DECREASED MOTOR INHIBITION IN TOURETTES DISORDER - EVIDENCE FROM TRANSCRANIAL MAGNETIC STIMULATION

Objective: Fluctuating motor and vocal ties are the diagnostic feature of Tourette's disorder. The pathophysiology of ties is still unclear. One major hypothesis is a deficient inhibitory control through the co rtical-striatal-thalamic-cortical motor loop. The authors tested this hypothesis by investigating motor cortex excitability through use of t he technique of transcranial magnetic stimulation . Method: Twenty pat ients with Tourette's disorder and a comparison group of 21 healthy su bjects were studied. Focal transcranial magnetic stimulation was appli ed to the left motor cortex, and surface electromyography (EMG) was re corded from the right abductor digiti minimi muscle. As measures of mo tor cortex excitability, motor threshold, cortical silent period, and intracortical inhibition and facilitation were studied. The peripheral silent period and the maximum M wave after supramaximal electrical st imulation of the ulnar nerve at the wrist were also determined. Result s: Motor threshold and peripheral motor excitability were normal in th e Tourette's disorder group, but the cortical silent period was shorte ned and the intracortical inhibition reduced. A subgroup analysis of t he patients with Tourette's disorder revealed that these abnormalities were seen mainly when ties were present in the EMG target muscle or i n patients without neuroleptic treatment. Age, sex, attention deficit hyperactivity disorder, obsessive-compulsive disorder, and sensory urg es had no significant effect on motor excitability. Conclusions: Findi ngs are consistent with the hypothesis that ties in Tourette's disorde r originate either from a primarily subcortical disorder affecting the motor cortex through disinhibited afferent signals or from impaired i nhibition directly at the level of the motor cortex or both.