The rise in autism and the role of age at diagnosis

Abstract

Background: Autism prevalence in California, based on individuals eligible for state-funded services, rose throughout the 1990s. The extent to which this trend is explained by changes in age at diagnosis or inclusion of milder cases has not been previously evaluated.

Methods: Autism cases were identified from 1990 through 2006 in databases of the California Department of Developmental Services, which coordinates services for individuals with specific developmental disorders. The main outcomes were population incident cases younger than age 10 years for each quarter, cumulative incidence by age and birth year, age-specific incidence rates stratified by birth year, and proportions of diagnoses by age across birth years.

Results: Autism incidence in children rose throughout the period. Cumulative incidence to 5 years of age per 10,000 births rose consistently from 6.2 for 1990 births to 42.5 for 2001 births. Age-specific incidence rates increased most steeply for 2- and 3-year olds. The proportion diagnosed by age 5 years increased only slightly, from 54% for 1990 births to 61% for 1996 births. Changing age at diagnosis can explain a 12% increase, and inclusion of milder cases, a 56% increase.

Conclusions: Autism incidence in California shows no sign yet of plateauing. Younger ages at diagnosis, differential migration, changes in diagnostic criteria, and inclusion of milder cases do not fully explain the observed increases. Other artifacts have yet to be quantified, and as a result, the extent to which the continued rise represents a true increase in the occurrence of autism remains unclear.

Figure 1

Annual incidence rates of autism based on the administrative database of the California Department of Developmental Services, 1990 through 2006, by quarter. Each point represents the number of new cases in each quarter divided by the population of children in the respective age group at the midpoint of the year (an estimate of the number of child-quarter-years), and then multiplied by 4 to obtain an annualized rate. The fitted lines represent smoothed loess curves and the vertical line at year 2003 quarter 3 is the time at which access to state services became more restrictive (see Discussion).

Figure 2

Cumulative incidence of autism by birth cohort from 1990 to 2003, at ages 1 through 9 years, California Departmental of Developmental Services database.

Figure 3

Age-specific incidence rates of autism, births 1990 through 2003, California Departmental of Developmental Services database.

Figure 4

Proportion of cases with first record of autism in the California Departmental of Developmental Services database at each age. Ages 1 through 9, by year of birth.