Restricted ablative lesions in motor portions of GPi in primates produce extensive loss of motor-related neurons and degeneration of the lenticular fasciculus.
Deep brain stimulation (DBS) of the internal pallidum (GPi) and the subthalamic nucleus and to a lesser extent, ablative lesioning, are broadly utilized to treat patients with medically intractable Parkinson's disease and other movement disorders. Beneficial outcomes however are not uniform and adverse cognitive and behavioral are significant risks. Surgical outcomes of GPi surgeries might be improved by approaches that better account for the course of motor and non-motor pallidothalamic projections. Although several studies, including our own tracer investigations, suggest that motor projections from GPi principally form the lenticular fasciculus and non-motor projections primarily contribute to the ansa lenticularis, other schemes have perpetuated. Presently, to corroborate the course of pallidothalamic projections and to assess the feasibility of selectively targeting the motor circuit of GPi, radiofrequency lesions were placed in the motor portion of GPi in monkeys. Degenerating pallidothalamic fibers were visualized with amino cupric staining techniques and regional cell counts in GPi were compared with control hemispheres. Lesions restricted to posterior motor portions of GPi produced selective degeneration of LF and only damaged AL if the lesions extended more anteriorly. Marked secondary neuronal loss occurred well beyond the principal lesions but was mainly confined to the posterolateral motor region of GPi. These findings corroborate the original pallidothalamic outflow scheme proposed by Ranson and Ranson. Conceivably, a restrictive lesion or a DBS probe could be placed in the centroposterior portion of GPi to selectively target both local motor-related neurons and traversing pallidothalamic fibers originating from more posterior and lateral motor regions.