Introduction
Mental illness constitutes one of the leading causes of disability in the United Kingdom (UK) [
1]. They are associated with several adverse trajectories, including unemployment, low income, homelessness, criminality, being assaulted or bullied, self-harm, suicide and other causes of premature death [
2]. It is known that psychiatric disorder in adulthood often begins in childhood and adolescence [
3]; it has been estimated that three-quarters of individuals who experience mental illness during adulthood will have met diagnostic criteria for a psychiatric disorder before reaching their 18th birthdays [
4]. Thus, early intervention, prevention and treatment are important to enhance long-term outcomes. In the UK, there has been a shift in the public discourse relating to mental illness, with psychological health and wellbeing during childhood and adolescence now at the forefront of the public health agenda. In 2016 the Mental Health Taskforce of NHS England published the report ‘The Five Year Forward View for Mental Health’, which emphasised children and young people as being a “priority group for mental health promotion and prevention” [
5] The UK Department of Health has also provided an additional £1.4 billion for Child and Adolescent Mental Health Services (CAMHS) between 2015 and 2020 [
6] .
Studies conducted in high-income countries indicate marked increases in numbers of children and adolescents diagnosed with neurodevelopmental disorders and mental illnesses during recent decades [
7]. Despite growing public concern that more children and adolescents in the UK may be affected by these conditions, there are no comprehensive and up-to-date published data on annual trends in the incidence rates in this demographic group. The most recent evidence reporting on the incidence of attention deficit/hyperactivity disorder (ADHD) [
8] and depression [
9,
10] indicate a stable trend or slight increases over time up until 2013. During the same period incidence of eating disorders appears to have remained stable in younger adolescents, but decreased in older adolescents [
11]. Evidence for temporal trends in incidence of anxiety disorders [
12] and autism spectrum disorder (ASD) [
13] is even more dated, describing trends in rates up until the year 2011. These studies also indicate a stable trend in incidence over time. Investigation of the incidence of self-harm, a behaviour that is strongly linked to poor mental health, has demonstrated an increase among adolescents in the UK [
14,
15] in recent years.
In addition to being somewhat dated, individual studies have drawn data from different populations and settings and have varying cohort inclusion criteria. They also differ in terms of diagnostic classification and follow-up duration and period, hindering inter-study comparison. To tackle these between-condition comparability issues and to address the gap in the evidence base, we conducted a comprehensive population-based investigation that examined temporal trends in the incidence of neurodevelopmental disorders, mental illnesses, and self-harm among young people in a single study cohort for the first time. This enabled us to assess age- and gender-specific temporal trends in incidence for multiple conditions with a uniquely high degree of comparability, as well as make use of the accurate routine primary and secondary care linkages that are available in the Clinical Practice Research Datalink (CPRD).
Discussion
In the absence of comprehensive up-to-date population-based epidemiological evidence, we examined temporal trends in the annual incidence of neurodevelopmental disorders, mental illnesses and self-harm in young people. Between 2003 and 2018 incidence rates for ADHD, anxiety disorders, autism, depression, and self-harm increased in both sexes. In some age groups the incidence more than doubled, with increases being particularly pronounced during the later years of the study’s observation period.
The temporal increases in incidence rates that we report for depression are, in terms of their magnitude, consistent with those reported from previous studies conducted in the UK [
22] and in Norway [
24]. In both sexes, incidence was higher at ages 13–16 and 17–19, but younger age groups saw larger relative increases. The increases in incidence of anxiety disorders followed a similar pattern, with the highest incidence recorded in the two oldest age groups, mirroring results reported from studies conducted in Wales [
12] and Sweden [
25]. The increases in incidence for depression and anxiety disorders that we observed were primarily accounted for by a growth in the number of individuals with recorded symptoms, rather than diagnostic codes, which too is consistent with evidence from the UK up until 2015 [
22]. Studies examining prescription rates of antidepressants also show large relative increases in recent years in Sweden [
26] and the UK [
22]. A recently conducted study based on CPRD GOLD investigating eating disorder incidence between 2004 and 2014 reported a stable trend for girls aged 11–15 [
11]. In our analysis, incidence rates nearly doubled in girls aged 13–16, with more modest increases in the 10–12 and 17–19 age groups. The increases in self-harm were of a similar magnitude to those observed for affective disorders. Girls aged 13–16 had the highest incidence, but rates increased at a faster rate in in the 10–12 group. Evidence from England & Wales report increases of a similar magnitude among adolescent girls aged 13–16 [
14] and 10–14 [
15] only, whereas we observed increases in both sexes and all age groups.
There is evidence for increases in prevalence of known risk factors for mental illnesses and self-harm. For example, recent data have demonstrated an increase in prevalence of maternal depression in the UK [
27], a well-established risk factor for anxiety disorder and depression among children and adolescents. Individuals who live in more socially deprived areas are more likely to have mental illnesses; deprivation levels in England have since 2004 increased in households with adolescents and young adults [
28]. There is also a mounting body of evidence for a dose-response relationship between social media usage and poor mental health outcomes in young people [
21,
29]. Greater social media use has been linked to online harassment, sleep deficits, low self-esteem and poor body image, which are associated with an increased risk for developing depression, anxiety disorders and eating disorders [
29]. Recent evidence indicates that suicide among 15–19-year-olds in England and Wales has increased between 2009 and 2017 [
30]. Whilst increased exposure to risk factors may explain some of the increases that we report for mental illnesses and self-harm, it is likely that we have largely uncovered previously unidentified need.
Compared to the other conditions examined, we observed a greater variety in the magnitude of temporal change in incidence of autism between boys and girls and specific age groups. The relative increase was particularly pronounced in females, with IRRs of 8.2, 10.2, and 20.5 in girls aged 6–9, 10–12, and 13–16, respectively. It is important to note that incidence rates among these groups were very low at the beginning of the study period. Thus, while the increases in relative terms are substantial, they are small in absolute terms. Autism incidence was highest among 1–5-year-old boys, which is slightly lower than previously reported in the UK [
31]. In terms of temporal change, studies from Sweden [
32,
33] and Denmark [
32] have reported prevalence increases in diagnosis that are of a comparable magnitude to the temporal trends that we observed. ADHD incidence increased in all age groups, except for among 1–5-year-olds, among whom rates decreased slightly over time in both sexes. Across most age groups studied, our findings were similar to those reported from a recent UK study showing stable incidence rates between 2003 and 2011, with an upwards trend in 2011–2013. Our results may be a continuation of this trend. Studies from the past decade show that the number of diagnoses and prescriptions of methylphenidate have increased by similar magnitudes in Denmark [
32,
34], Sweden [
32,
35] and the UK [
36]. Increases in parental age, in-utero exposure to antidepressants, or extremely low birth weight have been proposed as explanations for the increased incidence of ADHD and autism [
7], although raised prevalence of rare risk factors, or of those that are not strongly associated with the outcome, is unlikely to account for the large increases in incidence that we and other studies have observed. Moreover, both ADHD and autism have a strong genetic component, with an estimated heritability of 70–80% [
37,
38]. For these reasons, there is a growing consensus that increases in the incidence of neurodevelopmental illnesses observed over the last 20 years are mostly due to factors independent of aetiology, such as increased levels of awareness, detection, diagnosis and availability of services [
7].
Considerable caution is needed in making inferences regarding causal mechanisms from temporal trends in incidence rates at population level because these estimates to an unknown extent are influenced by factors that are independent of underlying disease aetiology. For example, increases over time likely reflect changes to ascertainment, service provision and treatment. Changes to diagnostic criteria may influence rates of diagnosis. However, clinicians in the UK primarily rely on the International Classification of Disease (ICD) system for diagnosis, which during the duration of our study has remained unchanged. Reductions to stigma associated with mental illnesses and increased awareness among teachers and parents may also contribute to increased help seeking. Estimates from studies that are less likely to be influenced by these factors report more modest increases in incidence over time. For example, results from the UK Child Adolescent Mental Health Survey conducted in 1999, 2004 and 2017, in which several thousand randomly sampled children and adolescents were assessed for mental illnesses and exposure to their risk factors, reported increases in the prevalence of mental illnesses among individuals aged 5–15 ranging from 9.7 to 11.2%. Moreover, whilst the proportion of self−/parent-reported mental illnesses has increased in the UK, the degree of reported psychological distress has remained stable [
39].
A potential limitation of our study is that we could not determine the validity or relative severity of diagnosed conditions and self-harm episodes. However, as several studies have demonstrated, there is a significant risk of under-ascertainment by adopting a case definition that is unduly stringent [
9,
22]. We invite interested readers to consider the supplementary files for a side-by-side comparison of results generated using case definitions of varying levels of sensitivity (Fig. S
4). Moreover, because the CPRD does not comprehensively capture detailed contextual information pertaining to neurodevelopmental disorders, mental illnesses and self-harm, we are unable to determine if the increases in incidence reflect true changes to underlying levels of psychological distress and psychopathology in the population. Whilst we implemented a 12-month clearance period to reduce the risk of including prevalent cases, it is possible that some prevalent cases were included as the CPRD does not have complete coverage across the entire study period. Our results reflect the incidence of recorded diagnoses of mental illnesses in primary care; cases recorded in other healthcare settings may therefore be missing. Finally, this study used data prior to the COVID-19 pandemic and resulting restrictions and change in service delivery.
Conclusions
In this study, we show that the number of recorded mental illness and self-harm-related primary care episodes have increased substantially among children and adolescents in the UK in recent years. These findings are consistent with evidence of increased demand for specialist mental health services [
40] and raise several important questions for researchers, commissioners, and policymakers. Secular trends of mental illness-related presentations in healthcare settings are influenced by a complex interaction of countervailing forces, many of which are independent of disease aetiology. Future research is needed to determine if increased demand for treatment reflects a real deterioration in psychological wellbeing in the population and, if so, to identify the underlying causes as this may lead to more targeted interventions. This could partly be accomplished by more frequent updates to the UK Child Adolescent Mental Health Survey, which provides prevalence estimates of psychiatric illnesses and their risk factors that to a lesser extent are influenced by factors unrelated to underlying morbidity. Identifying the most effective responses to the reported increase in demand represents a major challenge for policymakers and researchers alike. Whilst increasing access to treatment is a clear priority, it is evident that many of the solutions for reducing the burden of mental illnesses lie beyond child and adolescent mental health services, and preventive efforts will need to address underlying determinants, such as poverty, bullying, educational stressors, and parental mental illness. Because most psychiatric illnesses begin before adulthood, there is consensus among clinicians that preventative efforts targeting young people can lead to greater personal, social and economic benefit than interventions delivered at later stages of life [
41]. The ongoing public health response in the UK, which stresses the need for parental support and outlines plans for increased collaboration between schools and mental health services [
6], is consistent with this view. However, evidence from meta-analyses of school-based interventions is mixed [
42,
43], and more high quality evidence is needed to better understand which interventions are effective for whom and in what settings, and which factors are associated with their successful implementation.
Added value of this study
Using the UK Clinical Practice Research Link (CPRD) Aurum and GOLD datasets, we delineated a cohort study based on routinely collected primary care patient electronic health records. We combined the study cohorts and investigated temporal trends in the incidence of anxiety disorders, autism, attention-deficit hyperactivity disorder (ADHD), depression, eating disorders, and non-fatal self-harm in children and adolescents aged 1–19 years during 2003–2018. By presenting temporal trends in incidence rates of self-harm and several neurodevelopmental disorder and mental illness diagnostic categories in the same study cohort, we have addressed some of the key limitations that are inherent with inter-study comparisons, providing an up-to-date overview that hitherto has been lacking from the evidence base. Results indicate that an increasing number of children and adolescents are seeking help for neurodevelopmental disorders, mental illnesses and self-harm behaviour. For several age groups and diagnostic categories the incidence rose by two-fold or more, with especially steep increases occurring toward the end of the study’s observation period.
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