Abstract
This Research Letter examines cumulative incidence on autism spectrum disorder in Denmark using nationwide registered diagnosis data from 1980 to 2012 and assesses whether prevalence was stabilized.
The Centers for Disease Control and Prevention recently reported an autism prevalence of 1.68% among 8-year-old children in the United States in 2014,1 which is 14% higher than the reported rate for 8-year-old children in 2012 and 2010.2,3 Autism prevalence rates of 2% and 2.47% in school-aged children were previously estimated from parent report data in US national health surveys.4,5 These cross-sectional data may suggest that autism spectrum disorder (ASD) prevalence is reaching a peak. However, longitudinal data with follow-up into adulthood are needed to truly determine whether the prevalence has stabilized.
Methods
All live births in Denmark between 1980 and 2012 were identified in the Central Person Register and followed through 2016 for an ASD diagnosis (International Classification of Diseases, Eighth Revision codes 299.00, 299.01, 299.02, and 299.03; International Statistical Classification of Diseases and Related Health Problems, Tenth Revision codes F84.0, F84.1, F84.5, F84.8, and F84.9) via linkage with the Psychiatric Central Research and National Patient Registers containing diagnoses recorded by medical specialists (only inpatient contacts before 1995). Persons with suspected ASD receive a multidisciplinary evaluation at a psychiatric department and the final diagnosis is reported by a psychiatrist who has received mandatory registry-reporting training. Persons receiving the ASD subdiagnosis of pervasive development disorder, unspecified (International Statistical Classification of Diseases and Related Health Problems, Tenth Revision code F84.9) may be advised to return for re-evaluation.
We used competing-risks survival analysis6 to estimate ASD age-specific cumulative incidence in 2-year birth cohorts using age as the underlying time scale; persons were followed up until ASD diagnosis, death, emigration, or December 31, 2016, whichever came first (SAS version 9.4; SAS Institute Inc). In sensitivity analysis, we recalculated cumulative incidence based on the second ASD diagnosis for all persons with an initial diagnosis of pervasive development disorder, unspecified. The study was approved by the Scientific Ethics Committees of the Central Denmark Region and according to guidelines from the Danish Data Protection Agency. Under Danish law, register-based projects are exempt from obtaining informed consent.
Results
Autism spectrum disorder was reported for 31 961 persons among 2 055 928 live births. Each birth cohort cumulative incidence curve followed its own trajectory, with new case ascertainment into adulthood for older cohorts, and reaching a higher cumulative incidence with age than any of the earlier cohorts (Figure). The maximum value was 2.80% (95% CI, 2.57%-3.04%) at age 16 years for persons born in 2000-2001 (Table); it was 3.89% (95% CI, 3.52%-4.28%) in boys and 1.66% (95% CI, 1.41%-1.94%) in girls. The curves for persons born after 2001 indicate increasing cumulative incidence at younger ages (at age 10 years, 1.16% [95% CI, 0.95%-1.40%] in persons born in 2000-2001 vs 1.65% [95% CI, 1.38%-1.97%] for persons born in 2006-2007) (Table). The Figure inset for 1994-2012 births reveals shifts in the age and rate at which the curve for a given birth cohort begins to ascend more steeply than previous cohorts, yielding a cumulative incidence at age 4 of 0.11% (95% CI, 0.02%-0.40%) in persons born in 1994-1995 vs 0.37% (95% CI, 0.13%-0.88%) for persons born in 2012 (Table). In sensitivity analysis, the overall cumulative incidence pattern was unchanged but the maximum value declined (2.50% [95% CI, 2.29%-2.73%] at age 16 for persons born in 2000-2001).
Figure. Cumulative Incidence of Autism Spectrum Disorder by Age Through 2016 in Denmark Among 1980-2012 Birth Cohorts.
Each curve in the main body of the Figure corresponds to the autism spectrum disorder cumulative incidence through 2016 among persons in a 2-year birth cohort (beginning 1980-1981, bottom curve), except for the left-most curve that corresponds to the last cohort composed of a single birth year, 2012. The inset is a close-up view of the autism spectrum disorder cumulative incidence through age 5 years for each birth cohort, 1994-2012 (corresponding to births after the adoption of International Statistical Classification of Diseases and Related Health Problems, Tenth Revision autism spectrum disorder diagnostic criteria).
Table. Cumulative Incidence of Autism Spectrum Disorder Through 2016 for Persons Born Between 1980 and 2012 in Denmark.
| Birth Year | Autism Spectrum Disorder Cumulative Incidence by Age per 100 Persons, % (95% CI)a | ICD System | |||
|---|---|---|---|---|---|
| Age 4 y | Age 10 y | Age 16 y | Age 22 y | ||
| 1980-1981 | 0 (0-0)b | 0.03 (0.02-0.04) | ICD-8 onlyc | ||
| 1982-1983 | 0 (0-0)b | 0.05 (0.02-0.11) | |||
| 1984-1985 | 0 (0-0)b | ||||
| 1986-1987 | 0 (0-0)b | ||||
| 1988-1989 | 0 (0-0)b | ||||
| 1980-1981 | 0.09 (0.07-0.11) | 0.15 (0.13-0.17) | ICD-8 + ICD-10d | ||
| 1982-1983 | 0.20 (0.16-0.26) | 0.30 (0.25-0.35) | |||
| 1984-1985 | 0.06 (0.02-0.17) | 0.27 (0.21-0.35) | 0.40 (0.33-0.48) | ||
| 1986-1987 | 0.15 (0.09-0.26) | 0.39 (0.31-0.49) | 0.54 (0.45-0.64) | ||
| 1988-1989 | 0.26 (0.17-0.39) | 0.57 (0.47-0.69) | 0.79 (0.68-0.91) | ||
| 1990-1991 | 0 (0-0)b | 0.38 (0.27-0.52) | 0.76 (0.64-0.90) | 1.11 (0.98-1.25) | |
| 1992-1993 | 0.01 (0.0-0.0) | 0.45 (0.32-0.61) | 0.98 (0.84-1.14) | 1.47 (1.32-1.62) | |
| 1994-1995 | 0.11 (0.02-0.40) | 0.63 (0.48.0.81) | 1.43 (1.27-1.61) | 2.07 (1.90-2.25) | ICD-10 onlye |
| 1996-1997 | 0.14 (0.03-0.45) | 0.73 (0.56-0.93) | 1.78 (1.59-1.97) | ||
| 1998-1999 | 0.17 (0.05-0.50) | 0.90 (0.72-1.12) | 2.30 (2.09-2.51) | ||
| 2000-2001 | 0.18 (0.05-0.55) | 1.16 (0.95-1.40) | 2.80 (2.57-3.04) | ||
| 2002-2003 | 0.18 (0.04-0.64) | 1.33 (1.09-1.61) | |||
| 2004-2005 | 0.20 (0.04-0.70) | 1.44 (1.18-1.74) | |||
| 2006-2007 | 0.20 (0.04-0.75) | 1.65 (1.38-1.97) | |||
| 2008-2009 | 0.24 (0.06-0.76) | ||||
| 2010-2011 | 0.27 (0.07-0.79) | ||||
| 2012 | 0.37 (0.13-0.88) | ||||
Abbreviation: ICD, International Statistical Classification of Diseases and Related Health Problems.
Total ICD-8–diagnosed autism spectrum disorder cases: n = 184; total ICD-10–diagnosed autism spectrum disorder cases: n = 31 777.
Too few cases to calculate cumulative incidence.
Cumulative incidence based on ICD-8 diagnoses only (reported before 1994).
Cumulative incidence based on ICD-8 diagnoses (reported before 1994) and ICD-10 diagnoses (reported from 1994 onward).
Cumulative incidence based on ICD-10 diagnoses only (reported from 1994 onward).
Discussion
The Danish ASD trends are consistent with US cross-sectional data for similar birth cohorts.1,2,3,4,5 These populationwide data with follow-up into adulthood revealed no plateau in curves in more recent birth years, suggesting that ASD cumulative incidence has not stabilized. The ongoing increases at young ages in more recent cohorts suggest that future cumulative incidence could exceed 2.8%.
The age shifts at which the curve for a given birth cohort ascended more steeply than previous cohorts and the associated increases in cumulative incidence at young ages suggest that the services that support early detection of ASD expanded during this study’s observation period. However, new case ascertainment also continued well into adulthood in older cohorts. This ascertainment pattern likely reflects the complexity of the ASD phenotype, diverse age course of individual behavioral development, and family and community differences over time in ASD detection that can delay diagnosis into adulthood.
This study cannot delineate causes of the observed ASD trends or provide results by ASD characteristics that might aid services planning. Nevertheless, the results highlight the substantial public health challenges that lie ahead to meet the diverse support needs of persons with ASD across the life span.
Section Editor: Jody W. Zylke, MD, Deputy Editor.
References
- 1.Baio J, Wiggins L, Christensen DL, et al. . Prevalence of autism spectrum disorder among children aged 8 years: Autism and Developmental Disabilities Monitoring Network, 11 Sites, United States, 2014. MMWR Surveill Summ. 2018;67(6):1-23. doi: 10.15585/mmwr.ss6706a1 [DOI] [PMC free article] [PubMed] [Google Scholar]
- 2.Christensen DL, Baio J, Van Naarden Braun K, et al. ; Centers for Disease Control and Prevention . Prevalence and characteristics of autism spectrum disorder among children aged 8 years: Autism and Developmental Disabilities Monitoring Network, 11 Sites, United States, 2012. MMWR Surveill Summ. 2016;65(3):1-23. doi: 10.15585/mmwr.ss6503a1 [DOI] [PMC free article] [PubMed] [Google Scholar]
- 3.Developmental Disabilities Monitoring Network Surveillance Year 2010 Principal Investigators; Centers for Disease Control and Prevention . Prevalence of autism spectrum disorder among children aged 8 years: Autism and Developmental Disabilities Monitoring Network, 11 sites, United States, 2010. MMWR Surveill Summ. 2014;63(2):1-21. [PubMed] [Google Scholar]
- 4.Blumberg SJ, Bramlett MD, Kogan MD, Schieve LA, Jones JR, Lu MC. Changes in prevalence of parent-reported autism spectrum disorder in school-aged US children: 2007 to 2011-2012. Natl Health Stat Report. 2013;(65):1-11. [PubMed] [Google Scholar]
- 5.Xu G, Strathearn L, Liu B, Bao W. Prevalence of autism spectrum disorder among US children and adolescents, 2014-2016. JAMA. 2018;319(1):81-82. doi: 10.1001/jama.2017.17812 [DOI] [PMC free article] [PubMed] [Google Scholar]
- 6.Andersen PK, Geskus RB, de Witte T, Putter H. Competing risks in epidemiology: possibilities and pitfalls. Int J Epidemiol. 2012;41(3):861-870. doi: 10.1093/ije/dyr213 [DOI] [PMC free article] [PubMed] [Google Scholar]