Situating adult attention-deficit/hyperactivity disorder in the externalizing spectrum: Etiological, diagnostic, and treatment considerations.

Adult attention-deficit/hyperactivity disorder (ADHD) has a population prevalence of 5%. However, its prevalence is much higher in mental health and substance use treatment settings. It is associated with significant physical and psychiatric morbidity, as well as social, occupational, and legal consequences. Adult ADHD is considered to be a part of the externalizing spectrum with which it shares both homotypic comorbidity and heterotypic continuity across the lifespan. This is attributable to a shared genetic basis, which interacts with environmental risk factors such as nutritional deficiencies and psychosocial adversity to bring about epigenetic changes. This is seen to result in a lag in brain maturation particularly in the areas of the brain related to executive functioning (top-down regulation) such as the prefrontal and cingulate cortices. This delay when coupled with impairments in reward processing, leads to a preference for immediate small rewards and is common to externalizing disorders. Adult ADHD is increasingly understood to not merely be associated with the classically described symptoms of hyperactivity, impulsivity and inattention, but also issues with motivation, emotional recognition and regulation, excessive mind wandering, and behavioral self-regulation. These symptoms are also observed in other disorders which overlap with the externalizing spectrum such as oppositional defiant disorder, conduct disorder, antisocial and borderline personality disorder. It is therefore important to develop both broad-based and specific interventions to be able to target these deficits which can reduce the burden and improve outcomes.