Disturbed transcallosally mediated motor inhibition in children with attention deficit hyperactivity disorder (ADHD).

Objective: The aim of this study was to investigate mechanisms of motor-cortical excitability and inhibition which may contribute to motor hyperactivity in children with attention deficit hyperactivity disorder (ADHD).

Methods: Using transcranial magnetic stimulation (TMS), involvement of the motor cortex and the corpus callosum was analysed in 13 children with ADHD and 13 sex- and age-matched controls. Contralateral silent period (cSP) and transcallosally mediated ipsilateral silent period (iSP) were investigated.

Results: Resting motor threshold (RMT), amplitudes of motor evoked potentials (MEP) and cSP were similar in both groups whereas iSP-latencies were significantly longer (p<0.05) and their duration shorter (p<0.01) in the ADHD group. For the ADHD group iSP duration tended to increase and iSP latency to decrease with age (n.s.). Conners-Scores did neither correlate with iSP-latencies and -duration nor with children's age.

Conclusions: The shortened duration of iSP in ADHD children could be explained by an imbalance of inhibitory and excitatory drive on the neuronal network between cortex layer III-the projection site of transcallosal motor-cortical fibers-and layer V, the origin of the pyramidal tract. The longer iSP-latencies might be the result of defective myelination of fast conducting transcallosal fibers in ADHD. iSP may be a useful supplementary diagnostic tool to discriminate between ADHD and normal children.