Does irritability determine mood depending on age?

Irritability features prominently in many psychiatric disorders such as mania, disruptive mood dysregulation disorder, oppositional defiant disorder, generalised anxiety disorder and post-traumatic stress disorder, but its precise definition is widely debated. In the Diagnostic and Statistical Manual of Mental Disorders, Fifth Edition (DSM-5) irritability is defined as ‘easily annoyed and provoked to anger’, but interestingly, its status as a core symptom is dependent upon the nature of the encompassing mood episode (i.e. manic, depressed or mixed) and is curiously modulated by age. Hence, in this brief article, we describe the diagnostic status of irritability across different mood states and age groups and examine why its use is so variable, with a view to identifying areas for future research.

Irritability from adolescence to adulthood

Irritability is a key feature in adolescent depression. That is to say, according to DSM-5 diagnostic criteria, the core feature of low mood/feeling depressed can be substituted by irritability, and it is widely used in clinical practice to define adolescent depressive disorders. In essence, Diagnostic and Statistical Manual of Mental Disorders (DSM) assigns irritability in youth the same ‘weight’ as depressed mood, reflecting perhaps its significance in this age group. Conversely, irritability is not regarded as a key feature of adult depression despite research suggesting that it is commonly reported in this age group (Fava et al., 2010). And for reasons that are altogether unclear, irritability does not feature at all in DSM-5 Major Depressive Disorder (MDD). Instead, inability to enjoy or gain pleasure becomes a key feature alongside low mood. Additionally, irritability is not an item in the Hamilton Rating Scale for Depression or Montgomery-Asberg Depression Rating Scale.

Does the omission of irritability suggest that the transition from adolescence to adulthood in the context of a depressive illness somehow connotes a loss of irritability? One possible explanation for this is that symptoms of anxiety are prevalent as an admixture among depressive symptoms in adolescence, and perhaps irritability is tied to anxiety in this age group. Recent findings support the direct association between anxiety disorders in general and irritability over and above the presence of depressive or oppositional defiant disorder symptoms (Cornacchio et al., 2016).

Longitudinal and epidemiological research evidence indicates that anxiety is often a precursor and possibly an antecedent to depression. From a developmental perspective, studies have identified different irritability symptom trajectories measured from toddlerhood to middle childhood: low (60.8%) or moderate (21.0%) irritability at age 3 that reduces by age 9, high irritability at age 3 that remains stable through age 9 (10.6%), high irritability at age 3 that decreases by age 9 (5.4%) and high irritability at age 3 that increases significantly by age 9 (2.2%) (Wiggins et al., 2014). Given that irritability has been observed to vary with age, perhaps irritability diminishes with age, making it less likely to present in adulthood as compared to adolescence. It may be that because the neurobiology of depression is in a state of flux during adolescence, it is simply too difficult to pristinely identify a depressed state. Alternatively, irritability in youth may reflect difficulties in response to emotional stimuli and a more disorganised behavioural response to inner dysphoria predicated on immaturity of cognitive response capacity. It may lastly reflect the impact of temperamental or personality-based comorbidity.

Interestingly, recent research findings highlight that irritability at ages 8–9 years predicted anxious/depressive symptoms at age 13–14 years. Indeed, this was the strongest pathway between irritability and anxious/depressive symptoms, despite measuring the association from childhood through to young adulthood, suggesting that puberty may play an important role in the link between irritability and internalising symptoms. These findings further suggest that irritability may play a causal role in the development of internalising symptoms (Savage et al., 2015). However, once again, younger individuals may simply struggle to define their emotional state and instead feel perplexed and irritable – symptoms that are perhaps easier to understand and express.

Irritability and mood episodes in adulthood

Further complications arise because, although irritability is not used to define major depression in adults, it is used to define hypomania/mania, even though the symptom itself is not defined differently and is common in MDD. In DSM-5, irritability is in fact a key feature of mania/hypomania alongside elation/euphoria, and hence it is one of the key items in the Young Mania Rating Scale. One possible explanation is that irritability in mania/hypomania aims to capture and identify mixed states? But, if this is the case, it is puzzling why it remains in mania but is excluded as a feature of the newly introduced mixed features specifier. A number of research groups have recently shown that irritability is a key feature of mixed states (e.g. Malhi et al., 2015). But given current DSM-5 taxonomy, this complicates matters because despite being present in an array of divergent disorders, irritability can function as a core diagnostic feature of a manic episode but cannot be used as a feature to identify depressive symptoms in the context of hypomania/mania. Again, it is possible that in the context of both mania/hypomania and depression, coterminous irritability does indeed reflect anxiety co-mixed with mood symptoms (Malhi et al., 2015). However, although this theoretical explanation provides a reasonable rationale for understanding how the symptom migrates across diagnostic states within mood disorders and is used variably in classification and practice, it requires further investigation. Irritability spawning from within anxiety, and culminating in mixed states, signals that a better understanding of this phenomenology may provide new insights into the nature of mood disorders. Closer and clearer alignment with anxiety would perhaps be more useful, and its fluctuating role in defining mood disorders needs to be both clarified and simplified.

Future research in irritability

There is a notable lack of research examining irritability as an intrinsic construct. Instead, research to date has focused on irritability as symptomatic of particular disorders such as depression in youth or externalising disorders. Given its association with many mental disorders, it may be worthwhile considering irritability as a transdiagnostic dimensional construct underlying the development of psychopathology. Interestingly emerging findings suggest that pubertal cognitive, emotional and personality development may also be a key factor that can influence the association between irritability and psychopathology. Accordingly, this process should begin with an understanding of age-appropriate norms, allowing accurate definition of irritability, essentially delineating normal from abnormal irritability across key developmental periods through to adulthood. More persistent irritability may also result from traits such as low frustration tolerance, and high reward sensitivity and the role of temperament or personality needs to be delineated. Furthermore, inconsistent definitions of this construct have led to varying methods of measuring irritability, which in turn makes it difficult to compare results across research studies. It can occur at different levels of severity, frequency, either subjectively or more overtly, and as an underlying trait or state dependent phenomenon. Another important component to understanding irritability involves examining the underlying biological, cognitive and personality mechanisms that contribute to this emotional state. More broadly, this type of research could potentially allow us to identify individuals at risk of developing psychopathology and perhaps even identify the key developmental period for which age-appropriate interventions would be most relevant.