Aggression in children with attention-deficit/hyperactivity disorder

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    Review

    www.expert-reviews.com ISSN 1473-7175 2010 Expert Reviews Ltd 10.1586/ERN.10.146

    Attention-deficit/hyperactivity disorder (ADHD) is one of the most common and most studied neurodevelopmental disorders of childhood [13], affecting between 3 and 10% of the general childhood population [46]. The Diagnostic and Statistical Manual of Mental Disorders IV characterizes ADHD as a unitary construct that can be further divided into three subtypes [7]: predominately inattentive (ADHD-I); predomi-nately hyperactive/impulsive (ADHD-HI); and combined type (ADHD-C) [7,8]. To meet diag-nostic criteria for the disorder, individuals are required to exhibit developmentally inappropri-ate levels of inattention, hyperactivity and/or impulsivity prior to age of 7 years, and symptoms must be present for at least 6 months prior to the diagnosis. Symptoms should be present in more than one context (e.g., home and school), must be associated with significant impairment and should not be better accounted for by another disorder [7]. Aside from the inattentive, hyperactive and

    impulsive symptoms most commonly associated with ADHD, a key question for researchers and clinicians in the area is whether or not children with ADHD exhibit higher levels of aggressive behavior than typically developing children. This question is important because it can have

    implications for characterizing the disorder and for treatment development. Furthermore, research generally suggests that aggressive behav-ior in children with ADHD is not a spurious occurrence; rather, that children with ADHD tend to engage in high levels of aggression [4,9,10]. In fact, it is often aggressive behavior that drives treatment referrals to health and mental health practitioners and determines the type of treat-ment administered, especially when parents no longer feel able to manage their childs aggressive behavior [11,12]. Based on these findings, it has been suggested that problems with aggression may be specific to ADHD itself and not simply a result of comorbid conduct problems (CPs), such as oppositional defiant disorder (ODD) and con-duct disorder (CD) [13]. In sum, it is apparent that aggression is often an important consider-ation when assessing and treating children with ADHD. In fact, several leading researchers and clinicians have concluded that aggression, par-ticularly impulsive aggression, is a serious public health concern and that it should be a key target when treating children with ADHD [12,14]. The purpose of this article is to selectively

    review studies of aggression in children with ADHD. We first discuss how aggression is defined and conceptualized. Next, we discuss

    Sara King1,2 and Daniel A Waschbusch31Wood Street Centre, Nova Scotia, Canada 2Dalhousie University, Nova Scotia, Canada 3Florida International University, FL, USA Author for correspondence: Wood Street Centre, 225 Wood Street, Truro, Nova Scotia, B2N 7H8, Canada

    Tel.: +1 902 896 7040 sara.king@cehha.nshealth.ca

    Research shows that aggression is an important associated feature of attention-deficit/hyperactivity disorder (ADHD) and is important in understanding the impact of the disorder and its treatment. The occurrence of aggressive behavior in combination with ADHD does not appear to be spurious and the severity and/or presence of aggression and ADHD may significantly impact long-term prognosis. This article defines subtypes of aggression in relation to ADHD, identifies individual differences contributing to aggressive behavior in children with ADHD and discusses selected possible underlying mechanisms of aggression in ADHD, as well as current and emerging treatment approaches. Although aggressive behavior in children with ADHD is common, the reasons for this are not yet well understood. Multidisciplinary research should focus on investigating underlying mechanisms related to aggression in ADHD, as well as the utility of various treatment modalities.

    Keywords: ADHD aggression children comorbidity disruptive behavior

    Aggression in children with attention-deficit/hyperactivity disorderExpert Rev. Neurother. 10(10), 15811594 (2010)

    THeMed ArTICLe y ADHD

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  • Expert Rev. Neurother. 10(10), (2010)1582

    Review King & Waschbusch

    aggression in the context of ADHD, individual differences in aggression use in children with ADHD, proposed underlying mechanisms of aggression in this population and the treatment of aggressive behavior in children with ADHD. It is important to note that our intention is not a comprehensive review, but rather a selective one to provide an overview of the topic. Where appropriate, we have included references for more comprehensive reviews of specific topics in this area.

    Definition & conceptualization of aggressionHuman aggression can be defined as behavior with the proximate (i.e., immediate) intent of causing harm to another person [15,16]. Furthermore, the target must be motivated to avoid the aggressive behavior [15]. Although there are situations in which aggressive behavior can serve an adaptive function (i.e., all humans display aggression, ranging from mild irritation to verbal or physical aggression, at some point) [12], maladaptive aggression can have a serious impact on the individual and others in their environ-ment (e.g., family members and peers). Maladaptive aggression in children has been shown to have a significant impact on social development and is associated with negative outcomes, such as school difficulties [17,18], cognitive deficits [12], peer victimization and rejection [12], and antisocial behavior [1921]. Several taxonomies have been used to define subtypes of aggres-

    sion, including: reactive/proactive, in which reactive aggression can be defined as impulsive, angry, hot aggression used in response to a perceived threat and proactive aggression can be defined as nonangry, goal-oriented, cold aggression [2224]; hos-tile/instrumental, in which hostile aggression can be defined as aggression with the intention of inflicting pain or injury (with no benefit to the aggressor) and instrumental aggression can be defined as aggression that provides some advantage to the aggres-sor [15,16,25,26]; and overt/covert, in which overt aggression can be defined as visible aggression (e.g., physical assault or temper tantrums), whereas covert aggression can be defined as hidden aggression (e.g., cheating, lying or vandalism) [27]. Each of these taxonomic systems has proven to be useful in the study of aggres-sion; however, much research is needed to determine how these classification systems fit together.

    Aggressive behavior in children with ADHDDespite some debate concerning the best way to conceptualize human aggression in general [16] and in the context of ADHD, there is near universal agreement that it is essential to consider aggressive behavior when working with children with ADHD, both in research and clinical practice. This is because children with ADHD appear to be at risk for engaging in high levels of aggressive behavior, even when comorbid CPs, such as ODD and CD, are taken into account [4,9,10]. Furthermore, aggressive behavior that co-occurs with ADHD appears to be more debilitating compared with aggressive behavior that occurs without ADHD [28,29]. Whereas inattentivehyperactiveimpulsive symptoms are con-

    sidered to be the primary symptoms of ADHD, research sug-gests that the severity and/or presence of aggression also seriously impacts long-term development. Aggression and ADHD have

    historically been conceptualized as distinct but correlated dimen-sions of externalizing behavior, as reviewed by Hinshaw [28]. Other research, however, serves to further clarify the relation between the two dimensions by suggesting that emotional dys-regulation in children with ADHD is at least partially responsible for the aggressive behavior observed in this population [5,3032]. Specifically, Barkley asserts that emotional dysregulation in chil-dren with ADHD is a manifestation of impulsivity extended to another domain (i.e., emotions); just as children with ADHD are behaviorally and cognitively impulsive, so too are they emotion-ally impulsive [32]. Emotional impulsivity or emotional dysregu-lation is often expressed through maladaptive behaviors, such as frequent and intense temper tantrums and being quick to anger, which are in turn risk factors for aggression. Thus, there is clear evidence that ADHD and aggression are distinct constructs, but there is also evidence that the primary features of ADHD put children at high risk for problems with aggression. Further support for the hypothesis that at least some of the

    aggressive behavior observed in children with ADHD is related to impulsivity comes from studies of the dichotomies described above. Specifically, reactive aggression appears to be more strongly associated with ADHD than other types of aggression, suggesting that the impulsivity and dysregulation associated with the disor-der are partly responsible for aggression in this population. For example, a study conducted by Dodge and colleagues found that children who exhibit higher levels of reactive aggression also tend to exhibit more inattention and impulsivity problems than proac-tively aggressive children or nonaggressive children [33]. Likewise, another study examining the relationship between reactive aggres-sion and ADHD, ODD and CD [24] found that 6672% of reac-tively aggressive children met diagnostic criteria for ADHD plus ODD/CD, whereas 13% met criteria for ODD/CD only and 6% met criteria for ADHD only. Interestingly, those children who met the criteria for ADHD plus ODD/CD and reactive aggression were shown to be more impaired overall (e.g., poorer peer relationships and classroom behavior) than ADHD plus ODD/CD children who did not engage in reactive aggression. However, attention and impulse control difficulties do not fully account for differences between reactively aggressive children and other children on measures of psychosocial adjustment, social cognition or developmental history, lending further credence to the suggestion that reactive aggression and ADHD are related but separate constructs [4]. Laboratory studies have also highlighted individual differences

    in the use of aggression subtypes in children with ADHD. Results of a study examining aggression in children with ADHD only, ADHD plus ODD/CD and typically developing controls [34] indicated that all children, regardless of diagnostic group, used reactive aggression in response to high levels of provocation, but only boys with comorbid ADHD plus ODD/CD engaged in reac-tive aggression in response to low levels of provocation. Boys with the comorbid condition were also more likely to hold a grudge (i.e., they behave aggressively for a longer period of time) follow-ing provocation. Research using the hostile/instrumental aggres-sion dichotomy has also demonstrated that aggressive boys with

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  • www.expert-reviews.com 1583

    ReviewAggression & ADHD

    and without ADHD can be reliably distinguished from typically developing children [26]. Specifically, aggressive boys with and without ADHD have been shown to engage in higher levels of instrumental aggression compared with typically developing con-trol children; however, only those children with ADHD showed higher rates of hostile aggression. These findings suggest that children with ADHD plus ODD/CD may be more prone to using reactive aggression in response to provocation, and that this characteristic may be important in distinguishing this group from children who meet criteria for ADHD only or ODD/CD only. Additionally, results such as these suggest that the focus of treatment may need to be specifically tailored to the type of aggression children exhibit. Psychostimulant treatment was examined as part of a labora-

    tory study conducted by King and colleagues [35]. Specifically, the reactive/proactive and instrumental/hostile dichotomies were examined in a single study using a computerized analogue aggres-sion task similar to that used by previous researchers [9,26,36,37]. Results indicated that children with ADHD on placebo exhibited increased reactive aggression following high levels of provocation compared with control children, but children with ADHD on medication (methylphenidate [MPH]; Ritalin) did not. Over the last several trials of the task, only instrumental aggression dis-sipated in controls, whereas only hostile aggression dissipated in children in the placebo group. Both types of aggression dissipated for children in the medication group. Although preliminary, these results highlight the complex nature of aggression use in children with ADHD and make it clear that aggression use in this group depends on a large constellation of factors, a selection of which are described below.

    Factors contributing to development & maintenance of aggressive behavior in ADHDComorbid conduct problems Research suggests that the co-occurrence of comorbid ADHD and CPs is high, with findings indicating that the comorbid-ity between ADHD and CPs is more common than not [38]. Estimates suggest that the prevalence of comorbid ADHD plus ODD is approximately 60% in clinically referred children [39], and the prevalence of comorbid ADHD plus CD is approxi-mately 20% [39]; this co-occurrence is much higher than would be expected by chance [4]. Research consistently shows that the presence of comorbid CPs (e.g., ODD and CD) greatly increases the risk that children with ADHD will show frequent and severe aggression [4]. Research is less clear about whether aggression and other antisocial outcomes are a risk for children with ADHD after CPs are taken into account. However, at least some studies indicate that ADHD predicts aggression and antisocial outcomes even after controlling for co-occurring CPs [4044], although not all studies are consistent [4548]. Interestingly, the type of aggres-sion seems to be differentially associated with different types of CPs in youths with ADHD. That is, some research has found that adolescents diagnosed with ADHD plus CD tend to engage in more overt physical aggression compared with typical adolescents, whereas adolescents diagnosed with ADHD plus ODD are more

    likely to engage in verbal aggression compared with typical ado-lescents [49]. Therefore, the presence of comorbid CPs is a salient factor in understanding and predicting aggression use in children and adolescents with ADHD [49]. Along the same lines, studies have shown that children with

    ADHD plus CPs tend to use aggression differently than children who do not have comorbid diagnos...

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