Vim line technique thalamotomy for Parkinson tremor: Case series

Highlights • Vim thalamotomy is a usual procedure to control Parkinson tremor.• The ventral intermediate nucleus of the thalamus is difficult to identify.• The precision is crucial importance for the successful Vim thalamotomy.• Vim line technique (VLT) is useful for the determination of the vim location.


Introduction
Ventral intermediate (Vim) nucleus of the thalamus is reputed to be 'invisible' on the routinely available imaging techniques. Currently, there exists several hypotheses related to the estimation of the Vim location [6]. The use of magnetic resonance imaging (MRI) to identify critical structures near the stereotactic targets is crucial. The optic tract, internal segment of the globus pallidus, border between the Vim nucleus of the thalamus and the internal capsule must be evaluated precisely before some serious surgeries [5].
Several different surgical procedures have been proposed for the treatment of essential tremor, including deep brain stimulation (DBS), radiofrequency thalamotomy, gamma knife radio surgical thalamotomy and focused ultrasound thalamotomy [1]. Stereo-tactic lesioning surgeries such as thalamotomy and pallidotomy have been performed to control movement disorders. The thalamic Vim nucleus is a usual target for controlling the tremor. Tremordominant hemi-Parkinson's disease (PD) is a good indication of Vim thalamotomy, especially when tremor is a major complaint [5]. Some techniques have been developed to define the Vim location. For instance, the Guiot's technique uses 3/12 or 4/12 anterior from posterior commissure (PC) in anterior commissure-PC (AC-PC) line [6]. The coordinate base uses the brain atlas to define the Vim location [2]. New techniques are hence required to develop variation of the brain anatomy. Improvement after Vim thalamotomy in the control of tremor ranges between 74% and 90% [1]. VLT did not interfered by this variation. VLT can be used by neurosurgeons to determined the Vim location, including in different anatomical variation of the brain. Using VLT to determine Vim location should increase the outcome that can be measured by Unified PD Rating Scale (UPDRS) score. However, there is insufficient evidence to support or refute efficacy superiority of DBS or thalamotomy for treating essential tremor [7]. All of the procedures performed by the authors in this study involving human  participants were in accordance with the ethical standards of the institutional research committee. This work has been reported in line with the PROCESS guideline [8]. This work has been registered at http://www.researchregistry.com (researchregistry6183).

Case 1
A 30-year-old man presented with tremor in his right hand and a stiffness at his right side of the body. He was diagnosed with PD since the past 5 years and was receiving oral medication (Ldopa, trihexyphenidyl, pramipexole and ropinirole), but the tremor continued to recur.
The MRI and computerized tomography (CT) of the head is performed before conducting stereotactic surgery. The patient's UPDRS before surgery was ON 6 and OFF 33. The patient consented to undergo left Vim thalamotomy by the Vim line technique (VLT) using the Cosman G4 radiofrequency system (electrode active tip 4 mm, diameter 1.1 mm, 70 • C for 30 s) (Fig. 1). Evaluation after surgery revealed that tremor in the right hand and the stiffness of the right side of the body had improved. The patient's UPDRS score after surgery was ON 0 and OFF 7.

Case 2
A 59-year-old woman presented with a tremor at her left and right sides of the body. The left side tremor was dominant since past 15 years and the right side tremor since past 3 years. She also complained of rigidity and bradykinesia. Patient had ON-OFF response to oral medications (L-dopa, trihexyphenidyl, pramipexole and ropinirole).
The patient's head MRI and CT scan were performed before the stereotactic surgery. Her UPDRS score before the surgery was ON 15 and OFF 41. The patient consented to undergo the right Vim thalamotomy with VLT using the Cosman G4 radiofrequency system (electrode active tip 4 mm, diameter 1.1 mm, 70 • C for 30 s) (Fig. 2). Her evaluation after the surgery revealed that the tremor in the left side of the body had improved. Her UPDRS score after the surgery was ON 0 and OFF 7.

Case 3
A 57-year-old man presented with tremor in the left hand since the past 2 years. He had undergone left Vim thalamotomy 4 years ago for his right-side tremor. His head MRI and CT scan were performed before the stereotactic surgery. His UPDRS score  before the surgery was ON 13 and OFF 42 with oral medications (Ldopa, trihexyphenidyl, pramipexole and ropinirole). He consented to undergo right Vim thalamotomy with VLT using the Cosman G4 radiofrequency system (electrode active tip 4 mm, diameter 1.1 mm, 70 • C for 30 s) (Fig. 3). His evaluation after the surgery revealed that the tremor in the left hand had improved. His UPDRS score after the surgery was ON 4 and OFF 23.

Discussion
Thalamic stimulation and thalamotomy are equally effective in reducing drug resistant tremor, and stimulation has only a few adverse effects. The efficacy of stimulation ranges from 71% to 94% in PD patients and from 74% to 90% for essential tremor [1,4]. In patients with bilateral tremor, thalamic stimulation in the dominant hemisphere can be combined with thalamotomy for the non-dominant hemisphere; however, morbidity associated with thalamotomy probably outweighs any advantage of this approach [3].
The precise targeting of the Vim nucleus is crucial importance for the successful Vim thalamotomy. Targeting is usually conducted by using stereotactic coordinates that are specified in relation to a point on the AC-PC line based on the brain atlas or intraoper-ative stimulation techniques, although these methods sometimes need modification based on the anatomical brain variation. Since the identification of the thalamic nuclei on conventional imaging modalities is difficult, recently, MRI tractography was successfully used to investigate the dentatorubrothalamic tract that crosses the Vim nucleus [2]. Some of the stereotactic planning software available presently do not have the tractography facility.
VLT is a new technique that has been developed to determine the Vim location on MRI. This technique suitable for any brain anatomical variation. This technique is performed by drawing a line that connects both the external globus palidus posterior tips in the AC-PC plane. The Vim location is defined in 1.5-mm medial area from the cerebrospinal tract area border. Vim thalamotomy done with VLT using the Cosman G4 radiofrequency system (electrode active tip 4 mm, diameter 1.1 mm, 70 • C for 30 s).
A past study has reported improvement in the UPDRS scores in the long-term evaluation after Vim thalamotomy [4]. We noted symptom improvement in our cases as depicted by their UPDRS scores. Sixth months follow up of UPDRS were examined by same neurologist at the same hospital with constant score.

Conclusion
Our results depict that stereotactic thalamotomy using VLT can be used to determine the Vim location as a useful approach to reduce or control tremor in PD patients. However, further research is warranted to prove the effectiveness of the proposed approach.