Irritability: An inability,or an ability?

Irritability is an experience we are all familiar with. It occurs somewhat predictably with particular physiological triggers such as hunger, lack of sleep and withdrawal from substances such as nicotine and caffeine. It is also common in the context of psychological stressors such interpersonal conflict and bereavement. Irritability that emanates from these many factors is thought to lower the threshold for experiencing anger and increase our sensitivity to both internal and external stimuli (Toohey and DiGiuseppe, 2017).

As an experience, irritability is typically unpleasant for the individual and equally so for those around them, which begs the question – why it occurs in the first place? One possibility is that if one is under physiological or psychological stress, the experience of being irritable may serve as a signal, to both the individual and others, that something is wrong. In other words, irritability signifies an unhealthy or unwanted change and provides the individual with the ability to correct matters and restore homeostasis. Conceptualised as such, irritability, although unpleasant, serves a useful purpose – analogous to pain. But again, like pain, this does not appear to be the case when irritability occurs in the realm of psychopathology, where, as yet, it is inadequately understood and lacks a clear definition.

Irritability is truly transdiagnostic and features prominently within the criteria of many psychiatric disorders that vary considerably both in terms of their nature and when they occur developmentally (i.e. at what age). And, in these contexts, irritability is typically conceptualised as a symptom of the underlying disorder. Thus, it is regarded as a manifestation of pathophysiological processes – although as an experience, it remains a significant contributor to the functional impairment inflicted on the individual. Indeed, irritability is often the primary complaint of psychiatric patients seeking treatment (Judd et al., 2012), as it causes palpable distress due to both the subjective experience of being irritated, and the impact that this has on the individual’s interpersonal dealings and day-to-day functioning. At the same time, it is interesting to note that even though irritability features in a wide variety of psychiatric diagnoses, and seemingly lacks any specificity for any particular disorder or developmental age, it is often nominated as a key criterion for diagnosis, as discussed previously in the context of mood (Malhi and Bell, 2019). This is especially intriguing given that many of the psychiatric disorders within which irritability features have growing evidence that they may have distinct neurobiological and psychosocial drivers and origins.

As a consequence of this broader lack of specificity, irritability has been difficult to define and measure, and research examining the phenomenon is usually limited to the context of specific disorders within which it features. This means that irritability is studied within a particular disorder or environment, and even if this were to be understood fully, it would not necessarily inform our understanding of irritability more generally – as a construct that occurs in both disorders and health. Hence, evidence regarding its underlying mechanisms and how it manifests is scattered throughout different areas of psychiatry and the psychiatry of healthy behaviours, and this makes it difficult to unify. One potential way to approach this problem is to take a broad approach and to examine irritability across health and illness, to examine whether it is phenomenologically the same and importantly, whether it appears to serve the same ‘function’ in various contexts.

If irritability theoretically serves as a signal in healthy individuals that ‘something is wrong’, and enables them to correct the problem, then this may also be true for irritability in the context of psychiatric illness. It is possible that here too, irritability may signal ‘something is wrong’, perhaps at a neurobiological level, but, unlike in health, the underlying dysfunction is perhaps less amenable to correction. In other words, in addition to raising the alarm that ‘something is wrong’, irritability signals ‘an inability’ to rectify the underlying dysfunction. This then potentially explains why irritability perseveres and is such a persistent symptom once it emerges, and also why it is associated with significant impairment and distress. Of course, this is entirely speculative and theoretical, and discernible phenomenological differences between irritability in health and illness are yet to be determined. But the idea that a phenomenon that serves a useful purpose in health, instead causes impairment in illness, is not new, as exemplified by phantom pain (Costigan et al., 2009).

In healthy individuals, pain clearly serves a purpose: to signal that potential or actual damage to bodily tissues is occurring. Pain in this context is clearly and by design unpleasant and distressing, but this is necessary in order to override other signals, and the warnings conveyed by pain may be essential for survival. However, this function becomes problematic when it continues in the context of a body part that no longer exists, such as an amputated limb (Costigan et al., 2009). The phenomenon of phantom pain causes significant distress, as the sensations the pain is signalling have no value and cannot be addressed. The processes of phantom pain may serve as a model for the occurrence of irritability in the context of psychiatric illness and may help explain why irritability is not specific to any particular psychiatric disorder, or even type of disorder. Akin to phantom pain, irritability may be an aberration of communication. However, once again, it is important to emphasise that this is pure conjecture and that these ideas are posited solely to encourage discussion.

Such theorising is, however, necessary because understanding irritability and providing a basis for future treatments is essential, not only because of the impairment and distress that directly results from being irritable, but also because ‘solving irritability’ may provide key insights into the various psychiatric disorders within which it manifests. If irritability does indeed serve as a signal for an underlying dysfunction that leads to a psychiatric illness, it could serve as a marker for intervention both early in the course of the illness and preceding an acute episode. And this could provide significant benefit across a broad range of psychiatric illnesses spanning from childhood to adulthood. Hence, it is important to consider irritability as a cardinal symptom, and one that warrants closer scrutiny, because in addition to being an impairment (an inability), it may provide an ‘ability’ to intervene.