ADHD Prevalence Estimates Across Three Decades
ADHD Prevalence Estimates Across Three Decades
We have updated the two most comprehensive systematic reviews of studies addressing the prevalence of ADHD around the world and were able to include 135 studies published from 1985 to 2012 in our final model. Subsequently, we conducted a meta-regression analysis to test the effect of time on variability of estimates and updated previous analyses to explore the effect of methods and geographical location of studies. Our findings indicate that when controlling for study methods, prevalence estimates did not vary as a function of year of study during the past three decades. Diagnostic criteria, impairment criterion and source of information remained significantly associated with heterogeneity of estimates. Adjusting for methodological characteristics of studies, geographical location was not associated with heterogeneity of prevalence estimates.
As far as we are aware, this is the first study to directly address the question of a potential increase in prevalence rates of ADHD over time. Concerns were generated from a number of studies which showed that rates of diagnosis and treatment are in fact increasing. Two USA national landline telephone surveys conducted in 2003 and 2007 asked parents whether or not a doctor or other healthcare provider had ever told them that their child aged 4 to 17 years (randomly selected for the survey) had 'attention deficit disorder or attention deficit hyperactive disorder'. Interviews were conducted with about 102 353 and 73 123 children in each respective year. The percentage of children with a lifetime diagnosis of ADHD increased from 7.8% to 9.5% from 2003 to 2007, a 21.8% increase. In 2007, among those with a current ADHD diagnosis, 66.3% were taking medication for the disorder, representing 4.8% of all children aged 4 to 17 years. Another study using the medical records of a health plan in California reported a relative increase of 24% in the incidence of physician-diagnosed ADHD in children aged 5 to 11 years from 2001 to 2010. These figures are in accordance with other studies conducted in the USA, UK and Canada using administrative data from the 1990s and 2000s and reporting increasing rates of ADHD diagnosis and prescription of medications for the treatment over time. However, these data do not represent the rates of true prevalence of the disorder as they rely on administrative data, physicians or parents who report the diagnosis. True prevalence rates are estimated by standardized procedures in representative samples of the community.
This study aggregated original studies that estimated the prevalence of ADHD by using standard diagnostic criteria, and identified no effect of year of study on the variability of estimates. This indicates that, controlling for study methods, the true prevalence of the disorder did not increase from 1985 to 2012. Considering that ADHD is a chronic disorder, the absence of modification in prevalence estimates by year of study indirectly suggests that no increase in the incidence of ADHD occurred over the past three decades. Given our findings, the increasing rates of diagnosis of ADHD in clinical and administrative samples are probably related to increasing awareness and access to services. However, it is not possible to rule out potential changes in clinical practices over time. Thus, the validity of ADHD diagnoses in several clinical settings in certain countries like the USA should be carefully assessed in studies designed for this purpose. The estimate of the rates of diagnosis and treatment does not, in general, exceed the estimates of the prevalence of the disorder in the majority of countries where these data are available. In fact, in some countries, rates of diagnosis and treatment are below the estimated prevalence rate, indicating lack of recognition of the disorder and of access to resources. However, surveillance studies of medical and non-medical use of medications for the treatment of ADHD in different cultures are necessary in order to expand on this issue.
Further results of the meta-regression analysis indicated that study methods are associated with significant heterogeneity of estimates, corroborating previously reported results that were already extensively discussed. Additionally, with the extension of the number of studies included, the current analysis had more power to investigate the effect of geographical location on the variability of estimates, especially for continents less represented in the previous analysis. The estimate for North America remained no different from those of Europe, Oceania, South America and Asia. Estimates from Africa and the Middle East, which increased by 1 and 7 studies, respectively, became no significantly different from that of North America. This is consonant with numerous studies that have addressed cross-cultural differences in regard to symptom expression and structure, identification and treatment of ADHD, and that generally demonstrate invariance across cultures. Nevertheless, because the vast majority of studies did not ascertain representative samples of each country, it is not possible to exclude the possibility that broader social characteristics impact on the occurrence of the disorder. This issue needs to be further explored.
Our study must be understood in the context of its limitations. First, we analysed cross-sectional studies conducted at different time points and locations using different methods. To address the question of modification of the incidence of the disorder over time, higher-quality data would need to come from repeated cross-sectional studies (conducted in the same location with the same age range and using the same methods), successive birth cohorts, and incidence studies. Unfortunately, we were not able to identify such data at the present moment. Therefore, by conducting a meta-regression analysis with a large number of studies, we were able to hold constant the effect of methods and study location to estimate the effect of time. Second, because a substantial proportion of studies did not report years when the sample was assessed, we used year of publication as a proxy. However, because the implicit error in this proxy measure is in the same direction for all studies with a possible small variation among them, we understand that this may not have significantly affected the results. Third, although more studies were included in the present analysis, there are still substantially fewer studies in particular geographical locations, especially Africa and Oceania. More studies in these continents are needed. Fourth, we cannot exclude the possibility that other methodological characteristics not included in the model or not even reported by the studies would be associated with heterogeneity of results.
In spite of these limitations, our results have important implications. First, it is necessary to monitor rates of diagnosis and treatment of the disorder. Second, the stability of the prevalence rates estimated by standardized diagnostic procedures over time indicates that a potential increase of cultural pressure or social expectations over children has not driven an increase of real cases of ADHD across the past three decades. Third, study methods are consistently associated with heterogeneity of prevalence estimates, and further standardization across studies is necessary. Fourth, geographical location is not associated with variability of ADHD prevalence, which is consistent with the notion that cultural or social aspects are not implicated in the aetiology of the disorder.
This is the most comprehensive review on ADHD prevalence studies conducted to date. During the past three decades, prevalence estimates did not vary as a function of time. Increasing rates of diagnosis and treatment of ADHD are likely a reflection of increasing awareness, access to treatment or changing clinical practices. There is no evidence to suggest an increase in the number of children in the population who meet criteria for ADHD when standardized diagnostic procedures are followed.
Discussion
We have updated the two most comprehensive systematic reviews of studies addressing the prevalence of ADHD around the world and were able to include 135 studies published from 1985 to 2012 in our final model. Subsequently, we conducted a meta-regression analysis to test the effect of time on variability of estimates and updated previous analyses to explore the effect of methods and geographical location of studies. Our findings indicate that when controlling for study methods, prevalence estimates did not vary as a function of year of study during the past three decades. Diagnostic criteria, impairment criterion and source of information remained significantly associated with heterogeneity of estimates. Adjusting for methodological characteristics of studies, geographical location was not associated with heterogeneity of prevalence estimates.
As far as we are aware, this is the first study to directly address the question of a potential increase in prevalence rates of ADHD over time. Concerns were generated from a number of studies which showed that rates of diagnosis and treatment are in fact increasing. Two USA national landline telephone surveys conducted in 2003 and 2007 asked parents whether or not a doctor or other healthcare provider had ever told them that their child aged 4 to 17 years (randomly selected for the survey) had 'attention deficit disorder or attention deficit hyperactive disorder'. Interviews were conducted with about 102 353 and 73 123 children in each respective year. The percentage of children with a lifetime diagnosis of ADHD increased from 7.8% to 9.5% from 2003 to 2007, a 21.8% increase. In 2007, among those with a current ADHD diagnosis, 66.3% were taking medication for the disorder, representing 4.8% of all children aged 4 to 17 years. Another study using the medical records of a health plan in California reported a relative increase of 24% in the incidence of physician-diagnosed ADHD in children aged 5 to 11 years from 2001 to 2010. These figures are in accordance with other studies conducted in the USA, UK and Canada using administrative data from the 1990s and 2000s and reporting increasing rates of ADHD diagnosis and prescription of medications for the treatment over time. However, these data do not represent the rates of true prevalence of the disorder as they rely on administrative data, physicians or parents who report the diagnosis. True prevalence rates are estimated by standardized procedures in representative samples of the community.
This study aggregated original studies that estimated the prevalence of ADHD by using standard diagnostic criteria, and identified no effect of year of study on the variability of estimates. This indicates that, controlling for study methods, the true prevalence of the disorder did not increase from 1985 to 2012. Considering that ADHD is a chronic disorder, the absence of modification in prevalence estimates by year of study indirectly suggests that no increase in the incidence of ADHD occurred over the past three decades. Given our findings, the increasing rates of diagnosis of ADHD in clinical and administrative samples are probably related to increasing awareness and access to services. However, it is not possible to rule out potential changes in clinical practices over time. Thus, the validity of ADHD diagnoses in several clinical settings in certain countries like the USA should be carefully assessed in studies designed for this purpose. The estimate of the rates of diagnosis and treatment does not, in general, exceed the estimates of the prevalence of the disorder in the majority of countries where these data are available. In fact, in some countries, rates of diagnosis and treatment are below the estimated prevalence rate, indicating lack of recognition of the disorder and of access to resources. However, surveillance studies of medical and non-medical use of medications for the treatment of ADHD in different cultures are necessary in order to expand on this issue.
Further results of the meta-regression analysis indicated that study methods are associated with significant heterogeneity of estimates, corroborating previously reported results that were already extensively discussed. Additionally, with the extension of the number of studies included, the current analysis had more power to investigate the effect of geographical location on the variability of estimates, especially for continents less represented in the previous analysis. The estimate for North America remained no different from those of Europe, Oceania, South America and Asia. Estimates from Africa and the Middle East, which increased by 1 and 7 studies, respectively, became no significantly different from that of North America. This is consonant with numerous studies that have addressed cross-cultural differences in regard to symptom expression and structure, identification and treatment of ADHD, and that generally demonstrate invariance across cultures. Nevertheless, because the vast majority of studies did not ascertain representative samples of each country, it is not possible to exclude the possibility that broader social characteristics impact on the occurrence of the disorder. This issue needs to be further explored.
Our study must be understood in the context of its limitations. First, we analysed cross-sectional studies conducted at different time points and locations using different methods. To address the question of modification of the incidence of the disorder over time, higher-quality data would need to come from repeated cross-sectional studies (conducted in the same location with the same age range and using the same methods), successive birth cohorts, and incidence studies. Unfortunately, we were not able to identify such data at the present moment. Therefore, by conducting a meta-regression analysis with a large number of studies, we were able to hold constant the effect of methods and study location to estimate the effect of time. Second, because a substantial proportion of studies did not report years when the sample was assessed, we used year of publication as a proxy. However, because the implicit error in this proxy measure is in the same direction for all studies with a possible small variation among them, we understand that this may not have significantly affected the results. Third, although more studies were included in the present analysis, there are still substantially fewer studies in particular geographical locations, especially Africa and Oceania. More studies in these continents are needed. Fourth, we cannot exclude the possibility that other methodological characteristics not included in the model or not even reported by the studies would be associated with heterogeneity of results.
In spite of these limitations, our results have important implications. First, it is necessary to monitor rates of diagnosis and treatment of the disorder. Second, the stability of the prevalence rates estimated by standardized diagnostic procedures over time indicates that a potential increase of cultural pressure or social expectations over children has not driven an increase of real cases of ADHD across the past three decades. Third, study methods are consistently associated with heterogeneity of prevalence estimates, and further standardization across studies is necessary. Fourth, geographical location is not associated with variability of ADHD prevalence, which is consistent with the notion that cultural or social aspects are not implicated in the aetiology of the disorder.
This is the most comprehensive review on ADHD prevalence studies conducted to date. During the past three decades, prevalence estimates did not vary as a function of time. Increasing rates of diagnosis and treatment of ADHD are likely a reflection of increasing awareness, access to treatment or changing clinical practices. There is no evidence to suggest an increase in the number of children in the population who meet criteria for ADHD when standardized diagnostic procedures are followed.
Source...