In 1998, The Centers for Disease Control and Prevention (CDC) started tracking the prevalence rates of Autism (now called Autism Spectrum Disorders or ASD) along with the characteristics of those children diagnosed with with ASD in the United States. The first study conducted in Brick Township, New Jersey demonstrated a higher prevalence of ASD but similar characteristics compared with other studies. The second CDC study, which was conducted in Atlanta, Georgia showed similar estimates to other studies but a lower prevalence of ASD compared with the Brick Township study.
Hypothesizing that ASD rates could be different in different areas of the country, CDC established the Autism and Developmental Disabilities Monitoring (ADDM) Network to collect data in different states. The network was created in order to provide better estimates of the prevalence of ASD and other developmental disabilities in the United States. Currently, data is collected at 11 sites which include Arizona, Arkansas, Colorado, Georgia, Maryland, Minnesota, Missouri, New Jersey, North Carolina, Tennessee, and Wisconsin. All children included in the study are 8 years old since this represents the peak of when children with ASD are identified.
Tracking the prevalence of ASD poses unique problems due to the heterogeneity of symptoms, the lack of biologic diagnostic markers, and changing diagnostic criteria. Although symptoms may appear in early childhood, the social problems characteristic of the disorder may not be noticed until the child is old enough to display significant difficulties in normal life stage demands. Also, signs of ASD often overlap with those of other psychiatric disorders, making them difficult to distinguish.
CDC estimates of ASD prevalence among children aged 8 years in 11 U.S. communities have increased from approximately one in 150 children during 2000–2002 to one in 68 during 2010–2012. However, prevalence estimates varied widely based on the community, from a low of 13.1 in Arkansas to an high of 29.3 in New Jersey.
With rates more than doubling overall during this time, the need for continued tracking using reliable and consistent methods to evaluate the changing prevalence of ASD and the associated characteristics is clear. It will also be important to determine the reason significantly different rates have been observed in different communities.
CDC has also used ADDM data to determine what characteristics are associated with ASD in the U.S. and to describe how these features vary over time and among communities. Estimates have consistently estimated that there is a 4.5 to 1 male to female ratio of ASD during 2006–2012. Other characteristics that have remained relatively stable over time include the median age of earliest ASD diagnosis, which remained close to 53 months during 2000–2012 and the proportion of children receiving a comprehensive developmental evaluation by age 3 years, which remained close to 43% during 2006–2012.
In contrast, the ASD prevalence rate according to race/ethnicity has varied more over time in the 8 year olds sampled in the 11 communities. Although traditionally ASD prevalence estimates have been greater among white children compared with black or Hispanic children, the most recent CDC study reported that differences in prevalence rates between white and black children and white and Hispanic children have declined. This has been attributed to the greater increases in ASD prevalence rates among black and Hispanic children compared with the increase observed in white children. Previous reports from the ADDM Network estimated ASD prevalence among white children to exceed that among black children by approximately 30% in 2002, 2006, and 2010, and by approximately 20% in 2008 and 2012.
CDC estimates of ASD prevalence have also differed by socioeconomic status (SES). Consistently, there has been a higher estimate of the prevalence of ASDA identified for children coming from higher SES backgrounds. The prevalence of ASD has increased over time for all SES levels.
Finally, CDC estimates of ASD have shown greater increases in prevalence of these disorders in children with higher intellectual ability. The proportion of children with ASD whose intelligence quotient (IQ) scores fell within the range of scores which are defined as indicating intellectual disability has gradually decreased over time. During 2000–2002, about half of children in the sample with ASD had IQ scores in the range of intellectual disability, while during 2006–2008, this proportion was around 40 percent and during 2010–2012, the rate had decreased further to less than one third of the children with ASD scoring in the intellectual disability range. This decrease was more pronounced for girls as compared with boys. The proportion of males with ASD and intellectual disability decreased from about 40 percent during 2000–2008 to 30 percent during 2010–2012. The proportion of females with ASD and intellectual disability decreased from approximately 60% during 2000–2002 to 35% during 2010–2012.
This report provides the latest available ASD prevalence estimates from CDC based on the ADDM Network data underscoring the need for continued tracking of ASD prevalence rates and for improving early diagnosis of ASD. These data can be used to help plan services, guide research into risk factors and effective interventions, and inform policies that promote improved outcomes in health and education settings. It is important to remember however, that these statistics are based on data from a relatively small sample, consisting exclusively of 8 year olds in only 11 communities in the country. Future research should expand this population to include children of all ages in more communities to improve generalizability and appropriate methods of identification and treatment.
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