Neurosurgical interventions for movement disorders have increased in the last decade. Several disorders that are refractory to medical treatment are now being considered for surgery. Progress in understanding the underlying pathophysiology in these disorders and the anatomical and physiological relationships of basal ganglia components has helped identify potential targets for surgical interventions. In addition, advancement in neuroradiology, stereotactic localization, and intraoperative neurophysiological mapping has facilitated the localization of the targeted structures. This has resulted in improvement of surgical outcome and has made surgery a relatively safe and valuable option in the treatment of these disorders.

Currently, essential tremor (ET), Parkinson's disease (PD), and generalized dystonia (GD) are the three main movement disorders treated with stereotactic neurosurgical interventions. In this chapter, we will discuss indications for surgery, patient selection, rationale for surgery, and outcome for each of these movement disorders.

1.1 Essential Tremor

Essential tremor is a relatively common inherited disorder, with an estimated prevalence of 0.5 per 1000 persons older than the age of 40. Clinical symptoms usually start in early adulthood, with another peak of incidence in late ages. The illness is characterized by a 5 to 10 Hz tremor, which appears with maintenance of posture. Tremor amplitude may increase toward a target at the termination of movement (intention) [1]. The Essential Tremor Rating Scale is used to grade the severity of tremor: 0 = no tremor, 1 = slight, 2 = moderate amplitude, 3 = marked amplitude, 4 = severe amplitude [2]. Like most tremors, ET is worsened by emotion, fatigue, and exercise. It can be temporarily suppressed in the majority of patients by using oral ethanol. Beta blockers, such as propranolol, are usually helpful in controlling the tremor. Other drugs, including primidone and clonazepam, have also been effective [3]. Despite these medications, a small portion of ET patients continue to have severe tremor and significant motor disabilities. Patients with disabling tremor who have failed medical treatment are candidates for surgical intervention.

Neurons firing in synchrony with peripheral tremor are present in the ventral intermediate (Vim) thalamic nucleus. These so called ''tremor cells'' receive kinesthetic and cerebellar inputs and project primarily to the motor cortex. They can be identified intraoperatively through microelectrode recordings techniques. Intraoperative stimulation at sites with tremor cells will result in an immediate tremor arrest. Radiofrequency thalamotomy and tha-lamic deep brain stimulation (DBS) of the Vim nucleus are used in the treatment of ET.

Unilateral thalamotomy achieves 70% to 90% improvement in contralateral tremor as measured with the Tremor Rating Scale [4]. Reported complications of thalamotomy include paresthesia, dysequilibrium, and dysar-thria. Thalamic DBS has been used in an attempt to avoid these permanent complications. Unilateral DBS is reported to improve contralateral tremor by 68% to 89%. Bilateral thalamic DBS has been advocated to help patients with bilateral and axial tremor. Deep brain stimulation has many of the side effects of lesioning, but these effects can usually be diminished or eliminated by decreasing stimulation intensity. However, adjusting stimulation parameters may reduce the effectiveness of DBS in controlling tremor [5,6].

1.2 Parkinson's Disease

Parkinson's disease is a neurodegenerative disorder characterized mainly by loss of dopaminergic pigmented neurons in the substantia nigra pars compacta (SNc). The incidence of PD is 3/1000 and may reach as high as 3% in individuals older than age 65 [7]. Early in the course of the disease, patients with PD usually have good control of their symptoms with medical treatment. However, with time and disease progression, PD patients receive fewer benefits in response to medication, and significant adverse effects can appear. Further dose increases may not produce better control of symptoms or may induce important side effects, including drug-induced dyskinesias. Many patients continue to have profound motor disabilities despite the best available pharmacotherapy. The unpredictable dramatic switching between "on" (good motor function) and "off" (akinetic, rigid, and tremor) states, experienced by a large number of patients later in the course of PD complicates the illness [8].

Stereotactic surgery for PD is not new. Stereotactic thalamotomy was used to treat tremor in the 1950s and 1960s. However, the introduction of L-dopa in the late 1960s and its striking benefits in PD symptoms resulted in almost total disappearance of surgery. In the early 1990s, the surgical option was re-explored. The promising results of radiofrequency lesioning in the posteroventral part of the internal segment of globus pallidus (GPi) (pallidotomy) in improving motor symptoms of PD and controlling drug-induced dyskinesias opened the door for developments in surgical treatment of PD [9-11].

1.2.1 Selection of Patients

Surgery is indicated for patients with PD who continue to have significant motor disabilities despite best medical management. All efforts should be exercised by a specialist in movement disorders to optimize the medical treatment by adjusting doses and frequencies of medications before considering surgical intervention. Patients with idiopathic PD who respond to L-dopa are good potential candidates for surgery. Patients with so called "Parkinson plus'' syndromes, such as multiple systems atrophy or progressive supranuclear palsy, are often less responsive to L-dopa and are generally poor surgical candidates. Patients without significant disabling motor symptoms (grades 1 and 2) and those with end-stage (grade 5) PD are not the best candidates for surgery (Table 1) [12]. Cognitive and psychiatric disturbances, autonomic disturbances, and speech and swallowing difficulties are not uncommon in PD patients. These symptoms not only fail to improve, but may worsen after surgery. Younger patients derive more benefits from surgery, but advanced age is not a contraindication. Significant coexisting medical conditions, psychiatric disease, or focal abnormalities on brain images are relative contraindications.

1.2.2 Surgical Techniques

Magnetic resonance imaging (MRI) (the most commonly used image modality), computerized tomography (CT), or ventriculography can be used

Table 1 Hoehn and Yahr Staging of PD

Stage 1.0 = unilateral involvement only

Stage 1.5 = unilateral and axial involvement

Stage 2.0 = bilateral involvement without impairment of balance Stage 2.5 = mild bilateral involvement with recovery on retropulsion (pull) test

Stage 3.0 = mild to moderate bilateral involvement, some postural instability; still able to walk and stand unassisted Stage 5.0 = wheelchair bound or bedridden unless added

Abbreviation: PD, Parkinson's disease.

to localize surgical targets. Targets can be visualized directly on MRI or indirectly as a function of their relationship to the anterior and posterior commissures. A stereotactic frame is applied under local anesthesia before imaging is used to determine target coordinates. During the procedure, intraoperative mapping is used to verify the anatomical data obtained from images. Two forms of physiological mapping are available: macroelectrode stimulation and microelectrode recordings and stimulation. With macroelec-trode stimulation, a 1- to 2-mm diameter electrode is used to deliver a current to the chosen target. Using voltage output of approximately 1 to 4 volts at 2 to 300 Hz current, both clinical benefits and adverse effects of stimulation are noted. Microelectrode recordings and stimulation permit the acquisition of direct measures of the activity of single neurons. Spontaneous and evoked single-unit activity is amplified and displayed on an oscilloscope or heard on a speaker. This information permits unambiguous definition of axonal and neuronal territories. These data are used to determine the boundaries of the targeted structure and to help in placing the lesion or electrode more accurately. Stimulation can be used to identify important structures close to the target. In targeting thalamic Vim nucleus, the thalamic soma-tosensory relay nucleus (ventral caudalis) can be localized. For GPi, the corticospinal and corticobulbar tract are identified, as well as the optic tract. For the subthalamic nucleus (STN), oculomotor nerve, corticospinal, and medial lemniscal sensory tracts can also be identified [13].

The two widely used techniques in functional neurosurgery currently are lesioning and chronic deep brain stimulation. In lesioning, a radiofre-quency generator produces a lesion at a chosen target using the exposed electrode's tip (Fig. 1). The size of the lesion depends on the time of application (30 to 60 seconds), temperature at the electrode tip (45° to 90° C), and size of the electrode used (1 to 3 mm) [13,14]. In DBS, an electrode is

Figure 1 An axial cut T2 MRI scan for patient with DYT1 generalized dystonia treated with bilateral radiofrequency pallidotomy.

placed at the target and connected to a subcutaneous internal pulse generator fitted with a lithium battery (Figs. 2 and 3). Although radiofrequency le-sioning induce immediate effects, which may last for years, DBS has the advantage being adjustable and reversible. This is particularly important in case of bilateral procedures. On the other hand, DBS is more expensive and carries the risk of infection or mechanical failure like any other implanted foreign body [15,16]. The widely used targets in the treatment of PD are the thalamic Vim nucleus, GPi, and STN.

Thalamic Vim. Thalamotomy reduces or abolishes parkinsonian con-tralatral tremor in 80% to 90% of patients or, using a different measure, it improves tremor to grade 0 or 1 in about 75% of cases. Vim thalamotomy in PD is not effective for rigidity, bradykinesia, speech, or gait. Complica-

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