DBS targeting for essential tremor using intersectional dentato-rubro-thalamic tractography and direct proton density visualization of the VIM: technical note on 2 cases.

OBJECTIVE: Direct visualization of the ventral intermediate nucleus (VIM) of the thalamus on standard MRI sequences remains elusive. Therefore, deep brain stimulation (DBS) surgery for essential tremor (ET) indirectly targets the VIM using atlas-derived consensus coordinates and requires awake intraoperative testing to confirm clinical benefits. The objective of this study was to evaluate the utility of proton density (PD)-weighted MRI and tractography of the intersecting dentato-rubro-thalamic tract (DRTT) for direct "intersectional" targeting of the VIM in ET.

METHODS: DBS targets were selected by identifying the VIM on PD-weighted images relative to the DRTT in 2 patients with ET. Tremor reduction was confirmed with intraoperative clinical testing. Intended target coordinates based on the direct intersectional targeting technique were compared with consensus coordinates obtained with indirect targeting. Pre- and postoperative tremor scores were assessed using the Fahn-Tolosa-Marin tremor rating scale (TRS).

RESULTS: Planned DBS coordinates based on direct versus indirect targeting of the VIM differed in both the anteroposterior (range 0 to 2.3) and lateral (range -0.7 to 1) directions. For 1 patient, indirect targeting-without PD-weighted visualization of the VIM and DRTT-would have likely resulted in suboptimal electrode placement within the VIM. At the 3-month follow-up, both patients demonstrated significant improvement in tremor symptoms subjectively and according to the TRS (case 1: 68%, case 2: 72%).

CONCLUSIONS: Direct intersectional targeting of the VIM using PD-weighted imaging and DRTT tractography is a feasible method for DBS placement in patients with ET. These advanced targeting techniques can supplement awake intraoperative testing or be used independently in asleep cases to improve surgical efficiency and confidence.