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.