Editor’s Comments

The Walker nomenclature of the ventral lateral (VL) thalamus is less precise than the Hassler terminology (Table 11.2). The VL nucleus is roughly the equivalent of Hassler’s nucleus ventralis oralis anterior (Voa), nucleus ventralis oralis posterior (Vop), and Vim. Vim receives both vestibulo-thalamic and cerebellothalamic afferents and projects to the primary motor cortex (area 4) but not to area 3a of the sensorimotor cortex as suggested by Hassler. Vop may also receive cerebellothalamic afferents. Both Vop and Voa receive pallidothalamic fibers from the medial globus pallidus (GPi), and project to the precentral motor cortex and the supplementary motor cortex. Empirically, lesioning of the Voa/Vop is considered a target for control of rigidity and dyskinesia, whereas Vim is the best target for control of tremor. Despite multiple attempts to modify Hassler’s nomenclature to a more contemporary and accurate reflection of the neuroanatomy, functional neurosurgeons have kept it because the sensory and motor subdivisions matched what has been observed anatomically, electrophysiologically, and physiologically.¹⁷

These subnuclei are differentiated electrophysiologically (Fig. 11.4). Although most neurons in Vop and Vim fire in the 10 to 20 Hz range, there are tremor-synchronous neurons with rhythmic 5 to 7 Hz bursts. These tremor cells are recorded primarily in Vim, and to a limited extent in Vop. In PD, these tremor cells occur approximately four times as often as in ET and five times more often than in Holmes or cerebellar tremors.¹⁸ The highest percentage of nontremor bursting neurons occurs in Vop, but also occurs frequently in Voa. Neurons that alter their firing rate with joint movement or deep muscle pressure are called kinesthetic cells. These are very common in Vim (44%), but uncommon in Vop (16%),¹⁹ whereas neurons that change their firing rate with active movements are called voluntary cells and are common in Vop (70%) and less so in Vim (49%), but are rare in Voa. When transitioning into the thalamic sensory ventralis caudalis nucleus (Vc), there is a “shell” of deep sensory (proprioceptive) neurons (Vcae) that respond to deep pressure (66%) prior to the cutaneous responsive neurons (Vcpe).²⁰ Identifying these subdivisions then facilitates lead placement (Fig. 11.5).

There are multiple potential targets for DBS to affect tremor, including the pallidothalamic cells in the GPi,^21,22 the point where the pallidal and cerebellar fibers cross the subthalamic area around the zona incerta, the fields of Forel (subthalamic lesion or campotomy),^23–25 at their termination in Vim,^26–42 or downstream projections to the GPi from the subthalamic nucleus (STN).^21,22,43,44 Vim DBS is currently believed to be the best procedure for treating all forms of tremor and is the focus of this chapter. There have been several optimal thalamic locations for tremor suppression described in the past: anterior to where evoked potentials can be recorded in response to cutaneous stimulation of the thumb,⁴⁵ where there are hand kinesthetic cells and where electrical stimulation produces effects on tremor,⁴⁶ where electrical stimulation effects tremor and anterior to where electrical stimulation evokes sensations,⁴⁷ or where there is tremor cell activity.⁴⁸ These all appear to describe the same site. However, the optimal thalamic DBS site may be more anterior and dorsal⁴⁹ and is probably more medial⁵⁰ (i.e., the Vim/Vop boarder). DBS stimulation, unlike lesioning, alters the characteristics of the tremor.^51–53 PET consistently demonstrates Vim DBS tremor suppression is associated with decreased regional blood flow in multiple ipsilateral motor-related cortical and cerebellar areas suggesting modulation of the cerebellothalamocortical pathways.^11,54 Where the tremor (pathological or physiological) originates is unknown.

The focus of stereotactic surgeons in the past was primarily on tremor, especially parkinsonian tremor. Tremor, however, is usually not the most disabling symptom of PD. Akinesia is the most disabling feature of the disease and is not ameliorated by Vim DBS. The combination of both resting and intention tremors is not uncommon in severely affected PD patients and is likely to be disabling. However, at this point in the disease process, the other symptoms of PD often require treatment and would likely be better treated with stimulation of the STN or GPi. Reevaluation of appropriate targets has led to targeting of the STN or GPi more commonly to treat parkinsonian akinesia, rigidity, and gait disturbance of PD in addition to the tremor. Thalamotomy remains an option in the treatment of tremor for well-informed patients who decide against stimulation, in cases where DBS fails, where the cost is prohibitive, or in remote areas where mandatory follow-up for adjustments of pulse generators is not possible.⁵⁵ However, there are very few comparative studies and little randomized data upon which to scientifically base clinical practice.^29,33,38

If the tremor is unilateral or dramatically asymmetrical, the surgical recommendation is straightforward. If the tremor is bilateral but asymmetrical, there is more uncertainty as to the most effective method of treatment. Some stereotactic neurosurgeons would prefer treating the more severely affected side and hope postoperatively that medications will optimize the effectiveness. Others would concentrate on the dominant side to allow maximum rehabilitation from a unilateral procedure. In our opinion, the best choice depends on the degree of asymmetry. Patients with dramatic differences in side-to-side tremor should be operated on the more severely affected side. In those where asymmetry is less marked, we prefer operating on the dominant hemisphere to restore function to the dominant side. If the patient has symmetrical bilateral tremor, there is again less controversy, with the dominant side being operated on first to provide a unilateral procedure, which would be functionally beneficial. If at all possible, a second procedure should be avoided because the risk of complications increases, even with DBS. When a second-side procedure is unavoidable due to the severity of the symptoms, then we prefer to wait 3 to 6 months before the second side is operated upon. This will ensure that the second operation is necessary and allows one to better assess the potential for complications.

Fig. 11.5 Method of microelectrode mapping of the motor and sensory thalamus before placement of a stimulator for essential tremor. Segments of the track reconstructions are shaded to indicate caudate cells (open segment), nonmotor thalamus (stippled segment), motor thalamus (black segment), or sensory thalamus (gray segment). Locations of movement-related cells are indicated by triangles (leg), or circles (arm). (A) Detail of the first microelectrode track. The microstimulation threshold for sensory activation, marked “Vc stim,” was measured at the bottom of this track (frequency = 300 Hz, pulse width = 200 μs). (B) Three microelectrode tracks are superimposed on a scaled map of the thalamus and basal ganglia in a parasagittal plane (Schaltenbrand and Bailey Atlas parasagittal 15 plane) according to the surgical team’s judgment of the best fit of the tracks to the atlas. As illustrated here, the fit of actual micro-electrode tracks to a standard atlas map is frequently not perfect. The eventual location of the stimulator is shown as a dotted line. (Starr PA, Vitek JL, Bakay RAE. Deep brain stimulation for movement disorders. Neurosurg Clin N Am 1998;9:381–402.)

For initial target coordinates for stereotactic procedures, we use serial 1.5 mm contiguous, nonoverlapping, contrast-enhanced three-dimensional (3D) volumetric 1.5T MRI studies,66 which are performed through the entire brain (for our technique see Chapter 7). Direct identification of the Vim target remains very difficult but possible on occasion (Fig. 11.6). Use of planning stations allows calculation not only of the target but also of the entry point and trajectory to the target. Contrast allows cortical veins to be easily visualized, and trajectories can be customized for each patient to avoid deep blood vessels in the ventricle en route to the desired target. Many times, the ventricular system may be avoided as well by choosing a more lateral entry point. Proponents of this technique argue that penetration of the ventricular system causes increased brain shift due to excess cerebrospinal fluid (CSF) loss, potential damage to blood vessels, and alteration of microelectrode trajectory by passing through matter of variable densities. Opponents make the points that stereotactic atlases have been based upon rectilinear approaches to the deep brain targets, placing the guide tube through the ventricle avoids deflection, and blood vessels can be avoided. We prefer minimal angulations (1 to 8 degrees) to allow the most rectilinear approach possible (offset error of 1 mm through Vim) while avoiding the lateral ventricle most of the time. The entry point generally correlates to a ring angle of 60 to 70 degrees relative to the AC–PC line if the approach is applied in a rectilinear fashion to parallel the borders of the Vim.⁶⁷ An overlay of a Guiot derivation of the thalamic subnuclei location based on the AC–PC line and the height of the thalamus on MRI of a DBS lead illustrates this point (Fig. 11.7).

The target coordinates should be considered as only the initial starting point; physiological confirmation is required. Because the initial target point for many surgeons will vary from Vop to Vim, recommended target coordinates may vary from 1 to 7 mm posterior to the midpoint of the AC–PC line. Similarly, the lateral coordinates from the midline vary from 12 to 15 mm and the vertical coordinates vary from 0 to 3 mm above the AC–PC line. We measure 4 mm anterior to the PC along the intercommissural line to identify the border of the Vc. We prefer to use MER and neurological examination intraoperatively to identify the Vc nucleus first, and then move anteriorly to the appropriate position in the Vim (Fig. 11.8). The DBS lead needs to be placed 3 to 4 mm anterior to the Vc hand area. For direct targeting, the lateral distance depends on the observed location of the internal capsule on MRI, which is more accurate than ventriculography.⁶⁸ The lateral coordinate is selected as 3 mm from the edge of the internal capsule. For indirect targeting, the thalamotomy target is 14 to 16 mm lateral to the midline or 11.5 mm plus half the width of the third ventricle from midline and depth is at the level of the AC-PC plane. For DBS, the target (Vim/Vop border) is 11 to 14 mm or 10 mm plus half the width of the third ventricle from midline because the thalamic-capsular border moves medially. For predominantly head and neck tremor a more medial approach is used.

Because of the inherent variability in biological systems, radiographic determination of the target may be inaccurate, varying as much as several millimeters from physiologically determined sites. By combining MER and macroelectrode stimulation techniques with conventional radiographic methods, we believe accurate localization of the optimal target site can be greatly improved. Precise localization allows restriction of the size of the electrical field (lower voltage and longer battery life) while maintaining its effectiveness and minimizing potential complications. If electrophysiological recordings are initiated greater than 30 mm above target, identification of the caudate nucleus may be useful in plotting the tract on an atlas. Passage through the caudate nucleus will yield insertional activity (action potentials stimulated by MER movement that rapidly abates at rest) that would not be identified if the tract were too posterior or very lateral or if the nucleus were markedly atrophic.

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History of Surgery for Movement Disorders Efficacy and Complications of Deep Brain Stimulation for Movement Disorders Deep Brain Stimulation for Dystonia Gamma Knife Stereotactic Surgery with Microelectrode Recordings Deep Brain Stimulation Programming

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