Surgery of brain tumors asleep

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Introduction

In surgery of supratentorial brain tumors, prompt and complete resection offers the most favorable outcome; however, damage to essential cortico-subcortical structures may cause permanent neurological deficits . The surgeon faces a complex balance between radicality of resection and preservation of function: this is of particular importance in the surgery of gliomas, which exposes the patient to significant morbidity and, often, short life expectancy .

The seminal studies of Leyton and Sherrington and Broca led to the description of function in localized regions of the brain, and permitted the identification of classical eloquent areas, such as the precentral gyrus for motor function and a speech area in the left inferior frontal gyrus. However, it was with the prescient work of Wilder Penfield that this was translated into clinical practice through brain stimulation, and consequently, direct cortical stimulation (DCS) has been performed to recognize function, thus avoiding neurological injury .

In the past, most of the interest and research has been focused on cortical, not subcortical, mapping, due to the notion of the eloquent cortex, in which brain function lies in specific cortical regions. Classical mapping has been used to localize and avoid eloquent cortices considered “no entry zones,” in contrast with “noneloquent” areas that could be resected without incurring neurological deficits. This concept was clearly underlined in Penfield’s work, in which, referring to the precentral gyrus, he wrote, In the therapeutic approach, it should be pointed out that only very rarely has the Rolandic area been included in any excision and never has this region of the brain been touched unless a lesion was present that could be demonstrated grossly by operative inspection. This digression is made in the hope of discouraging surgical removal of normal brain from the Rolandic area, or elsewhere, whatever may have been the pattern of epileptic seizure . . The implication for clinical outcome was dramatic, as clinical procedure for lesions in so-called eloquent areas was mostly reoriented in favor of conservative treatment or biopsy . Recently, the reappraisal of classical cases, such as the one of Broca’s patient Leborgne , and the clinical evidence that resection of “noneloquent” areas incurred neurological deficit , while “eloquent areas” could be resected without permanent deficits , challenged this rigid cortical functional organization and caused renewed interest in brain associationism and disconnection syndromes. Accordingly, the concept of hodology—where brain function is composed of spatially distributed dynamic networks of subcortical pathways connecting distant cortices—has been proposed , moving the attention away from “eloquent cortices” to “essential networks,” from cortical to subcortical structures composed of white matter tracts .

When dealing with essential networks, the choice of mapping protocol cannot be underestimated. A recent European survey considering different mapping techniques for the surgery of tumors in eloquent areas demonstrated very different choices of mapping protocols among centers for similar type of pathology, with some centers performing almost all surgery asleep (asleepers) in contrast to other centers which performed almost all surgeries in awake fashion (awakers) . No data on functional outcome was reported in the survey, and it is difficult to prove that either one of the approaches is superior to the other, although it is expected that awake surgery remains essential whenever cognitive functions need to be tested.

While awake surgery is often advocated for preservation of cognitive functions as their mapping is only possible under local anesthesia, patients may become candidates for asleep procedures for a number of reasons—the surgical preference of individual center, conditions of scarce compliance, impaired function, or even anxiety.

Intraoperative neurophysiological monitoring (ION) has been used for more than 40 years and now is regarded as a well-established discipline able to predict and prevent neurological injury intraoperatively . It uses mapping and monitoring techniques: mapping techniques identify anatomically ambiguous neural structures, while monitoring techniques can assess the functional integrity of cortico-subcortical pathways . Some of these techniques are particularly valuable during supratentorial surgery to localize eloquent cortices and to identify and preserve subcortical connectivity: ION is the current gold standard for preservation of motor function and offers the possibility of monitoring sensory, motor, visual, and auditory pathways during the surgery.

We will discuss the impact of current ION techniques with particular regard to their application to surgical procedures for supratentorial surgery of brain tumors under general anesthesia.

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