A Review of Canadian Diagnosed ADHD Prevalence and Incidence Estimates Published in the Past Decade

(1) Background: ADHD is recognized as one of the most common neurodevelopmental disorders. The worldwide prevalence of ADHD is estimated at 5.3%; however, estimates vary as a function of a number of factors, including diagnostic methods, age, sex and geographical location. A review of studies is needed to clarify the epidemiology of ADHD in Canada. (2) Methods: A search strategy was created in PubMed and adapted for MEDLINE and PsycINFO. Papers were included if they examined diagnosed ADHD prevalence and/or incidence rates in any region of Canada, age group and gender. A snowball technique was used to identify additional papers from reference lists, and experts in the field were consulted. (3) Results: Ten papers included in this review reported on prevalence, and one reported on incidence. One study provided an overall prevalence estimate across provinces for adults of 2.9%, and one study provided an overall estimate across five provinces for children and youth of 8.6%. Across age groups (1 to 24 years), incidence estimates ranged from 0.4% to 1.2%, depending on province. Estimates varied by age, gender, province, region and time. (4) Conclusions: The overall Canadian ADHD prevalence estimate is similar to worldwide estimates for adults. Most studies reported on prevalence rather than incidence. Differences in estimates across provinces may reflect the varying number of practitioners available to diagnose and prescribe medication for ADHD across provinces. To achieve a more comprehensive understanding of the epidemiology of ADHD in Canada, a study is needed that includes all provinces and territories, and that considers estimates in relation to age, gender, ethnicity, geographical region, socioeconomic status and access to mental healthcare coverage. Incidence rates need further examination to be determined.


Introduction
Attention deficit hyperactivity disorder (ADHD) is recognized as the most common neurodevelopmental disorder of childhood. A frequently cited study by Polanszky of 2007 reported a worldwide ADHD prevalence estimate of 5.3% [1]. Studies carried out in countries other than Canada showed a range of ADHD estimates. For instance, estimates for children include 7.5% in Australia [2] and 10.2% in the United States [3]. Two studies on adolescents in China and Africa provided estimates of 6.3% [4] and 7.5% [5], respectively. One German study on children and adolescents combined provides an estimate of 6.1% [6]. Estimates are typically lower for adults than for children and adolescents [7]. A 2021 survey conducted in the United States, for example, found an ADHD prevalence rate for adults of 4.25% [8]. Findings from two recent worldwide meta-analyses suggest a prevalence of 3.4% in children and adolescents [9], and a lower prevalence in adults of 2.6% [10].
In addition to potential differences between countries, prevalence estimates may also vary due to the methods used and sample characteristics. Additionally, while prevalence estimates indicate the number of existing cases of a disorder in the population (existing + new cases over total population), incidence estimates indicate the number of new cases being added (i.e., new cases over total (susceptible) population). When the duration of a disorder is short, and new instances are constant or increasing, incidence can trend higher than prevalence. However, when the duration of a disorder tends to be lifelong (as is the case for ADHD), prevalence can trend higher than incidence because new cases are added and existing cases remain. Thus, prevalence and incidence provide unique information which combined, help to clarify the epidemiology of ADHD.
A few studies have examined the prevalence and/or incidence of ADHD specifically in the Canadian population, but these findings have not yet been summarized. To clarify the epidemiology of ADHD in Canada, a review of existing studies on the prevalence and incidence of diagnosed ADHD is needed. The aim of this review is to summarize Canadian ADHD prevalence and incidence estimates published in the past ten years and to consider these in relation to demographic (i.e., age and gender), geographic and methodological factors.

Materials and Methods
A search strategy to identify relevant literature using Boolean operators was developed using PubMed and adapted for Medline, and PsycInfo with the following terms: (ADHD or ADD or AD/HD or Attention *) and (Canada or Canadian or province * or territory or territory *) and (epidemiology, prevalence, incidence). All studies were screened, and data were abstracted by two reviewers. A snowball technique (i.e., examining reference lists of selected papers) and experts were consulted to identify additional relevant papers. Papers published within the past 10 years (2012 to 2022) were included if they provided estimates of diagnosed ADHD prevalence or incidence based on data from the Canadian population, regardless of age group, gender or geographical location examined. Papers that focused on symptoms of ADHD without a diagnosis (e.g., self-reported attention difficulties) and that focused on a particular environmental condition (e.g., ADHD with greenspace exposure or traumatic brain injury) were excluded.

Description of Included Papers
A total of ten papers are included in the review [11][12][13][14][15][16][17][18][19][20], all of which were published from 2012 to 2022 (see Table 1). Ten papers included prevalence estimates of ADHD in Canada [11][12][13][14][15][16][17][18][19][20]. Out of these papers, one also examined the incidence of ADHD in Canada [15]. One additional review paper [21] that was initially included, was ultimately excluded because it did not provide prevalence or incidence estimates. This paper was reviewed to ensure that any relevant studies mentioned were included here. See Appendix A for a PRISMA diagram [22] outlining results of the search strategy used to retrieve articles for the current paper.
Regarding gender, while all papers acknowledged gender differences in ADHD prevalence, six provided separate estimates for males and females [11,12,14,15,17,18]. The paper that included incidence estimates differentiated between males and females, although no exact estimates were provided [15]. No other non-binary gender identity was noted in any of the papers. The ethnic background of the participants was reported in only one paper; the majority were classified as "white" [13].

Overall ADHD Prevalence in Canada
None of the included studies provided an overall national prevalence estimate collapsed across age groups (i.e., children, adolescents and adults combined). However, one study provided an overall estimate of 2.9% across a large age range (i.e., 20 to 64 years of age), gender (i.e., male/female) and all provinces [14].
A few of the studies indicate that compared to adults and preschoolers, ADHD diagnosis is higher in school-aged children [11,12,15,17,20] and young adults [12]. In particular, one study found that the highest overall annual prevalence rates are in children 5 to 14 years of age, which held true across time (1999-2012) and province (Manitoba, Ontario, Quebec, Nova Scotia) [15].
None of the studies looked at differences in prevalence by ethnic background. One study compared differences between first and second-generation immigrants in British Columbia with a non-immigrant sample [20]. Prevalence was highest for the non-immigrant sample (9.2%), followed by the second-generation sample (5.9%). The lowest prevalence rate was found for the first-generation sample (4.3%).

Socioeconomic and Regional Differences
Only one study looked at Canadian ADHD prevalence rates in relation to socioeconomic status (SES) and region (urban versus rural) [16]. This study did not find an SES gradient in ADHD diagnosis, but when restricted by region, a small negative gradient was found for urban living and a small positive gradient for rural living. Additionally, adults in the highest income bracket were less likely than those in other income brackets to receive a diagnosis of ADHD before age 18.

Provincial Differences
One study looked at differences in estimates across provinces and indicated that ADHD prevalence may be higher in certain provinces compared to others [15]. Findings were that ADHD prevalence amongst children is higher in Nova Scotia (3.8%) and Quebec (3.8%) compared to Manitoba (2.8%) and Ontario (1.1%). Amongst adults, prevalence is also higher in Nova Scotia (1.7%) compared to Manitoba (0.8%), Quebec (0.7%) and Ontario (0.5%).

Differences across Time
A number of the studies reviewed here indicate that ADHD diagnosis and the number of patients prescribed ADHD medication are increasing over time [11,12,15,17]. One study reported that the prevalence of ADHD diagnosis in Canada has increased amongst all age groups-4 to 17-year-olds (6.9-8.6%), 18 to 34-year-olds (5.7-7.3%), and 35 to 64-year-olds (5.2-5.5%)-from 2008 to 2015, particularly for children and young adults [12]. Another study indicated that this upward trend in diagnosis holds across provinces. The highest increase in ADHD prevalence across time (1999 and 2012) was found for Quebec (3.5%) [15]. This study suggests that this increase was associated with a shift across time in practitioner type primarily diagnosing ADHD, from specialists to general practitioners [15].

Overall ADHD Incidence in Canada
None of the papers included in this review provided an incidence rate for ADHD across all age groups or using a national sample.

Age Differences
The one paper that examined ADHD incidence did so across a wide age range (1 to 24 years) [15]. Findings were that incident ADHD diagnosis is highest in 5 to 9-year-olds (from 0.8% in Ontario to 2.1% in Quebec) and 10 to 14-year-olds (0.5% in Ontario to 1.5% in Quebec) compared to all other age groups examined. This held true across year of study (1999 to 2012) and province examined (Manitoba, Nova Scotia, Ontario, Quebec).

Differences across Time
Based on one study, there is some indication that ADHD incidence is rising across time (1999 to 2012) in Manitoba, Nova Scotia and Quebec but remaining stable in Ontario [15].
See Table 1 for articles regarding the prevalence and incidence estimates of ADHD in Canada.

Which Practitioners Are Diagnosing and Treating ADHD in Canada
According to Vasiliadis et al. (2017) [15], who reported on the incidence of ADHD in Canada, ADHD diagnoses are primarily made by general practitioners in the three provinces examined (Nova Scotia, Ontario and Quebec). In Nova Scotia, diagnoses seem to be primarily performed by general practitioners (69%), followed by pediatricians (27%) and then psychiatrists (3%) [15]. In Ontario, diagnoses also seem to be primarily performed by general practitioners (52%), then psychiatrists (24%), pediatricians (23%), and other specialists (<1%) [15]. In Quebec, the primary sources of incident ADHD diagnoses seem to be general practitioners (46%) and pediatricians (43%), followed by psychiatrists (8%) and other specialists (3%) [15]. These statistics are displayed in Figure 1. Only one study in this review [17] examined medication treatment trends for ADHD by practitioner type (in Alberta), as shown in Figure 2. Findings from this study indicate that pediatricians most frequently prescribe stimulants (39.3%), followed by general practitioners (33.5%).
Psychiatrists prescribe stimulants least frequently (17.2%). This is in contrast to prescriptions for antidepressants, of which most are provided by general practitioners (48.1%), followed by psychiatrists (27.2%) and then pediatricians (11%).
Another study included in this review looked at medications prescribed to patients with ADHD aged 1 to 24 years, and found that the majority (70%) are prescribed stimulant medication [18]. The study indicated that psychiatric consultation is positively associated with antidepressant and antipsychotic medication prescriptions. The presence of a comorbidity (i.e., having a diagnosis of anxiety or depression) and increased age were also found to be positively associated with antidepressant prescriptions. Only one study in this review [17] examined medication treatment trends for ADHD by practitioner type (in Alberta), as shown in Figure 2. Findings from this study indicate that pediatricians most frequently prescribe stimulants (39.3%), followed by general practitioners (33.5%).
Psychiatrists prescribe stimulants least frequently (17.2%). This is in contrast to prescriptions for antidepressants, of which most are provided by general practitioners (48.1%), followed by psychiatrists (27.2%) and then pediatricians (11%).
Another study included in this review looked at medications prescribed to patients with ADHD aged 1 to 24 years, and found that the majority (70%) are prescribed stimulant medication [18]. The study indicated that psychiatric consultation is positively associated with antidepressant and antipsychotic medication prescriptions. The presence of a comorbidity (i.e., having a diagnosis of anxiety or depression) and increased age were also found to be positively associated with antidepressant prescriptions.

Discussion
The aim of this review was to examine the prevalence and incidence rates of diagnosed ADHD in Canada in the past decade. Ten papers were included in this review that covered all provinces and age groups from preschoolers to older adults. Based on one paper across five provinces, overall ADHD prevalence in children and youth (ages 4 to 17 years) was estimated to be 8.6% [12], and based on one paper across all provinces, overall ADHD prevalence in adults (ages 20 to 64 years) is estimated to be 2.9% [14], which is similar to worldwide estimates in adults [10]. Findings from this review indicate that:

•
The prevalence of diagnosed ADHD varies across provinces (from 0.5% in Ontario to 3.8% in Nova Scotia).

•
Prevalence is higher amongst children and adolescents than adults, both within individual provinces and across Canada [12,15,18]. • Prevalence is higher amongst males than females at all ages, although the disparity may decrease with age [11,17,18,20].

•
The incidence of diagnosed ADHD in children and youth varies across provinces (from 0.4% in Ontario to 1.2% in Quebec) [15] and has increased over time.
ADHD prevalence and incidence estimates require further examination, as only two studies in the past decade have examined prevalence across all provinces. No study covers all provinces and territories, nor all age groups. These are important gaps, as findings from the group of studies reviewed here highlight significant variability in estimates by province and age [e.g. , 9]. All studies included here reported information on prevalence, but only one reported on incidence. Further estimates are needed, particularly of incidence, to clarify the number of individuals affected and the rate of growth of individuals with ADHD in the population. For a comprehensive understanding of the epidemiology of ADHD in Canada, a study is needed that includes data from all provinces and territories, and that looks across all age groups, taking a lifespan approach.
There are a number of future directions and questions that should arise from this review. While some of the studies looked at estimates in relation to gender, only the dichotomy of males versus females was used. Additionally, only one study included here

Discussion
The aim of this review was to examine the prevalence and incidence rates of diagnosed ADHD in Canada in the past decade. Ten papers were included in this review that covered all provinces and age groups from preschoolers to older adults. Based on one paper across five provinces, overall ADHD prevalence in children and youth (ages 4 to 17 years) was estimated to be 8.6% [12], and based on one paper across all provinces, overall ADHD prevalence in adults (ages 20 to 64 years) is estimated to be 2.9% [14], which is similar to worldwide estimates in adults [10]. Findings from this review indicate that:

•
The prevalence of diagnosed ADHD varies across provinces (from 0.5% in Ontario to 3.8% in Nova Scotia).

•
The incidence of diagnosed ADHD in children and youth varies across provinces (from 0.4% in Ontario to 1.2% in Quebec) [15] and has increased over time.
ADHD prevalence and incidence estimates require further examination, as only two studies in the past decade have examined prevalence across all provinces. No study covers all provinces and territories, nor all age groups. These are important gaps, as findings from the group of studies reviewed here highlight significant variability in estimates by province and age, e.g., [9]. All studies included here reported information on prevalence, but only one reported on incidence. Further estimates are needed, particularly of incidence, to clarify the number of individuals affected and the rate of growth of individuals with ADHD in the population. For a comprehensive understanding of the epidemiology of ADHD in Canada, a study is needed that includes data from all provinces and territories, and that looks across all age groups, taking a lifespan approach.
There are a number of future directions and questions that should arise from this review. While some of the studies looked at estimates in relation to gender, only the dichotomy of males versus females was used. Additionally, only one study included here reported on ethnicity as a variable. Importantly, one study in this review highlighted variability in prevalence rates related to immigrant and refugee status [20], and another emphasized the need to consider socioeconomic and regional (urban vs. rural) differences [16]. Another important factor needing attention in epidemiological research is the influence of mental health insurance coverage on access to service access [16]. Further research on these demographic and socioeconomic variables will help clarify the diverse needs of individuals with ADHD across the country and inform service delivery. Consideration of comorbid conditions will also be important for future studies, as approximately 65-80% of children and 85% of adults with ADHD have at least one comorbid psychiatric disorder (anxiety, depression, conduct disorder, etc.). Individuals with ADHD have a 13-year shorter estimated life expectancy due to both psychiatric and medical comorbidity [23].
Canadian studies on prevalence in the past decade indicate an upward trend in ADHD diagnosis and medication treatment across time and provinces. One possible explanation is that variations in study methodology play a role, as found in a recent systematic review and meta-regression analysis of worldwide studies [21]. It was concluded that variations in estimates across those studies can be largely explained by methodological differences, rather than time or geographical location. Methodological variability is notable both within and across the studies included in this review, particularly with regard to data sources and case determination. A unique contribution of this review is that all studies examined diagnosed ADHD. Screening studies that asked about symptoms related to ADHD without a diagnosis by a health professional were excluded because the presence of self-reported symptoms alone does not confirm a diagnosis of ADHD and can result in inflated estimates [24]. However, a few of the studies included here relied on self-or other-reported diagnosis. Only six studies out of ten based the presence of ADHD on a diagnosis performed by a health professional and recorded in the health system [12,[15][16][17][18]20]. To achieve a more accurate reflection of the prevalence/incidence of ADHD in Canada, it is important that future studies include only confirmed and documented cases of ADHD.
Differences in taxonomy used to identify ADHD across studies and within studies, across time, may also influence estimates. Some research suggests that prevalence estimates based on the International Classification of Diseases (ICD) may underestimate ADHD prevalence compared to those that rely on Diagnostic and Statistical Manual of Mental Disorders (DSM) criteria [24][25][26][27][28]. Additionally, diagnostic criteria change across time, potentially influencing estimates, such as the change in symptom onset and exclusion criteria from DSM-IV to DSM-V. Important to note is that this review captured only the prevalence of individuals diagnosed with or treated for ADHD, rather than the prevalence of all diagnosed and undiagnosed individuals with ADHD. For instance, individuals diagnosed in the school and mental health systems would not be captured in the six studies included here that relied on health administrative datasets [12,[15][16][17][18]20]. As discussed by Vassiliadis et al., prevalence estimates based on physician claims may reflect a suspected or working diagnosis rather than a confirmed one. Administrative datasets may also miss ADHD diagnoses because comorbidities of anxiety and depression are often present with ADHD, and claims may be made under billing codes for these conditions instead of ADHD [15]. While the use of a wide range of health administrative datasets and publicly accessible surveys across the country highlights opportunities for gathering comprehensive epidemiological data on ADHD nationwide, it may be ideal to link data from multiple sources to most accurately inform case identification.
Provincial differences in ADHD prevalence/incidence and treatments (e.g., prescriptions of stimulants and antidepressants) raise several questions regarding the types of services and number of practitioners available to diagnose and treat ADHD. While provincial differences may be an important factor influencing prevalence, Canada also has an additional consideration given its geography, including large distances between communities and the resulting impact on access to services [29]. However, the one study in this review that looked at regional differences found that those in rural or lower-income communities are more likely to be diagnosed with ADHD and medicated [16]. This finding may depend, in part, on the extent to which ADHD is viewed as a medical condition across regions and provinces [11], as well the extent to which parents prefer accessing ADHD support through medical settings versus school and community settings [11].
Upward trends across time in ADHD diagnosis and treatment may reflect improvements in diagnosis and in ADHD medications (e.g., the introduction of long-acting stimulants in 2003), since the use of stimulants is considered first-line treatment for ADHD. This trend may also be associated with the increased availability of mental health training for primary care practitioners and practice guidelines supporting the successful management of ADHD in primary care [30,31]. Consistent with this possibility, findings from one study reviewed here found a shift across time and provinces away from ADHD diagnosis, primarily from specialists to general practitioners [15]. This shift also coincides with practitioner-reported improvements in comfort and willingness to diagnose and treat ADHD in primary care [11,12,17,18,31]. While combined medication and psychosocial treatment is recognized as an important treatment approach for ADHD, the insufficient availability of specialty care resources and publicly-funded psychosocial treatments in Canada limits treatment options, which may point to an important gap in equitable ADHD care access in Canada [11]. More data are needed on diagnosis and treatment trends over time, across Canadian provinces, territories and regions, to inform future efforts regarding training practitioners and ensuring access to services [29,31].

Conclusions
The prevalence of ADHD in Canada based on studies published in the past 10 years varies depending on age group, gender, location and methodological approach.
Future Canadian ADHD prevalence and incidence research would benefit from: • Overall estimates across provinces and territories. • Estimates moderated by age, gender, ethnicity, geographical location (e.g., province/territory and urban vs. rural), socioeconomic status and access to mental health care insurance. • Standardized methods for defining ADHD cases, particularly with regard to diagnostic criteria (e.g., DSM and ICD) and data sources (e.g., record of a diagnosis by a health professional) • ADHD case identification using a validated algorithm based on pre-established criteria (currently considered best practice for case determination when using large primary care datasets) [12]. • Incidence estimates tracked with demographic data.