Incidence Rates and Cumulative Incidences of the Full Spectrum of Diagnosed Mental Disorders in Childhood and Adolescence

Abstract

Importance: Knowledge about the epidemiology of mental disorders in children and adolescents is essential for research and planning of health services. Surveys can provide prevalence rates, whereas population-based registers are instrumental to obtain precise estimates of incidence rates and risks.

Objective: To estimate age- and sex-specific incidence rates and risks of being diagnosed with any mental disorder during childhood and adolescence.

Design: This cohort study included all individuals born in Denmark from January 1, 1995, through December 31, 2016 (1.3 million), and followed up from birth until December 31, 2016, or the date of death, emigration, disappearance, or diagnosis of 1 of the mental disorders examined (14.4 million person-years of follow-up). Data were analyzed from September 14, 2018, through June 11, 2019.

Exposures: Age and sex.

Main outcomes and measures: Incidence rates and cumulative incidences of all mental disorders according to the ICD-10 Classification of Mental and Behavioral Disorders: Diagnostic Criteria for Research, diagnosed before 18 years of age during the study period.

Results: A total of 99 926 individuals (15.01%; 95% CI, 14.98%-15.17%), including 41 350 girls (14.63%; 95% CI, 14.48%-14.77%) and 58 576 boys (15.51%; 95% CI, 15.18%-15.84%), were diagnosed with a mental disorder before 18 years of age. Anxiety disorder was the most common diagnosis in girls (7.85%; 95% CI, 7.74%-7.97%); attention-deficit/hyperactivity disorder (ADHD) was the most common in boys (5.90%; 95% CI, 5.76%-6.03%). Girls had a higher risk than boys of schizophrenia (0.76% [95% CI, 0.72%-0.80%] vs 0.48% [95% CI, 0.39%-0.59%]), obsessive-compulsive disorder (0.96% [95% CI, 0.92%-1.00%] vs 0.63% [95% CI, 0.56%-0.72%]), and mood disorders (2.54% [95% CI, 2.47%-2.61%] vs 1.10% [95% CI, 0.84%-1.21%]). Incidence peaked earlier in boys than girls in ADHD (8 vs 17 years of age), intellectual disability (5 vs 14 years of age), and other developmental disorders (5 vs 16 years of age). The overall risk of being diagnosed with a mental disorder before 6 years of age was 2.13% (95% CI, 2.11%-2.16%) and was higher in boys (2.78% [95% CI, 2.44%-3.15%]) than in girls (1.45% [95% CI, 1.42%-1.49%]).

Conclusions and relevance: This nationwide population-based cohort study provides a first comprehensive assessment of the incidence and risks of mental disorders in childhood and adolescence. By 18 years of age, 15.01% of children and adolescents in this study were diagnosed with a mental disorder. The incidence of several neurodevelopmental disorders peaked in late adolescence in girls, suggesting possible delayed detection. The distinct signatures of the different mental disorders with respect to sex and age may have important implications for service planning and etiological research.

Figure 1.. Sex- and Age-Specific Incidence Rates and Cumulative Incidences for Any Mental Disorder, Schizophrenia Spectrum Disorder, Mood Disorders, and Anxiety Disorder

Any mental disorder was identified as ICD-10 Classification of Mental and Behavioral Disorders: Diagnostic Criteria for Research (ICD-10-DCR), codes F00 to F99; schizophrenia spectrum disorder, as ICD-10-DCR codes F20 to F29; mood disorders, as ICD-10-DCR codes F30 to F39; and anxiety disorder, as ICD-10-DCR codes F40 to F48 and F93. Error bars show the 95% CIs in designated age ranges. Owing to the large sample size, the 95% CIs for the cumulative incidences are very close to the estimates and are therefore not shown. Because the cumulative incidences are estimated continuously with respect to age and the incidence rates are estimated in 1-year age intervals, the abscissa for the cumulative incidence measures the exact age, whereas the abscissa for the incidence rates measures the lowest cutoff point for the age interval. The y-axis scales differ by disorder to correspond to the range of observed outcomes.

Figure 2.. Sex- and Age-Specific Incidence Rates and Cumulative Incidences for Obsessive-Compulsive Disorder, Eating Disorders, Intellectual Disability, and Autism Spectrum Disorders

Obsessive-compulsive disorder was identified as ICD-10 Classification of Mental and Behavioral Disorders: Diagnostic Criteria for Research (ICD-10-DCR), code F42; eating disorders, as ICD-10-DCR code F50; intellectual disability, as ICD-10-DCR codes F70 to F79; and autism spectrum disorders, as ICD-10-DCR codes F84.x, excluding F84.2 to F84.4. Error bars show the 95% CIs in designated age ranges. Owing to the large sample size, the 95% CIs for the cumulative incidences are very close to the estimates and are therefore not shown. Because the cumulative incidences are estimated continuously with respect to age and the incidence rates are estimated in 1-year age intervals, the abscissa for the cumulative incidence measures the exact age, whereas the abscissa for the incidence rates measures the lowest cutoff point for the age interval. The y-axis scales differ by disorder to correspond to the range of observed outcomes.

Figure 3.. Sex- and Age-Specific Incidence Rates and Cumulative Incidences for Attention-Deficit/Hyperactivity Disorder (ADHD), ADHD-Inattentive Type, Oppositional Defiant Disorder/Conduct Disorder, and Tic Disorders

ADHD was identified as ICD-10 Classification of Mental and Behavioral Disorders: Diagnostic Criteria for Research (ICD-10-DCR), codes F90 and F98.8; ADHD-inattentive type, as ICD-10-DCR code F98.8; oppositional defiant disorder/conduct disorder, as ICD-10-DCR codes F91 plus F90.1; and tic disorders, as ICD-10-DCR code F95. Error bars show the 95% CIs in designated age ranges. Owing to the large sample size, the 95% CIs for the cumulative incidences are very close to the estimates and are therefore not shown. Because the cumulative incidences are estimated continuously with respect to age and the incidence rates are estimated in 1-year age intervals, the abscissa for the cumulative incidence measures the exact age, whereas the abscissa for the incidence rates measures the lowest cutoff point for the age interval. The y-axis scales differ by disorder to correspond to the range of observed outcomes.