A systematic review of global publication trends regarding long-term outcomes of ADHD

Abstract

There is increased global recognition of attention deficit hyperactivity disorder (ADHD) as a serious medical condition with long-term consequences. Although originally conceived of as a childhood disorder, ADHD is being increasingly recognized in adults. Individual geographic regions may have specific interests and objectives for the study of ADHD. A systematic review of long-term outcomes (LTOs) in ADHD was conducted to evaluate research on ADHD LTOs on a global scale. Studies that were at least 2 years in duration were examined. A total of 351 studies were identified in the final analysis. We identified nine outcomes of interest and classified studies by specific geographical regions, age groups studied and study design by region and over time. Published studies of LTOs in ADHD have increased in all geographical regions over the past three decades, with a peak number of 42 publications in 2008. This rise in publications on ADHD LTOs may reflect a rise in global interest and recognition of consequences and impairment associated with ADHD. Although many world regions have published on ADHD LTOs, the majority of studies have emerged from the US and Canada, followed by Europe. While investigators in the US and Canada were predominantly interested in drug addiction as a LTO, European researchers were more interested in antisocial behavior, and Eastern Asian investigators focused on both of these LTOs as well as self-esteem. Geographical differences in the focus of ADHD LTO studies may reflect regional variations in cultural values. Proportionally fewer prospective longitudinal studies and proportionally more retrospective and cross-sectional studies have been published in more recent decades. Finally, more studies focusing on ADHD in adolescents and adults have been conducted in recent years, and particularly adolescents in Eastern Asia. These changes in basic study design may reflect an increase in the recognition that ADHD is a lifetime chronic disorder. This systematic review analysis of publication trends in ADHD LTOs reflects geographically based interests that change over time.

Keywords: ADHD; TDAH; attention deficit; hyperactivity; hyperkinetic.

Figure 1

Total number of studies per year by geographical region. There was a marked rise in studies of long-term outcomes of ADHD published worldwide between 1980 and 2010. The number of long-term outcome studies published globally peaked in 2008 with 42 studies published that year. The number of studies published in all of three regions shown appears to be generally continuing to rise. Studies involving international collaborations started being published in 2002.

Figure 2

Interests of specific geographical regions. The total number of outcome results was highest for drug use/addictive behavior, followed by academic outcomes, antisocial behavior, social function, occupation, self-esteem, driving, services use, and obesity. In Europe, the most studies examined antisocial behavior; in Northern America non-medicinal drug use/addictive behavior; and in Eastern Asia self-esteem, non-medicinal drug use/addictive behavior, and antisocial behavior. The highest proportion of studies from all other regions together focused on academic outcomes.

Figure 3

Changes in study design over time. There are proportionally fewer and fewer prospective longitudinal studies being published. Proportionally more and more retrospective and cross-sectional studies are being published.

Figure 4

Participant ages by year. There are more and more studies of adolescents and adults being published, while the number of studies of children remains relatively stable. Adults are often studied retrospectively, which may explain the results shown in Figure 3.

Figure 5

Participant ages by region. The proportion of studies of different age groups is similar between Northern America and Europe, but studies from Eastern Asia and “Other” countries around the globe include a large proportion of adolescents and fewer children and adults.