Contralateral conjugate eye deviation during deep brain stimulation of the subthalamic nucleus

Object. Deep brain stimulation Of: the subthalamic nucleus (STN) in patients with Parkinson disease is often very effective for treatment of debilitating motor symptoms. Nevertheless, the small size of the STN and its proximity to axonal projections results ill Multiple side effects during high-frequency stimulation. Contralateral eve deviation is produced in a small percentage of patients, but the precise mechanism of this side effect is at present Poorly Understood. Methods. Contralateral eye deviation was produced by high-frequency stimulation of 22 contact sites in nine patients undergoing deep brain stimulation of the STN. The precise locations of these contacts were calculated and compiled in order to locate the stimulated structure responsible for eye deviation. Results. The mean x, y, and z coordinates associated with contralateral eye deviation were found to be 11.57, 2.03, and 3.83 mm lateral, posterior, and interior to the anterior commissure-posterior commissure midpoint, respectively. The point described by these coordinates is located within the lateral anterosuperior border of the STN. Conclusions. Given that stimulation of frontal eye field cortical regions produces similar contralateral conjugate eye deviation. these results tire best explained by electrical current spread to nearby frontal eye field axons coursing lateral to file STN within the internal capsule. Thus, placement of the implanted electrode in a more medial. posterior, and inferior position may bring resolution of these symptoms by reducing the amount of current spread to internal capsule axons.