Despite the heavy demands it places on the Australiancommunity and the health and educational systems, intellectualdisability has been a much neglected area of public health inthis country. The Australian Bureau of Statistics estimated thatalmost 1% of the Australian population had an intellectualdisability needing assistance in self-care, mobility and verbalcommunication.1 Wellesley et al.2 estimated a prevalence in 6 to 16-year-old children of 7.6 per 1000 live births in theWestern Australian (WA) cohort born between 1967 and 1976and, in a recent publication on disability in WA,3 a prevalenceof intellectual disability of 8.3 per 1000 live births in 1980–90in WA was reported using administrative data from the Dis-ability Services Commission (DSC). There are no other recentpopulation-based studies of intellectual disability in Australia,and no publications examining trends in intellectual disabilityover time in Australia.4

The causes for a sizeable proportion of intellectual disabilityare unknown and vary considerably according to study defini-tion and classification of intellectual disability.5,6 Based on theDSC data in WA for births 1980–89, half the registered caseswere of unknown cause.3

Genetic and chromosomal conditions contribute the largestproportion of known causes, although most of the 500 or so

known genetic causes are individually rare.7 Fragile X, consid-ered to be the commonest inherited cause of intellectualdisability,8 was found to account for about 0.5% of intellectualdisability in males in WA,3 while other genetic conditionssuch as phenylketonuria, Rett, Prader Willi and Angelmansyndromes,3,9 each have a prevalence of less than one in 10 000births in WA.

The exception is Down syndrome, the single most commonknown cause of intellectual disability, accounting for approx-imately 14–15% of all cases in WA.3,10 Over the past 15 years,there has been a major shift in the maternal age distribution.Women 35 years of age and over, who are at increased risk ofhaving a baby with Down syndrome, contributed only 4% of allbirths in 1980, but 13.4% in 1996.11 This has led to a rise inDown syndrome pregnancies, along with an increase in term-inations of Down syndrome-affected pregnancies (Fig. 1).However, it has had little impact on the liveborn prevalence ofDown syndrome, as most infants (over 60%) with Downsyndrome are born to women under 35 years of age. This maychange with the recently introduced maternal serum screeningprogram, which identifies women of all ages at risk of havingan infant with Down syndrome, and has been found to have asensitivity on average of about 64% in a series of demonstra-tion projects.12 The increased uptake of this screening test(47% of eligible women in WA participated in 199513) maylead to a fall in births with Down syndrome and hence have a major impact on the overall prevalence of intellectual dis-ability. However, this will be balanced to some extent by the improved survival of children with Down syndrome. For the1980–85 cohort of liveborn children with Down syndrome bornin WA, survival to 5 years of age was 80%; while for the1991–96 cohort, 93% survived to 5 years of age.14

Prenatal and postnatal infections with agents such as CMV,toxoplasma and herpes, were found to account for about 3% ofintellectual disability in WA.3 Before the introduction of vacci-nation for Haemophilus influenzae type b (Hib) infection in

J. Paediatr. Child Health (2000) 36, 213–215

Intellectual disability in Western Australia

C BOWER,1 H LEONARD1 and B PETTERSON2

1Division of Epidemiology, TVW Telethon Institute for Child Health Research, West Perth,2Disability Services Commission, West Perth, Western Australia, Australia

Objective: To investigate the prevalence of intellectual disability in Western Australia (WA), its causes, prevention, andtrends over time.Methodology: Data from an administrative database of intellectual disability in WA were used to report on the trends inintellectual disability in childhood.Results: The prevalence of intellectual disability was 8.3 per 1000 live births in 1980–90. For half the cases, there was noknown cause for the intellectual disability. Down syndrome accounted for 14 to 15% of all cases. Since the introduction ofnewborn screening, no WA-born child participating in the screening program has been diagnosed with intellectual disabilityas a result of either phenylketonuria or congenital hypothyroidism. The rate of autism spectrum disorders rose from three tosix per 10 000 in the 1980–83 WA birth cohort to 10–13 per 10 000 for the 1989–92 cohort.Conclusions: Recent linkage of this administrative database to the WA Maternal and Child Health Research Data Baseprovides a unique opportunity for more detailed investigation of intellectual disability and its risk factors in a large, well-ascertained population of children.

Key words: autism; Down syndrome; intellectual disability; record linkage; time trends.

Correspondence: Dr Carol Bower, TVW Telethon Institute for ChildHealth Research, PO Box 855, West Perth, Western Australia 6872,Australia. Fax: (08) 9489 7700; email: carolb@ichr.uwa.edu.au

C Bower, MBBS, MSc, PhD, Head, Division of Epidemiology. H Leonard, MB ChB, MPH, Research Officer. B Petterson, BSc, PhD,Research Officer.

This paper is dedicated to the memory of our colleague and friend,the late Dr Louisa Alessandri. Louisa was the principal author anddriving force of the overview of disability in Western Australia,Disability Counts, which has been the stimulus to undertake thisresearch into intellectual disability in Western Australia.

Accepted for publication 21 October 1999.

1993, 8% of surviving children with Hib meningitis in WAsuffered developmental delay.15 Given the effectiveness of thevaccine16,17 to prevent infection, Hib meningitis should nowmake a negligible contribution to intellectual disability. Insimilar fashion, congenital rubella syndrome, once a relativelycommon condition, and a cause of intellectual disability, hasnow become extremely rare following the introduction of theschoolgirl rubella vaccination program in Australia in 1971,and infant vaccination in 1990.18–20

Congenital hypothyroidism and phenylketonuria are alsovery rare causes of intellectual disability, now that screeningprograms identify these conditions in the newborn period,allowing for early intervention and prevention of subsequentintellectual disability. Since the newborn screening programwas instituted (in 1960 for phenylketonuria, and 1980 forcongenital hypothyroidism), no WA-born child participating inthe program has been diagnosed with intellectual disability as aresult of either of these two conditions.3 Vigilance however, isstill required, as women with treated phenylketonuria are nowreaching reproductive age and, if they do not have strict dietarycontrol during pregnancy, are at risk of having infants withdevelopmental delay.21

Preventive interventions such as newborn screening andearly treatment, prenatal diagnosis and termination of affectedpregnancies, and vaccination, the use of maternal immuno-therapy to prevent rhesus incompatibility, and increased peri-conceptional folate intake for the prevention of spina bifida(25% of WA children with spina bifida had intellectualdisability22), are all important individual strategies. However,they relate to conditions which are relatively uncommon, andhence are unlikely to have a major impact on the overall preva-lence of intellectual disability.

In contrast, Western Australia, like many other regions of the world, has seen a marked increase in the rate of diagnosisof autism and related conditions since the mid-1980,23,24 and ahigh proportion (approximately 70%) of children with theseconditions have intellectual disability.25 Three to six childrenper 10 000 born in WA between 1980 and 1983, were diagnosed

with autism spectrum disorders, and the rate rose to 10–13 per10 000 for the 1989–92 birth cohort. Analysis showed an asso-ciation of the timing of increases in prevalence with changes indiagnostic criteria. This, together with the marked reduction inthe age at diagnosis over time, suggests that the rising preva-lence may be primarily due to the method of diagnosis and theincreasing awareness of the condition among health profes-sionals and the general community (G Chaney, pers. comm.,1999). Although there is general agreement that genetic factorsplay a part in the aetiology of autism,26 the precise nature of thegenetic and environmental contributions remains an enigma. Itis hoped that the recent establishment of the WA Registry ofAutism Spectrum Disorders will lead to a better understandingof this puzzling condition.

Although the causes of most cases of intellectual disabilityare unknown, risk factors or characteristics of the mother,pregnancy and infant have been documented in the ‘unknowncause’ category. These include maternal exposures such assmoking27 and alcohol ingestion during pregnancy,28 pretermbirth,29 low birthweight30,31 and multiple births.31 A majorpredictor of intellectual disability is thought to be of socio-economic disadvantage, and it has also been suggested thatinfants already at biological risk of intellectual disability aremore susceptible to adverse social and environmental factors.30

Family clustering of intellectual disability has been found inabout one-quarter of cases of mild mental retardation ofunknown aetiology in two European studies.32,33

For many of these potential risk factors, there have beenmarked changes in prevalence in recent years, which may alsohave an effect on trends in intellectual disability. In the lastdecade in WA there has been an increase in multiple births,perhaps related to the increase in maternal age,12 as well as theincrease in pregnancies resulting from assisted reproduction.34

There has also been an increase in the survival of infants bornpreterm and of low birthweight.35

The only reported research to examine the contributions ofthese different factors (taken together, and over time) to theaetiology of intellectual disability, related to births in Finland1967–81.36 However, as well as being based on data which arenow quite old, the prevalence of intellectual disability in thisstudy was low (1.8 per 1000) and the study was small (339children with intellectual disability).

In WA, we are in a unique position to investigate intellectualdisability and its risk factors in a recent, well-ascertained, andlarge population of children, using the Maternal and ChildHealth Research Data Base (MCHRDB). This database providesdetails of the pregnancies and births relating to every infantborn in WA since 1980,37 and is linked to the Birth DefectsRegistry, the Cerebral Palsy Register, to hospitalisations anddeaths and to pooled census data on socio-demographic indi-cators based on geographic indices. All births to each womanin the database are linked to create sibships. The MCHRDB is also now linked to the Western Australian IntellectualDisability Database2,3 which is a comprehensive database ofmedical and demographic information on children and adultswith intellectual disability. There are few comprehensivepopulation-based datasets on intellectual disability anywherein the world and, to our knowledge, none linked to a resourceas valuable or extensive as the MCHRDB. This linked databasewill allow investigation of the contribution of factors such asmaternal age, multiple births, birthweight, gestational age, intra-uterine growth, mode of delivery, race, socio-economic status,familial clustering of intellectual disability, and geographical

214 C Bower et al.

Fig. 1 Trends in Down syndrome pregnancies and survivors, WesternAustralia 1981–91. (r), Number of Down syndrome pregnancies per1000 births; (j), number of children with Down syndrome per 1000 births; (m), number of pregnancies terminated because of Downsyndrome per 1000 births.

location to the aetiology and prevalence of intellectual dis-ability, and trends over time. It will also provide the opportunityto investigate and quantify the role of post-natal events such asinfections, road traffic accidents and non-accidental injury inthe aetiology of intellectual disability.

This investigation should lead to a better understanding ofthe causes of intellectual disability, and assist in determiningwhat proportion may be due to factors amenable to healthpromotion interventions, and hence may be preventable. Ananalysis of trends in intellectual disability and risk factors overtime will assist in predicting the burden of intellectualdisability in the future.

ACKNOWLEDGEMENTS

We are grateful to Lynda Blum, the staff of the DisabilityServices Commission and the Birth Defects Registry for assis-tance with data; and to the other members of the SteeringCommittee for Disability Counts (H Brown, L Alessandri,J Barratt, E Blair, L Blum, P Montgomery, C Rook, P Ruthven,R Shean, C Spencer, F Stanley, S Zubrick). Part of this workwas funded by a grant from the Apex Foundation for Researchinto Intellectual Disability.

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