deficit
6.38 (3.28) 7.14 (2.40) 3.11 (2.63)
4.67 (3.00) 7.12 (2.20) 2.38 (2.18)
2.92 (p < .01) .05
3
5.70 (3.27) 7.11 (2.31) 2.86 (2.50)
6.55 (3.53) 8.09 (2.02) 4.02 (2.69)
5.62 (3.25) 6.67 (2.32) 2.11 (2.06)
S.D. in parentheses.
1.76
t .90 2.53 (p < .05) 4.49 (p < .001)
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Table 2 Associations between ADHD/hyperactivity ratings and adaptive behavior VABS score
Research samples 2 (Hyperactivity)
Mental age Communication Socialization Daily living skills *
.243* .141 .057 .127
3 (Attention Deficit) .203* .074 .024 .131
p < .05.
more hyperactivity symptoms than their siblings in the same family (mean = 2.09, S.D. = 2.01) (t(74) = 15.34, p < .001). Given the impact of chronological and mental age on hyperactivity ratings for the children with intellectual disabilities, this difference may have been due to chronological or mental age differences between the siblings. To explore these possibilities, hyperactivity scores were predicted for the children with intellectual disabilities using regression equations and then compared to their siblings’ measured scores. For chronological age, a regression equation was derived from the data on children with intellectual disabilities [SDQ hyperactivity score = (CA .198) + 9.068)]. The chronological age of the sibling was entered into the equation to calculate a predicted hyperactivity score for the child with intellectual disabilities (i.e., as if the children were of the same age). These predicted scores were compared with the measured scores of the siblings using a related samples t-test. The children with intellectual disabilities still had significantly higher scores than their siblings (t(74) = 19.89, p < .001). A similar procedure was repeated for mental age. No data on sibling mental age was available, so the assumption was made that their chronological and mental ages were equivalent. Thus, the chronological age of the sibling was entered into the regression equation for mental age as a predictor of hyperactivity in the children with intellectual disabilities [hyperactivity = (MA .217) + 8.019]. Thus, a comparison could be made as if the children were of the same mental age. Again, a related samples t-test comparing siblings’ measured scores with predicted scores for the children with intellectual disabilities still showed significantly higher scores in the children with intellectual disabilities after controlling for mental age (t(74) = 14.65, p < .001).
4. Discussion The present analyses have led to consistent results using different samples and different measures of ADHD/hyperactivity. As with children without disabilities, chronological age was negatively correlated with mothers’ reports of symptoms of ADHD/hyperactivity for children with intellectual disabilities. However, in contrast to studies of children without disabilities (Gaub & Carlson, 1997), there was little evidence that the child’s gender affected mothers’ ratings of their symptoms. Finally, consistent with previous studies of children with disabilities, a diagnosis of Autism was found to be associated with more ADHD/hyperactivity symptoms. In terms of overall level of functioning, the results were
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also consistent across the samples explored in this research. First, we found that functioning across the domains of the VABS was not associated with the scores on ADHD/ hyperactivity scales. Second, there were small but significant negative associations between symptom ratings and overall mental age. Our final analyses confirmed that children with intellectual disabilities are at increased risk for ADHD/hyperactivity and that this seems to be independent of the effects of chronological and mental age. Focusing on the main significant findings first, there are a number of implications of the present research for future studies and for clinical practice. First, we did find that overall mental age was associated with symptoms of ADHD/hyperactivity in children with intellectual disabilities. The lack of a relationship in previous research studies may be due to the fact that only crude comparisons between levels of intellectual disability have been used (e.g., mild versus severe intellectual disability). Much more research is needed to explore the impact of mental age on ADHD/hyperactivity symptoms, with a particular need to differentiate types of symptoms (e.g., inattention, impulsiveness, and overactivity). The data presented here suggest that it is important to consider mental age when diagnosing ADHD in children with intellectual disabilities. However, our data also suggest that ADHD should not be rejected as a diagnosis simply because a child has an intellectual disability. Rather, children with intellectual disabilities do seem to have an increased risk for ADHD/hyperactivity behaviors over and above that associated with their developmental delays. Our second clear finding was that a reported clinical diagnosis of Autism was associated with the increased presence of ADHD/hyperactivity behaviors. There has been research interest in the pharmacological treatment of such symptoms in children with pervasive developmental disorders (Aman & Langworthy, 2000) but little research concerned with explaining their apparently increased risk for ADHD. The DSM diagnostic criteria suggest that ADHD/hyperactivity symptoms in Autism may be better explained by the disorder itself than through the application of the ADHD label. However, more basic research is needed to understand the underlying disorder. In particular, we could find no studies comparing neuropsychological performance on inhibition and attention tasks for children with Autism who meet with diagnostic criteria for ADHD and those who do not. Such research would more clearly help to delineate the nature of the disorder of ADHD in Autism than reliance on ratings of behavioral symptoms alone. Our third main finding was based on a within-family design that confirmed an increased risk for ADHD/hyperactivity symptoms in children with intellectual disability. This design had the advantage of controlling for the overall family environment (the children and their siblings lived in the same family home) and the rater (both children were rated by the same mother). However, there may also be disadvantages with such a design. For example, we might speculate that mothers will tend to over-estimate the behavior problems of their children with intellectual disabilities (perhaps because these behaviors are stressful and/or because they wish to attract support services for their child) and under-estimate those of their children without disabilities (perhaps because of a reluctance to suggest that the sibling is negatively affected by having a brother or sister with a disability). A further strength was that we were able to conduct exploratory analyses to control for the effects of both chronological and mental age when comparing the two groups of children. This is a methodological approach that has not been adopted in previous studies comparing the
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presence of ADHD/hyperactivity in children with and without intellectual disabilities. Future research should seek to employ more careful matching to understand the limits of the apparent increased risk for children with intellectual disabilities. A limitation of the present research is that a different measure was used in each analysis. Although each individual scale seems to perform well in terms of internal consistency reliability, there are few data to support their diagnostic utility specifically with children with intellectual disabilities. Most significant is probably the question of whether these scales capture the nature of ADHD/hyperactivity in children with intellectual disabilities. Until further research confirms whether ADHD presents in a similar manner in children with and without intellectual disabilities, measurement issues cannot be advanced. At present, any clinician considering a diagnosis of ADHD or Hyperkinesis in a child with intellectual disability should probably collect multiple sources of data (several rating scales, observations), consider carefully the impact of the presence of Autism, consider the potential impact of low mental age, and ideally include a test of performance on some experimental tests found to be reliable in other populations. Tests that have been shown to differentiate children with and without ADHD include the Stop-Signal Task and the Choice-Delay Task (Solanto et al., 2001). Many questions are left unanswered in relation to ADHD/hyperactivity in individuals with intellectual disabilities. The first substantial problems to be solved are the measurement, diagnosis, and understanding of the phenomenology of the disorder in children with intellectual disability. There are then questions about the treatment of the disorder. Assuming that new measurement is used in future pharmacological studies to confirm existing positive results, there is likely to be an important role for drug treatment. However, a coherent effort should also be directed at psycho-social interventions. Recent data on parent behavior management interventions for pre-school children with ADHD suggest that such interventions can lead to clinically and statistically significant outcomes (Bor, Sanders, & Markie-Dadds, 2002; Sonuga-Barke, Daley, Thompson, Laver-Bradbury, & Weeks, 2001). Similar intervention methods might be successful with parents of school age children with intellectual disabilities, given the children’s developmental delays. Nonpharmacological interventions may be attractive especially for children with intellectual disabilities who do not benefit from drug interventions, require high doses to have an effect, or who are at risk for more negative side effects from stimulant medication. A final set of questions relate to the presence of high levels of hyperactivity symptoms in many populations of adults with intellectual disabilities (Fox & Wade, 1998). ADHD is increasingly recognized as a disorder across the lifespan (Faraone, Biederman, Feighner, & Monuteaux, 2000; Young, Toone, & Tyson, 2003) and it is important that developments in research, theory, and treatment for adults consider the particular needs of people with intellectual disabilities.
Acknowledgement Preparation of this manuscript was supported in part by the Health Foundation (grant number 1583/1475). Any opinions expressed herein are those of the authors and are not necessarily endorsed by the Health Foundation.
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