Original ArticleAnatomic Targeting of the Optimal Location for Thalamic Deep Brain Stimulation in Patients with Essential Tremor
Introduction
Long-term stimulation of the ventral intermediate nucleus (Vim) of the thalamus has been shown to be effective for the surgical treatment of medically refractory tremor.1, 2 The Vim as a target was initially selected for lesioning and subsequently for deep brain stimulation (DBS).3 In this thalamic region, movement-related kinesthetic and tremor cells coexist,4, 5, 6 rendering electrophysiology a main strategy for targeting the Vim.2, 7 Precisely targeting the Vim is crucial for long-term improvement of tremor. Previous studies have shown that even small deviations from the optimal location may result in a suboptimal outcome.8, 9
As currently used in the clinic, structural magnetic resonance imaging (MRI) is not able to visualize thalamic nuclei, even with specialized sequences or diffusion imaging.10, 11, 12 Commonly used approaches to target the Vim are based on formulaic methods that derive coordinates relative to the anterior commissure (AC) and posterior commissure (PC).8, 13, 14 However, such approaches are insensitive to the variability of thalamic morphology and the size of the third ventricle. Both factors can contribute to errors in correctly identifying the Vim, which may ultimately lead to a suboptimal outcome. Our aim was to determine the optimal target for chronic DBS for essential tremor (ET) and construct an anatomic targeting method based on this probabilistic map to overcome the limitations in identifying thalamic subdivisions on MRI sequences.
Section snippets
Study Population
The study was approved by the institutional review board of the University Health Network. Records of patients with ET treated with Vim DBS at Toronto Western Hospital within the last 5 years were reviewed. To develop our targeting-based system, we selected patients who met 3 inclusion criteria. First, patients needed to have disabling medically refractory ET, as determined by a movement disorders neurologist. Second, intraoperative microelectrode recording had to show the presence of
Optimal Location of Stimulation
At 1 year, the distribution of the active contacts with reference to the electrode array in the patients included in our initial analysis was as follows: contact 0 in 1 patient, contact 1 in 5 patients, contact 2 in 2 patients, and contact 3 in 2 patients. Stimulation settings were 2.6 V ± 0.9, 66 μs ± 13, and 155 Hz ± 27. The optimal stimulation area based on the estimated volume of activation that maximized clinical tremor reduction and minimized side effects is shown in Figure 1. This
Discussion
This study shows that the optimal location for long-term DBS in ET comprises a region that is likely within the Vim near its anterior border. Based on a probabilistic map, we designed an anatomic FX method similar in principle to the anatomic targeting that has been used to locate the subthalamic nucleus. In the latter instance, a more easily identifiable structure on MRI, the red nucleus, was used to approximate the less readily identifiable subthalamic nucleus.22 In our analysis, the FX
Conclusions
We compared several methods that can be used to target the optimal location for long-term stimulation for tremor. The newly described FX method seems to be more precise, corresponds well to the DRT, and can be easily translated into current clinical practice. Although further validation is required, the FX method may be a complementary strategy to help surgeons targeting the optimal location for placement of DBS electrodes in patients with tremor.
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Cited by (0)
Conflict of interest statement: C.H. has received honoraria from Medtronic and St. Jude Medical, Inc. A.F. has given expert testimony for Medtronic; has served on scientific boards of AbbVie, Inc., and UCB; and has received honoraria from Teva Pharmaceuticals, Ltd., UCB, Medtronic, AbbVie, Inc., Boston Scientific, and Chiesi Farmaceutici. A.M.L. receives consulting fees from Medtronic, Boston Scientific, St. Jude Medical, Inc., and Eli Lilly and Company and holds an interest in Functional Neuromodulation, Inc. The remaining authors have no conflicts to report.