prevalence of intellectual disability in western australia

58

©

Blackwell Publishing Ltd.

Paediatric and Perinatal Epidemiology 2003,

17,

58–67

Blackwell Science, LtdOxford, UKPPEPaediatric and Perinatal Epidemiology0269-5022Blackwell Publishing Ltd, 200317Original Article

Intellectual disability in W. AustraliaH. Leonard

et al.

Correspondence:

Dr Helen Leonard, Telethon Institute for Child Health Research, PO Box 855, West Perth, WA 6872, Australia.E-mail: [email protected]

Prevalence of intellectual disability in Western Australia

Helen Leonard

a,b

, Beverly Petterson

b

, Carol Bower

a

and Richard Sanders

c

a

Centre for Child Health Research, The University of Western Australia, Telethon Institute for Child Health Research,

b

Disability Services

Commission, and

c

Department of Education, Western Australia

Summary

Record linkage of multiple sources was used to ascertain children with intellectualdisability born in Western Australia between 1983 and 1992. The prevalence was 14.3per 1000, 10.6 per 1000 for children with mild or moderate and 1.4 per 1000 for thosewith a severe level of intellectual disability. Prevalence was greater in males (preva-lence ratio 1.6) and in children of Aboriginal mothers (prevalence ratio 2.3). Althoughprevalence of intellectual disability has been found to vary according to study location,method of ascertainment and criteria used, these estimates are similar to estimatesfrom several recent studies.

Introduction

Prevalence of intellectual disability varies widelydepending on study location, methods of case ascer-tainment and criteria for case inclusion, with most esti-mates in the range 5–30 per 1000.

1–3

The proportion ofchildren with different degrees of intellectual disabilityalso varies.

1

Studies using only providers of medicaland allied services to ascertain cases tend to report alower prevalence than those using education facilitiesas an additional source, with the latter reporting alower proportion of severe intellectual disability.

1

Age-specific prevalence also varies, with maximum valuesreported in the school-age groups. The lower values inpreschool children result from underascertainment ofmilder cases, whereas in the post-school age group,the reduced prevalence has been attributed to twofactors: a higher mortality rate in the intellectually dis-abled population, and a lesser likelihood of identifyingpeople in the community with mild intellectual dis-ability than in the formal school environment.

3

The state of Western Australia is geographically iso-lated with nearly three-quarters (73.4%) of its popula-tion now residing in the metropolitan area.

4

There islow outward migration of 13% by the age of 14 years.A population-based study of intellectual disability inbirths from 1967 to 1976 in Western Australia found aprevalence of 7.6/1000

5

using multiple sources of ascer-tainment. As part of a report on disability in Western

Australia, the prevalence of intellectual disability inmore recent years using just administrative data wasfound to be 8.3/1000.

6,7

In order to investigate this morethoroughly, the current study was undertaken to defineand describe a cohort of children with intellectual dis-ability born in Western Australia between 1983 and1992. It was considered important to include cases iden-tified through educational facilities as well as admin-istrative sources in order to maximise ascertainment ofmilder intellectual disability, which may be moreamenable to both prevention and early intervention.

Methods

Children with intellectual disability born in WesternAustralia 1983–92 inclusive and known to be alive atthe end of 1999 formed the case group for this study.The primary source of cases was the Disability ServicesCommission, a multidisciplinary body, one of whosefunctions is to co-ordinate the medical and therapeuticsupport and accommodation needs of people withintellectual disability in Western Australia. Secondarysources were cases registered in 1999 with three agen-cies (one government and two non-government) pro-viding educational support for Western Australianchildren with intellectual disability (Department ofEducation, Catholic Education and Association ofIndependent Schools).

Intellectual disability in W. Australia 59

©



17,

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Record linkage with the Maternal and Child HealthResearch Database (MCHRDB) was used to obtainbasic demographic information not available from theother sources. The MCHRDB is a population-baseddata set of all Western Australian births from 1980,

8

which is also linked to death registrations. Using prob-ability matching based on full or part names, date ofbirth and gender,

9

linkage was attempted betweeneach data source and the MCHDRB for all live births1983–92, not known to have died before 2000. Manualchecking of data from the Disability Services Commis-sion paper files was undertaken for unlinked recordsto obtain further information, including the place ofbirth if recorded. This resulted in additional linkages.Manual checking was not possible for the secondarydata sources, as there was no provision for access toadditional data. The linkage allowed information to beobtained on mother’s race and place of residence at thetime of birth for each case. Other variables that are alsoavailable relating to the pregnancy and birth were notused in this analysis, but will form the basis of furtherstudy.

For the purposes of this study, cases identified fromthe Disability Services Commission were considered tohave intellectual disability if: (1) their IQ was

<

70 onformal testing; (2) they had a condition known to beassociated with intellectual disability (e.g. Down’s syn-drome); or (3) they were clearly documented as havingintellectual disability in their Disability Services Com-mission records. The Department of Education definedchildren as having intellectual disability ‘if they dem-onstrate significant deficits in adaptive behaviour andacademic achievement and demonstrate intellectualfunctioning two or more SD below the mean on anapproved measure of cognitive functioning’.

10

Thisdefinition was used to define cases obtained from thethree educational sources.

For cases identified from the Disability ServicesCommission, the level of severity of intellectual dis-ability was obtained from IQ testing where available,and otherwise was based on the level assigned by themedical officer. Cases are classified in line with DSMIV recommendations

11

(according to which psycholog-ical test was used) as ‘mild’ (IQ 50–55 to 69), ‘moder-ate’ (IQ 35–40 to 40–54) and ‘severe (includingprofound)’ (IQ

<

35 or 40). Similar severity levels wereused by the educational sources but, in most of thedata they provided, mild and moderate were collapsedinto one category. Hence, for ease of comparison ofdata from the different sources in this study, the sever-

ity of intellectual disability was categorised as mild/moderate, severe and unspecified (for those childrenfor whom no severity level was assigned).

Data were obtained from the MCHRDB on all West-ern Australian live births between 1983 and 1992 sur-viving to the end of 1999, by gender, race and maternalresidence at birth (fully available on 99.8%), and usedas the denominator. STATA

12

was used for statisticalanalysis to calculate prevalence per 1000, prevalenceratios (PR) and 95% confidence intervals [CI] using thePoisson distribution.

Results

A total of 1534 cases of intellectual disability in chil-dren born in Western Australia between 1983 and 1992were identified from the Disability Services Commis-sion and linked to the MCHRDB. A further 26 casesdid not link, but were known to have been born inWestern Australia, giving a total of 1560 cases. Fromthe Department of Education, the records of 3507 chil-dren with intellectual disability, born between 1983and 1992, were identified. Of these, 2614 linked to theMCHRDB, and 915 of these were also ascertained fromthe Disability Services Commission. Thus, there were1699 cases unique to the Department of Education, ofwhom 11.3% had been referred to the Disability Ser-vices Commission but did not meet the defined criteriafor case inclusion from this source. Of the 322 casesfrom Catholic Education and the 110 cases from theAssociation of Independent Schools, 230 and 81 cases,respectively, linked to the MCHRDB. A total of 108cases from Catholic Education and 36 from the Asso-ciation of Independent Schools were also identified ineither the Disability Services Commission or in othereducation data sets (or both), leaving 122 unique toCatholic Education and 45 unique to the Associationof Independent Schools. Among the cases ascertainedfrom the educational sources that did not link to theMCHRDB, it was not known what proportion wereborn in Western Australia.

The final data set of 3426 cases for this study thuscomprised 1560 identified from the primary datasource, the Disability Services Commission (including26 known to have been born in Western Australia butthat did not link to the MCHRDB), 1699 uniquely iden-tified from the Department of Education, 122 uniqueto Catholic Education and 45 unique to the Associationof Independent Schools. Thus, the prevalence of intel-lectual disability in 1999 for children born in Western

60

H. Leonard

et al.

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17,

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Australia 1983–92 was 14.3 per 1000 [CI 13.8, 14.7](Table 1). The prevalence of intellectual disability bybirth year increased by 3% per year on average [CI1.8%, 4.2%] from 11.2 per 1000 for 1983 to a peak of16.6 per 1000 for 1991, and fell slightly to 15.2 per 1000for 1992. The increase was greatest (6.8% [CI 3.7%,10.0%]) in children whose level was unspecified, was2.4% [CI 1.0%, 3.8%] in those with mild or moderateand 1.5% [CI

-

2.2%, 5.4%] in those with a severe intel-lectual disability. The increase was greater in males(4% [CI 2.6%, 5.7%]) than in females (1% [CI

-

1.2%,2.7%]). The prevalence for Western Australian-bornchildren ascertained from the Disability Services Com-mission was 6.5 per 1000 [CI 6.2, 6.8] and 7.8 [CI 7.4,8.1] per 1000 for cases uniquely ascertained from theeducation sources (Table 2). The rate of increase inprevalence by birth year was greater (5% [CI 3.4%,6.7%]) for children uniquely ascertained through edu-cational sources than for children ascertained throughthe Disability Services Commission (1% [CI 1.0%,2.5%]).

Overall, 10.6 per 1000 children had intellectual dis-ability of a mild or moderate level of severity, 1.4 per1000 were classified as severe and, for 2.3 per 1000, thelevel of severity was unspecified (Table 3). Of the 1560children ascertained from the Disability Services Com-mission, 70.1% were classified as mild or moderate,

15.1% as severe or profound and 14.9% as unspecified.Severity level was available from IQ testing for only535 of these children (34.3%), the remainder beingidentified from the category assigned by the medicalofficer attending the child. In order to compare theseverity level assigned by the medical officer withthat obtained on IQ assessment, a kappa statisticmeasure of 0.35 was obtained on the 381 caseswhere data from both sources were available. For the1866 children with intellectual disability ascertainedonly through the secondary sources, 77.6% were mildor moderate, 5.2% were severe, and 17.3% wereunspecified.

The prevalence of intellectual disability in males was17.4 [CI 16.7, 18.2] per 1000. Thus, boys were

>

1.5 timesmore likely to have an intellectual disability (preva-lence ratio (PR)

=

1.6 [CI 1.5, 1.7]) than girls. Theincreased risk was similar for mild/moderate(PR

=

1.6) and severe intellectual disability (PR

=

1.5)and for the different sources of ascertainment (Tables 2and 3). Maternal race was available for 99% of casesand categorised as Aboriginal in 398 (11.6%). The prev-alence of intellectual disability was 30.8 [CI 27.8, 33.9]per 1000 for all children of Aboriginal mothers(Tables 2 and 3). These children were over twice aslikely to have an intellectual disability as children ofCaucasian mothers (PR

=

2.3 [CI 2.1, 2.6]). Although

Table 1.

Denominator data, number of cases and prevalence of intellectual disability per 1000 by year of age, Western Australian-bornchildren 1983–92

Age

a

Year

b

All ID Mild and moderateSevere andprofound Unspecified

n

c

n

d

Prev

e

n

d

Prev

e

n

d

Prev

e

n

d

Prev

e

6 1992 381 24 988 15.2 [13.8, 16.9] 267 10.7 [9.4, 12.0] 32 1.3 [0.9, 1.8] 82 3.3 [2.6, 4.1]7 1991 408 24 648 16.6 [15.0, 18.2] 290 11.8 [10.4, 13.2] 41 1.7 [1.2, 2.2] 77 3.1 [2.5, 3.9]8 1990 377 25 626 14.7 [13.3, 16.3] 275 10.7 [9.5, 12.1] 38 1.5 [1.0, 2.0] 64 2.5 [1.9, 3.2]9 1989 354 25 118 14.1 [12.7, 15.6] 270 10.7 [9.5, 12.1] 32 1.3 [0.9, 1.8] 52 2.1 [1.5, 2.7]

10 1988 346 24 748 14.0 [12.5, 15.5] 252 10.2 [9.0, 11.5] 34 1.4 [1.0, 1.9] 60 2.4 [1.9, 3.1]11 1987 389 23 766 16.4 [14.8, 18.1] 306 12.9 [11.5, 14.4] 34 1.4 [1.0, 2.0] 49 2.1 [1.5, 2.7]12 1986 320 23 461 13.6 [12.2, 15.2] 242 10.3 [9.1, 11.7] 36 1.5 [1.1, 2.1] 42 1.8 [1.3, 2.4]13 1985 307 22 900 13.4 [11.9, 15.0] 244 10.7 [9.4, 12.1] 27 1.2 [0.8, 1.7] 36 1.6 [1.1, 2.2]14 1984 291 22 510 12.9 [11.5, 14.5] 225 10.0 [8.7, 11.4] 29 1.3 [0.9, 1.9] 37 1.6 [1.2, 2.3]15 1983 253 22 593 11.2 [9.9, 12.7] 169 7.5 [6.4, 8.7] 29 1.3 [0.9, 1.8] 55 2.4 [1.8, 3.2]

All ages All years 3426 240 358 14.3 [13.8, 14.7] 2540 10.6 [10.2, 11.0] 332 1.4 [1.2, 1.5] 554 2.3 [2.1, 2.5]

a

Age in years at end of 1998 school year.

b

Year of birth.

c

Number of cases.

d

Number of Western Australian live births in year group (survivors to end of 1999).

e

Prevalence per 1000 [95% CI].


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17,

58–67

the risk of intellectual disability was significantlyincreased for Aboriginal children ascertained fromboth the Disability Services Commission and the edu-cational sources, it was much greater from the latter(Table 2). Children of mothers of other racial back-grounds had a similar risk of intellectual disability tochildren of Caucasian mothers overall, and in casesascertained from both the Disability Services Commis-sion and educational sources. The increased risk for

Aboriginal children was greater for mild/moderate(PR

=

2.6) and unspecified (PR

=

3.5) intellectual dis-ability than for severe disability (PR

=

1.7) (Table 3).The prevalence of intellectual disability was no differ-ent in children whose mother was resident in a ruralarea at the time of the child’s birth (PR

=

1.0 [CI 0.9,1.1]) compared with a metropolitan residence. Thiswas little affected by either the source of ascertainmentor the severity of intellectual disability (Tables 2 and 3).

Table 2.

Prevalence of intellectual disability in Western Australia by gender, maternal race, place of residence at birth according to sourceof ascertainment

Total DSC Educational sources

n

a

Prev

b

PR

c

n

a

Prev

b

PR

c

n

a

Prev

b

PR

c

Female 1277 10.9 1.0 Reference 599 7.8 1.0 Reference 678 5.8 1.0 ReferenceMale 2149 17.4 1.6 [1.5, 1.7] 961 5.1 1.5 [1.4, 1.7] 1188 9.6 1.7 [1.5, 1.8]

Caucasian 2812 13.2 1.0 Reference 1322 6.2 1.0 Reference 1490 7.0 1.0 ReferenceAboriginal 398 30.8 2.3 [2.1, 2.6] 109 8.4 1.4 [1.1, 1.6] 289 22.3 3.2 [2.8, 3.6]Other 180 12.6 1.0 [0.8, 1.1] 99 6.9 1.1 [0.9, 1.4] 81 5.7 0.8 [0.6, 1.0]Missing 36 30 6

Metro 2280 14.1 1.0 Reference 1054 6.5 1.0 Reference 1226 7.6 1.0 ReferenceRural 1110 14.2 1.0 [0.9, 1.1] 476 6.1 0.9 [0.8, 1.0] 634 8.1 1.1 [1.0, 1.2]Missing 36 30 6

All 3426 14.3 [13.8, 14.7] 1560 6.5 [6.2, 6.8] 1866 7.8 [7.4, 8.1]

DSC, Disability Services Commission.

a

Number of cases.

b

Prevalence per 1000.

c

Prevalence ratio [95% CI].

Table 3.

Prevalence of intellectual disability in Western Australia by gender, maternal race, place of residence at birth according toseverity

Total Mild to moderate Severe Unspecified

n

a

Prev

b

PR

c

n

a

Prev

b

PR

c

n

a

Prev

b

PR

c

n

a

Prev

b

PR

c

Female 1277 10.9 1.0 Reference 949 8.1 1.0 Reference 131 1.1 1.0 Reference 197 1.7 1.0 ReferenceMale 2149 17.4 1.6 [1.5, 1.7] 1591 12.9 1.6 [1.5, 1.7] 201 1.6 1.5 [1.2, 1.8] 357 2.9 1.7 [1.4, 2.0]

Caucasian 2812 13.2 1.0 Reference 2073 9.7 1.0 Reference 280 1.3 1.0 Reference 459 2.1 1.0 ReferenceAboriginal 398 30.8 2.3 [2.1, 2.5] 324 25.0 2.6 [2.3, 2.9] 29 2.2 1.7 [1.2, 2.5] 45 3.5 1.6 [1.2, 2.2]Other 180 12.6 0.9 [0.8, 1.0] 123 8.6 0.9 [0.7, 1.1] 23 1.6 1.2 [0.8, 1.9] 34 2.4 1.1 [0.8, 1.6]Missing 36 20 0 16

Metro 2280 14.1 1.0 Reference 1655 10.2 1.0 Reference 237 1.5 1.0 Reference 388 2.4 1.0 ReferenceRural 1110 14.2 1.0 [0.9, 1.1] 865 11.1 1.1 [1.0, 1.2] 95 1.2 0.8 [0.6, 1.0] 150 1.9 0.8 [0.7, 1.0]Missing 36 20 0 16

All 3426 14.3 [13.8, 14.7] 2540 10.6 [10.2, 11.0] 332 1.4 [1.2, 1.5] 554 2.3 [2.1, 2.5]

a

Number of cases.

b

Prevalence per 1000.

c

Prevalence ratio [95% CI].

62

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et al.

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17,

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Discussion

Accurate estimates of the prevalence of intellectual dis-ability are vital for both planning purposes and serviceprovision. A major strength of this study is the use ofrecord linkage to maximise ascertainment of intellec-tual disability by combining information from severalexisting sources. Using data from the state agency thatprovides medical and other services to children withan intellectual disability and three agencies providingeducational services, the prevalence of intellectual dis-ability in children born in Western Australia in thedecade from 1983 to 1992 was 14.3 per 1000. Findingsfrom seven recent overseas population-based studiesand two from Western Australia (including the presentone) are summarised in Table 4. Similar rates of 12.0and 12.2/1000 were reported in two US studies.

3,13

Oneused both educational and medical administrativesources to ascertain intellectual disability in 10-year-old children in metropolitan Atlanta;

13

the other esti-mated the prevalence in US children aged 6–16 yearsusing the Disability Supplement of the NationalHealth Interview Survey.

3

A Swedish study

14

usingboth administrative and education sources found theprevalence of mild intellectual disability (IQ 50–72) tobe 12.8 per 1000 in 9- to 15-year olds. A lower preva-lence of intellectual disability of 5.2 per/1000 wasfound in Californian children between 4 and 12 yearswho were registered with a state agency providingdiagnostic and remedial services.

15

Two Scandinavianstudies,

16–18

using multiple sources, including recordlinkage in one, reported a similar prevalence to Croen

et al

.

15

(6.1–6.2 per 1000). According to the authors, theyoung age profile (

<

7 years) of the Finnish populationmight account for the lower than expected estimate.

17

The reasons for the variation in prevalence are multi-ple and include: true population variation, to whichsocio-economic factors have been shown to contrib-ute;

14,19–21

criteria for case inclusion; and the age rangeof cases.

3

It seems that studies that use multiplesources of ascertainment, especially those that includethe education system, report higher prevalence.

In the present study, 54% of children were ascer-tained only through the education system. Althoughthe Disability Services Commission and educationalcriteria for determining intellectual disability wouldappear to be similar, it is important to take into accountthe reason behind assessment when making inferencesabout epidemiological data.

22

The availability of extrafunding for intellectually disabled children and the

likelihood that early intervention may improve long-term outcome could influence the less restrictiveinterpretation of eligibility criteria in the educationsystem.

Record linkage provides a cost-effective method ofmaximising ascertainment of intellectual disability, butit restricts criteria for case inclusion and categorisationto those available from the source databases. Depen-dence on the source databases for case definition isthus an unavoidable weakness of studies using recordlinkage. In the current study, if case status was uncer-tain using information from the Disability ServicesCommission database, a manual review of files wasundertaken to validate criteria. Such a review was notpossible for children ascertained through the educa-tion systems and is often not possible in record linkagestudies.

15

Although 35.3% of the 2614 cases identifiedfrom the state education system were ascertained fromother sources, we were dependent on educationdepartment criteria for defining intellectual disabilityin the remaining 1699 children. Other than our findingthat 11.3% of these 1699 (or 7.3% of all children knownto the Department of Education) had been referred tothe Disability Services Commission but did not meetthe defined criteria for this study, we had no means ofvalidation. Changes in status of individual childrenover time or insufficient information available on theDisability Services Commission files to meet the crite-ria for case inclusion in the study could account forsome of the discrepancies.

About a quarter of the cases identified through thestate education department could not be linked tobirth data. This is much higher than the proportion(9%) of resident Western Australian school-aged chil-dren estimated to be born outside the state. It is likelythat some linkages were missed, possibly because ofname changes and typographical errors in names anddates of birth. Some state education cases actually bornin Western Australia but not able to be linked to theMCHRDB may be represented in the 547 childrenapparently unique to the Disability Services Commis-sion. The latter number is also higher than would beexpected because all children apart from those who aretaught at home, permanently hospitalised or havemoved out of state should be receiving educationalservices.

Currently, there are no universally accepted stan-dards for grouping cases of intellectual disabilityaccording to severity level. In their review of the prev-alence of intellectual disability, Roeleveld

et al

.

23


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17,

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Tab

le 4

.

Prev

alen

ce o

f in

telle

ctua

l dis

abili

ty in

a s

elec

tion

of

rece

nt p

opul

atio

n-ba

sed

stu

die

s Aut

hor

and

ref

. no.

Wel

lesl

ey

et a

l

. 5M

urph

y

et a

l

.13

Fern

ell 1

4Fe

rnel

l 24

Gis

sler

et a

l

.17

,18

Stro

mm

e an

dV

alva

tne

16C

roen

et a

l

.15

Lar

son

et a

l

.3

Leo

nard

et a

l

. T

his

pape

r

Year

of

publ

icat

ion

1992

1995

1996

1998

1998

, 199

919

9820

0120

01B

irth

yea

rs o

f st

udy

popu

lati

on19

67–7

619

85–8

719

79–8

519

79–9

219

8719

80–8

519

87–9

4N

/A

1983

–92

Cou

ntry

Aus

tral

ia,

WA

USA

, Atl

anta

Swed

en,

Stoc

khol

m(B

otky

rka)

Swed

enSt

ockh

olm

(Bot

kyrk

a)

Finl

and

Nor

way

,A

kerh

usco

unty

USA

,C

alif

orni

aU

SAA

ustr

alia

, WA

Size

of

stud

ypo

pula

tion

210

789

89 5

3463

9714

138

59 1

3830

037

4 59

0 33

311

6 00

0m

embe

rs o

f46

000

hou

seho

lds

240

358

Met

hod

of

case

Stat

e se

rvic

ere

gist

ry a

ndot

her

sour

ces

Scho

ol

reco

rds

and

othe

r so

urce

s

Ad

min

istr

ativ

e,ho

spit

al a

ndsc

hool

rec

ord

s

Ad

min

istr

ativ

ere

cord

sSc

hool

, chi

ldsu

ppor

t an

dre

gion

alin

telle

ctua

ld

isab

ility

regi

ster

s (r

ecor

dlin

kage

)

Mul

tipl

em

edic

alan

d e

duc

atio

nal

sour

ces

Stat

ese

rvic

ere

gist

ry

Surv

ey o

fas

cert

ainm

ent

popu

lati

on(n

on-

inst

itut

iona

lised

)

Ad

min

istr

ativ

ean

d e

duc

atio

nre

cord

s(r

ecor

d li

nkag

e)

Age

ran

ge o

f st

udy

popu

lati

on (

year

s)6–

1610

9–15

3–16

78–

134–

126–

176–

15(i

nclu

sive

)Se

vere

ID

per

1000

live

bir

ths

1.6

(IQ

<

35)

3.6

(IQ

<

50)

1.5

(IQ

<

35)

4.5

(IQ

<

50–

55)

0.68

1.2

(IQ

< 3

5)1.

1

a

(IQ

< 5

0)1.

4(I

Q <

40)

Mild

and

mod

erat

e ID

per

100

0 liv

ebi

rths

5.4

(IQ

35–

69)

8.4

(IQ

50–

70)

12.8

(IQ

50–

72)

2.64

5.0

(IQ

35–

70)

2.9

a

(IQ

50–

70)

10.6

(IQ

40–

69)

Lev

el I

D u

nspe

cifi

edpe

r 10

00 li

ve b

irth

s0.

82.

81.

2

a

2.3

Tota

l ID

per

1000

live

bir

ths

7.6

(IQ

< 7

0)12

.0 (

IQ <

71)

6.1

(IQ

<

70)

6.2

(IQ

< 7

1)5.

2

b

(IQ

< 7

1)12

.2(o

pera

tion

ald

efini

tion

)

14.3

(IQ

<

70)

Mal

e to

fem

ale

rati

o1.

5 (I

Q <

70)

1.5

(IQ

<

35)

1.6

(IQ

<

71)

1.3

(IQ

50–

72)

1.7

(IQ

< 5

0–55

)1.

431.

3 (I

Q

<

71)

1.9

c

ad

just

ed(I

Q

<

71)

1.6

(IQ

<

70)

Rac

e ra

tio

1.5

1.7

adju

sted

1.5

c

ad

just

ed2.

3A

bori

gina

l to

Cau

casi

an(I

Q

<

71)

Afr

ican

–A

mer

ican

to C

auca

sian

(IQ

<

71)

Afr

ican

–A

mer

ican

to C

auca

sian

(IQ

<

70)

Abo

rigi

nal t

oC

auca

sian

a

Est

imat

ed f

rom

dis

trib

utio

n of

sev

erit

y in

Tab

le 1

(C

roen

et a

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© Blackwell Publishing Ltd. Paediatric and Perinatal Epidemiology 2003, 17, 58–67

dichotomised severity levels to mild (IQ 50–70) andsevere (IQ < 50), and reported a relatively stable prev-alence of severe intellectual disability with a meanvalue of 3.8/1000. Studies using the same method ofcategorisation and published since this review15,24

(Table 4) reported the prevalence of severe intellectualdisability in the range 0.68–4.5 per 1000 live births,reflecting varying methods of case ascertainment andage ranges. Two studies, also summarised in Table 4,reported the prevalence of those with an IQ < 35 at1.513 and 1.2 per 1000.16 Data sources used in our studyclassified severe intellectual disability as IQ < 35–40,and the prevalence of children in this group was 1.4/1000 live births. Standardised categories would facili-tate comparisons between populations, especially insituations, such as the present study, where analysiswas restricted to the categories available in the originaldata source. Because of their need for services otherthan educational, it is not surprising that the propor-tion of children with a severe level of intellectual dis-ability was higher among those receiving services fromthe Disability Services Commission (15.5%) than thoseascertained only through the education system (5.2%).This difference highlights the advantage of using mul-tiple sources of ascertainment to include the wholespectrum of intellectual disability.

The excess of males in this population present incases ascertained from both medical and educationalsources was consistent with a large number of previ-ous reports (e.g. Table 1). The inclusion of comprehen-sive information on race was made possible in our dataset by the use of record linkage to obtain the maternalrace recorded at the birth of each child. Whereas theprevalence of intellectual disability was similar in chil-dren of Caucasian and other racial backgrounds, theprevalence in Aboriginal children was 2.3 [CI 2.1, 2.6]times those of Caucasian origin. The difference wasgreatest in the children with mild/moderate disability,rate ratio 2.6 [CI 2.3, 2.9], and those only ascertainedfrom education sources, rate ratio 3.2 [CI 2.8, 3.6]. Sev-eral US studies have reported a higher prevalence ofintellectual disability in African–American childrenthan in children of Caucasian origin.15,25 This was trueof all intellectual disability25 and intellectual disabilityof unknown cause15 after adjustment for confounderssuch as sex, maternal age, birth order, maternal educa-tion and socio-economic status. Moreover, in WesternAustralia, other poor perinatal and childhood out-comes are also more common in Aboriginal than innon-Aboriginal children.26,27

Social and environmental factors such as socio-economic status, maternal education and housing maybe contributing to the ethnic differences that we iden-tified in our study. However, some of the other prena-tal and postnatal factors postulated to be related to theincreased risk of intellectual disability in African–American children15,25 also occur in the AustralianAboriginal population. These include an increasedprevalence of maternal diabetes, hypertension andchronic renal disease28 and fetal alcohol syndrome.29

Speculated postnatal child factors included elevatedlead levels and anaemia,15,25 with the latter found to bepresent in 39% of children in remote Aboriginal com-munities in Australia’s Northern Territory.30 A recentstudy using record linkage31 found that, for each dec-rement in haemoglobin, the risk of mild to moderateintellectual disability increased by 1.28-fold after con-trolling for birthweight, maternal education, sex,maternal age, race and child’s entry age into the pro-gramme. Further aetiological pathways for the racialdifferences we have found may involve intergenera-tional factors, as has also been postulated in the Afri-can–American population.32 On the other hand, it isalso important to be aware that children from minoritycultures may be more likely to be labelled intellectuallydisabled because of cultural differences includingsocially different behaviour and inappropriatetests.25,33,34 The work of Kearins34 has provided strate-gies that will help psychologists to ensure that theirassessment of Aboriginal children does reflect theunderlying true cognitive ability.

The methods used in this study are different fromthose used in a previous study of this population.5

Although the Disability Services Commission was theprimary source of ascertainment, the proportion ofcases collected from this source varied markedly fromthe present study because of differing methods of caseidentification. The study by Wellesley et al.5 reportedthe prevalence of intellectual disability in an earliercohort of the Western Australian population, bornfrom 1967 to 1976, as 7.6/1000 live births. The Disabil-ity Services Commission provided 80% of cases for thisstudy, with a further 10% unique to the education sys-tem. This was in contrast to the current study, in which54% of cases were provided by the education system,although the prevalence in cases ascertained throughthe Disability Services Commission was similar in thetwo studies (6.1/1000 for 1967–76 and 6.5/1000 for1983–92 births). The gender and race ratios were alsosimilar. The rural excess found in the earlier study was



not replicated in the current study with present resultsmore consistent with previous literature.35 A precursorto the present study, which used only administrativedata from the Disability Services Commission,6 foundthe prevalence of intellectual disability to be 8.3/1000live births in the cohort born from 1980 to 1990. How-ever, this study had only limited information on placeof birth (and thus was unable to differentiate childrenborn in and outside Western Australia), and no manualchecks were conducted to validate criteria for caseinclusion.

In the present study, the prevalence of intellectualdisability was greater in younger children. This is incontrast to the usual pattern, where prevalence usuallypeaks in the 10–14- or 15–19-year-old age group.35 Inour study, the increase in younger children was moremarked in boys and in children ascertained onlythrough the education system. There are a numberof factors that may have contributed to these findings.We only have cross-sectional data from one year(1999) from the educational system. If the interpreta-tion of eligibility criteria by the education system hasbecome more inclusive over time, this could have con-tributed to the increase in prevalence that we see inyounger children. Such a pattern may also have beeninfluenced by improving community acceptance overtime of people with disability of all types and hence areduced reluctance to label children as intellectuallydisabled.

The risk of autism is about four times higher inboys than in girls, and intellectual functioning is nor-mal only in 20% of cases.36 Although there is no evi-dence for a secular trend in the incidence of autismworldwide, prevalence has increased because ofchanging diagnostic criteria and diagnosis at an earlierage.36,37 For the same reasons and because of greatercommunity awareness, there may have been morecomprehensive identification of children with bothautism and intellectual disability by providers ofeducational services in recent years in WesternAustralia.

Ascertainment of children known to the educationsystems during only one year means that we wereunable to quantify other factors relating to changesin prevalence over time. First, we could not takeaccount of the increasing cumulative identificationof intellectual disability, especially the milder forms,during the school years. Secondly, although retentionof children within the education system is thoughtto be high for those with an intellectual disability, it

was not possible for us to monitor children whomay have left school during their teenage years. It isclear that there are many parallels between the meth-odological difficulties in measuring secular trends inthe prevalence of intellectual disability and those dis-cussed by Fombonne36 in relation to the incidence ofautism. Our data limitations do not allow us to providetemporal trends and, unfortunately, providing thisimportant information has been a problem univer-sally.38 There is an urgent need for more complexmonitoring and surveillance systems in which casedefinition and ascertainment methods, particularlybetween health and education systems, are keptconstant or at least comparable for all neurodevelop-mental disorders including intellectual disability. Iden-tical case definitions and the use of a single identifierin the health and education systems would greatlyfacilitate this.

Provision of support services for people with intel-lectual disability places a heavy burden on familiesand community health and education systems. Thisstudy, based on multiple record-linked sources for caseascertainment and a large and relatively stable popu-lation, has provided a precise estimate of the preva-lence of intellectual disability. It will form the basis forfurther investigation of intellectual disability in West-ern Australia, which may lead to the identificationof preventive strategies and opportunities for earlyintervention.

Acknowledgements

We are grateful to the Disability Services Commission,Telethon Institute for Child Health Research, Depart-ment of Education, Catholic Education Office, Associ-ation of Independent Schools Western Australia andthe Birth Defects Registry for assistance with data col-lection and other aspects of the study including organ-isational support. We would also particularly like tothank Kate Rowell, Emma Glasson, Elvira Edwards,Jane Pavledis, Mairead McCoy, Tessa Vincent, ChrisGostello, Maureen Thomson, Audrey Jackson, PeterCosgrove and Huan Ngyuen. We would also like toacknowledge the statistical advice provided by Asso-ciate Professor Nick de Klerk. This work was fundedpartly by a grant from the Apex Foundation forResearch into Intellectual Disability and by theNational Health and Medical Research Council (pro-gram grant 003209).

66 H. Leonard et al.


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