Irritability in children: what we know and what we need to learn

Irritability can be defined as increased proneness to anger, relative to peers. Clinically, it manifests as developmentally inappropriate temper outbursts and sullen, grouchy mood; thus, it includes both behavioral and mood components. Related constructs are mood dysregulation, which is broader than irritability, and aggression, which encompasses only behavioral manifestations. Anger proneness has a defined developmental trajectory, peaking in the preschool period and declining thereafter, with a modest increase during adolescence. Irritability is a common reason for mental health evaluation in children, and pediatric irritability is associated with both concurrent and future impairment. In the 1990s, American researchers suggested that pediatric bipolar disorder does not present with distinct manic episodes as in adults, but instead with severe, chronic irritability. However, post-hoc analyses of epidemiological studies found associations between pediatric irritability and risk for subsequent anxiety and depression, but not for bipolar disorder. Similarly, in studies comparing the two dimensions of oppositional defiant disorder (ODD) (i.e., irritability and headstrong behavior), irritability predicts subsequent anxiety and depression, while headstrong behavior predicts attention-deficit/hyperactivity disorder (ADHD) and conduct disorder. Thus, the diagnosis of bipolar disorder should be reserved for youth (and adults) with distinct manic episodes, rather than chronic irritability. Genetically informative studies link irritability and depression. Twin studies document that longitudinal associations between irritability and both anxiety and depression have a genetic component. These studies also find that the heritability of irritability is approximately 40-60%, similar to anxiety or unipolar depression. Irritability is a diagnostic criterion for multiple disorders in youth, including anxiety disorders, major depressive disorder, and ODD. It is also common in youth with ADHD, bipolar disorder, conduct disorder, and autism. However, for children and adolescents, the validity and clinical utility of a diagnostic category characterized primarily by irritability remains an important and unanswered question. Historically, that category has been ODD, which is conceptualized as a disruptive behavior disorder. However, ODD consists of two dimensions, only one of which, irritability, has genetically mediated longitudinal associations with depression and anxiety. Also, severe irritability has significant cross-sectional associations with anxiety disorders. These considerations call into question the appropriateness of combining irritable and headstrong features into one disorder, and of categorizing a diagnosis characterized primarily by irritability as an externalizing, disruptive behavior disorder, rather than as a mood disorder. Given these complexities, it is not surprising that DSM-5 and ICD-11 take different approaches to diagnosing youth whose primary problem is severe irritability. Reflecting the American controversy about pediatric bipolar disorder, the mood disorder section of DSM-5 includes a new diagnosis, disruptive mood dysregulation disorder (DMDD), characterized by severe, chronic irritability. DMDD captures youth whose irritability causes impairment comparable to that of youth with bipolar disorder and, hence, is more severe than that of most youth with ODD. Given overlap between DMDD and ODD, ICD-11 instead includes a specifier to the diagnosis of ODD denoting chronic irritability. To evaluate these different nosologic strategies, future research should focus on their utility in predicting treatment response. Two neuroscience-based formulations can guide research on the pathophysiology of irritability. One conceptualizes irritability as an aberrant response to frustration, the emotion elicited when goal attainment is blocked, as when an expected reward is withheld. The second conceptualizes irritability as an aberrant approach response to threat: whereas healthy organisms approach a threat (i.e., attack) only when it is inescapable, irritable individuals may attack in a broader range of contexts. In an animal model with translational potential for studying irritability, threat and frustration were found to interact in determining an animal’s behavior. Specifically, compared to non-frustrated rodents, those who experienced “frustrative non-reward” (i.e., did not receive an expected reward) showed increased motor activity and were more likely to attack a conspecific. Such hyperactivity and increased propensity for aggression may be analogous to the behavior exhibited by a frustrated child experiencing a temper outburst. Functional magnetic resonance imaging research has examined associations between irritability and neural responses to frustration (e.g., rigged games) and threat (e.g., angry faces). Work with frustrating tasks shows associations between irritability and dysfunction in striatum, anterior cingulate cortex, amygdala, and parietal lobe, consistent with irritable youths’ deficits in reward processing and in maintaining attentional control when frustrated. Irritable youth are also more likely than their non-irritable peers to view ambiguous faces as angry and, like youth with anxiety disorders, to attend preferentially to angry faces. One direction for future research would be identifying the extent to which abnormalities in reward or threat processing differentiate subtypes of irritable youth. Also, an important question is whether the brain mechanisms mediating irritability vary across diagnoses and/or when irritability co-occurs with another trait, such as anxiety. Early data suggest that the pathophysiology of irritability varies across such contexts. Given the very recent inclusion of DMDD in DSM-5, limited controlled trials exist. However, tentative recommendations stem from studies focused on treating irritability in the context of other disorders, including ADHD and major depressive disorder. Considerable data indicate that stimulants reduce irritability in youth with ADHD. This suggests that, while