Asherson et al (Reference Asherson, Chen and Craddock2007) raise some important issues regarding adult attention-deficit hyperactivity disorder (ADHD). They state that some symptoms of bipolar disorder are similar to those of ADHD but the distinction is not difficult. However, although ADHD and classic euphoric mania (bipolar I) may be distinct, differentiation of ADHD and bipolar disorder may be difficult, especially in bipolar II, bipolar-spectrum disorder and episodes of mixed symptomatology. At times, it may be almost impossible to discriminate solely by symptoms. Irritability, excessive activity, impulsive behaviour, poor judgement and denial of problems are characteristic of both ADHD and bipolar disorder, thus making diagnosis difficult. The two also clearly occur together in some individuals: the reported overall lifetime prevalence of comorbid ADHD in people with bipolar disorder is 9.5% (Reference Nierenberg, Miyahara and SpencerNierenberg et al, 2005); comorbidity with unipolar disorder is also frequent.

Asherson et al state that ADHD is a persistent trait whereas bipolar disorder is episodic. However, inter-episodic symptoms are common in bipolar disorder and the course of both bi- and unipolar disorder is frequently chronic; for example, up to 13% of people with bipolar disorder report continuous cycling without a well phase and 54% are not fully euthymic between episodes (Reference Kupka, Bolen and AltshulerKupka et al, 2001).

Children of mothers with bipolar I disorder have increased rates of both unipolar disorder and ADHD, further suggesting a neurobiological overlap of these three diagnoses. Hirshfeld-Becker et al (Reference Hirshfeld-Becker, Biederman and Henin2006) report significantly higher rates (23.5%) of ADHD in offspring of parents with bipolar disorder compared with psychiatric comparison parents (8.4%) and non-psychiatric comparison parents (4.2%).

Drug treatments also overlap. Stimulant-type medication has been used in bipolar depression, and newer medications such as atomoxetine have similar pharmacological characteristics to some antidepressants (Reference Lydon and El-MallakhLydon & El-Mallakh, 2006). Catecholaminergic antidepressants are not only potentially of benefit in ADHD but may be less likely to destabilise bipolar disorder.

There is thus a clinical and neurobiological overlap between ADHD, bipolar and unipolar disorder. Asherson et al's timely editorial has reminded us that ADHD in adults should not be overlooked and that further research is needed to clarify its impact on other adult psychopathology and comorbidity, particularly in mood disorders.