Trends in Autism Spectrum Disorder Diagnoses in Japan, 2009 to 2019

A recent large-scale birth cohort study in Denmark reported that the diagnosis of autism spectrum disorder (ASD) is increasing and that its future cumulative incidence could exceed 2.8%.¹ In Japan, 3 recent cohort studies^2-4 have consistently reported prevalence or incidence of ASD exceeding 3%. The question may arise: do these relatively high figures compared with worldwide data (eTable in the Supplement) represent the nationwide incidence in Japan? To answer this question, we analyzed the data from the National Database (NDB) of Health Insurance Claims of Japan.⁵ We examined the geographical variations in the incidence of ASD and calculated the nationwide cumulative incidence of ASD in Japan.

This cohort study was approved by the ethics committee of Shinshu University School of Medicine. Informed consent was not required because of the anonymous nature of the data. This study was reported following the Strengthening the Reporting of Observational Studies in Epidemiology (STROBE) reporting guideline. No statistical tests were conducted in this study. A detailed description of the methods is shown in the eAppendix in the Supplement. Data on children born in fiscal years (April 1 to March 31) 2009 to 2016 and diagnosed with ASD (International Statistical Classification of Diseases, Tenth Revision [ICD-10] code F84) in fiscal years 2009 to 2019 were retrieved from the NDB. Extracted information included sex, the year and age at diagnosis, and the prefecture where ASD was diagnosed. Data analysis was conducted between December 2020 and March 2021.

Because the annual number of births in each fiscal year was not available, the number of births in each calendar year was used to estimate the lifetime cumulative incidence by dividing the number of children diagnosed in a fiscal year by the number of births for that calendar year. The difference in birth numbers between fiscal years and calendar years is likely to be less than 1% (eAppendix in the Supplement).

We extracted data on 313 353 children (236 386 [75.4%] boys and 76 967 [24.6%] girls) born in fiscal years 2009 to 2016 and diagnosed with ASD in fiscal years 2009 to 2019. The number of children born in Japan between 2009 and 2014 was 6 262 731, of whom 172 276 (131 117 [76.1%] boys and 41 159 [23.9%] girls) were diagnosed with ASD in or before the fiscal year 5 years after their birth year. Therefore, the estimated nationwide 5-year lifetime cumulative incidence of ASD in children born between fiscal 2009 and 2014 was 2.75%. The lifetime cumulative incidence for each birth year cohort (Figure 1) showed a steady increase, from 2.23% for the 2009 cohort to 3.26% for the 2014 cohort. Diagnostic incidence per year increased substantially from 2 years after birth and decreased slightly after 6 years after birth in both boys and girls. For example, for the 2009 to 2011 cohorts, the mean incidence within 2 years after birth was 0.0014% per year for boys and 0.0006% per year for girls; the mean incidence from 2 to 6 years after birth was 0.0109% per year for boys and 0.0035% per year for girls; the mean incidence from 7 to 8 years after birth was 0.0086% per year for boys and 0.0033% per year for girls. Figure 2 shows the 5-year lifetime cumulative incidence of ASD in each of the 47 prefectures in Japan. The 5-year lifetime cumulative incidence in each prefecture ranged from 0.9% to 7.9%, with a median of 2.4%. The 5-year lifetime cumulative incidence exceeded 2.0% in 29 prefectures (62%).

This study found that the nationwide cumulative incidence of ASD was comparable with what has been reported in local cohorts^2,4 and that the diagnosis of ASD increased in Japan between 2009 and 2019. Expanding public awareness may have contributed to the increased nationwide incidence, whereas the regional variation may be because of other etiological and nonetiological (eg, accessibility to services) factors. Our findings indicate an important need for further health services and etiologic research.

Several limitations must be noted. First, our calculations of cumulative incidence did not assume migration. Second, Rett syndrome was counted as ASD because of the use of ICD-10 code F84. However, the effect of these limitations on the nationwide cumulative incidence is likely to be negligible due to the low rate of migration abroad and the low incidence of Rett syndrome.⁶ In contrast, migration between prefectures may have caused some inaccuracy in the cumulative incidence for each prefecture. Furthermore, the number of births per calendar year was used for analyses, most likely leading to a minor underestimation of the cumulative incidence.

Despite the limitations, this study found that the incidence of ASD in Japan was higher than what has been reported worldwide. The results bring attention to the necessity of developing support systems to meet the needs of an increasing number of individuals diagnosed with ASD.

Accepted for Publication: March 17, 2021.

Published: May 4, 2021. doi:10.1001/jamanetworkopen.2021.9234

Correction: This article was corrected on June 14, 2021, to fix an error in Figure 1.

Open Access: This is an open access article distributed under the terms of the CC-BY License. © 2021 Sasayama D et al. JAMA Network Open.

Corresponding Author: Daimei Sasayama, MD, PhD, Department of Psychiatry, Shinshu University School of Medicine, 3-1-1, Asahi, Matsumoto, Nagano 390-8621, Japan (sasayama@shinshu-u.ac.jp).

Author Contributions: Dr Sasayama had full access to all of the data in the study and takes responsibility for the integrity of the data and the accuracy of the data analysis.

Concept and design: All authors.

Acquisition, analysis, or interpretation of data: Sasayama.

Drafting of the manuscript: Sasayama, Honda.

Critical revision of the manuscript for important intellectual content: Kuge, Toibana, Honda.

Statistical analysis: Sasayama.

Obtained funding: Sasayama.

Administrative, technical, or material support: Kuge.

Supervision: Toibana, Honda.

Conflict of Interest Disclosures: Dr Sasayama reported receiving personal fees from Otsuka Pharmaceutical, Janssen Pharmaceuticals, Takeda Pharmaceutical, Shionogi Pharma, Eli Lilly Japan, and Pfizer Japan outside the submitted work. Dr Kuge reported receiving personal fees from Otsuka Pharmaceutical, Shionogi Pharma, and Mochida Pharmaceutical outside the submitted work. Dr Honda reported receiving grants from Otsuka Pharmaceutical, Shionogi Pharma, Pfizer Japan, Astellas Pharma, and Eli Lilly Japan and receiving personal fees from Otsuka Pharmaceutical, Shionogi Pharma, Shire Japan, Janssen Pharmaceuticals, Sumitomo Dainippon Pharma, Merck Sharp and Dohme, Meiji Seika Pharma, AstraZeneca, Towa Pharmaceutical, and Yoshitomiyakuhin Corporation outside the submitted work. No other disclosures were reported.

Funding/Support: This study was funded by grant-in-aid for scientific research H30-shintai/chiteki-ippan-002 from the Ministry of Health, Labour, and Welfare.

Role of the Funder/Sponsor: The funder had no role in the design and conduct of the study; collection, management, analysis, and interpretation of the data; preparation, review, or approval of the manuscript; and decision to submit the manuscript for publication.