Attention Deficit Disorder Without Hyperactivity:
ADHD, Predominantly Inattentive Type

By Jennifer Wheeler, M.A., and Caryn L. Carlson, Ph.D.
The University of Texas at Austin

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In 1980, the third edition of the Diagnostic and Statistical Manual of Mental Disorders (DSM-III) [American Psychiatric Association (APA), 1980] introduced a major change in the conceptualization of the childhood disorder that had previously been called "hyperkinesis" or "hyperactivity". Because experts had begun to speculate that attention deficits, rather than high activity level, might play a greater role in the problems of these children, the term "attention deficit disorder" (ADD) replaced the earlier diagnostic terminology. Along with this shift in diagnostic emphasis came the recognition that attention deficits could exist even in the absence of high activity level, and thus two ADD subgroups were defined: ADD with hyperactivity (ADDM) and ADD without hyperactivity (ADD/WO). While the ADD/H category was fairly consistent with previous definitions, the latter subtype represented essentially a new category, for which there was no previous counterpart. Thus, almost everything we know about ADD/WO is based on research conducted since 1980.

From the start, the inclusion of ADD/WO as a valid diagnostic category has been the subject of considerable debate. Because of controversy surrounding how useful or "real" the subtype was, the 1987 revision of the DSM-III (DSMIII-R; APA 1987) excluded ADD/WO all together. Instead, the idea of a single type of ADD was reestablished and the term "Attention-deficit Hyperactivity Disorder" (ADHD) was introduced. Although the DSM-IIIR did include an "Undifferentiated ADD (UADD)" category, this was essentially a residual grouping for which there were no formal diagnostic criteria. In retaining this diagnosis, DSM-III-R acknowledged the fact that researchers and clinicians might wish to continue to identify children diagnosed as ADD/WO so that more research could be obtained to evaluate the utility of its diagnosis.

However, such a classification scheme led to problems in the field as it was inconsistent with previous research supporting a behavioral distinction between the ADD subtypes. In addition, it was problematic clinically, in that there was no particular symptom constellation required for a diagnosis of ADHD, thus allowing children who were previously diagnosed as having ADD/WO to be inappropriately included in the ADHD category¹. In response to such difficulties, the most recent edition of the DSM (DSM-IV; APA 1994) returned to DSM-III-type terminology. Current criteria delineates two separate symptom lists, inattention and hyperactivity-impulsivity, and allows children to be classified into one of three categories based on the presence or absence of symptoms in each of these areas: ADHD, Combined Type (ADHD,C), which requires children to display at least six out of nine inattentive and six out of nine hyperactive-impulsive symptoms; ADHD, Predominantly Inattentive Type (ADHD,IA), which requires at least six out of nine inattentive symptoms and less than six hyperactive-impulsive symptoms; and ADHD, Hyperactive-Impulsive Type (ADHD,HI), which requires the presence of six out of nine hyperactive-impulsive symptoms and less than six inattentive symptoms. The ADHD,IA category is presumed to identify children diagnosed with DSM-III as ADD/WO, ADHD,C is thought to identify children given DSM-III ADDM or DSM-III-R ADHD diagnoses, and ADHD,HI is essentially a new category, presumed to identify extremely active children who do not display gross inattention. Although some of the research discussed below was conducted with children diagnosed as ADDM, ADD/WO, and ADHD, in this paper we will use the terms ADHD,C and ADHD,IA to refer to all attention-disordered children with and without hyperactivity, respectively.

If the above discussion confuses you, you are not alone. There has been a great deal of debate and disagreement among professionals about the nature of the ADHD,IA subtype. However, despite the changes in nomenclature, a good deal of research progress has been made. The most common research strategy has been to compare children with ADHD,IA to children with ADHD,C on important cognitive and behavioral dimensions. In these studies, questions regarding the similarities and differences between the subtypes have been addressed. In particular, research has focused on how we might expect children with ADHD,IA to develop, what problems they might face, and what treatments will be the most effective.

Activity Level

One area in which fairly consistent findings have emerged involves the "behavioral activity style" demonstrated by ADHD,IA children. While children with ADHD,C are often described as boisterous, impulsive, excitable, and overactive, children with ADHD,IA are much less likely to receive such labels. In fact, studies have found that children with ADHD,IA are often described as "hyperactive", i.e., they are likely to show "underactivity" and be described as sluggish, lethargic, and daydreamy². This unexpected finding has generated some interesting speculation about the ADHD,IA subgroup and further distinguished them from their ADHD,C peers. While children with ADHD,IA typically do not differ in activity level from nondisordered children, they are most notably not impulsive, a key characteristic of children with ADHD,C. The so called impulsivity of children with ADHD,IA is typically more related to disorganization than the physical impulsivity of children with ADHD,C. For example, raters are more likely to endorse items such as an inability to complete tasks and poor organizational skills when rating children with ADHD,IA, and items such as acts without thinking, shifts from one activity to another, and frequently interrupts others when rating children with ADHD,C³. This distinction was clarified in DSM-IV when a factor analysis⁴ revealed that many of the DSM-III "impulsivity" items clustered with "hyperactivity", and others (e.g., "difficulty organizing tasks") clustered with inattention. This led to the current use of a single-symptom list for hyperactivity-impulsivity versus two separate listings (impulsivity and hyperactivity) used in DSM-III.

Accompanying Disorders

Research has shown that accompanying behavior disorders are likely to be found in both ADHD,C and ADHD,IA children. Thus, children with ADHD,IA are less likely to display "externalizing" problems, such as aggression and conduct disorders⁵. Some research also finds that among clinic-referred children, ADHD,IA children are more likely than ADHD,C children to display "internalizing" problems, such as depression and anxiety⁶. This theory is supported by research conducted by Barkley and his colleagues (1990) who, using a clinic-referred sample, found that the relatives of children with ADHD,C were more likely to suffer problems with substance abuse, aggression, and ADHD,C, than the relatives of children with ADHD,IA and nondisordered controls. In contrast, the relatives of children with ADHD,IA were more likely to have anxiety disorders than the relatives of children in the other groups. Still, findings on co-occurring internalizing problems between the ADHD subtypes are inconsistent. For example, a recent large-scale school-based study⁷ found that teachers rated ADD,C children as showing more anxiety/depression than ADHD,IA children. This finding led the authors to speculate that it may be only among ADHD children who are referred to clinics that this pattern of greater internalizing problems among ADHD,IA children is displayed⁸. Thus, it may be that children with ADHD,IA who also display high levels of anxiety and/or depression are more likely to be referred to clinics than children with ADHD,IA alone. However, when identified in a general population, ADHD,IA children may not necessarily show greater levels of anxiety/depression than children with ADHD,C.

Peer Relationships

Since the problematic peer relationships of children with ADHD,C has been consistently found, it has been of interest to researchers to examine what, if any, peer relationship problems might characterize children with ADHD,IA. Of the studies that have examined this issue, the overall conclusion appears to be that both ADHD subtypes are less popular with their peers; however, the nature of their unpopularity seems slightly different⁹. For example, there is some evidence that the peer relationship problems of children with ADHD,C are more severe than those of children with ADHD,IA¹⁰. When rated by their peers, some studies find that children with ADHD,C are more "actively disliked" than children with ADHD,IA and nondisordered controls, whereas children with ADHD,IA are more "socially withdrawn". Based on these findings, some researchers have suggested that the nature of the ADHD subtypes' social deficits may, in fact, be qualitatively different. Using Gresham's (1988) model of social skills, Wheeler and Carlson (1994) proposed that children with ADHD,C may suffer from social performance deficits (i.e., they know what to do but do not use this knowledge appropriately), whereas children with ADHD,IA may suffer from deficits in both social performance and knowledge (i.e., they don't know what to do in social situations, thus they cannot use this knowledge appropriately). The authors further speculate that these deficits may be mediated by so-called interfering responses, which correspond to symptoms typically associated with the disorder. Thus, impulsivity and hyperactivity may prevent a child with ADHD,C from waiting his turn in line, even though he knows he is supposed to do so, and "sluggishness" and anxiety may prevent a child with ADHD,IA from participating in enough social interactions to learn the rules of the game. While this hypothesis has not been empirically tested, it clearly has important treatment applications. For example, if children with ADHD,C are found to possess adequate social knowledge, attention should be directed at decreasing what prevents them from interacting appropriately, rather than focusing exclusively on training in social skills. In contrast, if children with ADHD,IA are found not to know what to do in social situations, social skills training will be the most useful.

School Performance

We have evidence that children with ADHD,IA, like those with ADHD,C, often experience school learning problems. Indeed, some research has indicated that children with ADHD,IA may show elevated rates of school failure and are rated by teachers as having greater problems in learning, relative to nondisordered controls. One relatively recent finding is the strong relationship between inattention and academic problems. Studies examining the behavioral correlates of the ADHD subtypes have consistently shown greater academic difficulties in children diagnosed with ADHD,C and ADHD,IA than children with excess motor activity/impulsivity (ADHD,HI) alone¹¹. With respect to intellectual functioning, there is little evidence for significant IQ differences between the ADHD groups, however, there is some research in support of a higher rate of learning disabilities in children with ADHD,IA¹². In addition, children with ADHD,IA are typically more similar behaviorally to children with learning disabilities than are children with ADHD,C¹³. However, the specific nature of the relationship between learning disabilities and ADHD has not been clearly established. More research needs to be conducted before any firm conclusions can be drawn.

Etiology

The precise etiology, or cause, of ADHD is unknown. Numerous theories have been proposed, including those relating to abnormal brain development (e.g., an "immature brain"), neurochemical abnormalities, exposure to environmental toxins, and deficient childrearing practices. Although research has been conducted in all of these areas, no firm conclusions cam yet be drawn. Also, most studies have been done with children with ADHD,C, resulting in even less knowledge about the etiology of ADHD,IA. Some researchers have speculated ADHD,IA and ADHD,C are entirely different disorders, in which case they may stem from different causes. The most promising theories to date include exposure to various agents that can lead to brain injury (e.g., trauma, disease, fetal exposure to environmental toxins), diminished brain activity, and heredity¹⁴.

Research on children with ADHD,C has found that they seem to have underactive orbital-frontal regions, the part of the brain which is thought to be responsible for sustaining attention, inhibiting behavior, self-control, and planning¹⁵. This finding has been documented in several studies in which children with ADHD were shown to have diminished electrical activity and blood flow in this region, relative to nondisordered controls.

Again, less research exploring the possible cause of ADHD,IA has been conducted. Given the "sluggish" cognitive tempo and frequent achievement problems typically occurring in this subtype, some researchers have speculated that children with ADHD,IA may suffer from posterior, rather than frontal, right hemispheric dysfunction¹⁶. However, studies using neuropsychological measures that have examined this hypothesis have failed to find the predicted deficits¹⁷. Thus, research involving more sensitive measures of neurological processes are needed before any firm conclusions regarding etiology can be drawn.

It should also be noted that many previous theories of ADHD have not been supported. Thus, there is no conclusive evidence that ADHD can be caused by diet, hormones, lighting, motion sickness, or bad parenting¹⁸. Given the exciting new advances in our ability to examine the way the brain works, we can be hopeful that continuing research in these areas will soon expand our understanding of the causes of ADHD.

Treatment Considerations

One of the most pressing issues in developing effective treatments for children with ADHD,IA has been whether or not these children will show similar responses to stimulant medication as do children with ADHD,C. The best evidence that we have concerning this issue comes from a study in which the responses of children with ADHD,C and children with ADHD,IA were evaluated to 5, 10, and 15 mg doses of methylphenidate (Ritalin)¹⁹. While the groups did not significantly differ on any of the outcome measures, children with ADHD,IA were more likely to be nonresponders (24%) or to respond best to the lowest dose (35%) as compared to children with ADHD,C. In contrast, 95% of the ADHD,C children were judged to be positive responders, with the majority (71%) recommended to receive a moderate or high dose. Thus, it may be that at least a portion of children with ADHD,IA respond favorably to stimulant medication, although at a lower dosage than children with ADHD,C. One factor that may mediate this subgroup's responsiveness is the presence of co-occurring internalizing disorders (e.g., anxiety) when they exist. There is some evidence that children with ADHD who display accompanying internalizing symptoms are less likely to respond positively to stimulant medication than are children who suffer from ADHD alone²⁰. In any case, stimulant responsiveness is clearly a matter in which more research needs to be done. For any given ADHD child, decisions about the usefulness of medication should be made based on am individual child's responsiveness.

It is likely that the development of other effective treatments for children with ADHD,IA will depend upon the individual pattern of the accompanying problems they display. The information reviewed above suggests several areas of functioning that might be considered relevant for evaluation, including school functioning, the presence of other disorders (e.g., anxiety or depression), and peer relationship problems. Based on assessment in each of these areas, treatment programs can be individually tailored to meet each child's specific needs. As is becoming increasingly clear with children with ADHD,C, it is likely that children with ADHD,IA will be heterogeneous in many ways and that there will be wide individual variation in the types of accompanying problems they display. While we are still in the early stages of understanding the disorder, heightened interest in this area and the recent publication of DSM-IV should lead to increasing knowledge about its causes and responsiveness to treatments.

Conclusions

The above review addresses research in many areas affecting children with ADHD. While few studies have been conducted using DSM-IV terminology, preliminary reports suggest that the findings are consistent. According to these studies, both ADHD subtypes are generally less popular than their peers, suffer difficulties academically, and frequently display accompanying problems. Given the relatively recent identification of ADHD,IA, it is mandatory that further research be conducted. Although these children generally show fewer overt behavior problems than their ADHD,C peers, they are just as in need of diagnosis and treatment.

Author's Note: This article is a revised version of one published previously in two ADHD parent support group newsletters, the ADDA (Winter, 1993) and HAAD ENOUGH (Jan/Feb 92).

Jennifer Wheeler is a graduate student in clinical psychology at The University of Texas at Austin. Her research and clinical interests include social skills and their application to children with ADHD.

Dr. Caryn L. Carlson received her Ph.D. from The University of Georgia in 1984 and is currently an Associate Professor of Psychology at the University of Texas in Austin. She has published work related to the assessment and treatment of children with ADHD.

Footnotes:

¹ Lahey, Carlson & Frick in press Lahey Schaughency Hynd, Carlson, & Nieves, 1987 Lahey et al., 1990

² Lahey, Schaughency, Strauss, & Framo, 1984; Lahey et al., 1987

³ Stanford & Hynd, 1994

⁴ Lahey et al., 1988

⁵ Baumgaertel, Wolraich, & Dietrich, 1995: Gaub & Carlson, 1996; Lahey et al., 1987; 1994; McBurnett, Pfiffner, Swanson, Ottoni, & Tamm, 1995

⁶ Lahey et al., 1987

⁷ Gaub & Carlson, 1996

⁸ Gaub & Carlson, 1996

⁹ King & Young, 1982; Carlson Lahey, Frame, Walker, & Hynd, 1987;1989

¹⁰ Carlson et al., 1987;1989, Gaub & Carlson, 1996

¹¹ Baumgaertel et al, 1995; Gaub & Carlson, 1996; Lahey et al., 1994; McBurnett et al., 1995

¹² Goodyear & Hynd, 1992

¹³ Stanford & Hynd, 1994

¹⁴ Barkley, 1995 ¹⁵ Barkley, 1995 ¹⁶ Schaughency & Hynd, 1989 ¹⁷ see Goodyear & Hynd, 1992 for a review ¹⁸ see Barkley, 1995, for a more comprehensive review ¹⁹ Barkley, DuPaul, & McMurray, 1991

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