Deficient Motor Cortex Inhibition in Tourette Syndrome. Evidence from Transcranial Magnetic Stimulation
Ulf Ziemann, Walter Paulus, Aribert Rothenberger
Thursday 9:15:00 AM / Room 309
To test the hypothesis that tics in Tourette syndrome (TS) are due to impaired inhibitory mechanisms at the level of the motor cortex.
Fluctuating motor and vocal tics are the diagnostic feature of TS. The pathophysiology of tics is still unclear. One major view is that tics are due to a deficient inhibitory control of the motor cortex through the striato-thalamo-cortical loop.
Twenty patients with TS ( mean age 28.5±7.0 years, range 16-43 years) and 21 age matched healthy volunteers were investigated. Written informed consent was obtained from all subjects and the study was approved by the local ethics committee. Focal transcranial magnetic stimulation (TMS) was applied to the left motor cortex, surface electromyography was recorded from the right abductor digiti minimi muscle. As parameters of motor cortex excitability, motor threshold, cortical silent period duration, and intracortical inhibition and facilitation (paired stimulus paradigm of Kujirai et al. 1993; J Physiol 471:501-519) were studied. Peripheral motor excitability was tested by the maximum M wave and peripheral silent period after supramaximal electrical stimulation of the ulnar nerve at the wrist.
Motor threshold, intracortical facilitation and peripheral motor excitability were normal in the TS group, while the mean cortical silent period was shortened (TS vs control, 127±50ms/166±30ms.
Findings indicate that tics in TS are associated with deficient motor inhibition, which is compatible either with a primarily non-motor cortex (e.g. basal ganglia) disorder affecting the motor cortex through disinhibited afferents and/or with impaired inhibition originating in the motor cortex directly.