Chapter 16: ADHD and Executive Function Disorders

INTRODUCTION

Attention deficit disorder (with or without hyperactivity) first appeared in the Diagnostic and Statistical Manual of Mental Disorders (DSM) III in 1980,¹ though reports of pathological inattention in children have appeared as early as the 18th century by physicians Melchior Weikard in Germany and Alexander Crichton in Scotland.² Sir Alexander Crichton wrote in 1798 about patients who are, from birth, “incapable of attending with constancy to any one object of education.”³ Of the disorder, he wrote: “…but, it seldom is in so great a degree as totally to impede all instruction; and what is very fortunate, it is generally diminished with age.”³ In time, the diagnosis evolved through subsequent DSM editions to be divided into three subtypes: predominantly inattentive, predominantly hyperactive-impulsive, and combined type. The diagnostic criterion regarding age of symptom onset was most recently expanded from 7 years to 12 years old.⁴ Given the natural variability of attentional traits and abilities between individuals,^5,6 ongoing debate exists as to what severity of deficits “cross the threshold” to become a psychiatric illness.⁷

This chapter will review aspects of executive function disorders with a main focus on attention-deficit hyperactivity disorder (ADHD). Executive functioning refers to the brain processes that integrate, control, and regulate cognitive, affective, and behavioral functions. Although some models⁸ consider attention to be in the domain of executive functioning, in this chapter we separate attention from executive function in a clinically meaningful way. Attention, at the basic level, starts with a modicum level of arousal that allows an individual to focus on a stimulus. Attention may be thought of as a prerequisite for most other cognitive abilities. More complex aspects of attention include where to pay attention in the visual world, what to pay attention to, the ability to divide attention, the sustaining of one’s focus or attention for periods of time, and the ability to switch attention, with much overlap between these functions. Deficits in various aspects of attention are associated with ADHD, although problems with attention also commonly result from many other developmental and acquired conditions.

EPIDEMIOLOGY

The DSM-5 reports a prevalence of ADHD in about 5% of children and 2.5% of adults based on a meta-analysis of global epidemiologic studies.^4,9,10 Prevalence rates in Europe and North America are comparable, while the Middle East and Africa share lower rates, possibly due to smaller sample sizes and/or sociocultural factors.¹⁰ National registries and insurance provider databases tracking mental health disorders show significant increases in the number of ADHD diagnoses in the past 50 years, while epidemiologic studies remain mixed about whether prevalence, itself, has increased.^10,11 This increase in diagnoses with an unclear increase in prevalence established from epidemiologic studies suggests a trend toward overdiagnosis of ADHD in children and adolescents in developed countries, possibly explained by misinterpretation of normal developmental behavior as pathology, misleading information from caregivers’ ...