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The review of deep brain stimulation (DBS) for tremor in patients with multiple sclerosis by Wishart and colleagues1 was a good summary of the current literature, its shortcomings, and the problems associated with this type of surgery. We have recently published a report on the difficulties invloved2 and would like to add a comment about targeting the site of DBS implantation in the mesodiencephalon in this patient group.
An earlier review of stereotactic ablative and DBS surgery showed that a range of different thalamic subnuclei and mesodiencephalic areas has been targeted, with variable success.3 Although a target in the thalamic nucleus ventrointermedius (Vim) is often cited, we have found—like Aziz’s group4,5—that a more anterior and ventral electrode placement was most likely to reduce the tremor. In the 12 patients implanted in our series,6 the median coordinates of the site of optimal intraoperative tremor suppression were 13.5 mm lateral to the midline, 2 mm behind the AC–PC (anterior commissural–posterior commissural) midpoint, and 2.5 mm deep to the AC–PC plane. These coordinates suggest a subthalamic–zona incerta target, which would interrupt the dentato–Vim projections. The deepest of the quadripolar electrodes was inserted at this site, suggesting that the remaining rostral electrodes straddle the Vim or nucleus ventro-oralis posterior, which lies anterior to the Vim.
Although our targets are not dissimilar to those reported by Aziz’s group,4,5 we have not done intraoperative microelectrode recordings or postoperative magnetic resonance imaging to confirm our intraoperative targeting. Furthermore, most patients with tremor caused by multiple sclerosis have major brain distortions because of demyelination, plaque formation, and ex vacuo hydrocephalus when they come to stereotactic surgery. It is difficult, therefore, to know how their mesodiencephalic anatomy conforms to a stereotactic atlas. This may explain why, in our experience, targeting in patients with multiple sclerosis is considerably more demanding than in patients with either Parkinson’s disease or essential tremor.
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