Surgical Strategies A Preoperative

The preoperative investigation of patients with cerebral lesions falls into two general categories. The first is targeting areas for the purpose of stimulation or ablation. These circumstances occur in patients with movement disorders in which parts of the basal ganglia or other subcortical regions are selected for lesioning (e.g., pallidotomy) as a means of improving symptoms. Lesioning using stereotactic focal radiation or direct surgical ablation, by heating or freezing, are of interest for the treatment of cerebral neoplasms and vascular malformations. In a similar fashion, stimulating electrodes are now being employed in the treatment of Parkinson's disease and certain types of tremors.

The exact location for the placement of these electrodes requires knowledge of the structural anatomy and local electrophysiology or function to obtain maximal therapeutic benefit.

1. Targeting

Brain mapping techniques have already been employed for targetting. Currently, these approaches are limited to obtaining better definition of a patient's structural anatomy and developing better atlases to identify selected portions of the brain, given the individual variability among patients. With highresolution structural imaging, specific locations for potential lesions can be identified anatomically and a frameless stereotactic approach can be used to direct an ablation probe or stimulation electrode. Once located, electrophysiological recordings can be used to verify the local functional environment of a given anatomical site.

Functional activation of deep brain sites (e.g., medial globus pallidus) is an important area of current investigation. Ideally, such sites should be located both structurally and functionally to identify a surgical target more accurately preoperatively. Such a facility would mean less retargeting and repositioning of electrodes and probes, thus reducing operating room time and morbidity, resulting in a higher success rate. Currently, there are no validated clinical examples of such an approach.

2. Differentiation of Normal from Abnormal Brain

Another important aspect of presurgical investigation of patients with brain mapping techniques is the identification of normal cortex or deep brain structures so that they may be avoided during surgical resection or ablation of cerebral lesions. The goal is to remove an abnormality in its entirety without removing normal brain tissue. Preoperative evaluation with PET, SPECT, fMRI, or transcranial magnetic stimulation may be employed to identify the functional anatomy of an individual's brain to determine the safety of surgery and a strategy for reaching an abnormal brain region to remove pathologic tissue.

Such functional imaging techniques may ultimately replace procedures such as the Wada test or reversible pharmacologic interruptions of brain function. The selective administration of barbiturates to brain regions that produce transient deficits has long been used to determine the relative safety associated with removal of a portion of the brain. Nevertheless, such tests are difficult to perform, particularly in younger children, and the exact distribution of the pharmaco-logic agent can be difficult to verify. If the same information can be obtained through presurgical use of functional imaging, it is hoped that both the accuracy and the ease of obtaining such data will be enhanced (Fig. 11).

The combined use of all of the noninvasive scanning methods, both current and experimental, resulting in integrated and composite images linked to interactive graphics stations in the operating room or the interventional neuroradiological suite, will surely become a part of such interventional procedures in the future. To be successful, these approaches need to be less costly, more accurate, and associated with lower eventual morbidity than conventional surgery without such ancillary noninvasive procedures.

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