IJE Advance Access originally published online on September 7, 2009
International Journal of Epidemiology 2009 38(5):1245-1254; doi:10.1093/ije/dyp260
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Autism spectrum disorders in young children: effect of changes in diagnostic practices
Telethon Institute for Child Health Research, Centre for Child Health Research, The University of Western Australia, West Perth, WA, Australia.
* Corresponding author. Dr Helen Leonard, Telethon Institute for Child Health Research, Centre for Child Health Research, The University of Western Australia, West Perth, WA, Australia. E-mail: hleonard{at}ichr.uwa.edu.au
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Abstract |
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Background It is unclear whether the increase in autism over the past two decades is a real increase or due to changes in diagnosis and ascertainment of autism spectrum disorders (ASDs), which include autism, Asperger syndrome and pervasive developmental disorder not otherwise specified (PDD-NOS). The aim of this study was to examine the trends in ASD over time in Western Australia (WA) and the possible effects and contribution of changes in diagnostic criteria, age at diagnosis, eligibility for service provision based on ASD diagnoses and changes in diagnostic practices.
Methods A population-based study was conducted among the cohort of children born in WA between 1983 and 1999 and diagnosed with ASD between the age of 2 and 8 years up to December 31, 2004. The trend in ASD diagnosis over the study period was assessed by investigating birth cohort and period effects, and examining whether these were modified by age of diagnosis. ASD diagnosis corresponding with changes in diagnostic criteria, funding and service provision over time were also investigated. A subgroup analysis of children aged 
5 years was also conducted to examine trends in the incidence and age of diagnosis of ASD and intellectual disability (ID) and to investigate the role of changes in diagnostic practices.
Results The overall prevalence of ASD among children born between 1983 and 1999 and diagnosed by age 8 was 30 per 10 000 births with the prevalence of autism comprising 21 per 10 000 births. The prevalence of ASD increased by 11.9% per annum, from 8 cases per 10 000 births in 1983 to 46 cases per 10 000 births in 1999. The annual incidence of ASD, based on newly diagnosed ASD cases in each year from 1985 to 2002, increased over the study period. The increase in incidence of ASD appeared to coincide with changes in diagnostic criteria and availability of funding and services in WA, particularly for children aged <5 years. The age-specific rates of autism and PDD-NOS increased over time and the median age of diagnosis for autism decreased from 4 to 3 years of age throughout the 1990s. For children aged 5 years the incidence of ASD diagnosis increased significantly from 1992, with an average annual increase of 22%. Similar findings were found for autism. In the corresponding years the incidence of diagnosis of severe ID fell by 10% per annum and mild–moderate ID increased by 3% per annum.
Conclusions The rise in incidence of all types of ASDs by year of diagnosis appears to be related to changes in diagnostic and service provision practices in WA. In children aged 5 years, diagnosis of severe ID decreased, but mild–moderate ID increased during the study period. A true increase in ASD cannot be ruled out.
Keywords Autism, autism spectrum disorders, intellectual disability, prevalence
Accepted 24 June 2009
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