To assess nationwide cumulative incidence and temporal trends in newly diagnosed autism spectrum disorder (ASD) and attention-deficit/hyperactivity disorder (ADHD) in Japan, including age, sex, and comorbidity patterns.
Approach:
Design and data: Cohort study using Japan’s National Database of Health Insurance Claims and Specific Health Checkups, covering approximately 99% of the population. Newly diagnosed cases from fiscal years 2010–2023 were analyzed after a 1-year washout.
Measures: Annual incidence proportions were calculated using census population denominators. Cumulative diagnosis risk through age 49 was projected using 2023 rates.
Key Findings:
The analysis included 1,621,682 newly diagnosed patients with ASD and 1,483,470 with ADHD; diagnoses for both conditions peaked at ages 5–9 years.
Among people aged 0–19 years, ASD diagnoses consistently outnumbered ADHD diagnoses. Among adults, ADHD diagnoses exceeded ASD diagnoses beginning in 2013 for females and 2014 for males.
Females had a secondary ADHD incidence peak at ages 20–24 years that emerged around 2017. In 2020, rates among young adult females neared the intensity of the childhood peak; this pattern was not observed in males.
Projected cumulative diagnosis risk by age 49, based on 2023 incidence rates, was 15.7% for ASD and 14.2% for ADHD. Estimates were higher in males than females for both conditions.
Approximately 46.3% of comorbid cases received both diagnoses simultaneously. Among sequential diagnoses, ASD-first sequencing predominated across age groups.
Increasing trends for both disorders were interrupted during 2019–2020 and rebounded in 2021.
Interpretation:
The authors report increasing neurodevelopmental diagnoses in Japan, with adult diagnoses shifting toward ADHD and a distinct rise among young adult women. They note that the reasons for the trends and the post-2021 rebound remain uncertain.
Limitations:
The database captures treated cases rather than community prevalence, and diagnostic validity depends on clinical coding.
Anxiety and mood disorder codes were unavailable and could confound observed trends.
Record-linkage failures after simultaneous changes in name and insurer may slightly inflate incidence estimates, particularly among women aged 25–34 years.
Cumulative risks were projected from 2023 incidence rates using a synthetic cohort, an approach that violates stationarity assumptions amid changing diagnostic patterns.
Conclusion:
The authors state that ongoing surveillance and further investigation of prognostic factors and implications are warranted.
