Evidence of functional somatotopy in GPi from results of pallidotomy

Abstract

The objective of this study was to explore the functional anatomy of the globus pallidus internus (GPi) by studying the effects of unilateral pallidotomy on parkinsonian 'off' signs and levodopa-induced dyskinesias (LID). We found significant positive correlations between the preoperative levodopa responsiveness of motor signs and the levodopa responsiveness of scores in timed tests (Core Assessment Program for Intracerebral Transplantations) in the contralateral limbs and the improvement in these scores after surgery, whereas there was no correlation with the improvement in LID. We also found a highly significant correlation (P: < 0.0001, r = 0.8) between the volume of the ventral lesion in the GPi and the improvement in LID in the contralateral limbs, whereas there was no correlation between the ventral volume and the improvement in parkinsonian 'off' signs. The volumes of the total lesion cylinder and the dorsal lesion did not correlate with the outcome of either dyskinesias or parkinsonian 'off' signs. The differential predictive value of levodopa responsiveness for the outcome of parkinsonian 'off' signs and LID and the different correlations of ventral lesion volume with dyskinesias and parkinsonian 'off' signs indicate that different anatomical or pathophysiological substrates may be responsible for the generation of parkinsonian 'off' signs and dyskinesias. Whereas cells in a wider area of the GPi may be implicated in parkinsonism, the ventral GPi seems to be crucial for the manifestation of LID. We suggest that our observations are additional proof of the functional somatotopy of the systems within the GPi that mediate parkinsonism and dyskinesias, especially along the dorsoventral trajectory used in pallidotomy. The outcome of pallidotomy in which the lesion involves the ventral and dorsal GPi could be the net effect of alteration in the activity of pathways which mediate different symptoms, and hence could be variable.

The Functional Organization of the Sensorimotor Region of the Subthalamic Nucleus

“…Several clinical reports have confirmed the efficacy of sensorimotor STN stimulation in alleviating parkinsonian symptoms [7,8,35]. Similar results have been obtained with GPi DBS or lesions, which are far more effective on parkinsonian symptoms when placed in the core of the sensorimotor region [27,[36][37][38][39].…”

Four year follow-up study after unilateral pallidotomy in advanced Parkinson's disease

“…This finding could be related to the particular somatotopy of the systems within the medial pallidum that mediate parkinsonism and dyskinesias [2]. Antiakinetic effects of levodopa are blocked by pallidal stimulation when the poles used for stimulation are the more ventral [4,11,12,13]. By contrast, the dorsal poles produce a reversal on akinesia and induce dyskinesias, an effect that may reflect the segregation of the pallidofugal fibers from the outer portion of the medial pallidum forming the ventral ansa lenticularis and the inner portion forming the dorsal lenticular fasciculus.…”

Deep brain stimulation

“…About half of the STN neurons show a response to passive joint movements (Magarinos-Ascone et al 2000;Abosch et al 2002;Benazzouz et al 2002). A somatotopic organization in PD patients was shown for the GPi (Taha et al 1996;Kishore et al 2000), whereas the studies questioning human STN somatotopy yielded controversial results, one study showing a somatotopic organization (Rodriguez-Oroz et al 2001) similar to the findings in primate STN (Nambu et al 1996), while another study failed to detect consistent somatotopy . There is no consensus regarding the need of microrecordings in targeting for DBS.…”