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Symptoms of Attention Deficit HyperactivityDisorder in Children and Adults withIntellectual Disability: A ReviewColin Reilly* and Niamh Holland�
*School of Education, University College Dublin, Belfield, Dublin 4, Ireland; �Psychology Department, St. Michael’s House,
Dublin 9, Ireland
Accepted for publication 18 October 2010
Background Despite a reported excess of attention deficit
hyperactivity disorder (ADHD) symptoms in individu-
als with intellectual disability, it has been argued that
ADHD symptoms have been under diagnosed and inad-
equately treated in individuals with intellectual disabil-
ity.
Materials and methods Published studies focussing on the
level of ADHD symptoms in children and adults with
intellectual disability are reviewed. Issues around screen-
ing for ADHD symptoms in individuals with intellectual
disability and the validity of the diagnosis of ADHD are
considered. Interventions including pharmacological
studies are discussed with respect to published research.
Results Reported prevalence rates of ADHD symptoms
in individuals with intellectual disability vary signifi-
cantly depending on instruments and diagnostic prac-
tices employed. Published research on interventions for
individuals with ADHD has primarily focussed on phar-
macological interventions.
Conclusion Much less is known about ADHD in individ-
uals with intellectual disability than about ADHD in
individuals without intellectual disability. There is an
urgent need to clarify baseline rates of ADHD in indi-
viduals with intellectual disability and to develop effica-
cious interventions to support affected individuals and
their families.
Keywords: ADHD, intellectual disability, intervention,
prevalence
Introduction
Attention deficit hyperactivity disorder (ADHD) is a
condition first manifest in childhood characterized by
severe and persistent symptoms of inattention, over-
activity, and impulsiveness and is associated with long-
term educational and social disadvantage (Swanson
et al. 1998). ADHD as described in the Diagnostic and
Statistical Manual of Mental Disorders-Fourth Edition-
Text Revision (DSM-IV-TR) (American Psychiatric Asso-
ciation (APA) (2000) is closely related to the category of
‘Hyperkinetic Disorders’ as outlined in the International
Classification of Diseases-10th Revision (ICD-10) (World
Health Organization (WHO) 1992), with the category of
‘Hyperkinetic Disorders’ often seen as a more severe
form of ADHD. The symptoms of the disorders in both
diagnostic systems are very similar, with requirements
relating to pervasiveness and diagnostic profile varying
somewhat between the ICD-10 and DSM taxonomies
(Swanson et al. 1998). For example, according to DSM-IV
criteria, it is possible to have ADHD without being inat-
tentive. However, inattentiveness is a necessary require-
ment for a Hyperkinetic Disorder according to ICD-10
criteria (Taylor 1998).
It has been argued that ADHD remains largely undi-
agnosed and insufficiently treated in individuals with
intellectual disability (Rose et al. 2009), and historically,
children with intellectual disability have been specifi-
cally excluded from studies of children with ADHD
(Handen et al. 1998). In the large Multimodal Treatment
Study of Children with ADHD (MTA Cooperative
Group 1999) in the US, an IQ score below 80 was an
exclusion criterion for participation. It may be that one
of the reasons it can be difficult to diagnose ADHD in
individuals with intellectual disability is ‘diagnostic
overshadowing’ (Reiss et al. 1982; Joop & Keys 2001;
Journal of Applied Research in Intellectual Disabilities 2011, 24, 291–309
� 2011 Blackwell Publishing Ltd 10.1111/j.1468-3148.2010.00607.x
Published for the British Institute of Learning Disabilities
Mason & Scior 2004), a phenomenon which highlights
that clinicians may emphasize a client’s intellectual
disability and underemphasize psychopathology (White
et al. 1995). This assumption that intervention is not
needed because the abnormal or atypical behaviour is
an inherent part of intellectual disability may lead to
disorders being missed and untreated (Santosh & Baird
1999). The fact that there are overlaps among symptoms
of ADHD and features of intellectual disability (Deb
et al. 2008) may lead to diagnostic overshadowing in the
case of ADHD among those with intellectual disability.
As a result it has been reported that psychiatrists may
be less confident in making a diagnosis of ADHD in
children and adults with intellectual disability (Buckley
et al. 2006). It is also argued that ADHD is a develop-
mental disorder not a psychiatric disorder and as such
it is expected that children with intellectual disability
will show symptoms that will overlap with the symp-
toms of ADHD (Deb et al. 2008). Therefore, delineating
attention deficits against a backdrop of generalized intel-
lectual impairment has not been without controversy,
and it remains contentious whether the symptoms of
ADHD reflect delayed development or deviant develop-
ment in those with intellectual disability (Deutsch et al.
2008).
This article reviews ADHD as it has been studied in
individuals with intellectual disability. The use of diag-
nostic criteria for ADHD from international classification
systems is discussed with reference to the applicability
of such systems to individuals with intellectual disabil-
ity. The prevalence of ADHD symptoms and correlates
of ADHD in individuals with intellectual disability are
examined with reference to published studies. Published
studies evaluating interventions for individuals with
ADHD and intellectual disability are reviewed. The arti-
cle concludes with a summary of current knowledge
and suggestions for future directions in the area.
Methodology
Literature searches using PsychINFO, ERIC, and
PubMed were conducted using the keywords
ADHD + intellectual disability, ADHD + Mental Retar-
dation, and ADHD + learning disability, from 1980 to
June 2010. This resulted in a very large number of arti-
cles (1800+) primarily due to the inclusion of the term
‘learning disability’ which has different meanings in the
UK and US. Using the terms ADHD + mental retarda-
tion, and ADHD + intellectual disability as keywords in
PsychINFO and ERIC, and as search terms in ‘Title ⁄Abstract’ in PubMed resulted in 51 articles in ERIC, 239
in PsychInfo and 139 in PubMED. Many of the results
were replicated across the three databases and there
were approximately 240 unique results. The full article
or abstracts of these were reviewed and 69 were
included for review based on inclusion criteria of: pri-
marily focussing on ADHD in intellectual disability with
respect to prevalence, interventions or manifestation.
Articles focussing on case studies or studies where there
were <10 participants, or where the primary focus was
not on ADHD and intellectual disability were excluded.
To garner other relevant articles, reference lists from per-
tinent articles and chapters in books were examined to
further identify relevant studies and reviews. A number
of journals focusing on intellectual disability and child
psychology ⁄ psychiatry were also hand searched includ-
ing the Journal of Intellectual Disability Research, Journal of
Applied Research in Intellectual Disabilities, Research in
Developmental Disabilities, Intellectual and Developmental
Disabilities (formerly Mental Retardation), American Journal
of Intellectual and Developmental Disabilities (formerly
American Journal on Mental Retardation), Journal of Child
Psychology and Psychiatry, and Journal of the American
Academy of Child and Adolescent Psychiatry. After this pro-
cess articles were chosen based on the previous inclusion
criteria. This process yielded 31 further articles and pri-
marily included articles which focussed on psychopa-
thology ⁄ mental health difficulties (including ADHD) in
individuals with intellectual disability
Diagnostic Criteria and Validity of ADHDin Individuals with Intellectual Disability
Neither ICD-10 (WHO 1992) or DSM-IV-TR (APA 2000)
have given detailed consideration to the diagnosis of
ADHD ⁄ Hyperkinetic disorder among people with intel-
lectual disability (Seager & O’Brien 2003), and in earlier
editions of the DSM, low IQ was an exclusion criteria
for a diagnosis of ADHD (Antshel et al. 2006a). The
DSM-IV-TR describes the essential features of ADHD as
a continual pattern of inattention, and ⁄ or hyperactivity–
impulsivity, starting in early childhood with the pattern
more frequently displayed and more severe than is typi-
cally observed in individuals with a similar level of
development (APA 2000). In children with intellectual
disability it is recommended that an additional diagno-
sis should only be given, if the symptoms of inattention
or hyperactivity are excessive for the child’s mental age
(APA 2000). Although the DSM clearly highlights the
importance of a differential ADHD diagnosis in children
with low IQ, the manual provides limited direction on
how to determine if the symptoms are excessive (Ant-
292 Journal of Applied Research in Intellectual Disabilities
� 2011 Blackwell Publishing Ltd, 24, 291–309
shel et al. 2006a). In DSM-IV-TR three subtypes of
ADHD are recognized; ADHD-Combined subtype in
which inattention and hyperactivity–impulsivity are
present, ADHD-Predominantly Inattentive subtype, and
ADHD-Predominantly Hyperactive-Impulsive subtype.
The Hyperactive–Impulsive subtype appears rare in
comparison to the other two subtypes (Wolraich et al.
1998). The ICD-10 classificatory system includes the
diagnoses of ‘disturbance of activity and attention’ and
‘hyperkinetic conduct disorder’ under the category
‘Hyperkinetic Disorders’. Both ICD-10 diagnoses require
the presence of impaired attention with overactivity,
which should be ‘judged with reference to developmen-
tally appropriate norms’ (Seager & O’Brien 2003). It has
been argued that the presentation of developmental and
psychiatric disorders is essentially the same for people
with mild intellectual disability and probably at the
upper levels of the moderate range of intellectual
disability (Brown et al. 2004). However, the applicability
of classification systems like the DSM-IV and ICD-10 to
an intellectual disability population is still debated. The
usefulness of both ICD-10 and DSM-IV systems in rela-
tion to adult ADHD has also been questioned as both
systems reflect child not adult behaviour and adult life-
styles (e.g. referring to school and play). The set of 18
DSM-IV symptoms characterizing the differing ADHD
types was originally developed for use in child psychia-
try (Rosler et al. 2006), and no validation studies in
adults have been performed, and thus the question
remains as to whether DSM-IV or ICD-10 criteria are
adequate to characterize adult ADHD.
Two alternative classification systems have been
published for use by professionals working with indi-
viduals who have intellectual disability. The Diagnostic
Criteria for Psychiatric Disorders for use with adults
with Learning disabilities ⁄ Mental retardation (DC-LD)
was developed by the Royal College of Psychiatrists
(2001) in the UK to improve upon psychiatric classifica-
tion in adults with intellectual disability. The Diagnostic
Manual-Intellectual Disability: A Textbook of Diagnosis
of Mental Disorders in Persons with Intellectual Disabil-
ity (DM-ID, National Association for the Dually Diag-
nosed 2007) was designed to be an adaptation of the
DSM-IV-TR (APA 2000) for persons with intellectual dis-
ability, with the goal of facilitating more accurate psy-
chiatric diagnosis of people with intellectual disability
(Fletcher et al. 2007).
The principal frame of reference to DC-LD is ICD-10
and correspondingly, the term ‘Hyperkinetic Disorders’
is used as opposed to ADHD. Two subtypes are
included under ‘Hyperkinetic Disorders’ in DC-LD,
namely, ‘attention deficit hyperactivity disorder of
adults (Childhood onset)’ and ‘attention deficit hyperac-
tivity of adults (age of onset unknown)’. DC-LD recog-
nizes: ‘that the diagnosis only applies where the overall
picture is in excess of that which might be expected on
the basis of general intelligence, or severity of learning
disabilities’. Six criteria for either subtype, all of which
must be met for a diagnosis to be made, are included in
DC-LD. The six include: age of onset (symptoms present
before 7 or in the case of lack of developmental history
symptoms must be long standing), aetiology (disorder
must not be attributable to another psychiatric or physi-
cal disorder such as delirium, dementia, manic episode
or hyperthyroidism), nature of disorder and develop-
mental level (symptoms not attributable to level of intel-
lectual disability), pattern of action which is impatient
and impulsive (e.g. acting without due care and atten-
tion to consequences for themselves and others), motor
over-activity and inability to keep still, and pervasive
and persistent (disorder is pervasive across situations
and longitudinally over time). The DC-LD (like the
ICD-10) does not have a diagnostic category equivalent
to the DSM-IV-TR diagnosis of ADHD – Predominantly
Inattentive Type (Seager & O’Brien 2003).
The principal frame of reference for DM-ID is
DSM-IV-TR. It is noted that three of the criteria for
ADHD in DSM-IV-TR imply the use of language and
these criteria would not be applicable for individuals
with no expressive or receptive language (Lee & Fried-
lander 2007). The adapted criteria for ADHD recom-
mended by DM-ID include assessing attention by
comparing with peers of comparable intellectual and
chronological age (Lee & Friedlander 2007). With regard
to hyperactivity–impulsivity, it is suggested that the
child’s symptoms should be compared with peers of
comparable developmental and chronological age, rather
than with younger typical children of comparable devel-
opmental age (Lee & Friedlander 2007). The other main
adaptation included refers to the need for the presence
of clear evidence of impairment that is clinically signifi-
cant, in the areas of social, academic, or occupational
functioning, and is related to inattention, hyperactivity,
or impulsivity and not just intellectual disability (Lee &
Friedlander 2007). It is not clear how widespread the
use of these alternative classification systems is in clini-
cal practice.
One question often asked by professionals working
with individuals with intellectual disability concerns the
validity of a diagnosis of ADHD in individuals with
intellectual disability. Antshel et al. (2006a) have carried
out a comprehensive review of the validity of the
Journal of Applied Research in Intellectual Disabilities 293
� 2011 Blackwell Publishing Ltd, 24, 291–309
ADHD in individuals with intellectual disability. They
point out a fundamental limitation of research into
ADHD in individuals with intellectual disability, namely
that much of the research in the field assumes that a
diagnosis of ADHD can be made in individuals with
intellectual disability in a manner similar to the way it
is made in individuals without intellectual disability. As
ADHD symptoms are considered common in individu-
als with intellectual disability assessing for a clinically
significant disorder in this population may be more
complex (Antshel et al. 2006a). Antshel et al. (2006a)
quote Meehl and Rosen (1955) in asserting that assessing
the predictive power of various diagnostic instruments
relies on knowing the prevalence or base rate of the
condition in the population being tested. As most of the
instruments for assessing ADHD excluded individuals
with intellectual disability in their normative and valida-
tion samples, established base rates for ADHD in the
population with intellectual disability via standardized
instruments do not exist (Antshel et al. 2006a). As a
result this can hinder the judgement of whether ADHD
symptoms are over and above symptoms due to the
level of an individual’s intellectual disability. Antshel
et al. (2006a) conclude that despite the lack of research
in the area of ADHD in individuals with intellectual
disability in comparison with individuals without intel-
lectual disability, there is preliminary evidence to
suggest that ADHD is a valid disorder in individuals
with intellectual disability.
Prevalence
The prevalence of ADHD in school-aged children with-
out intellectual disability is estimated at 3–7% (APA
2000). Periodic changes in the nosological systems make
it difficult to compare prevalence rates for ADHD over
time and inter-rater agreement may be inadequate when
clinicians make DSM-IV and ICD-10 diagnoses in chil-
dren with intellectual disability (Dykens 2000). The base
rate of ADHD symptoms for children with low IQ has
never been firmly established (Antshel et al. 2006a)
although a number of studies have sought to identify
the prevalence of ADHD in samples of individuals with
intellectual disability. Table 1 displays rates of preva-
lence of ADHD in published studies in individuals with
intellectual disability since 1980. In some of the earlier
studies prevalence rates for a diagnosis of ADHD are
not reported, but the equivalent diagnoses are reported.
Some of the studies reported the prevalence of ADHD
symptoms alone whereas other studies examined the
prevalence of a range of psychiatric ⁄ behavioural disor-
ders including ADHD. Studies primarily focusing on
the rates of ADHD in individuals with a genetic syn-
drome associated with intellectual disability (e.g. Down
syndrome, fragile X syndrome) are not included in
Table 1, but are discussed later in this review.
The prevalence rates indicated in Table 1 would
suggest that the prevalence of ADHD in individuals
with intellectual disability is significantly higher than
the prevalence rate of the disorder in those without
intellectual disability. However, reported prevalence
rates vary significantly across studies. This is likely to
be due to the range of instruments used, the popula-
tions sampled, and the diagnostic criteria employed in
identifying ADHD. While some studies employed rele-
vant DSM or ICD criteria, others used clinical cut-offs
on specific ADHD checklists, the older studies tended
to rely on clinical interviews and a review of medical
records and ⁄ or databases with diagnoses made on the
basis of clinical judgement. The range of instruments
and measures used make any comparisons across stud-
ies very difficult. In some of studies the samples were
drawn from community settings, whilst in some of the
studies samples were to drawn from more specialist
settings, but no clear pattern emerges regarding the
comparative prevalence of ADHD symptoms in either
type of settings. In all of the studies, the reported prev-
alence rates are higher than the reported prevalence
rates in individuals without intellectual disability.
However, some have argued that it is still unclear
whether there is a higher prevalence of ADHD among
children and adults with intellectual disability or the
higher rates reflect difficulties with scales used to
screen for or diagnose the disorder in this population
(Miller et al. 2004a).
Dekker & Koot (2003) reported one of the lowest
prevalence rates of 6.8% if their more stringent criteria
of ADHD are used. To meet the more stringent criteria
in this study, an individual must have significant
impairment due to their ADHD symptoms. This study
was the only study to explicitly include ‘impairment’
criteria and it is possible that some of the higher
reported prevalence rates reflect the fact that instru-
ments used are validated in a population without intel-
lectual disability. A number of the studies compared the
rates of ADHD in individuals with intellectual disability
and those without intellectual disability using the same
instruments and noted significantly higher ADHD
symptoms in the individuals with intellectual disability
(e.g. Simonoff et al. 2007). Emerson (2003) and Emerson
& Hatton (2007) reported a nearly a 10-fold increase in
294 Journal of Applied Research in Intellectual Disabilities
� 2011 Blackwell Publishing Ltd, 24, 291–309
Tab
le1
Pre
val
ence
of
AD
HD
and
⁄or
AD
HD
sym
pto
ms
inin
div
idu
als
wit
hin
tell
ectu
ald
isab
ilit
yin
pu
bli
shed
stu
die
ssi
nce
1980
Au
thor
N(G
ende
r)
Mea
nag
ein
year
s(a
gera
nge
inye
ars)
Lev
elof
inte
llec
tual
disa
bili
tyC
rite
ria
for
AD
HD
Ove
rall
prev
alen
ce
Sta
nda
rdiz
ed
inst
rum
ents
⁄Mea
sure
s
Bak
eret
al.
(201
0)
95(5
7M,
38F
)N
R(3
–5)
Bo
rder
lin
e(3
2)
Mil
d(6
3)
DS
M-I
VM
ale
40.4
%
Fem
ale
36.8
%
DIS
C
Oes
ebu
rget
al.
(201
0)
1074
(626
M,
448
F)
15.4
(12–
18)
Bo
rder
lin
e⁄M
ild
(785
)
Mo
der
ate
(253
)
Sev
ere
⁄Pro
fou
nd
(39)
AD
HD
bas
edo
n
par
enta
lre
po
rt
21.1
%N
on
e
La
Mal
faet
al.
(200
8)
46(3
0M,
16F
)37
.6(N
R)
Mil
d(9
)
Mo
der
ate
(20)
Sev
ere
(14)
Pro
fou
nd
(3)
CA
AR
S-O
:SV
(Cu
t-o
ff>
70)
19.6
%C
AA
RS
-O:S
V
Bu
ckle
yet
al.
(200
8)
84(5
3M,3
1F)
10.5
(5–1
8)M
od
erat
eto
sev
ere
CT
RS
(Cu
t-o
ff>
70)
27.4
%A
DH
DIn
dex
45.2
%C
og
nit
ive
Pro
ble
ms
⁄In
atte
nti
on
Ind
ex
28.6
%H
yp
erac
tiv
ity
Ind
ex
CT
RS
DiN
uo
vo
&
Bu
on
o(2
007)
184
(103
M,
F81
)17
.1(6
–50)
Mil
d(1
24)
Mo
der
ate
(60)
DS
M-I
V7.
36%
No
ne
Sim
on
off
etal
.
(200
7)
192
(125
M67
F)
83C
hil
dre
n
IQ<
70
13.7
(11.
9–16
)M
ild
SD
QH
yp
erac
tiv
ity
item
s
(IQ
<60
)12
%
(IQ
60–6
9)6%
SD
Q
Em
erso
n&
Hat
ton
(200
7)
641
(NR
)10
.1(5
–15)
NR
ICD
-10
8.3%
DA
WB
A(i
ncl
ud
ing
stru
ctu
red
par
enta
lin
terv
iew
and
teac
her
qu
esti
on
nai
re)
Vo
igh
tet
al.
(200
6)
19(7
M,
12F
)(6
–18)
1M
ild
DS
M-I
Vb
ased
on
clin
ical
dia
gn
osi
s
31.6
%N
R
Has
tin
gs
etal
.
(200
5)
75(4
8M,
26F
)29.
75(3
–19)
Mil
d⁄M
od
erat
e(5
2)
Sev
ere
⁄Pro
fou
nd
(23)
SD
Qcu
t-o
ff60
%ab
ov
ecu
t-o
ffS
DQ
Dek
ker
&K
oo
t
(200
3)
474
(293
M,
181F
)
12.9
(6–1
8)B
ord
erli
ne
tom
od
erat
eD
SM
-IV
14.8
%(6
.8%
had
sig
nifi
can
t
imp
airm
ent)
DIS
C-I
V-P
Cli
nic
alIn
terv
iew
Em
erso
n(2
003)
264
(192
M,
72F
)N
R(5
–15)
NR
ICD
-10
8.7%
DA
WB
A(i
ncl
ud
ing
stru
ctu
red
par
enta
lin
terv
iew
and
teac
her
qu
esti
on
nai
re)
Ko
sken
tau
sta
etal
.(2
002)
155
(92M
,63
F)
9.7
(6–1
3)M
ild
(87)
Mo
der
ate
(30)
Sev
ere
(18)
Pro
fou
nd
(20)
ICD
-10
bas
edo
n
exam
inat
ion
of
case
no
tes
6–14
%N
on
e
Journal of Applied Research in Intellectual Disabilities 295
� 2011 Blackwell Publishing Ltd, 24, 291–309
Tab
le1
(Co
nti
nu
ed).
Au
thor
N(G
ende
r)
Mea
nag
ein
year
s(a
gera
nge
inye
ars)
Lev
elof
inte
llec
tual
disa
bili
tyC
rite
ria
for
AD
HD
Ove
rall
prev
alen
ce
Sta
nda
rdiz
ed
inst
rum
ents
⁄Mea
sure
s
Str
om
me
&
Dis
eth
(200
0)
178
(104
M,
74F
)
NR
(8–1
3)M
ild
(99)
Mo
der
ate
⁄Sev
ere
and
pro
fou
nd
(79)
ICD
-10
16%
No
ne
Fo
x&
Wad
e
(199
8)
86(6
1M,
25F
)
41.4
(21–
72)
Mea
nIQ
22.2
(IQ
ran
ge
12–4
9)
DS
M-I
Van
da
cut-
off
of
15o
n
CR
S
55%
DS
M-I
V
Inat
ten
tiv
e
15%
DS
M-I
V
Hy
per
acti
ve–
Imp
uls
ive
20%
CR
SH
yp
erac
tiv
ity
Ind
ex
Hy
per
acti
vit
y
Ind
exo
fC
RS
Har
dan
&S
ahl
(199
7)
233
(168
M,
65F
)
9(3
.2–1
9.4)
1B
ord
erli
ne
(36)
Mil
d(3
9)
Mo
d(3
3)
Sev
ere
⁄Pro
fou
nd
(22)
DS
M-I
II-R
AD
HD
bas
edo
ncl
inic
al
dia
gn
ose
s
Bo
rder
lin
e44
%
Mil
d41
%
Mo
d42
%
Sev
ere
⁄Pro
fou
nd
36%
No
ne
Joh
nso
net
al.
(199
5)
169
(136
M,
33
F) 60
%o
f
sam
ple
had
inte
llec
tual
dis
abil
ity
.
Ch
ild
ren
wer
e
clin
icre
ferr
als
4.75
(2.6
–5.1
1)1
Bo
rder
lin
e(2
1)
Mil
d(4
0)
Mo
d(2
4)
Sev
ere
⁄Pro
fou
nd
(17)
Rev
iew
of
reco
rds
and
DS
M-I
IIan
d
DS
M-I
II-R
crit
eria
Bo
rder
lin
e47
.6%
Mil
d37
.5%
Mo
der
ate
25%
Sev
ere
⁄Pro
fou
nd
5.9%
No
ne
Kin
get
al.
(199
4)25
1(1
57M
94F
)36
.3(2
–81)
1S
ever
e⁄P
rofo
un
dD
SM
-III
-Rcr
iter
ia.
Ob
serv
atio
ns.
Rev
iew
of
char
ts
15.5
4%N
on
e
Ep
stei
net
al.
(198
6)
245
(137
M,
108F
)
NR
(12.
26)
Mea
nIQ
65.4
7(I
Qra
ng
e
50–7
0)
CT
Qcu
t-o
ff18
%C
TQ
Gil
lber
get
al.
(198
6)
83(5
4M
29F
)N
R(1
3–17
)M
ild
Hy
per
acti
ve
Dis
ord
er–
Par
enta
lIn
terv
iew
and
Ch
ild
Ob
serv
atio
n
11%
No
ne
Qu
ine
(198
6)39
9(2
45M
154
F)
NR
(0–1
4)S
ever
eO
ver
acti
ve
–S
tru
ctu
red
Inte
rvie
w
Mal
e25
%
Fem
ale
14%
DA
S
Jaco
bso
n(1
982)
3211
2N
R(0
–21)
Mil
dto
pro
fou
nd
Hy
per
acti
vit
y–
Tak
enfr
om
dat
abas
ein
div
idu
als
wit
h
inte
llec
tual
dis
abil
ity
7.4%
No
ne
296 Journal of Applied Research in Intellectual Disabilities
� 2011 Blackwell Publishing Ltd, 24, 291–309
the risk for ‘Hyperkinesis’ among children with intellec-
tual disability.
Gender Ratio in Individuals withIntellectual Disability and ADHD
In individuals without intellectual disability ADHD is
reported to be more prevalent in males with a male-to-
female ratio ranging from 2 : 1 to 9 : 1 depending on the
type of ADHD (APA 2000), with the gender ratio likely
to be less pronounced for the Predominantly Inattentive
subtype. However, in a number of the studies reported
in Table 1 there was not a statistically significant differ-
ence between the prevalence of ADHD among males
and females (Epstein et al. 1986; Dekker & Koot 2003;
Buckley et al. 2008; La Malfa et al. 2008; Baker et al.
2010). Fox & Wade (1998) found that males were signifi-
cantly more likely to meet the DSM-IV Predominantly
Inattentive criteria than females although a significant
difference between the genders was not found for
DSM-IV Hyperactive ⁄ Impulsive criteria. Hardan & Sahl
(1997) reported that males were significantly more likely
to have ‘poor concentration’, and they reported that the
prevalence of ADHD in favour of males was approach-
ing significance. King et al. (1994) reported that the
gender ratio of the diagnosis of ADHD was 1.7 : 1 in
favour of males. Gillberg et al. (1986) found a male
female ratio of 3.5 : 1 with regard to the diagnosis of
‘Hyperactive Disorder’ in their sample of individuals
with mild intellectual disability. Quine (1986) also
reported that significantly more males than females had
ADHD type difficulties with statistically more males
presenting as ‘overactive’.
Association between Level of IntellectualDisability and ADHD Symptoms
A number of published studies have reported the rates
of ADHD symptoms at different levels of intellectual
disability or have reported on the association between
ADHD symptoms and level of disability. La Malfa et al.
(2008) found a statistically significant influence of the
level of IQ on the subscales ‘Hyperactivity’, ‘ADHD
Total’, ‘ADHD index’ on the Conners Adult ADHD
Rating Scale-Observer Screening Version (CAARS-O:SV;
Conners et al. 1998a), with the adults in the severe or
profound range of intellectual disability having higher
subscale scores than individuals with moderate or mild
levels of intellectual disability. However, a statistically
significant effect of IQ on the ‘Inattentive’ subscale of
the CAARS-O:SV was not found in this study. Dekker &Tab
le1
(Co
nti
nu
ed).
Au
thor
N(G
ende
r)
Mea
nag
ein
year
s(a
gera
nge
inye
ars)
Lev
elof
inte
llec
tual
disa
bili
tyC
rite
ria
for
AD
HD
Ove
rall
prev
alen
ce
Sta
nda
rdiz
ed
inst
rum
ents
⁄Mea
sure
s
Rei
d(1
980)
60(3
4M,
26F
)>
16(1
3.3)
41M
ild
6M
od
erat
e
13S
ever
e
Hy
per
kin
etic
syn
dro
me-
bas
ed
clin
ical
op
inio
n
and
med
ical
reco
rds
15%
No
ne
1A
ge
ran
ge
of
tota
lsa
mp
le.
2G
end
ero
fo
ne
ind
ivid
ual
no
tre
po
rted
.
NR
,N
ot
Rep
ort
ed;
DIS
C,
Dia
gn
ost
icIn
terv
iew
Sch
edu
lefo
rC
hil
dre
n(C
ost
ello
etal
.19
85);
CA
AR
S-O
:SV
,C
on
ner
sA
du
ltA
DH
DR
atin
gS
cale
-Ob
serv
erS
cree
nin
gV
er-
sio
n(C
on
ner
set
al.
1998
a);
CT
RS
-R,
Co
nn
ers
Tea
cher
Rat
ing
Sca
le–R
evis
ed(C
on
ner
set
al.
1998
c);
SD
Q,
Str
eng
ths
and
Dif
ficu
ltie
sQ
ues
tio
nn
aire
(Go
od
man
1997
);
DIS
C-I
V-P
,D
iag
no
stic
Inte
rvie
wS
ched
ule
for
Ch
ild
ren
–P
aren
tV
ersi
on
(Sh
affe
ret
al.
2000
);D
AW
BA
,D
evel
op
men
tan
dW
ell
Bei
ng
Ass
essm
ent
(Go
od
man
etal
.20
00);
CR
S,
Co
nn
ers’
Rat
ing
Sca
les
(Co
nn
ers
1990
);C
AT
RS
,C
on
ner
sT
each
erQ
ues
tio
nn
aire
(Co
nn
ers
1973
);D
AS
,D
isab
ilit
yA
sses
smen
tS
ched
ule
(Ho
lmes
etal
.19
82);
DS
M-
III,
Dia
gn
ost
ican
dS
tati
stic
alM
anu
alo
fM
enta
lD
iso
rder
s,3r
ded
n.
(AP
A19
80);
DS
M-I
I-R
,D
iag
no
stic
and
Sta
tist
ical
Man
ual
of
Men
tal
Dis
ord
ers,
3rd
edn
rev
ised
(AP
A19
87).
Journal of Applied Research in Intellectual Disabilities 297
� 2011 Blackwell Publishing Ltd, 24, 291–309
Koot (2003) did not find a difference in the rates of
ADHD among individuals with an IQ below 60 and an
IQ between 60 and 80. Fox & Wade (1998) found that
individuals with IQ of 20 or below were more likely to
meet DSM-IV Predominantly Inattentive criteria than
individuals with IQ 21 or greater. However, no signifi-
cant relationships were found between the two IQ
groupings for DSM-IV Hyperactive–Impulsive criteria.
However, Hardan & Sahl (1997) reported broadly simi-
lar levels of prevalence across the four levels of intellec-
tual disability.
Johnson et al. (1995) reported that in their sample of
preschool children, there was a tendency for a diagnosis
of ADHD not to be used with children functioning in
the severe or profound range of intellectual functioning,
and as a result the prevalence of ADHD declined with
the severity of disability. In Gillberg et al.’s (1986) study
11% of individuals with mild intellectual disability (IQ
score between 50 and 70) had ‘Hyperactive Disorder’,
while in the group with ‘severe intellectual disability’
(IQ below 50) none of the children were assigned a
‘Hyperactive Disorder’ diagnosis, although it was noted
that nine children in this group were ‘pervasively
hyperactive’. Hastings et al. (2005) reported overall
mental age was significantly negatively associated with
ADHD symptoms (Hastings et al. 2005), with more
ADHD symptoms at lower mental ages. In Jacobson’s
(1982) study the trend was for an increase in the rates of
‘hyperactivity’ in both adults and children as the sever-
ity of disability increased.
Association between Age and Symptomsof ADHD
The prevalence rate of ADHD in adults without intellec-
tual disability who meet full DSM-IV criteria for ADHD
falls between 1% and 4% (Farone et al. 2003). In the non-
disabled population, there appears to be a trend
towards a decline in hyperactivity and impulsivity, but
difficulties with attention tend to be more persistent
(Biederman et al. 2000). Xenitidis et al. (2010) compared
adults with ADHD and an IQ score below 80, to adults
with ADHD and an IQ score above 80. They found that
ADHD symptoms in the lower functioning adults were
more severe and there was a less favourable pattern of
improvement in the adults in the lower IQ group.
Buckley et al. (2008) reported that children younger than
10 years of age scored higher than children over
10 years on the ‘Hyperactivity’ and ‘ADHD index’
subscales of the Conners Teacher Rating Scale–Revised
(CTRS-R; Conners et al. 1998c) in their study of school-
aged children. However, there was not a significant
difference between the younger and older age group on
the ‘Cognitive Problems ⁄ Inattention’ subscale in this
study. Hastings et al. (2005) noted a statistically signifi-
cant pattern of reducing ADHD symptoms with age.
Dekker & Koot (2003), Emerson (2003) and Epstein et al.
(1986) did not find significant differences in the preva-
lence of ADHD symptoms among different age groups
in their samples.
Aman et al. (1996) reported on 26 children with intel-
lectual disability and ADHD who were followed up
4 years after they participated in an initial study.
Although 80% of the children continued to have high
scores on the Irritability, Stereotypic Behavior, and
Hyperactivity subscales of the Aberrant Behavior Check-
list (ABC; Aman et al. 1985), parents rated the children
as having significantly lower scores on all subscales
with the exception of the Stereotypic Behavior subscale
after 4 years. Aman et al. (2002) reported on 20 children
with ADHD and IQs below 85 who were followed up
four and half years after originally participating in a
study examining the impact of pharmacology on ADHD
symptoms. Both teacher and parent ratings on the ABC
Hyperactivity scale were significantly lower at the
second time of contact.
Co-morbidity with Psychiatric andBehavioural Disorders
School-aged children with ADHD who do not have
intellectual disability often present with co-morbid
conduct problems typically characterized by aggression,
defiance and disobedience (Young & Amarasinghe
2010), while older children with ADHD without intellec-
tual disability are at increased risk for depression, anxi-
ety, antisocial personality characteristics, and substance
abuse problems (Fischer et al. 2002). The majority of the
published studies on ADHD in individuals with intellec-
tual disability do not report on rates of co-morbidity.
With regard to diagnosis of conduct disorder there is an
implied assumption that person understands appropri-
ate social norms with this diagnosis (Lee & Friedlander
2007). This cannot be assumed in many individuals with
intellectual disability and as a result there may be less
likelihood of a child with intellectual disability and
ADHD receiving a diagnosis of conduct disorder. It has
been suggested that there is a similar prevalence of
affective disorders in children with intellectual disability
compared with rates in children with ADHD without
intellectual disability (Johnson et al. 1994), although few
published studies have addressed this issue. In a study
298 Journal of Applied Research in Intellectual Disabilities
� 2011 Blackwell Publishing Ltd, 24, 291–309
by Aman et al. (2002) parents reported that a significant
number of children with intellectual disability and
ADHD presented with enuresis (40%), encopresis (35%),
and motor and ⁄ or vocal tics (25%). A high level of anxi-
ety spectrum disorder (50%) was also reported in the
children with ADHD and intellectual disability in this
study. The authors suggested that the spectrum of
psychopathology in children with ADHD and intellec-
tual disability may be qualitatively different than for
children with ADHD without intellectual disability.
Co-morbidity with Autism SpectrumDisorder (ASD)
A significant minority of individuals with intellectual
disability may have ASD. DeBildt et al. (2005) estimated
that 9.3% of individuals with mild intellectual disability
and 26.1% of individuals in a combined moder-
ate ⁄ severe ⁄ profound intellectual disability group met
the criteria for ASD. As a result, it is possible that some
individuals with intellectual disability and significant
ADHD symptoms may also have ASD. A substantial
number of children and adolescents with ASD also exhi-
bit significant levels of inattention, impulsivity, and
over-activity, behaviours typically found in children
with ADHD (Handen et al. 2000; Bradley & Isaacs 2006).
Up to 12% of children with ASD are prescribed stimu-
lant medication to treat such behaviours (Aman et al.
1995). However, ADHD is still an exclusion criterion for
ASD in ICD-10 and DSM-IV-TR and the DSM diagnostic
criteria suggest that ADHD ⁄ hyperactivity symptoms in
ASD may be better explained by the disorder itself than
through the application of the ADHD label (Hastings
et al. 2005). Much controversy still remains in the field
regarding whether ADHD symptoms are simply associ-
ated features of ASD or whether ADHD is an indepen-
dent co-morbid condition (Antshel et al. 2006a).
Although DSM-IV hierarchical rules enforce a classifica-
tion of ASD it may be clinically relevant to assign both
diagnoses when children show a combination of difficul-
ties in the triad of impairments associated with ASD,
and significant difficulties with inattention and ⁄ or
hyperactivity ⁄ impulsivity (Luteijn et al. 2000). The Diag-
nostic Criteria for Psychiatric Disorders for use with
Adults with Learning Disabilities ⁄ Mental Retardation
(DC-LD; Royal College of Psychiatrists 2001) permits the
coding of ASD in addition to ADHD, if both sets of
diagnostic criteria are met (Seager & O’Brien 2003).
The majority of published studies of the prevalence of
ADHD symptoms in individuals with intellectual
disability do not report on the prevalence of ASD symp-
toms or diagnoses of ASD in individuals who received a
diagnosis of ADHD or scored above cut-offs for ADHD
on standardized instruments. Children with a diagnosis
of ASD were reported to have significantly more symp-
toms of ADHD than children without ASD in two of the
three samples of children with intellectual disability
reported on by Hastings et al. (2005). However, this
pattern of a relationship between ADHD symptoms and
ASD symptoms was not observed by Simonoff et al.
(2007). A study by Bradley & Isaacs (2006) found that
while teenagers with ASD and intellectual disability were
significantly more likely to meet DSM or ICD criteria for
ADHD than teenagers with intellectual disability without
ASD, a subgroup of those with intellectual disability and
ASD displayed very few ADHD behaviours.
The Prevalence of ADHD in GeneticSyndromes Associated with IntellectualDisability
In a number of genetic syndromes associated with intel-
lectual disability there have been reports of significant
ADHD symptoms over and above the level of symp-
toms found in the general population (Turk 2009). Simi-
lar to the issue of the validity of the diagnosis of ADHD
in individuals with intellectual disability without a
genetic aetiology, there is also an issue in relation to the
validity of diagnosing ADHD in genetic syndromes
associated with intellectual disability, and it has been
posited that the utility of an ADHD diagnosis in chil-
dren with complex behavioural phenotypes needs
further evaluation (Zwaigenbaum et al. 2006).
Table 2 illustrates prevalence rates of ADHD in a num-
ber of selected genetic syndromes associated with intel-
lectual disability. Studies were only included if they
included specific measures of ADHD. The lowest
reported prevalence rates are for individuals with Down
syndrome and it has been suggested that individuals
with Down syndrome exhibit maladaptive behaviour or
psychopathology less often than others with intellectual
disability (Dykens et al. 2000; p.79). With regard to a com-
parison of ADHD symptoms across syndromes Einfield
et al. (2007) compared the evolution of ADHD symptoms
(at four separate times) via the 6-item ADHD scale on the
Developmental Behaviour Checklist (DBC; Einfield &
Tonge 2002) in children with Prader-Willi syndrome,
Fragile X syndrome, Down syndrome, ASD, Williams
syndrome and an epidemiological group with mixed aeti-
ology for their disability. For scores on the DBC ADHD
scale, the children with ASD and Williams syndrome had
the highest initial scores and while scores declined over
Journal of Applied Research in Intellectual Disabilities 299
� 2011 Blackwell Publishing Ltd, 24, 291–309
time for both groups, both groups still had higher mean
ADHD scores than all of the other groups at the final
assessment. The mean scores for children with Down syn-
drome were the lowest at all four times of assessment.
Although many genetic syndromes associated with intel-
lectual disability are characterized as having attentional
problems, these difficulties may differ significantly when
studied in any depth (Cornish et al. 2007), and as a result
it is argued that a cross syndrome perspective is needed
to capture the subtleties of cross syndrome comparisons
and similarities (Cornish et al. 2007). Other genetic syn-
dromes associated with intellectual disability with a
reported high association with ADHD symptoms include
Tuberous Sclerosis Complex (Prather & de Vries 2004),
Rubinstein-Taybi syndrome (Hennekam et al. 1992),
Smith-Magenis syndrome (Dykens & Smith 1998) and 5p
syndrome (Dykens & Clarke 1997).
Screening Instruments for ADHDSymptoms in Individuals with IntellectualDisability
Most instruments for measuring symptoms of ADHD
lack reliability, validity and normative data for people
with intellectual disability (Miller et al. 2004b). Antshel
et al. (2006a) conclude that with regard to assessment,
none of the commonly used instruments for assessing
symptoms of ADHD in those with intellectual disability
have been validated for use in the intellectual disability
population, with the exception of the Hyperactivity
subscale of the ABC (Aman et al. 1985). The most
comprehensive examination of the validity of a range
of ADHD specific instruments in a population with
intellectual disability was carried out by Miller et al.
(2004a). This study involved 48 children (28 males and
20 females) aged between 5 and 12 years with mild to
moderate intellectual disability, and parents, teachers
and teaching assistants completed a range of instru-
ments to assess ADHD symptoms. Observational data
were collected on all children via the Classroom Obser-
vation Code (Abikoff & Gittelman 1985). Overall
teacher ratings of ADHD factors revealed only a few
significant correlations with classroom observations.
The study provides the most support for use of the
ABC as it was the only scale to demonstrate significant
correlations with classroom observations on all factors.
The study also provided support for the use of teach-
ing assistants in the assessment of ADHD in children
Table 2 Reported prevalence rated of ADHD ⁄ ADHD symptoms in selected genetic syndromes associated with intellectual disability
Genetic syndrome Author
Reported
prevalence
rates of ADHD Notes
Down syndrome Gath & Gumley (1986)
Myers & Pueschel (1991)
2.4–6.1% Rate of ADHD symptoms appears lower than
other genetic syndromes
Fragile X syndrome Hagerman (1987)
Bregman et al. (1988)
Baumgardner et al. (1995)
Backes et al. (2000)
Freund et al. (1993)
Male 71–73%
Female 35%
Decline in hyperactive symptoms with age
Females likely to be less hyperactive than males
Williams syndrome Leyfer et al. (2006) 64.7% Predominantly inattentive subtype is the most
common
Difficulties with hyperactivity tend to decline
with age but difficulties with inattention seem
to persist into adulthood
Velo-cardio-facial
syndrome ⁄ 22q11.2
syndrome
Papolos et al. (1996)
Gothelf et al. (2003)
Gothelf et al. (2004)
Niklasson et al. (2005)
Antshel et al. (2006b)
Niklasson et al. (2009)
36–55% ADHD predominantly inattentive type, the most
common subtype of ADHD
In those with ADHD high co-morbidity with
ASD and psychiatric disorders
Prader-Willi syndrome Gross-Tur et al. (2001)
Wigren & Hansen (2005)1
26–72% Attention deficits and impulsivity more common
symptoms than excessive motor activity
1Not based on actual diagnoses but on score on Conners’ Parent Rating Scale.
300 Journal of Applied Research in Intellectual Disabilities
� 2011 Blackwell Publishing Ltd, 24, 291–309
with intellectual disability (Miller et al. 2004a). In terms
of reliability, the ABC was the only measure on which
the inter-rater reliability was adequate for clinical pur-
poses (Miller et al. 2004b).
Some of the most commonly used instruments in mea-
suring ADHD symptoms in children with intellectual
disability are the Conners’ rating scales. However, there
is some evidence to suggest that the use of these instru-
ments in children with intellectual disability is not effica-
cious. Deb et al. (2008) compared clinical cut-off scores
on the Conners’ rating scales-revised, with children who
had intellectual disability and a clinical diagnosis of
ADHD based on DSM-IV-TR and children with intellec-
tual disability who did not have ADHD. The versions of
the Conners’ rating scales used were the Conners’ Parent
Rating Scales-Revised (CPRS-R) (Conners et al. 1998b)
and Conners’ Teacher Scales-Revised (CTRS-R) (Conners
et al. 1998c). There was a poor correlation between scores
on the CPRS-R and CTRS-R scores for 71 children who
were rated. Furthermore, scores on the CTRS-R were not
significantly different for children with a diagnosis of
ADHD and children without this diagnosis. Scores on
the CPRS-R were significantly different on a number of
subscales between those with a diagnosis of ADHD and
those who did not have ADHD. It was concluded that it
would be difficult to recommend use of the CTRS-R total
score for screening for ADHD among children with intel-
lectual disability due to poor sensitivity and specificity.
Teachers chose not to answer a number of questions on
the CTRS-R presumably due to perceived unsuitabil-
ity ⁄ validity for children with intellectual disability partic-
ularly children with moderate intellectual disability. The
authors cautioned that use of the CTRS-R with children
with intellectual disability may be unsuitable for children
with significant intellectual disability and ⁄ or those with-
out functional speech (Deb et al. 2008).
Interventions in Children with IntellectualDisability and ADHD
The majority of published research on interventions for
ADHD symptoms in individuals with intellectual
disability has focussed on the effectiveness of psycho-
pharmacological intervention for children. Pharmaco-
therapy is commonly used to treat ADHD, and
stimulants are the primary intervention with approxi-
mately 80–90% of individuals with ADHD without intel-
lectual disability receiving stimulants at some point in
their lives (Pelham 1993). Treatment with stimulant
medication improves the core symptoms of ADHD in
individuals without intellectual disability and has
resulted in a positive response in >75% of children
(APA 2000; Safer 2000; Poulton & Cowell 2003). The
majority of published studies of psychopharmacology
for individuals with ADHD and intellectual disability
have involved methylphenidate. There have also been
published reports on comparisons between methylphe-
nidate and risperidone (Correia Filho et al. 2005), meth-
ylphenidate and thioroidazine (Aman et al. 1991), and
methylphenidate and fenfluramine (Aman et al. 1993,
1997), and one study on the effect of clonidine (Agarwal
et al. 2001). In terms of the level of disability of partici-
pants, most published studies included individuals who
were predominantly operating in the mild or moderate
range of intellectual disability. Where individuals with
severe or profound levels of intellectual disability were
included the response rates for these individuals were
lower than those with higher levels of intellectual func-
tioning (Aman et al. 1991).
Aman (1996) calculated that the response rate to stimu-
lants in the studies of individuals with ADHD and intel-
lectual disability to be 54% in comparison to the rate of
75% reported for individuals without intellectual disabil-
ity. Studies since Aman’s review suggest similar and in
some cases higher response rates. However, response
rates have primarily been calculated on the basis of
parent and teacher ratings on behavioural scales, and
improvements on these scales have not always been
accompanied by improvements on cognitive tasks.
Assessing cognitive changes in individuals with ADHD
as well as behavioural changes is important as improve-
ments in behaviour are not always associated with
improvements in cognitive tasks (Pearson et al. 2004b),
and reported decreases in activity level and impulsivity
on behaviour checklists may be partly due to increased
sedation (Handen et al. 2000). In terms of dosage, in some
studies higher response rates were associated with higher
doses (e.g. Pearson et al. 2004a), but higher doses have
resulted in increased side effects (e.g. Handen et al. 1999),
and in some cases these side effects have been significant,
and resulted in medication being stopped. It is not clear if
reported improvements in symptoms are maintained over
time and if there are changes in the frequency and sever-
ity of side effects over time as most published studies in
individuals with intellectual disability have focussed on
periods of between 4 and 6 weeks. Studies on the effec-
tiveness of risperidone in reducing the level of disruptive
behaviours in children with IQ below 85 have found that
the use of risperidone has also significantly reduced
ADHD symptoms as measured by checklists including
the Aberrant Behaviour Checklist (Aman et al. 2004) and
it has been suggested that the addition of risperidone in
Journal of Applied Research in Intellectual Disabilities 301
� 2011 Blackwell Publishing Ltd, 24, 291–309
children already taking stimulants resulted in signifi-
cantly reduced levels of hyperactivity than was achieved
via stimulants alone (Aman et al. 2004). However, a
review of the use of risperidone in the treatment of
ADHD in individuals with intellectual disability suggests
that there is no evidence from randomized controlled tri-
als that it is effective in the treatment of ADHD in those
with intellectual disability (Thomson et al. 2009).
The use of stimulants with children with genetic
syndromes associated with intellectual disability has not
been extensively studied. Hagerman et al. (1998) com-
pared the effectiveness of two stimulant medications,
methylphenidate and dextroamphetamine in 15 children
(13 males, two females) with fragile X syndrome. In
terms of attention span and socialization skills, seven
children showed significant improvement on methylphe-
nidate, and three showed significant improvement on
dextroamphetamine. There have been published reports
of stimulant use in individuals with Williams syndrome
suggesting a mixed response (Bawden et al. 1997; Power
et al. 1997; Zwaigenbaum et al. 2006). Reported side
effects in the Zwaigenbaum et al. study included
noticeable changes in behavioural irritability and mood
stability. With regard to Velo-Cardio-Facial ⁄ 22q11.2 syn-
drome, a 2003 study noted that significant clinical
improvement occurred in nine of 12 children prescribed
methylphenidate (Gothelf et al. 2003) Reported side-
effects in this study included poor appetite, irritability,
sadness, stomach aches, irritability, talking little with
others and proneness to crying. The side effects were
reportedly not significant enough to warrant discontinu-
ation of the medication. One published study has specif-
ically focussed on children with ASD and intellectual
disability who had significant ADHD symptoms, and
the response rate in this study was similar to that of
other studies of children with ADHD and intellectual
disability (Handen et al. 2000).
In the UK, the National Institute for Health and Clini-
cal Excellence (NICE) (2009) published ADHD clinical
guidelines. These guidelines endorse the use of non-
pharmacological treatments for children, young people,
and adults with ADHD and recommended that pharma-
cological treatments should always form part of a
comprehensive intervention plan that includes psycho-
logical, behavioural, and educational advice and inter-
ventions (Young & Amarasinghe 2010). Parents may
have concerns about pharmacological interventions and
may prefer non-medication alternatives for children
with ADHD (Bussing & Gary 2001). There is no compel-
ling evidence that stimulant medication is capable of
improving long-term prognosis of ADHD (MTA Coop-
erative Group 1999), and research findings have raised
the possibility of stimulant medication having a ‘shelf-
life’, in that gains accrue on a relatively short-term basis
(Jensen et al. 2007). Thus, stimulant medication alone as
‘standalone’ intervention may not adequately address
the complexity and range of mental health needs and
pervasive impairments associated with ADHD (Young
& Amarasinghe 2010).
Despite the recognition of the importance of non-
pharmacological interventions in individuals with
ADHD there have been few reported studies of the effi-
cacy of non-pharmacological interventions with children
or adults with intellectual disability who also have
significant ADHD symptoms. It is likely that classroom
interventions commonly used for children with intellec-
tual disability without ADHD, and for children with
ASD, such as the use of behavioural techniques and
environmental accommodations are already been used
with children with intellectual disability and significant
ADHD symptoms although there is little published
research in this area. Denckla (2008) argues that applied
behaviour analysis (at any age or intellectual level)
ought to be considered as the infrastructure that
supports other aspects including medication in interven-
tions for ADHD. Results from the 36-month follow up
of the MTA (Jensen et al. 2007) make clear that behav-
iour modification is an essential component of interven-
tion for ADHD life outcomes (not just symptom
ratings), and may emerge as even more important when
long-term treatment is considered (Denckla 2008).
There are an increasing number of studies focussing
on supporting parents of young children with intellec-
tual disability who have behavioural difficulties. These
studies have included adaptations of existing parenting
programmes to take into account the needs of children
with intellectual disability and have included modifica-
tions to the Positive Parenting Programme (Plant &
Sanders 2007), the Stepping Stones Triple P (Roberts
et al. 2006) and the Incredible Years Parent Training
(IYPT) (McIntyre 2008). Although these studies have not
specifically focussed on children with intellectual
disability and ADHD they do suggest that parent train-
ing can be effective for reducing behavioural difficulties
in young children with intellectual disability.
Discussion
This review has attempted to identity key issues with
regard to the prevalence of ADHD symptoms and inter-
vention for such symptoms in individuals with intellec-
tual disability. International classification systems (e.g.
302 Journal of Applied Research in Intellectual Disabilities
� 2011 Blackwell Publishing Ltd, 24, 291–309
DSM-IV, ICD-10) designed for use with individuals
without intellectual disability, have limitations with
regard to the identification of ADHD in individuals with
intellectual disability. While the development of parallel
systems for individuals with intellectual disability in the
form of DC-LD and DM-ID may be helpful, these
systems do not yet appear to enjoy widespread use and
have not been accompanied by the development of stan-
dardized instruments based on the criteria outlined in
the systems. The validity of the diagnosis of ADHD in
some individuals with intellectual disability is likely still
to be queried and be the subject of discussion until
progress is made with regard to agreed upon diagnostic
criteria, the development of instruments validated in a
population of individuals with intellectual disability,
and the development of a range of efficacious interven-
tions for individuals in this population.
The reported prevalence rate of ADHD symptoms in
individuals with intellectual disability is higher than in
individuals without intellectual disability. However,
prevalence studies continue to be limited by the variety
of assessment methods used. Most of the prevalence
studies have not assessed the rate of clinical diagnosis
of ADHD among individuals with intellectual disability
(Deb et al. 2008), but have identified a group of individ-
uals with ADHD symptoms as measured by behavioural
checklists. In their study Dekker & Koot (2003) included
reference to the fact that meeting the symptom criteria
for a DSM-IV disorder did not always coincide with
significant impairment with respect to everyday func-
tioning, and this may be something than diagnosing
clinicians and teams, and researchers should consider
with regard to making a diagnosis and estimating prev-
alence rates.
It is not clear from the published research if ADHD
symptoms increase with severity of intellectual disabil-
ity, or how ADHD symptoms change from childhood to
adulthood. Although some studies suggest a decline in
the levels of ADHD symptoms with age in individuals
with intellectual disability, there is a paucity of studies
addressing this issue. It does appear from studies which
have attempted to assess levels of ADHD or symptoms
of ADHD in adulthood (e.g. La Malfa et al. 2008) that
ADHD symptoms continue to a significant extent in
some adults with intellectual disability. The study of
ADHD symptoms in adults with intellectual disability is
still its infancy and there is a need for more studies
focusing on the prevalence and nature of ADHD symp-
toms throughout adulthood in this population. The issue
of the relationship between ADHD and level of disabil-
ity is complicated by the fact that in clinical practice
there is a reported reluctance to classify individuals
with intellectual disability in the severe or profound
ranges as having ADHD, and it is not clear how mani-
festations of ADHD symptoms might change as the
severity of intellectual disability increases. With regard
to the gender ratio, the available evidence suggests that
the pattern of a much higher risk for males found in the
non-intellectually disabled population is not replicated
in the population with intellectual disability, although
there still may be an excess in the number of males
diagnosed in comparison with females. Rates of co-mor-
bidity with other behavioural and psychiatric disorders
have not been extensively studied in individuals with
ADHD and intellectual disability, but it is possible that
the effects of intellectual disability may lead to a differ-
ent pattern and manifestation of co-morbidities.
There is some evidence that ADHD symptoms are
more prevalent in some genetic syndromes associated
with intellectual disability (e.g. Williams syndrome) in
comparison to other syndromes (e.g. Down syndrome).
The use of ADHD as a broad umbrella term may fail to
capture the subtlety of symptom differences across
syndromes. Studies of ADHD symptoms have often
excluded children and adults with genetic syndromes,
and subtyping individuals with intellectual disability by
genetic syndromes or by level of ASD symptoms may
potentially elucidate distinctive behavioural phenotypes
of attention deficits (Deutsch et al. 2008).
It is not clear if scales designed for measuring ADHD
symptoms in children without intellectual disability are
as useful in this population and if these scales capture
the nature of ADHD in children with intellectual dis-
ability. Some of the scale items in these instruments
seem less useful as the severity of intellectual disability
increases. The Hyperactivity scale of the ABC has been
cited as the most valid and reliable instrument in this
population. There would appear to be a need for the
development of a new comprehensive instrument
tapping the range of symptoms of ADHD in individuals
with intellectual disability. However, until research
confirms whether ADHD presents in a similar manner
in children with and without intellectual disability, and
agreed upon prevalence estimates can be established,
measurement issues may not be significantly advanced
(Hastings et al. 2005).
Research suggests that some individuals with ADHD
and intellectual disability respond to stimulant medica-
tion with regard to outcome measures such as behavio-
ural checklists, but that the response rate is lower than
with individuals without intellectual disability, and
side-effects may be greater for individuals with intellec-
Journal of Applied Research in Intellectual Disabilities 303
� 2011 Blackwell Publishing Ltd, 24, 291–309
tual disability. Thus, there appears to be even greater
heterogeneity in stimulant response in children with
ADHD and intellectual disability compared with chil-
dren with ADHD without intellectual disability. There is
a particular need for more studies on the effects of
medication in this population over significant periods of
time. While some studies suggest a lower response rate
for individuals with severe and profound levels of
disability as opposed to individuals with mild and
moderate levels of disability, the issue of response to
stimulants in relation to severity of intellectual disability
needs further clarification (King 2007). There is also a
need for more specific, treatment sensitive outcome
measures (e.g. social functioning, adaptive behaviours,
quality of life measures etc.) rather than relying primar-
ily on rating scale scores (Antshel et al. 2006a). There
has been little research to date on the effectiveness of
pharmacological interventions for adults with intellec-
tual disability and ADHD symptoms, and although
there have been reports of a similar response rate to that
of children with intellectual disability (e.g. Jou et al.
2004), there is a need for studies to determine if adults
with intellectual disability and significant ADHD symp-
toms will respond at a similar level to children with
intellectual disability and ADHD or adults with ADHD
without intellectual disability.
There is a paucity of information regarding interven-
tions or practices in schools regarding supports for chil-
dren with intellectual disability who have significant
ADHD symptoms, and as such there is a need for
research on teacher directed interventions in the school
environment. Behavioural interventions in this specific
population have received little attention to date, and
research on early intervention for young children with
significant ADHD symptoms and developmental delay
via parents’ education and training would appear
particularly important. Research on multimodal
approaches involving the concurrent use of pharmaco-
logical and non-pharmacological interventions for indi-
viduals with intellectual disability and ADHD would
yield valuable data on whether individuals with ADHD
respond in a similar manner to individuals without intel-
lectual disability. Given the heterogeneity of individuals
with intellectual disability there is an urgent need to
identify what children and what adults with significant
ADHD symptoms benefit most from what treatments,
and what modifications to existing efficacious treatments
in the non-disabled population need to be made.
In conclusion, much less is known about ADHD in
individuals with intellectual disability than about
ADHD in individuals without intellectual disability.
Much of the assessment and intervention approaches
used in the population with intellectual disability have
been borrowed from the non-intellectually disabled
population often without adaptation. As a result there
are still no accurate estimates of the prevalence of
ADHD in individuals with intellectual disability and
interventions have been less successful than those in the
non-intellectually disabled population. It is hoped that
future research will identify more clearly the extent of
the phenomenon and what can be done to ameliorate its
impact in individuals with intellectual disability.
Correspondence
Any correspondence should be directed to Colin Reilly,
School of Education, University College Dublin, Belfield,
Dublin 4, Ireland (e-mail: [email protected]).
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