By: Adele Diamond
Most studies of attention-deficit/hyperactivity disorder (ADHD) have focused on the combined type and emphasized a core problem in response inhibition. It is proposed here that the core problem in the truly inattentive type of ADHD (not simply the subthreshold combined type) is in working memory. It is further proposed that laboratory measures, such as complex-span and dual-task dichotic listening tasks, can detect this. Children with the truly inattentive type of ADHD, rather than being distractible, may instead be easily bored, their problem being more in motivation (under-arousal) than in inhibitory control. Much converging evidence points to a primary disturbance in the striatum (a frontal–striatal loop) in the combined type of ADHD. It is proposed here that the primary disturbance in truly inattentive-type ADHD (ADD) is in the cortex (a frontal–parietal loop). Finally, it is posited that these are not two different types of ADHD, but two different disorders with different cognitive and behavioral profiles, different patterns of comorbidities, different responses to medication, and different underlying neurobiologies.
I join the growing chorus of those who argue that attention-deficit/hyperactivity disorder (ADHD) of the “truly” inattentive subtype (what I will call “attention-deficit disorder” [ADD]) is a different disorder from ADHD where hyperactivity is present (e.g., Barkley, 2001; Cantwell, 1983; Carlson, 1986; Carlson & Mann, 2000; Goodyear & Hynd, 1992; Hynd, Lorys, Semrud–Clikeman, Nieves, Huettner, & Lahey, 1991; in particular see the outstanding paper by Milich, Balentine, & Lynam, 2001). Not only is “ADHD without hyperactivity” (ADHD of the predominantly inattentive type) an awkward locution, but it also tries to squeeze ADD into a box in which it does not belong. The term ADHD should be reserved for when hyperactivity is present (as the term implies), regardless of whether inattention is also present.
The points I make in this paper include the following: many individuals currently diagnosed with the inattentive subtype of ADHD appear to be misdiagnosed. ADD appears to be an instance of childhood-onset “dysexecutive syndrome.” ADD and ADHD are characterized by dissociable cognitive and behavioral profiles, different patterns of comorbidities, different responses to medication, and different underlying neurobiological problems. The core cognitive deficit of ADD is in working memory. Contrary to what many have claimed (that laboratory tests cannot capture the core cognitive deficits in ADD), I argue that complex-span and dual-task dichotic listening tasks can indeed capture them. The working memory deficit in many children with ADD is accompanied by markedly slowed reaction times, a characteristic that covaries with poorer working memory in general. Individuals with ADD are not so much distractible as easily bored and underaroused. I hypothesize that the DAT1 gene will be found to be more closely linked to ADHD than to ADD, whereas the DRD4 gene will be found to be more closely linked to ADD than to ADHD, and that the primary neural circuit affected in ADHD is frontal–striatal, whereas the primary neural circuit affected in ADD is frontal–parietal.
The current DSM-IV (American Psychiatric Association [APA], 1994) diagnostic guidelines list three subtypes of ADHD: (a) primarily inattentive, (b) primarily hyperactive and impulsive, or (c) both combined. ADHD conceived in this manner is by far the most commonly diagnosed psychological/behavioral disorder of childhood (e.g., Barkley, DuPaul, & McMurray, 1990; Szatmari, 1992; Weiss & Hechtman, 1979).
Individuals with ADHD of the inattentive subtype tend to be disorganized, easily pulled off course, forgetful, and inattentive (DSM-IV; APA, 1994). They tend to be disorganized mentally and physically. They tend to make careless mistakes, and are not good at paying close attention to detail. They have difficulty organizing their work, setting priorities, planning out a strategy, and remembering to do all required tasks. They have difficulty organizing their things and tend to be sloppy. They have trouble keeping track of their belongings and forget where they have put them, in part because they rarely put things away. If multiple items are needed for an assignment or task, they will typically forget one or more. They have trouble keeping track of multiple things held in mind, which can make arithmetic calculation, reading, or abstract problem solving difficult.
Individuals with ADHD of the inattentive type also tend to have a hard time sustaining focused attention on a task or activity. They are quite poor at following through on something to completion. They tend to get bored with a task fairly quickly and often abandon a task unfinished, bouncing from one partially begun project to another. They may have a hard time keeping their mind on any one thing at a time. When doing homework or reading, they often find their minds wandering. Because focusing deliberate, conscious attention on completing a task is so arduous or aversive for individuals with the inattentive subtype of ADHD, they tend to try to avoid beginning a task, procrastinate, may forget to write an assignment down, forget to bring home the materials needed to complete an assignment, or lose materials needed for an assignment.
In 1986 Baddeley coined the term dysexecutive syndrome to refer to adults who seem to have a deficient “central executive” and who thus appear to be disorganized, easily pulled off course, forgetful, and inattentive. As far as I know, dysexecutive syndrome has always been used with reference to adults. I would like to suggest that children with ADHD of the inattentive subtype provide an instance of the dysexecutive syndrome in children. Dysexecutive syndrome patients may go off on tangents or lose their train of thought. Individual skills, such as encoding an item into memory or retrieving an item from memory, are intact. However, dysexecutive patients “have problems in initiating [a chore], monitoring their performance, and . . using such information to adjust their behavior. As most tests concentrate on the building blocks or component skills and are less concerned with the integration of these skills into real-life tasks, many [dysexecutive] patients … perform adequately on frontal lobe tasks … In contrast, many everyday activities involving executive abilities require patients to organize, or plan their behavior over longer time periods or to set priorities in the face of two or more competing tasks” (Wilson, Evans, Emslie, Alderman, & Burgess, 1998, p. 214). It is on such everyday activities that the dysexecutive deficit is most evident. Dysexecutive patients often start out performing a task well, but quickly become sidetracked. It is hard for them to stay focused on the task at hand, and they commonly must be reminded of what it was they were supposed to be doing.
The DSM-IV cutoffs for the inattentive, hyperactive, and combined subtypes of ADHD were derived largely from research with young males, who are more prone to hyperactivity and impulsivity than are girls or older males or females. Hence, some individuals get miscategorized as inattentive-type ADHD, despite being hyperactive for their gender or age, because they are not significantly more hyperactive or impulsive than young boys (e.g., Carlson & Mann, 2002; deHaas & Young, 1984; Milich et al., 2001; Weiss, Worling, & Wasdell, 2003). Such individuals should be considered the combined type. In this article, I focus on individuals with ADD (those who meet the criteria for inattentive-type ADHD and who are not hyperactive, excluding those with significant hyperactivity even if subthreshold for a combined-type diagnosis according to current DSM criteria). There is considerable overlap between what I am calling “ADD” and what others have called “slow cognitive tempo” (SCT; e.g., Milich et al., 2001), but SCT includes additional features that characterize only a subset of children with ADD. I reserve use of the term ADHD for ADHD that includes prominent hyperactivity (which for all practical purposes means combined-type ADHD because predominantly hyperactive ADHD is so rare after the age of 6).
Children with combined-type ADHD have many of the above symptoms, but they also have great difficulty sitting still (APA, 1994). They are overactive (motor and verbal), restless, and always on the go. They are overly talkative, fidgety, and squirmy. They often do repetitive motions like wiggling their feet or tapping their pencil. They get up when remaining seated is expected. They can talk incessantly and have difficulty playing quietly.
They also tend to be impulsive (APA, 1994) and are inclined to be very disorganized and sloppy, because they are often too impatient to carefully attend to detail or to put things away. They can have trouble waiting their turn, may blurt out an answer before hearing the whole question, and may interrupt others. They may intrude on others’ conversation or game, without considering beforehand that it might be inappropriate. Because they tend to act impulsively, they may run into the street without looking or grab a toy from another child.