ADHD is defined in DSM-5 by developmentally inappropriate and impairing levels of inattentive and hyperactive-impulsive symptoms. However, emotional dysregulation is considered to be an associated feature of the disorder that supports the diagnosis of ADHD. The common co-occurrence of emotional liability (EL) in ADHD raises the question of whether EL should be viewed as a component of ADHD or reflecting a comorbid condition.

To address the question of whether EL should be viewed as a third dimension of ADHD.

We investigated the association of EL with ADHD and impairment scores, in a sample of adults with ADHD that had been carefully selected for absence of comorbid conditions that could give rise to EL using both rating scale and experience sampling methods to measure emotional instability. We reviewed the effects of stimulants and atomoxetine on EL and the covariation of EL with ADHD inattention and hyperactivity-impulsivity. We further considered the phenotypic and genetic association of EL with ADHD using population twin data.

From these studies, we found that EL is strongly associated with ADHD even in non-comorbid cases and gives rise to additional impairments after ADHD symptoms are controlled for in the analysis. Stimulants and atomoxetine both improve EL and these improvements are correlated with changes in ADHD symptoms, indicating a shared treatment response. Genetic model fitting suggests a common pathway model, consistent with a single genetic liability for inattention, hyperactivity-impulsivity and EL.

Taken together these findings suggest that EL can be viewed as a third dimension of ADHD. Patients presenting with chronic emotional instability should always be screened for ADHD.

The author has not supplied his declaration of competing interest.