Motor cortex excitability and comorbidity in Gilles de la Tourette syndrome

Gilles de la Tourette syndrome (GTS) is often complicated by comorbid attention-deficit/hyperactivity disorder (ADHD) or obsessive-compulsive disorder (OCD). This study examines whether motor cortex excitability differs between uncomplicated GTS patients and those complicated by ADHD or OCD.

Motor thresholds, short-interval intracortical inhibition (SICI) and intracortical facilitation (ICF), and short latency afferent inhibition (SAI) were measured using transcranial magnetic stimulation (TMS) in 29 untreated GTS patients (18 uncomplicated, six with comorbid ADHD, five with comorbid OCD) and 24 healthy subjects. Tic severity was rated with standard clinical scales.

Patients had slightly higher resting (RMT) and active motor thresholds (AMT). The threshold of SICI and ICF expressed as a percentage of each individual's AMT was similar in controls and GTS patients. Above threshold, GTS patients had less SICI and more ICF. SICI was similar in all subgroups, but ICF differed significantly between them. Patients with GTS+ADHD had more ICF than controls, uncomplicated GTS patients or GTS+OCD patients; ICF was similar in these other groups. GTS patients as a whole had reduced SAI. Uncomplicated GTS patients or GTS+ADHD patients had less SAI than controls or GTS+OCD patients.

GTS with ADHD comorbidity is associated with more extensive changes in the excitability of motor cortex circuits than uncomplicated GTS or GTS+OCD. The extent to which various different neuronal circuits are affected may be relevant for the phenotype of Tourette spectrum disorders.