Thalamotomy and Pallidotomy

Indications

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Pallidotomy
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  Parkinson disease : Complications of advancing disease and medical therapy including tremor, wearing off, motor fluctuations, and dyskinesia, in patients with a good response to levodopa therapy. Pallidotomy should preferably be unilateral in Parkinson’s disease patients.
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  Dystonia : Disabling symptoms nonresponsive to medical therapy, including anticholinergics, benzodiazepines, and botulinum toxin. In certain cases, pallidotomy may be performed bilaterally in dystonic patients.
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Thalamotomy
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  Essential tremor : Disabling, predominantly upper extremity, unilateral kinetic tremor despite optimum medical therapy, including beta blockers, primidone, and benzodiazepines
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  Parkinson disease : Unilateral rest tremor resistant to medical therapy in tremor-dominant disease
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  Cerebellar outflow tremor : Medically intractable unilateral kinetic, postural, or rest tremor secondary to multiple sclerosis or traumatic brain injury
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Contralateral to deep brain stimulation (DBS) system
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  In patients requiring bilateral surgery (e.g., patients with Parkinson disease, essential tremor), pallidotomy and thalamotomy may be performed contralateral to a DBS system.
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  Dystonia may be treated with bilateral pallidotomy or unilateral pallidotomy contralateral to a globus pallidus internus (GPi) DBS system.
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In place of DBS system
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  In patients who have undergone DBS (GPi, ventralis intermedius [Vim], possibly subthalamic nucleus), but in whom there have been hardware-related complications, such as chronic infection, the DBS may be removed and radiofrequency lesioning performed.

Contraindications

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Unstable medical condition precluding awake stereotactic surgery
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Neuropsychiatric conditions, including untreated depression, psychotic symptoms (unless resulting from medical therapy such as dopamine agonists), and cognitive decline
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Multisystem atrophy in parkinsonian patients
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Poor response to levodopa therapy in Parkinson disease (except tremor)
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Contralateral homotopic lesion (except for dystonia)
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Need for general anesthesia during surgery
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  With rare exception, radiofrequency lesions should be created with constant neurologic evaluation, necessitating an awake patient.
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  General anesthesia can be used and the patient reversed for the lesioning part of the procedure, if necessary.

Planning and positioning

Frame application

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The stereotactic frame is usually affixed under local anesthesia with or without conscious sedation on the morning of surgery.
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Because of severe movement disorder (e.g., dystonia, tremor) or anxiety, some patients may need the frame affixed or imaging obtained under general anesthesia for comfort or to obtain adequate imaging.
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The use of frameless systems developed after the era during which most pallidotomies and thalamotomies were performed, but these systems are adaptable for use with these procedures.

Imaging

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Volumetric imaging is obtained with the stereotactic frame and fiducial marker in place.
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T1-weighted magnetic resonance imaging (MRI) (with contrast agent to visualize venous anatomy) is sufficient.
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Stereotactic computed tomography (CT) scan also may be obtained and coregistered to MRI; in this case, MRI may be performed before frame application.
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In patients with contraindications to MRI (e.g., pacemaker), stereotactic CT alone may be used.

Target and entry planning

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Preplanning
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  The scan is loaded into a neuronavigation workstation; multiple scans, if obtained, are coregistered and checked for alignment.
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  Fiducial markers are indicated and checked for rectilinearity.
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  The locations of the anterior and posterior commissures (AC, PC) and midline points are marked, and the image sets are reformatted (automatically) orthogonal to the intercommissural line (ICL).
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Target planning
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  Pallidotomy : The target in the posteroventral GPi is selected using consensus coordinates for indirect targeting (e.g., 20 mm lateral, 2 mm anterior to mid-commissural point, 4 mm ventral to ICL). Alternatively, the target may be selected or refined by direct visualization of the GPi, facilitated by inversion recovery imaging.
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  Thalamotomy : The target in the Vim nucleus is selected using consensus coordinates for indirect targeting (e.g., 11.5 mm lateral to ventricle wall, 6 mm anterior to PC, at vertical level of AC-PC line.) No direct visualization of Vim is currently routinely possible.
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Entry planning
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  A precoronal entry point is selected that avoids venous, sulcal, or ventricular penetration, which may necessitate a double-oblique trajectory.

Stereotactic frame adjustment

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The stereotactic frame coordinates are adjusted to the target selected and checked.
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If available, a phantom is similarly adjusted, and the accuracy of the delivery of a test mandrel to the target is checked.

Patient positioning

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The patient is positioned supine with slight neck flexion and counterflexion at the knees to prevent slipping down (and resultant airway compromise) during the procedure.
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Transparent drapes are used to increase visibility of and by the patient.
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The C-arm is aligned transversely at the patient’s head.

Figure 41-1:

Target and entry planning. A, Volumetric T1 and inversion recovery images are imported into a neuronavigational functional neurosurgery platform (in this case, Stealth Station with Framelink software [Medtronic, Minneapolis, MN]) and merged (coregistered). B, The frame fiducial markers are indicated, and rectilinearity of the frame is checked. C, Internal landmarks are marked (AC, PC, and midline points) for reformatting of the examination along the plane of the ICL. D-F, Identification of the initial target in the GPi or Vim nucleus of thalamus can be done automatically using the software containing preloaded targeting formulas. D, GPi 20 mm lateral, 4 mm anterior to the mid-commissural point (MCP), 4 mm below the ICL. F, Vim 11.5 mm lateral to the third ventricle wall, 0.25 mm × ICL length anterior to the PC, at the level of the ICL. These targets can be modified as needed, which is facilitated by overlaying of the Schaltenbrand and Wahren stereotactic atlas. Entry point is selected at the appropriate angle of approach to the GPi or Vim (approximately 30 degrees from the vertical in the sagittal plane, lateral to the vertical in the coronal plane so as to miss penetrating the ventricle), modified to avoid prominent cerebral veins and sulci. The final stereotactic coordinates and angles are generated by the software (E) .

Figure 41-2:

Adjustment of the stereotactic frame. The planned target coordinates are used to adjust the stereotactic frame ( X, Y, and Z —lateral, anteroposterior, and vertical). Ideally, a phantom base is available, as with the Cosman-Robert-Wells (CRW) frame (Integra, Plainsboro, NJ), to check the accuracy of the physical setting of the frame coordinates and the stereotactic accuracy of the frame, which must be carefully maintained and calibrated.

Figure 41-3:

Patient positioning and room arrangement. The patient is positioned supine with neck flexion of 45 degrees or less, and the frame is affixed to the table through the Mayfield adapter. Knees are slightly flexed to provide comfort and countertraction to prevent the patient’s torso from slipping toward the feet, which may lead to head extension and airway compromise. The C-arm is positioned before draping the patient so that it can be draped out of the sterile field.