The ipsilateral silent period in boys with attention-deficit/hyperactivity disorder

OBJECTIVE Characterize maturation of transcallosal inhibition (ipsilateral silent period [iSP]) in attention deficit/hyperactivity disorder (ADHD) using transcranial magnetic stimulation (TMS). BACKGROUND Maturation of the iSP is related to acquisition of fine motor skills in typically developing children suggesting that dexterous fine motor skills depend upon mature interhemispheric interactions. Since neuromotor maturation is abnormal in boys with ADHD we hypothesized that iSP maturation in these children would be abnormal. We studied iSP maturation in 12 boys with ADHD and 12 age-matched, typically developing boys, 7-13 years of age. METHODS Surface electromyographic activity was recorded from right first dorsal interosseus (FDI). During background activation, focal TMS was delivered at maximal stimulator output over the ipsilateral motor cortex. RESULTS Maturation of finger speed in boys with ADHD was significantly slower than that in the control group. The iSP latency decreased with age in the control group but not in the ADHD group. CONCLUSIONS These findings suggest the presence of a complex relationship between abnormalities of certain interhemispheric interactions (as represented by iSP latency) and delayed maturation of neuromotor skills in boys with ADHD. SIGNIFICANCE These data provide preliminary physiologic evidence supporting delayed or abnormal development of interhemispheric interactions in boys with ADHD.

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