Transcranial magnetic stimulation in Gilles de la Tourette syndrome

The cause of Gilles de la Tourette syndrome (GTS), a chronic motor and vocal tic disorder of childhood onset, remains unknown. Abnormalities in basal ganglia-thalamo-cortical circuits presumably play an important role in the pathophysiology underlying the involuntary tics. The use of transcranial magnetic stimulation (TMS), a noninvasive and painless tool to examine the excitability of several different circuits in the human motor cortex has advanced our understanding of the pathophysiology. Motor thresholds are similar in GTS and healthy subjects; in the resting state, recruitment of motor evoked potentials (MEPs) above threshold is more gradual in patients than controls. In contrast, recruitment of MEPs during preactivation is similar in both groups, as is the duration of the cortical silent period. This suggests that the distribution of excitability in the corticospinal system in patients at rest is different to that in healthy individuals. Importantly, correlation analysis showed that reduced levels of excitability at rest relate, in pure GTS patients, to video ratings of complex tics, and hand and finger tics, with less excitability predicting fewer tics. The correlations disappear for measures made during voluntary activation. This suggests that this is an adaptive response to abnormal basal ganglia-motor cortex inputs in an effort to reduce unwanted movements, a notion supported by electroencephalography-coherence studies that show increased cortico-cortical coupling. Compared to the healthy control group, short intracortical inhibition (SICI) thresholds are similar. However, above-threshold SICI recruitment and sensory afferent inhibition (SAI), a paradigm to examine sensory motor integration, are reduced in patients. This is consistent with the suggestion that reduced excitability of cortical inhibition is one factor that contributes to the difficulty that patients have in suppressing involuntary tics. In addition the reduced SAI indicates that impaired intracortical inhibition may not be limited to the motor cortex but also involves circuits linking sensory input and motor output. GTS with attention deficit hyperactivity disorder comorbidity is associated with more extensive changes in the excitability of motor cortex circuits than pure GTS or GTS+obsessive-compulsive disorder. The extent to which various different neuronal circuits are affected may be relevant for the phenotype of Tourette spectrum disorders.