19
Symptoms of Attention Deficit Hyperactivity Disorder in Children and Adults with Intellectual Disability: A Review Colin 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 Ó 2011 Blackwell Publishing Ltd 10.1111/j.1468-3148.2010.00607.x Published for the British Institute of Learning Disabilities

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Page 1: Symptoms of Attention Deficit Hyperactivity Disorder in Children and Adults with Intellectual Disability: A Review

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

Page 2: Symptoms of Attention Deficit Hyperactivity Disorder in Children and Adults with Intellectual Disability: A Review

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

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Page 3: Symptoms of Attention Deficit Hyperactivity Disorder in Children and Adults with Intellectual Disability: A Review

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

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Page 4: Symptoms of Attention Deficit Hyperactivity Disorder in Children and Adults with Intellectual Disability: A Review

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

Page 5: Symptoms of Attention Deficit Hyperactivity Disorder in Children and Adults with Intellectual Disability: A Review

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

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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

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296 Journal of Applied Research in Intellectual Disabilities

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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

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ort

ed;

DIS

C,

Dia

gn

ost

icIn

terv

iew

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edu

lefo

rC

hil

dre

n(C

ost

ello

etal

.19

85);

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AR

S-O

:SV

,C

on

ner

sA

du

ltA

DH

DR

atin

gS

cale

-Ob

serv

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cree

nin

gV

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n(C

on

ner

set

al.

1998

a);

CT

RS

-R,

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nn

ers

Tea

cher

Rat

ing

Sca

le–R

evis

ed(C

on

ner

set

al.

1998

c);

SD

Q,

Str

eng

ths

and

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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

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ng

Ass

essm

ent

(Go

od

man

etal

.20

00);

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S,

Co

nn

ers’

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ing

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les

(Co

nn

ers

1990

);C

AT

RS

,C

on

ner

sT

each

erQ

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(Co

nn

ers

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);D

AS

,D

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ched

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M-

III,

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gn

ost

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dS

tati

stic

alM

anu

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fM

enta

lD

iso

rder

s,3r

ded

n.

(AP

A19

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DS

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,D

iag

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stic

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tist

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ual

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tal

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ord

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3rd

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rev

ised

(AP

A19

87).

Journal of Applied Research in Intellectual Disabilities 297

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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

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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

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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.

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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

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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.

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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-

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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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