Cortical excitability in adult patients with attention-deficit/hyperactivity disorder (ADHD)

Attention-deficit/hyperactivity disorder (ADHD) is more and more focused on, and the awareness of adult patients with ADHD increases. Deficits in inhibitory processes in cortical brain areas are discussed as possible causes for ADHD. An easy measurement of these processes is provided by transcranial magnetic stimulation (TMS). We applied single- and double-pulse TMS to the left motor cortex while an electromyogram (EMG) was taken at the abductor pollicis brevis muscle (APB) of the right hand. Intracortical inhibition (SICI) and facilitation (ICF) were measured in ten adult ADHD patients and ten healthy participants using inter-pulse intervals of 2 and 3ms (SICI), and 8 and 15ms (ICF). Furthermore, resting motor threshold (RMT) and latency of the motor evoked potential (MEP) following magnetic stimulation were compared. t-Tests were calculated for statistical analysis. TMS measurements resulted in impaired inhibition in ADHD patients, whereas there were no differences in facilitation, RMT and MEP-latency between groups. Large variability in the patient group was found. This study expands the findings of deficits in inhibition described in earlier studies in children to an adult population, which could be a hint for similar neurophysiological mechanisms underlying ADHD symptomatology in children and adults.