Somatosensory evoked potentials in attention deficit/hyperactivity disorder and tic disorder

Objective Both attention deficit/hyperactivity disorder (ADHD) and chronic tic disorder (TD) are hyperkinetic disorders. These disorders often coexist with each other and frequently have sensory components. Therefore, we hypothesized that they might have a common pathophysiology involving the somatosensory system, especially hyper-excitabilities of primary somatosensory area. Methods To evaluate sensory system excitability, we examined somatosensory evoked potentials (SEP) elicited by median nerve stimulation in 18 children with ADHD and 18 children with TD. Results Three children with ADHD and 8 children with TD showed giant SEP and the peak-to-peak amplitude for N20–P25 was also significantly greater than that obtained from normally developing children (P < 0.05 for ADHD and P < 0.01 for TD). Children with TD had significant left-ward asymmetry of N20–P25 (P < 0.01) and higher left-hemispheric N20–P25 than children with ADHD (P < 0.05). Conclusions Although hyper-excitability of the primary somatosensory area is a common characteristic for ADHD and TD, its severity, especially in the left-hemisphere, differs (i.e. TD has left-ward hyper-excitability). Significance The possibility remains that hyper-excitability of the primary somatosensory area is a reason why these disorders often coexist with each other and left-ward hyper-excitability of the primary somatosensory area is a unique feature of TD described for the first time.