This section addresses Subsection (3) of 399DD: "Information on the incidence of autism spectrum disorder (ASD) and trend data on such incidence since the date of enactment of the CAA of 2006." Information on the prevalence of ASD is provided by the CDC.
Centers for Disease Control and Prevention (CDC)
The CDC surveillance efforts reported in this chapter provide current ASD prevalence information rather than incidence. Incidence measures are typically more limited and indicate the number of newly diagnosed people with a disorder during a specific time period. In comparison, prevalence data indicate the total number of people affected at any given time in the population studied. Given the variability in the age of diagnosis of ASD and that not all children identified by the CDC surveillance efforts have a documented diagnosis, capturing prevalence, rather than incidence, is a more complete method for estimating the impact of ASDs in the population. CDC has been tracking the prevalence of developmental disabilities since the early 1980s and of ASD since 1996. To maintain the ability to track trends over time and to ensure the most complete count of the disorder among the population, CDC continues to report prevalence rather than incidence of the condition.
CDC's autism surveillance provides important information on the numbers of children affected with ASD and helps describe the characteristics of the population. Surveillance tracks trends over time, and is an essential building block for population-based research—providing clues about potential risk factors that warrant further study. Surveillance also provides critically important data for communities to use when planning for services. Ultimately, surveillance helps evaluate the progress of prevention and intervention programs.
In 2007, CDC's Autism and Developmental Disabilities Monitoring (ADDM) Network published its first report of ASD prevalence. In this study, six ADDM sites (Arizona, Georgia, Maryland, New Jersey, South Carolina and West Virginia) reported the prevalence for children who were 8 years old in 2000 (born in 1992).38 An additional eight sites (Alabama, Arkansas, Colorado, Missouri, North Carolina, Pennsylvania, Utah, and Wisconsin) participated in the second study year—determining the prevalence for children who were 8 years old in 2002 (born in 1994). The reported prevalence estimates ranged from 1 in 300 to 1 in 100, with an average ASD prevalence of 1 in 150 children.39 In December 2009, CDC and the ADDM Network published an updated report of ASD prevalence estimates. ASD prevalence in 2006 was approximately 1 in 110 8-year-old children living in the 11 communities participating in the 2006 surveillance year.40 This report also indicated that there were 4.5 times as many boys with ASD as girls. Among children aged 8 years, ASD prevalence was 14.5 per 1,000 males compared with 3.2 per 1,000 females. ADDM also reported that about 40 percent of the children with ASD also had an intellectual disability.
In March 2012, CDC and the ADDM Network published a third report of ASD prevalence estimates. ASD prevalence in 2008 was approximately 1 in 88 among 8-year-old children living in the 14 communities that participated in the 2008 surveillance year.41 This report also indicated that more children were diagnosed at earlier ages—a growing number of them by 3 years of age, even though most children were not diagnosed until after age 4, and that ASD prevalence among boys was almost five times as high as it was for girls (1 in 54 compared to 1 in 252).
The prevalence of ASD over time (2002-2008 study years)
Data from the 13 ADDM sites with results from both 2002 and 2008 surveillance years indicated a significant increase (78 percent) in identified ASD prevalence over this 6 year period. Combining data from these 13 sites, ASD prevalence increased across all gender, racial/ethnic, and cognitive functioning subgroups. For the 2008 surveillance year, a wider range of site-specific ASD prevalence estimates were identified than in previous ADDM Network surveillance years.41 Variation in estimated ASD prevalence was associated primarily with sites having access to education records and also appeared to be associated with improved identification among children without intellectual disability. No single factor explains the changes identified in ASD prevalence over time and much needs to be done to understand the relative contribution of the multiple factors involved, although some of these increases can be accounted for by improved identification and awareness. Understanding the relative contribution of multiple factors is important and efforts to further understand these contributions are underway. Whether identified ASD prevalence estimates will plateau, decrease, or continue to increase is unknown and continued examination of ASD prevalence changes is important.
Disparities in identification
Access to appropriate educational and health services improves a child's chance of being identified by the ADDM Network surveillance system, and disparities may be reflected in the observed prevalence among some groups. Since the earliest ADDM Network report on ASD prevalence, some but not all sites have identified significantly higher ASD prevalence among White children compared to Black or Hispanic children. One report on ADDM data published in 2010 suggests that these disparities in identified prevalence may be associated with socioeconomic status.42 Differences in heritable factors as well as exposure to various risk and protective factors may also influence differences in identified prevalence among certain groups, so ongoing monitoring is needed to better understand these factors.
Looking towards the future for ASD surveillance
CDC has been working to develop methods to track the population characteristics of ASD in younger children. In 2010, CDC funded six sites to expand ADDM methodology to younger cohorts. Through Early ADDM surveillance of ASD among 4-year-old children, CDC can better understand the population characteristics of young children affected by ASD and better inform early identification efforts. To understand ASD prevalence, it is important to identify children who have the diagnosis and children who have the symptoms (but do not have a diagnosis). The ADDM Network uses a retrospective record review methodology and casts a wide net to ensure the most accurate estimate of prevalence. Expanded surveillance of 4-year-olds can provide a more complete picture of the impact of ASD; however, it is particularly important to note that because methods are dependent on record review, prevalence estimates are impacted by the age at which children receive services. Continued monitoring of ASD among 8-year-olds, which we know is "peak prevalence," or the age at which most children have been identified, is essential to understanding the complex changes occurring in ASD over time.
Changes in diagnostic criteria
In May 2013 the American Psychiatric Association (APA) released an updated edition of the Diagnostic and Statistical Manual of Mental Disorders (DSM). The new edition, the DSM-5, recommends a single diagnosis of "Autism Spectrum Disorder" as a replacement to the broader category of Pervasive Developmental Disorders (PDD) and five separate autism diagnoses, such as Asperger syndrome and PDD-not otherwise specified (PDD-NOS). The diagnostic criteria were modified to place the emphasis on the core elements of ASD–deficits of social communication and reciprocity, and presence of repetitive behaviors and restricted interests (rather than verbal abilities as was done in the DSM-IV definition). In addition, several clinical specifiers (e.g., intellectual disability, language delay, medical conditions such as seizures and pattern of onset) were added to help measure each individual's level of impairment and facilitate more customized approaches to treatment and interventions.
Investigators from CDC and the ADDM Network recently completed an evaluation of the new (DSM-5) diagnostic criteria for ASD to estimate the impact on prevalence estimates originally based on the last iteration (DSM-IV-TR) criteria. Findings from this evaluation were accepted for publication in 2013. In the future, CDC will be able to apply both the previous (DSM-IV) and current (DSM-5) diagnostic criteria to generate prevalence estimates based on the different definitions and evaluate the impact on prevalence trends and diagnostic practices. Thus CDC will be able to provide comparable trend data despite changes in diagnostic criteria.