Abstract
The ventral intermediate nucleus (VIM) of the thalamus is an established surgical target for stereotactic ablation and deep brain stimulation (DBS) in the treatment of tremor in Parkinson's disease (PD) and essential tremor (ET). It is centrally placed on a cerebello-thalamo-cortical network connecting the primary motor cortex, to the dentate nucleus of the contralateral cerebellum through the dentato-rubro-thalamic tract (DRT). The VIM is not readily visible on conventional MR imaging, so identifying the surgical target traditionally involved indirect targeting that relies on atlas-defined coordinates. Unfortunately, this approach does not fully account for individual variability and requires surgery to be performed with the patient awake to allow for intraoperative targeting confirmation. The aim of this study is to identify the VIM and the DRT using probabilistic tractography in patients that will undergo thalamic DBS for tremor. Four male patients with tremor dominant PD and five patients (three female) with ET underwent high angular resolution diffusion imaging (HARDI) (128 diffusion directions, 1.5 mm isotropic voxels and b value = 1500) preoperatively. Patients received VIM-DBS using an MR image guided and MR image verified approach with indirect targeting. Postoperatively, using parallel Graphical Processing Unit (GPU) processing, thalamic areas with the highest diffusion connectivity to the primary motor area (M1), supplementary motor area (SMA), primary sensory area (S1) and contralateral dentate nucleus were identified. Additionally, volume of tissue activation (VTA) corresponding to active DBS contacts were modelled. Response to treatment was defined as 40% reduction in the total Fahn-Tolosa-Martin Tremor Rating Score (FTMTRS) with DBS-ON, one year from surgery. Three out of nine patients had a suboptimal, long-term response to treatment. The segmented thalamic areas corresponded well to anatomically known counterparts in the ventrolateral (VL) and ventroposterior (VP) thalamus. The dentate-thalamic area, lay within the M1-thalamic area in a ventral and lateral location. Streamlines corresponding to the DRT connected M1 to the contralateral dentate nucleus via the dentate-thalamic area, clearly crossing the midline in the mesencephalon. Good response was seen when the active contact VTA was in the thalamic area with highest connectivity to the contralateral dentate nucleus. Non-responders had active contact VTAs outside the dentate-thalamic area. We conclude that probabilistic tractography techniques can be used to segment the VL and VP thalamus based on cortical and cerebellar connectivity. The thalamic area, best representing the VIM, is connected to the contralateral dentate cerebellar nucleus. Connectivity based segmentation of the VIM can be achieved in individual patients in a clinically feasible timescale, using HARDI and high performance computing with parallel GPU processing. This same technique can map out the DRT tract with clear mesencephalic crossing.
Highlights
The ventral intermediate nucleus (VIM) of the thalamus is an established surgical target, for stereotactic ablation and deep brain stimulation (DBS) in the treatment of tremor in Parkinson's disease (PD), essential tremor (ET) and multiple sclerosis (Benabid et al, 1989, 1991, 1993; Berk et al, 2004; Hariz et al, 2007; Pahwa et al, 2001; Pollak et al, 1993; Schuurman et al, 2008)
Connectivity based segmentation of the VIM can be achieved in individual patients in a clinically feasible timescale, using high angular resolution diffusion imaging (HARDI) and high performance computing with parallel Graphical Processing Unit (GPU) processing
Connectivity based segmentation of the VIM can be achieved in individual patients in a clinically feasible timescale, using HARDI and high-performance computing with parallel GPU processing
Summary
The ventral intermediate nucleus (VIM) of the thalamus is an established surgical target, for stereotactic ablation and deep brain stimulation (DBS) in the treatment of tremor in Parkinson's disease (PD), essential tremor (ET) and multiple sclerosis (Benabid et al, 1989, 1991, 1993; Berk et al, 2004; Hariz et al, 2007; Pahwa et al, 2001; Pollak et al, 1993; Schuurman et al, 2008). Identifying the nucleus traditionally involves indirect targeting relying on atlas-defined coordinates in relation to the anterior commissure (AC) – posterior commissure (PC) points as landmarks, along with other identifiable structures such as the lateral thalamic/ internal capsule border (Schaltenbrand et al, 1977). Needless to say, this approach does not fully account for individual variability. Surgery often needs to be performed with the patient awake to allow for intraoperative confirmation of targeting, increasing patient discomfort (Gross et al, 2006). Intraoperative confirmation is not always readily feasible e.g. when performing a thalamotomy using Gamma Knife (Witjas et al, 2015) or focused ultrasound (Elias et al, 2016)
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