Technical Considerations in Movement Disorders Surgery

Philip Starr

University of California San Francisco, San Francisco, California, U.S.A

1 INTRODUCTION

The major subcortical structures targeted for deep brain stimulation (DBS) or lesioning for the treatment of movement disorders include the nucleus ventralis intermedius (Vim) of the thalamus, the globus pallidus internal segment (GPi), and the subthalamic nucleus (STN). The major technical goal during surgery for movement disorders is to maximize both precision and safety. The methods for localization of the Vim, GPi, and STN are evolving and vary significantly between centers. Three types of methods may be used to determine target location before lesioning or chronic stimulator placement: image-guided stereotactic localization, microelectrode mapping, and intraoperative test stimulation through the lesioning or DBS electrode, often called "macrostimulation." The first of these is based on anatomy, whereas the latter two are based on physiology.

2 IMAGE-GUIDED LOCALIZATION 2.1 Targeting from the Commissures

Classically, image-guided localization has been based on identification of internal landmarks, usually the anterior and posterior commissures (AC and

PC), in reference to a coordinate system provided by a stereotactic frame rigidly fixed to the patient's head. The AC and PC may be visualized on ventriculography, computed tomography (CT), or magnetic resonance imaging (MRI). Anatomical targets within motor thalamus, GPi, and STN may all be localized indirectly by measuring fixed distances from these landmarks, based on the location of the targets with respect to the AC and PC.

Table 1 provides reasonable initial anatomical coordinates for each target, determined from standard brain atlases and clinical studies in which the anatomical target coordinates have been verified physiologically [1-4]. Lateral coordinates for Vim and Gpi are best expressed in terms of distance from the third ventricular wall rather then the midline. These anatomical coordinates must be considered only an approximate guideline, however, for the following reasons: (1) There is significant individual variability in the spatial position of these targets in AC-PC based coordinates [1,3]; (2) The optimal target point within each nucleus has not been determined with certainty, as very few studies correlate location of lesions or electrode leads with outcome [5,6]; (3) Optimal location within a given target may be different for lesioning versus chronic stimulation [7]; (4) The exact initial target position may also depend on the patient's symptomatology. In targeting for

Table 1 Approximate Anatomic Coordinates for Vim, GPi, and STN with Respect to the Commissures

Target nucleus

Coordinates in mm

Corresponds to:

References

Lateral = 11 from third ventricle wall AP = 6 anterior to PC

Anteroventral border of Vim, in the arm territory

[3,4]

Gpi

Vertical = -2 to -8 Lateral = 18 from third ventricular wall AP = 2 anterior to MCP

Inferior border of motor territory of Gpi, immediately superior to optic tract

[1,15,50, 89]

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