Article

Intellectual Disability in Children with Attention Deficit Hyperactivity Disorder

Child and Adolescent Psychiatry Section, Institute of Psychological Medicine and Clinical Neurosciences, Cardiff University School of Medicine, Cardiff, United Kingdom.
The Journal of pediatrics (Impact Factor: 3.79). 04/2013; 163(3). DOI: 10.1016/j.jpeds.2013.02.043
Source: PubMed

ABSTRACT

Objective
To determine whether children with attention deficit hyperactivity disorder (ADHD) and mild intellectual disability (ID) are a clinically distinct ADHD subgroup.

Study design
This was a cross-sectional study comparing clinical characteristics (ADHD subtypes, total number of symptoms, and rates of common comorbidities) between children with ADHD and mild ID and those with ADHD and IQ test scores >70, and also between children with ADHD and ID and a general population sample of children with ID alone. The sample comprised a clinical sample of children with ADHD with ID (n = 97) and without ID (n = 874) and a general population sample of children with ID and without ADHD (n = 58).

Results
After correcting for multiple statistical tests, no differences were found between the 2 ADHD groups on any measure except the presence of conduct disorder (CD) symptoms and diagnoses. Children with ADHD and ID had higher rates of both (OR, 2.38; 95% CI, 1.71-3.32 and OR, 2.69; 95% CI, 1.69-4.28, respectively). Furthermore, children with ADHD and ID had significantly higher rates of oppositional defiant disorder (OR, 5.54; 95% CI, 2.86-10.75) and CD (OR, 13.66; 95% CI, 3.25-57.42) symptoms and a higher incidence of oppositional defiant disorder diagnoses (OR, 30.99; 95% CI, 6.38-150.39) compared with children with ID without ADHD.

Conclusion
Children with ADHD and mild ID appear to be clinically typical of children with ADHD except for more conduct problems. This finding has implications for clinicians treating these children in terms of acknowledging the presence and impact of ADHD symptoms above and beyond ID and dealing with a comorbid CD.

Full-text

Available from: Joanna Martin, Aug 14, 2014
Intellectual Disability in Children with Attention Decit
Hyperactivity Disorder
*
Alka Ahuja, MRCPsych
1,2,
*, Joanna Martin, BSc (Hons)
2,3,
*, Kate Langley, PhD
2,3
, and
Anita Thapar, FRCPsych, FMedSci, PhD
2,3
Objective To determine whether children with attention deficit hyperactivity disorder (ADHD) and mild intellectual
disability (ID) are a clinically distinct ADHD subgroup.
Study design This was a cross-sectional study comparing clinical characteristics (ADHD subtypes, total number
of symptoms, and rates of common comorbidities) between children with ADHD and mild ID and those with AD HD
and IQ test scores >70, and also between children with ADHD and ID and a general population sample of children
with ID alone. The sample comprised a clinical sample of children with ADHD with ID (n = 97) and without ID (n = 874)
and a general population sample of children with ID and without ADHD (n = 58).
Results After correcting for multiple statistical tests, no differences were found between the 2 ADHD groups on
any measure except the presence of conduct disorder (CD) symptoms and diagnoses. Children with ADHD and
ID had higher rates of both (OR, 2.38; 95% CI, 1.71-3.32 and OR, 2.69; 95% CI, 1.69-4.28, respectively). Further-
more, children with AD HD and ID had significantly higher rates of oppositional defiant disorder (OR, 5.54; 95% CI,
2.86-10.75) and CD (OR, 13.66; 95% CI, 3.25-57.42) symptoms and a higher incidence of oppositional defiant dis-
order diagnoses (OR, 30.99; 95% CI, 6.38-150.39) compared with children with ID without ADHD.
Conclusion Children with AD HD and mild ID appear to be clinically typical of children with ADHD except for more
conduct problems. This finding has implications for clinicians treating these children in terms of acknowledging
the presence and impact of ADHD symptoms above and beyond ID and dealing with a comorbid CD. (J Pediatr
2013;163:890-5).
A
ttention deficit hyperactivity disorder (ADHD) is a disabling condition, affectin g 1.4%-6% of children.
1
Little is known
of the clinical presentation and etiology of ADHD in children with intellectual disability (ID), because those with lower
cognitive abilit y (IQ scores <70) are often excluded from studies of ADHD,
2
despite evidence that ADHD is more com-
mon in children with ID, and that the risk increases with increasing severity of ID.
3
It has been suggested that ADHD does not occur in children with ID, and that any inappropriate behavior in children with ID
is secondary to “mental impairm ent.”
4
That view is not supported by current evidence, however. Studies have shown that
ADHD occurs more commonly in these children but may be underdiagnosed owing to such issues as “diagnostic overshadow-
ing,” the tendency of clinicians to overlook additional psychiatric diagnoses after a diagnosis of ID is made, or “masking,” in
which the clinical characteristics of a mental disorder are masked by a cognitive, language, or speech deficit.
5
A populati on-based study estimating the prevalence of psychiatric diagnoses in children with ID identified hyperkinetic dis-
order as the most common psychiatric disorder.
6
Studies of children with mild and borderline ID have identified ADHD in 8%-
39% of cases.
7-9
A crucial clinical issue is whether or not the clinical pattern of comorbidity in this group is the same as that seen
in children with ADHD but without ID. This is important in determining the level and type of servic es and clin ical care required
for this subgroup.
In the present study, we compared the rates of comorbid problems and ADHD symptom levels in 2 groups of children
with ADHD, 1 group with ID (ADHD + ID group) and the other group without
ID (ADHD-only group). Consistent with previous studies of ADHD, we defined
ID is an IQ test score <70. We hypothesized that the ADHD profiles in the 2
groups (ADHD + ID [IQ <70] vs ADHD-only [IQ $70]) would be highly similar
From the
1
Ty Bryn Unit, St Cadocs Hospital, Newport,
United Kingdom; and
2
Child and Adolescent Psychiatry
Section, Institute of Psychological Medicine and Clinical
Neurosciences, and
3
Medical Research Council Center
for Neuropsychiatric Genetics and Genomics, Cardiff
University School of Medicine, Cardiff, Uinted Kingdom
*Contributed equally.
Clinical ADHD sample was funded by the Baily Thomas
Charitable Trust, Action Medical Research, and the
Wellcome Trust. The ALSPAC sample was funded by the
UK Medical Research Council, the Wellcome Trust
(092731), and the University of Bristol. The authors de-
clare no conflicts of interest.
0022-3476/$ - see front matter. Copyright ª 2013 The Authors.
Published by Elsevier Inc.
http://dx.doi.org/10.1016/j.jpeds.2013.02.043
*
This is an open access article under the CC BY license (http://
creativecommons.org/licenses/by/3.0/).
ADHD Attention deficit hyperactivity disorder
ALSPAC Avon Longitudinal Study of Parents and Children
ASD Autism spectrum disorder
CAPA Child and Adolescent Psychiatry Assessment
CD Conduct disorder
CNV Copy number variant
DSM-III-R Diagnostic and Statistical Manual of Mental Disorders, 3rd edition revised
DSM-IV Diagnostic and Statistical Manual of Mental Disorders, 4th edition
ID Intellectual disability
ODD Oppositional defiant disorder
890
Page 1
in terms of symptoms, rates of subtypes, and patterns of co-
morbid problems (ie, oppositional behaviors, conduct disor-
der [CD], anxiety, and depression).
Methods
Participants were recruited from more than 30 child and
adolescent mental health services or comm unity pediatric
outpatient clinics in Wales, England, and Scotland for a ge-
netic study of ADHD. Given this study’s focus on evaluat-
ing for the presence of nonsyndromal ID in children with
ADHD, International Statistical Classification of Diseases
and Related Health Problems, 10th revision
10
and Diagnostic
and Statistical Manual of Mental Disorders, 4th edition
(DSM-IV)
11
exclusion criteria were used. Children with
a known diagnosis of schizophrenia, autism spectrum disor-
der (ASD), bipolar disorder, Tourette syndrome, epilepsy,
brain damage, or any other neurologic or genetic disorder
were excluded. Information on these conditions was derived
from a questionnaire completed by the referring cli nician,
diagnostic interview information obtained from parents,
and quality control of genetic data performed as part of
the genetic study. Children with IQ <50 were also excluded,
because the study focused on mild ID, and the assessment
measures have not yet been validated in individuals with se-
vere ID.
A total of 971 children met the inclusion criteria and had
sufficient data for analysis. All of these children met the
DSM-IV
11
or Diagnostic and Statistical Manual of Mental
Disorders, 3rd edition revised (DSM-III-R)
12
criteria for a di-
agnosis of ADHD, which was confirmed through research
diagnostic interviews.
13
The children ranged in age from 5
to 17 years (mean age, 10.1 2.8 years), and included
148 females (15.2%). The study received ethical approval
from the North West England and Wales Multicentre Re-
search Ethics Committees. For all subjects, written informed
consent was obtained from parents and assent/consent from
children.
Cognitive ability was assessed using the Wechsler Intelli-
gence Scale for Children versions III (n = 381) and IV (n =
590)
14,15
to obtain an estimate of full-scale IQ (using all re-
quired subtests). Two versions of this assessment tool were
used because version IV was released during the study period.
The assessment was performed by trained psychologists. In
children who had recently undergone IQ assessment in
school, that score was use d to determine ID status. In accor-
dance with International Statistical Classification of Diseases
and Related Health Problems, 10th revision and DSM-IV cri-
teria, children with an IQ score of 50-69 were considered to
have mild mental retardation/ID and classified in the
ADHD + ID group. Children with an IQ score $70 were clas-
sified in the ADHD-o nly group.
ADHD symptoms, impairment, and diagnoses were con-
firmed using the Child and Adolescent Psy chiatry Assessment
(CAPA),
13
a research diagnostic interview with parents.
Interviews were performed by trained psychologists super-
vised weekly by a child psychiatrist. Interrater reliability for
ADHD was perfect (k = 1.0). Information on ADHD symp-
toms and school impairments was obtained using the Child
ADHD Teacher Telephone Interview,
16
the DuPaul teacher
rating scale,
17
or the Conners teacher rating scale.
18
A diag-
nosis of ADHD required that the child have symptoms meet-
ing DSM-IV or DSM-III-R criteria, substantial impairment
from symptoms at home, and pervasive symptoms and im-
pairment in the school setting.
The CAPA was also used to assess current symptoms, im-
pairment, and DSM-IV diagnoses of comorbid oppositional
defiant disorder (ODD), CD, anxiety disorders (ie, general-
ized anxiety disorder, social anxiety, and separation anxiety),
depression, and mania. Comorbid symptoms were also as-
sessed using the child version of the CAPA
19
for children
aged $12 years. Comorbid anxiety or depression symptoms
were endors ed if reported by the parent or child. Owing to
the scarcity of anxiety and depression diagnoses in the sam-
ple, only symptoms of these disorders could be analyzed. In-
terrater reliability for parent-rated CD symptoms was ve ry
good (intraclass corr elation, 0.98).
Avon Longitudinal Study of Parents and Children
To compare clinical variables found to be associated in the
primary analysis in the ADHD + ID and ADHD-only
groups, we turned to the Avon Longitudinal Study of Par-
ents and Children (ALSPAC), a large, well-characterized
longitudinal dataset. Details of the study metho dology are
available elsewhere.
20
Ethical approval for all aspects of
the study was obtained from the ALSPAC Law and Ethics
Committee and the local Research Ethics Committees. Par-
ents provided written consent and the children provided as-
sent at each assessment. IQ had been assessed at age 8 years
using the Wechsler Intelligence Scale for Children version
III.
14
Children who scored between 50 and 69 on the IQ
test and had no diagnosis of ADHD or ASD were included
in our analysis. A total of 74 children (1.2% of the ALSP AC
sample with complete data on these measures) met these
criteria. Data on ADHD, ASD, ODD, and CD symptoms
and diagnoses were collected from participants at age 128
months, using the parent and teacher Development and
Well-Being Assessment.
21
Complete clinical data were avail-
able for 58 children, who constituted the ID-only group.
These children were 10-11 years old at the time of clinical
assessment (mean, 10.8 0.1 years), and 27 were female
(46.6%).
Statistical Analyses
The ADHD clinical sample was divide d into those with ID
(ADHD + ID; n = 97) and those without ID (ADHD-only;
n = 874). The 2 groups were compared on each of the clinical
factors identified. All descriptive statistics are presented as
raw scores for ease of interpretation. Where a variable was
nonnormally distributed, the scores were naturally logarith-
mically transformed, and analyses were run on transformed
scores.
Clinical predictor variables w ere used to predict binary
outcomes (ADHD + ID or ADHD-only) using regression
Vol. 163, No. 3 September 2013
891
Page 2
analyses. All analyses included child’s age at the time of
assessment as a covariate. Sex was not included as a covar-
iate, because it was not associated with the presence or ab-
sence of ID. Clinical variables were assessed both
categorically and continuously, whenever relevant. All
analyses were performed using SPSS version 16 (IBM, Ar-
monk, New York). To take into account multiple testing,
Bonferroni correction for the number of variables tested
was used, a was set at P = .003 (0.05/15) for the 15 tests
performed.
Based on our results, a hypothesis-driven comparison of
the ADHD + ID and ID-only samples was performed for
rates of diagnoses and symptom counts for ODD and CD, ad-
justed for the covariates age, sex, and IQ. CD items available
in both datasets were summed and used to generate the CD
diagnoses (lying, fighting, breaking curfew, stealing, truancy,
running away from home, and bullying).
Results
Sample Description: Clinical ADHD Sample
At the time of assessment, 74.3% of the children (n = 721)
met the criteria for DSM-IV ADHD Combined type, 6.0%
(n = 58) met the criteria for DSM-IV ADHD Inattentive
type, 9.6% (n = 93) met the criteria for DSM-IV ADHD
Hyperactive-Impulsive type, and the remaining 10.2%
(n = 99) met the criteria for DSM-III-R ADHD. The rates
of comorbid disorders were 45.2% (n = 435) for DSM-IV
ODD, 17.6% (n = 171) for DSM-IV CD, 6.1% (n = 59)
for any anxiety disorder, and 1.1% (n = 11) for any de-
pressive disorder. The mean IQ test scores were 61.8
5.4 for the ADHD + ID group and 87.8 11.4 for the
ADHD-only group; the range of scores was 50-69 for the
ADHD + ID group and 70-139 for the ADHD-only group.
The scores were normally distributed for the sample as
a whole.
Sample Description: ID-Only ALSPAC Sample
The mean IQ score of the ID-only group was 64.8 4.5. At
the 128-month assessment, 2 of the children in the ID-only
group met the criteria for DSM-IV ODD (3.4%), and none
met the criteria for DSM-IV CD.
Analysis of Clinical Features in Child ren with ADHD
The ADHD + ID group was older than the ADHD-only
group, but the 2 group s did not differ in terms of sex distri-
bution. Table I presents the descriptive statistics and results
of regression analyses with age included as a covariate.
Although a trend for the ADHD + ID group to be more
likely to have the DSM-IV Combined ADHD subtype was
seen, this result did not withstand correction for multiple
testing. Otherwise, the 2 groups of children were similar in
terms of ADHD subtypes and Inattentive, Hyperactive-
Impulsive, and total ADHD symptoms. The 2 groups also
had similar rates of ODD diagnoses and of anxiety and
depression symptoms. There was a trend for children with
ADHD + ID to have on more ODD symptoms on average.
The ADHD + ID group had more symptoms of CD and
were more likely to have a diagnosis of CD; these
associations remained after multiple testing was taken into
consideration.
Analysis of Clinical Features in Children
with ID-Only
Results of the comparison of ADHD + ID and ID-only
groups (clinical sample vs population sample) are pre-
sented in Table II. The 2 groups differed significantly in
terms of sex (more boys in the ADHD + ID group), but
not in age at assessment. Although the range of IQ scores
was similar in the 2 groups, the ADHD + ID group had
significantly lower sco res (OR, 0.88; 95% CI, 0.82-0.95;
P = .001). After adjusting for sex, age, and IQ score, the
children in the ADHD + ID group were significantly
more likely to have a diagnosis of ODD, and had
Table I. Clinical features of the ADHD + ID and ADHD-only groups
Variable
ADHD + ID (n = 97) ADHD only (n = 874) Statistics*
n (%) Mean (SD) n (%) Mean (SD) OR 95% CI P
Age, y 11.4 (3.0) 10.0 (2.7) 1.19 1.11-1.28 4.6E-06
Male sex 84 (86.6) 739 (84.6) 0.85 0.46-1.56 .60
DSM-IV Combined ADHD diagnosis 79 (82.3) 631 (72.6) 1.96 1.12-3.40 .02
DSM-IV Inattentive ADHD diagnosis 5 (5.2) 51 (5.8) 0.62 0.23-1.62 .32
DSM-IV Hyperactive-Impulsive ADHD diagnosis 6 (6.2) 87 (10.0) 0.59 0.25-1.41 .23
DSM-III-R ADHD diagnosis only 5 (5.2) 89 (10.2) 0.49 0.19-1.26 .14
DSM-IV ODD diagnosis 36 (37.5) 399 (46.1) 0.74 0.48-1.14 .17
DSM-IV CD diagnosis 35 (36.1) 136 (15.6) 2.69 1.69-4.28 2.8E-05
ADHD symptoms: inattentive 7.5 (1.5) 7.3 (1.7) 1.25 0.75-2.09 .39
z
ADHD symptoms: hyperactive-impulsive 7.8 (1.4) 7.8 (1.5) 1.38 0.79-2.40 .25
z
ADHD symptoms: total 15.4 (2.1) 15.1 (2.4) 1.08 0.98-1.18 .12
DSM-IV ODD symptom count 4.1 (2.3) 3.8 (2.4) 1.09 0.99-1.19 .08
DSM-IV CD symptom count 2.1 (2.2) 1.0 (1.5) 2.38 1.71-3.32 2.5E-07
z
DSM-IV anxiety symptoms 1.2 (2.0) 1.1 (1.9) 1.21 0.86-1.70 .28
z
DSM-IV depression symptoms 1.8 (1.8) 1.3 (1.3) 1.26 0.84-1.89 .26
z
*All clinical analyses were adjusted for the covariate child’s age.
†Critical P value corrected for multiple testing: P < .003.
zTransformed.
THE JOURNAL OF PEDIATRICS www.jpeds.com Vol. 163, No. 3
892 Ahuja et al
Page 3
significantly more symptoms of ODD and CD. Statistical
assessment of the between-group difference in the rate of
CD diagnoses was not possible, given the rate of 0 in the
ID-only group.
Discussion
In our clinical sample of children with ADHD, those with and
without mild ID (IQ score 50-69) exhibited similar patterns
of ADHD subtypes, total number of ADHD symptoms, and
comorbidity with ODD, anxiety, and depression. The chil-
dren in the ADHD + ID group did have higher rates of CD
symptoms and diagnoses, however. To explore these results
further and test whether these differences were related to
an increase in behavioral problems in children with ID in
general, we compared our ADHD + ID group with a popula-
tion-based sample of children with ID but without ADHD.
We found significantly higher rates of ODD diagnoses and
ODD and CD symptoms in the ADHD + ID group, suggest-
ing that the combination of ADHD and ID results in in-
creases in the rates of comorbid CD and ODD beyond the
rates of these disorders in individuals with ID only or
ADHD only. The mean IQ score was lower in the ADHD +
ID group compared with the ID-only group. This difference
in IQ may be related to selective attrition in the AL SPAC
sample,
22
clinical ascertainment effects, or the effects of
ADHD on IQ test performance. Regardless, when we
matched the groups on IQ scores by selecting a subsample
of children with ADHD + ID (n = 58, to match the number
of children in the ID-only group), we obtained the same pat-
tern of results (Tab le III; available at www.jpeds.com ).
ADHD is one of the most common forms of psychopa-
thology in children with ID.
7
Generally, children with ID
are neglected in the medical field, perhaps in part because
of their poor ability to communicate and social disadvan-
tages.
2
They are frequently excluded from clinical, etiologic,
and treatment studies,
2
and thus few studies to date have
examined the clinical presentation, etiology , patterns of ser-
vice use, and treatment of ADHD in children with ID.
Moreover, there are few standardized norms or guidelines
in the classification sy stems for identifying “normal” or
“usual” amounts of inattention, overactivity and impulsiv-
ity in persons with ID.
Studies with clinic-referred children and community and
population samples have shown consistently higher rates of
ADHD in children with ID and higher rates of ID in children
with ADHD.
23-25
There has been less work on whether the
clinical features and levels of ADHD symptomatology differ
between children with ID and those without ID. One study
found equivalent levels of ADHD symptoms in preschool-
age children with ADHD and normal IQ and tho se with
ADHD and ID.
26
A population study assessed overactivity,
inattention, and impulsivity symptoms using the Strengths
and Difficulties Questionnaire in children with IQ <70 and
those with IQ $70 and found no differences between the 2
groups.
24
A recent longi tudinal study compared children
with ADHD with IQ <85 (indicating ID or borderline IQ)
and those with IQ $85 and found similar inattentive and
hyperactive-impulsive ADHD symptom trajectories over
a 3-year period.
27
An important di fference noted in that sam-
ple was that the childr en with lower IQ tended to meet the
diagnostic criteria for ADHD at an earlier age and to have
more diagnostic stability than the children with higher IQ,
indicating a more severely impairing form of ADHD. A study
that addressed this question from the other direction found
that although the children with developmental delay (IQ
<85) had a higher rate of ADHD than the typically develop-
ing children (IQ $85), the pattern of ADHD subtypes and
levels of inattentive and hyperactive-impulsive symptoms
were similar in the 2 groups.
9
The few studies examining whether rates of comorbid psy-
chiatric disorders in ADHD differ in children with and with-
out ID have shown mixed results. Some studies have found
higher rates of comorbid impairments in social skills, con-
duct problems, aggression, and noncomp liance in children
with ADHD and ID compared with children with ADHD
and normal IQ.
26,28
In contrast, in community samples, the
profiles of comorbidity with emotional and conduct prob-
lems in children with ADHD symptoms did not differ ac-
cording to the presence of mild ID (IQ <70)
24
or using
a cutoff of IQ <85.
29
A third study found higher rates of non-
compliance, anxiety, depression, and social problems in
those with ADHD and ID compared with those with ID
alone.
30
A limitation of previous research is that questionnaires
generally have be en used to assess comorbidity rather than
Table II. Clinical features of children with ADHD + ID (clinical sample) and those with ID only (ALSPAC sample)
Variables
ADHD + ID (n = 97) ID-only (n = 58) Statistics
n (%) Mean (SD) n (%) Mean (SD) OR 95% CI P
Age, y 11.4 (3.0) 10.8 (0.1) 1.10 0.96-1.27 .16
Male sex 84 (86.6) 31 (53.4) 0.18 0.08-0.39 1.4E-05
ODD symptom count 4.1 (2.3) 0.2 (0.7) 5.54 2.86-10.75 4.0E-07*
CD symptom count (out of 7) 1.2 (1.4) 0.0 (0.3) 13.66 3.25-57.42 3.6E-04*
ODD diagnosis 47 (49.0) 2 (3.4) 30.99 6.38-150.39 2.0E-05*
CD diagnosis 16 (16.7) 0 (0.0) NA NA NA
NA, not applicable.
*All clinical analyses adjusted for the covariates child’s age at time of assessment, sex, and IQ.
†Because there were no CD diagnoses in the ID-only group, statistical calculation was not applicable.
September 2013 ORIGINAL ARTICLES
Intellectual Disability in Children with Attention Deficit Hyperactivity Disorder
893
Page 4
the more in-depth standardized diagnostic assessment
methods used in the present study. Another strength of the
present study is the relatively large sample size of the
ADHD group compared with previous studies. The IQ cut-
point used to delineate comparison groups in the literature
varies, such that in some studies, children with borderline
ID (IQ 70-85) are included in the group of children with
mild ID. Although the present study focused on comparing
children based on an IQ cutpoint of 70, reanalysis of the
data after dividing the children into 3 groups (mild ID: IQ
50-69 [n = 97], borderline ID: IQ 70-84 [n = 380], and typ-
ically developing: IQ $85 [n = 494]) showed a similar pat-
tern of results (Tab le IV; available at www.jpeds.com ).
An important limitation of the present study is that the
presence of ID was defined based primarily on IQ score, be-
cause no measure of adaptive functioning was available. An-
other limitation is that disharmonic IQ profiles were not
considered, and thus children with significant performance
IQ and verbal IQ discrepancies might not necessarily be con-
sidered to have ID in clinical practice. However, given that IQ
alone is often the basis for exclusion criteria in ADHD re-
search, the implications of our findings are valid in this con-
text. Samples are also likely to be heterogeneous because of
ascertainment differences in referred samples. Our sample
comprised referred cases, and thus the higher rate of CD in
our cohort may be related to the fact that these children
were identified with ADHD and referred. Because this sub-
group of children with ADHD + ID are excluded from virtu-
ally all clinical, etiologic, and treatment studies, further work is
needed to verify our findings. A further limitation of this study
is that we were unable to take into account the variable effects
of medication timing and dosage. However, in those children
receiving a stimulant medication, whether or not they took the
medication on the day of testing had no effect on whether they
were classified as ADHD + ID or ADHD-only.
A related question that is beyond the scope of the present
study is the extent to which the presence of ID in children
with ADHD indexes a differe nt etiology. Previous work has
shown that children with ADHD + ID are significantly
more likely to have large, rare structural deletions or duplica-
tions of DNA, called copy number variants (CNVs).
31
Im-
portantly however, CNVs are also associated with ADHD
without ID.
32
In addition, the presence of such CNVs does
not appear to index a distinct pattern of etiologic correlates,
in the form of various prenatal and perinatal risk factors, or
a distinct clinical profile in children with ADHD with and
without ID.
33
Thus, whether the etiology and risk correlates
of ADHD are substantially different in affected children
with and without ID remains uncle ar, and further work is
needed to explore this question.
Our results, together with other findings, suggest that ex-
cluding children with ADHD from services and interventions
on the basis of the presence of mild ID is clinically unwar-
ranted, given that children with ADHD and ID do not
seem to differ from those without ID in terms of ADHD sub-
type and number of ADHD symptoms. They are more likely
to have CD, however. It also appears that they differ from
children with ID alone, suggesting that ID does not drive
the link to conduct problems. Thus, services that deal with
ADHD should be well placed to manage ADHD in children
with mild ID; however, they will need access to the types of
social and clinical interventions that will also help manage as-
sociated conduct problems.
n
We thank the families, pediatricians, and Child and Adolescent Men-
tal Health Service clinicians who supported this project. We also
thank the field team members for clinical sample collection and Mi-
chael O’Donovan, Michael Owen, Peter Holmans, Lindsey Kent, and
Sharifah Syed for assistance with the dataset collected for the genetic
study. We also thank all of the families who took part in the ALSPAC
study, the midwives for their help in recruiting these participants, and
the entire ALSPAC team, including interviewers, computer and lab-
oratory technicians, clerical workers, research scientists, volunteers,
managers, receptionists, and nurses.
Submitted for publication Jun 22, 2012; last revision receive d Jan 9, 2013;
accepted Feb 22, 2013.
Reprint requests: Kate Langley, PhD, Cardiff University School of Medicine,
Department of Psychological Medicine, 4th Floor Main Building, Heath Park,
Cardiff CF14 4XN, UK. E-mail: langleyk@cf.ac.uk
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September 2013 ORIGINAL ARTICLES
Intellectual Disability in Children with Attention Deficit Hyperactivity Disorder
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Table III. Clinical features of children with ADHD + ID (clinical sample) and those with ID only (ALSPAC sample) after
matching the samples based on IQ
Variables
ADHD + ID (n = 58) ID-only (n = 58) Statistics
n (%) Mean (SD) n (%) Mean (SD) OR 95% CI P
Age, years 10.6 (2.9) 10.8 (0.1) 0.96 0.80-1.15 .64
Male sex 50 (86.2) 31 (53.4) 0.18 0.07-0.46 2.5E-04
ODD symptom count 4.1 (2.3) 0.2 (0.7) 5.08 2.63-9.79 1.2E-06*
CD symptom count (out of 7) 1.1 (1.5) 0.0 (0.3) 11.41 2.82-46.13 6.4E-04*
ODD diagnosis 28 (48.3) 2 (3.4) 32.09 6.26-164.56 3.2E-05*
CD diagnosis 16 (16.7) 0 (0.0) NA NA NA
*A adjusted for the covariates: child’s age at time of assessment, sex, and IQ.
†Because there were no CD diagnoses in the ID-only group, statistical calculation was not applicable.
Table IV. Clinical features of children with ADHD + ID compared with those with IQ 70-84 and those with IQ $85
Variables
Group 2: IQ 70-84
(n = 380)
Statistics*
(group 2 compared
with ADHD + ID)
Group 3: IQ 85
(n = 494)
Statistics*
(group 3 compared
with ADHD + ID)
n (%) Mean (SD) OR 95% CI P
n (%) Mean (SD) OR 95% CI P
Age, years 10.2 (2.8) 0.87 0.80-0.94 .0004 9.8 (2.7) 0.82 0.76-0.88 4.5E07
Male sex 317 (83.4) 1.28 0.68-2.44 .45 422 (85.4) 1.10 0.58-2.08 .76
DSM-IV Combined ADHD diagnosis 277 (73.5) 0.54 0.30-0.96 .04 354 (72.0) 0.49 0.28-0.87 .01
DSM-IV Inattentive ADHD diagnosis 21 (5.5) 1.44 0.52-3.99 .49 30 (6.1) 1.79 0.66-4.87 .25
DSM-IV Hyperactive-Impulsive ADHD diagnosis 41 (10.8) 1.86 0.76-4.54 .17 46 (9.3) 1.55 0.63-3.76 .34
DSM-III-R ADHD diagnosis only 34 (9.0) 1.78 0.67-4.69 .25 55 (11.2) 2.23 0.86-5.77 .10
DSM-IV ODD diagnosis 160 (42.6) 1.19 0.75-1.90 .45 239 (48.8) 1.51 0.95-2.37 .08
DSM-IV CD diagnosis 82 (21.6) 0.55 0.34-0.89 .02 54 (10.9) 0.25 0.15-0.41 7.8E08
ADHD symptoms: inattentive 7.3 (1.7) 0.79 0.46-1.36 .40
z
7.3 (1.7) 0.80 0.47-1.36 .41
z
ADHD symptoms: hyperactive-impulsive 7.8 (1.3) 0.81 0.45-1.45 .48
z
7.7 (1.6) 0.66 0.37-1.18 .16
z
ADHD symptoms: total 15.2 (2.4) 0.94 0.85-1.04 .21 15.1 (2.4) 0.92 0.83-1.01 .09
DSM-IV ODD symptom count 3.9 (2.4) 0.95 0.86-1.05 .33 3.7 (2.3) 0.89 0.81-0.99 .02
DSM-IV CD symptom count 1.3 (1.76) 0.53 0.37-0.75 .0003
z
0.9 (1.3) 0.34 0.24-0.48 1.5E09
z
DSM-IV anxiety symptoms 1.1 (1.9) 0.84 0.58-1.20 .33
z
1.1 (2.0) 0.82 0.57-1.18 .29
z
DSM-IV depression symptoms 1.2 (1.4) 0.72 0.47-1.11 .13
z
1.3 (1.3) 0.86 0.57-1.31 .49
z
*All clinical analyses are adjusted for the covariate child’s age.
†Critical P value corrected for multiple testing: P < .003.
zTransformed.
THE JOURNAL OF PEDIATRICS www.jpeds.com Vol. 163, No. 3
895.e1 Ahuja et al
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  • Source
    • "Significantly higher prevalence of obesity was observed in patients with a comorbid diagnosis of adjustment disorder and mild mental retardation. Previous studies have shown that ADHD occurs in 8% to 39% of patients with mild to moderate mental retardation (Ahuja, Martin, Langley, & Thapar, 2013). Children with disabilities, such as mental retardation or ADHD often have weight problems (Reinehr, Dobe, Winkel, Schaefer, & Hoffmann, 2010; J. H. Rimmer, Yamaki, Davis, Wang, & Vogel, 2011 ). "
    [Show abstract] [Hide abstract] ABSTRACT: Assessment of the prevalence of overweight and obesity in children and adolescents with ADHD with emphasis on pharmacological treatment and comorbid disorders. We analyzed 408 medical records of patients with ADHD aged 7 to 18. The prevalence of overweight (14.71% vs. 12.83%, χ(2) = 3,586.43, p < .001) and obesity (6.37% vs. 3.45%, χ(2) = 3,588.19, p < .001) was significantly higher in children with ADHD compared with the population. There was significantly higher incidence of obesity in patients with comorbid diagnosis of adjustment disorder (22.22% vs. 4.42%, χ(2) = 5.66, p = .02) and mental retardation (19.05% vs. 4.42%, χ(2) = 7.63, p = .005). Pharmacological treatment was associated with a higher incidence of obesity (8.37% vs. 2.76%, χ(2) = 4.92, p = .03). Standardized body mass index (BMI), prevalence of overweight, and obesity was higher in patients with ADHD compared with the population. Higher incidence of obesity was shown in patients with analyzed comorbidities. © 2015 SAGE Publications.
    Full-text · Article · Apr 2015 · Journal of Attention Disorders
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    • "In a clinical sample, children with IQs < 70 showed more severe ADHD symptoms than children with IQ > 70 [6]. Lower IQ is also associated with higher levels of externalising and behavioural problems in individuals with ADHD [7]. IQ has been shown to positively impact the response to pharmaceutical treatment in ADHD891011. "
    [Show abstract] [Hide abstract] ABSTRACT: While the negative association between ADHD symptoms and IQ is well documented, our knowledge about the direction and aetiology of this association is limited. Here, we examine the association of ADHD symptoms with verbal and performance IQ longitudinally in a population-based sample of twins. In a population-based sample of 4,771 twin pairs, DSM-IV ADHD symptoms were obtained from the Conners’ Parent Rating Scale-Revised. Verbal (vocabulary) and performance (Raven’s Progressive Matrices) IQ were assessed online. ADHD symptom ratings and IQ scores were obtained at ages 12, 14 and 16 years. Making use of the genetic sensitivity and time-ordered nature of our data, we use a cross-lagged model to examine the direction of effects, while modelling the aetiologies of the association between ADHD symptoms with vocabulary and Raven’s scores over time. Although time-specific aetiological influences emerged for each trait at ages 14 and 16 years, the aetiological factors involved in the association between ADHD symptoms and IQ were stable over time. ADHD symptoms and IQ scores significantly predicted each other over time. ADHD symptoms at age 12 years were a significantly stronger predictor of vocabulary and Raven’s scores at age 14 years than vice versa, whereas no differential predictive effects emerged from age 14 to 16 years. The results suggest that ADHD symptoms may put adolescents at risk for decreased IQ scores. Persistent genetic influences seem to underlie the association of ADHD symptoms and IQ over time. Early intervention is likely to be key to reducing ADHD symptoms and the associated risk for lower IQ.
    Full-text · Article · Apr 2015 · PLoS ONE
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    • "Två svenska studier har visat att 40 procent av de intagna på anstalt inom kriminalvården hade ADHD [10, 25] Tidig debut av rökning, olika former av missbruk, kriminalitet och problem i sociala relationer samt tidiga graviditeter och svårigheter i föräldra rollen är också vanligt förekommande … » ADHD bör uppmärksammas mer – tidiga insatser spar lidande [27]. Vid epilepsi [28], cerebral pares [29] och utvecklingsstör- ning [30] förekommer ADHD i ökad omfattning. I kliniska grupper inom vuxenpsykiatrin förekommer ofta ADHD, t ex hos drygt 20 procent i en studie av öppenvårdspa- tienter [31]. "
    [Show abstract] [Hide abstract] ABSTRACT: ADHD is a common neurodevelopmental/neuropsychiatric disorder affecting about 5 percent of children. About 2-3 percent meet diagnostic criteria in adulthood as well. The core symptoms include inattention with or without hyperactivity/restlessness and impulsivity. The main cognitive deficit involves executive functions, probably related to a weak reward system. Symptoms will affect daily functioning at home, among friends and at school/work. In girls and women particularly, a correct diagnosis of ADHD is often late, or is not at all appropriately considered. Co-existing disorders are common; dyslexia, developmental coordination disorder, emotional lability, conduct disorder, autistic symptoms, obsessive compulsive disorder, depression, bipolar disorder, Tourette syndrome, eating disorder, sleeping disorder, and substance abuse. Extensive research in ADHD has increased knowledge in genetics, neurobiology, neuropsychology, intervention, and treatment. Despite this, many individuals with ADHD are not offered a correct assessment, and accordingly, not given appropriate support and treatment.
    Full-text · Article · Sep 2014 · Lakartidningen
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