Neurophysiology

The use of simultaneous MEP and SSEP monitoring has been suggested for this purpose and has been studied to a limited extent. Nagle et al. reviewed a series of 116 cases involving surgical manipulation of the spinal cord or column in which simultaneous intraoperative SSEP and MEP monitoring was utilized. Significant intraoperative changes in both SSEP and MEP patterns occurred in eight of these patients. An additional patient had isolated MEP changes. All patients with intraoperative changes awoke with post-operative deficit. Therefore, the authors support simultaneous use of both MEP and SSEP monitoring to achieve parallel, independent monitoring of spinal function [18]. Additional data from another series of patients undergoing surgical correction of spinal deformity suggested that relevant intraoperative changes are acquired in a more timely fashion with MEP than with SSEP [15].

Cortical Mapping Techniques

Functional areas other than the primary motor cortex can be localized with electrically or magnetically driven cortical mapping techniques. Preservation of language function is a primary concern when performing dominant temporal lobe resections. Investigations that have mapped cortical regions subserving functional speech have demonstrated considerable variability in the specific location of these areas along the superior and medial temporal gyri. Resections of the dominant temporal lobe using standardized strategies have the potential to significantly damage the patient's ability to speak or, alternatively, underestimate the potential limits of resection, depending on the exact location of language areas.

The most reliable and widely used technique for identifying cortical language areas involves direct electrical stimulation of cortex that is putatively involved in functional speech. Numerous studies have shown that electrical stimulation of speech-related cortex will interfere with language tasks, generally resulting in anomia or a complete abrogation of speech. Electrical-stimulation language mapping is typically carried out in awake patients. When circumstances dictate that a resection be carried out under general anesthesia, an initial cran-iotomy can be performed to place indwelling surface electrode grids over the brain regions to be mapped. Following recovery from this initial procedure, detailed language-mapping protocols are carried out via the externalized electrical leads. After language mapping has been completed, with all functionally important cortical sites identified, the patient is returned to the operating room for electrode removal and an appropriately guided resection under general anesthesia.

Safe and effective language mapping is accomplished via direct electrical stimulation of the cortex at strengths that are below the afterdischarge (AD) threshold. ADs are abnormal cortical discharges that are evoked by focal electrical stimulation and which persist beyond the period of stimulation. Electrocorticographic recording electrodes must be positioned immediately adjacent to the site of electrical stimulation in order to detect ADs. Stimulation strengths with the potential to evoke ADs are also capable of evoking local seizure activity. This can render stimulation mapping uninter-pretable or, at worst, precipitate a generalized seizure. Typically, electrical stimuli are delivered via a hand-held probe and consist of pulse trains of charge-balanced square waves (0.2 ms duration, 50 Hz). The patient is instructed to carry out a variety of language tasks (e.g. object identification, word repetition, counting, execution of verbal commands) as disruptive electrical stimuli are delivered to various cortical surface sites. Sites that are associated with changes in speech and comprehension are identified and can be spared during the subsequent surgical resection.

Microelectrode Recording/Stimulation

Neurosurgical treatment options for the various movement disorders have been under investigation for a number of decades. Parkinson's disease has perhaps received the greatest amount of attention, partly owing to unsatisfactory long-term outcomes with current medical therapies, improved understanding of the pathophysiolog-ical connections relevant to Parkinson's disease, and advances in monitoring techniques relevant to these procedures. These cases generally involve either ablation or deep-brain stimulation of the motor thalamus, the globus pallidus or the subthalamic nucleus via stereotactic electrode placement; more recently, stem cell transplantation has emerged as a potential treatment option. Precise advancement and the final position of electrodes used for ablation or stimulation are of paramount importance in reducing post-operative morbidity as well as ensuring the best chance for therapeutic success. The primary method used over the last decade for this purpose is electrophysiological monitoring of the brain structures that are traversed by the microelectrode during the procedure.

The microelectrode mapping technique relies on the development of a "physiological map" based on the known spontaneous firing rate and pattern of particular neuronal groups and the predictable pattern changes that are related to various stimuli. The internal capsule and optic tract can be identified through the recording of firing changes related to sensory stimuli such as limb movement or flashes of light, respectively. As the recording/stimulating electrode is advanced, the physiological map that is developed can be correlated to a standardized stereo-tactic atlas or thin-slice high-resolution MRI images from the particular patient. Once this map is developed, lesion or stimulation electrode placement can occur with maximum precision.

The proven utility of microelectrode recording for increasing accuracy and decreasing morbidity in ablation or deep-brain stimulator placement is somewhat unclear. Although the vast majority of surgeons performing these procedures utilize microelectrode guidance, a critical review of the relevant literature compiled by Hariz and Fodstad questioned this practice. They noted that rates of severe complications and mortality appeared to be higher when microelectrodes were used, rather than MRI-based guidance, for either ablative or stimulatory purposes, while concomitant gains in accuracy and efficacy of the procedure were not seen. Their final conclusion focused on the need for a prospective, randomized trial comparing micro- and macroelectrodes in movement disorder surgery [19].

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