Neuromodulation in Attention-Deficit/Hyperactivity Disorder: Toward a Precision Psychiatry Approach

Attention-deficit/hyperactivity disorder (ADHD) is a highly heritable and common neurodevelopmental disorder, defined by age-inappropriate levels of inattention, hyperactivity, and impulsivity interfering with the individual functioning, which typically emerges in childhood but persists over the lifespan. The clinical presentation of ADHD is very heterogeneous, as other aspects characterize its phenotype, including emotional dysregulation, mind wandering, and sleep difficulties. Moreover, ADHD symptomatology partially overlaps with that of other conditions and is very frequently associated with other psychiatric disorders. Even though the current systems of classification in psychiatry have been of value in facilitating communication between clinicians and researchers, they failed in establishing the validity of their diagnostic categories beyond the clinical level. In this context, the approach proposed by the National Institute of Mental Health (NIHM) called Research Domain Criteria (R-Do-C) emerged as a useful framework, as it assumes that mental disorders are biological conditions involving brain circuits that implicate specific domains of cognition, emotion, and behavior, which cannot be constrained by the current DSM categories. In this perspective, ADHD should be considered as a dimensional disorder affecting several brain circuits, and its treatment should target all dimensions involved. Even though treatment with stimulants is a mainstay of treatment of ADHD, their use is still controversial because of concerns for potential side effects that may lead to their interruption. In this context, Noninvasive Brain Stimulation (NIBS) appears as a promising tool, useful for improving some ADHD dimensions of pathology. In fact, both transcranial magnetic stimulation (TMS) and transcranial direct current stimulation (tDCS) can modulate cortical excitability and brain network activity. In this chapter, we summarize the available data in the literature regarding the therapeutic applications of TMS and tDCS in ADHD treatment, and their potential in addressing the underlying nervous system dysfunction characterizing ADHD.