Autism Spectrum Disorder: Incidence and Time Trends Over Two Decades in a Population-Based Birth Cohort

Abstract

We retrospectively identified autism spectrum disorder (ASD) incident cases among 31,220 individuals in a population-based birth cohort based on signs and symptoms uniformly abstracted from medical and educational records. Inclusive and narrow research definitions of ASD (ASD-R_I and ASD-R_N, respectively) were explored, along with clinical diagnoses of ASD (ASD-C) obtained from the records. The incidence of ASD-R_I, ASD-R_N, and ASD-C increased significantly from 1985 to 1998, then ASD-R_I and ASD-R_N plateaued while the rate of ASD-C continued to increase during 1998-2004. The rising incidence of research-defined ASD may reflect improved recognition and documentation of ASD signs and symptoms. Although the frequency of threshold ASD symptoms stabilized, the rate of ASD-C continued to increase, narrowing the gap between clinical ascertainment and symptom documentation.

Keywords: Autism spectrum disorder; Epidemiology; Incidence; Time trends.

Fig. 1.

Steps in identifying ASD incident cases.

Fig. 2.

(A) Incidence by age 21 (per 1000 person-years) of research-identified inclusive (ASD-R_I) and narrow (ASD-R_N) autism spectrum disorder and clinically diagnosed autism spectrum disorder (ASD-C), according to calendar year of first meeting criteria and averaged across gender. See body of text in the Results section for explanation of the horizontal dotted lines used to draw statistical inference with respect to trends. (B) Cumulative incidence by age 21 (×100) of research-identified autism spectrum disorder (ASD-R_I and ASD-R_N) and clinically diagnosed autism spectrum disorder (ASD-C), according to birth year and averaged across gender. The solid curves for both (A) and (B) are the respective posterior mean estimates for ASD-R_I, ASD-R_N and ASD-C incidence, while the bands are 95% simultaneous credible bands; 95% of curves from the posterior distribution will fall entirely within these bands. (C) Numerical data corresponding to (A). (D) Numerical data corresponding to (B).

Fig. 3.

(A) Rate ratio of ASD-C incidence to ASD-R_I and ASD-R_N incidence, respectively, according to calendar year. (B) Rate ratio of ASD-C cumulative incidence to ASD-R_I and ASD-R_N cumulative incidence, respectively, according to birth year. The solid curves are the respective posterior mean estimates for ASD-R_I and ASD-R_N incidence, while the bands are 95% simultaneous credible bands; 95% of curves from the posterior distribution will fall entirely within these bands.

Fig. 4.

The age-specific incidence per 1,000 person-years for five separate birth periods (averaged across calendar year) for (A) ASD-R_I and (B) ASD-R_N. The solid curves are the respective posterior mean estimates for ASD-R_I and ASD-R_N incidence, while the bands are 95% simultaneous credible bands; 95% of curves from the posterior distribution will fall entirely within these bands.