Cortical evoked potentials from pallidal stimulation in patients with primary generalized dystonia.

Deep brain stimulation (DBS) of globus pallidus internus (GPi) has emerged as an effective treatment for primary generalized dystonia. However, the physiological mechanisms of improvement are not fully understood. Cortical activity in response to pallidal stimulation was recorded in 6 patients with primary generalized dystonia >6 months after bilateral GPi DBS. Scalp electroencephalogram was recorded using 60 surface electrodes during 10 Hz bipolar pallidal DBS at each electrode contact pair. Anatomical position of the electrode contacts in relation to the GPi, medial medullary lamina and globus pallidus externus (GPe) was determined from the postoperative stereotactic MRI. In all six patients an evoked potential (EP) was observed with average onset latency of 10.9 ms +/- 0.77, peak latency 26.6 ms +/- 1.6, distributed mainly over the ipsilateral hemisphere, maximal centrally. The mean amplitude of this potential was larger with stimulation in posteroventral GPi than in GPe (3.36 microV vs. 0.50 microV, P < 0.0001). The EP was absent in one patient-side, ipsilateral to a previous thalamotomy. Low frequency GPi stimulation produces an EP distributed centrally over the ipsilateral hemisphere. The latency and distribution of the EP are consistent with stimulation of pallidothalamic neurons projecting to the sensorimotor cortex. Because the EP is larger and more consistently present with stimulation of posteroventral GPi than GPe, it may provide a physiological tool to identify contacts within the optimal surgical target.