Kuan & Young point out that further research into the role of mood symptoms in attention-deficit hyperactivity disorder (ADHD) is essential. In a recent study of 141 adults with ADHD, 95% were found to have mood symptoms, chiefly mood instability (Reference KooijKooij, 2007). We observe that in adult ADHD mood instability frequently responds to stimulants over the same time course as core ADHD symptoms, an observation reported by others. This has led to the suggestion that mood dysregulation might represent a core impairment in adult ADHD, perhaps related to the same processes that cause dysregulation of other executive processes.

Despite these observations the relationship of ADHD to mood disorders is controversial. The controversy has arisen in the context of paediatric bipolar disorder, where the distinction from ADHD is made difficult if one chooses to view irritability as a sufficient manifestation of bipolar disorder and if the requirement for episodicity is not strictly applied. However, available validation studies for the construct of paediatric bipolar disorder use elation and/or grandiosity as cardinal symptoms, rather than irritability. Narrowly defined paediatric bipolar disorder can be differentiated from ADHD, shows longitudinal stability and has plausible familial aggregation patterns (Reference Geller and TillmanGeller & Tillman, 2005; Reference Geller, Tillman and BolhofnerGeller et al, 2006). Recent evidence suggests that the narrowly defined disorder can be distinguished at the behavioural and electro-physiological level from broadly construed disorder (Reference Rich, Schmajuk and Perez-EdgarRich et al, 2007). Conversely, it has been argued that the intensity of irritability (Reference Mick, Spencer and WozniakMick et al, 2005) and its temporal pattern (chronic or episodic) can distinguish paediatric bipolar disorder from ADHD (Reference Leibenluft, Cohen and GorrindoLeibenluft et al, 2006). The family study of Hirschfeld-Becker et al (2006) is intriguing, yet the sample size is small (12 families with bipolar I disorder, 11 with bipolar II disorder), and further work is needed to clarify the rates of ADHD among relatives with narrowly defined v. broadly defined bipolar disorder.

One of the main questions to be addressed relates to how valid a diagnostic concept broadly defined bipolar disorder is, or whether mood instability/irritability in the presence of ADHD may be more adequately described by a new dimension, such as mood dysregulation (Reference Brotman, Schmajuk and RichBrotman et al, 2006). Until the relevant empirical data become available, we see merit in maintaining the classic definition of mania, so that a diagnosis of bipolar disorder requires euphoria, grandiosity and episodicity, and the differential between ADHD and bipolar disorder remains explicit.