Introduction: ADHD in an Adolescent Finnish Population

Abstract

Attention-deficit/hyperactivity disorder (ADHD) is a common, chronic behavioral disorder characterized by inattention, hyperactivity, and impulsivity. A substantial amount of controversy still surrounds ADHD as questions regarding its diagnostic validity continue, and others concerning medication treatment of the syndrome are voiced. The current special section brings to light several new findings that may provide new insight into thinking about ADHD. First and foremost, ADHD is a clearly recognizable condition with a relatively stable prevalence throughout the world. Using DSM-IV diagnostic criteria in a population-based birth cohort of adolescents (ages 16Y18) in northern Finland, the estimated prevalence of ADHD is 8.5%. The clinical characteristics of ADHD in this population isolate in Finland are remarkably comparable to that found in other adolescent samples drawn from around the world with more males than females affected, persistence into adolescence (from childhood) in some two thirds of cases, comparable subtype distributions, and strong associations with other psychiatric disorders, including anxiety, depression, substance abuse and dependence, and conduct and oppositional disorders. Smalley et al. (2007) describe these findings along with a detailed description of the birth cohort and sampling methods. The second major point to be highlighted by the articles in this special section is that an analysis of ADHD symptomatology at a population level, although varied in level of impairment, comorbidity, and other features, reveals ADHD to be best described as a unitary construct rather than being composed of multiple subtypes. Previous studies suggesting that numerous latent classes were necessary to capture symptom data may have been limited because of the statistical methodologies used. The development of a robust statistical procedure that incorporates both factor analysis and latent class analysis (factor mixture modeling) enables a more rigorous evaluation of putative classes or groups (subtypes) in ADHD. At least by the time of adolescence, there appears to be only a need for two classes (affected and unaffected) at a population level based on behaviors reported using the Strengths and Weaknesses of ADHD-Symptoms and Normal-Behavior (SWAN) survey, and no need for specific subtypes based on inattentive or hyperactiveimpulsive symptoms, a crucial distinction for genetic studies. These data presented by Lubke and colleagues (2007) demonstrate the powerful role newly developed statistical approaches can have on interpreting data. The third major point revealed in this series of articles is the striking evidence that executive function deficits are by no means ubiquitous in ADHD and that although they occur with greater frequency (~50%) in that sample than among normal controls (~8%), they are not evident for about half of the adolescents with ADHD. These data, presented by Loo and colleagues (2007), highlight the need for further research on the cognitive endophenotypes in ADHD and highlight the