Stereotactic targeting of the globus pallidus internus in Parkinson's disease: Imaging versus electrophysiological mapping

OBJECTIVE: The reintroduction of pallidotomy for the treatment of Parkinson 's disease (PD) has generated various opinions regarding the ideal anatomic or physiological location of the target within the globus pallidus. The ro le of microelectrode recording guidance in pallidotomy for the treatment of advanced PD is presently under debate. The purpose of this study was twofo ld. The first goal was to determine the degree of accuracy in the targeting of the globus pallidus internus (GPi) with magnetic resonance imaging (MRI ), by comparing these results with the final placement of the thermolytic l esions (as defined by electrophysiological assessment). The second goal was to ascertain the somatotopic arrangement of the GPi in PD. METHODS: The analysis involved 50 patients with PD who underwent microrecor ding-guided pallidotomy. The theoretical coordinates for lesioning were cal culated after definition of the intercommissural line by MRI. The actual pl acement of the lesions was determined after mapping of the GPi by microreco rding, using stimulation to identify the sensorimotor region and its somato topic organization, RESULTS: In most cases, the lesions were placed posterior and lateral to th e targets chosen by MRI. Mapping by microrecording revealed differences of 2.3 +/- 1.55 mm and 3 +/- 1.9 mm in the mediolateral and anteroposterior co ordinates, respectively. The actual lesion overlapped the theoretical targe t for only 45% of the patients. The somatotopic organization of the GPi was analyzed. Most of the units with sensorimotor activity or tremor-related a ctivity were in the lateral portion of the nucleus. Upper limb and axial un its were in the most lateral region and mainly in the ventral one-third of the nucleus. Lower limb responses were recorded mainly in the dorsal one-th ird of the nucleus. Tremor-related cells were found throughout the sensorim otor region of the nucleus. CONCLUSION: These results indicate that lesion targeting based on MRI alone is not sufficiently accurate to guarantee placement of the lesion in the s ensorimotor region of the GPi.