**Vignette 1. Intraoperative cortical electrical stimulation**

It is the most widely used technique in awake patient surgery to delimit essential brain regions for some functions such as movement and language. It consists of the administration of an electric current in milli-amperes directly on the cerebral cortex (authors recommend bipolar stimulation, short train, 1 ms duration and 200 Hz frequency, with 5-20 mA intensity) in order to cause depolarization of a group of neurons belonging to a cerebral system to produce a positive symptom (such as a muscle contraction) or negative (such as the arrest during number counting).

Thanks to this technique, the organization of the representation of the body in the cerebral cortex was described by the eminent neurosurgeon Wilder Penfield in the 1950s and later important contributions were made on the organization of language in the brain by George Ojemann.

Among the most important advantages is the speed with which a wide region of the cerebral cortex can be mapped, in cases where the neoplasm delimited in the cortex can be observed, the edges of the lesion can be delimited. In the same way, it is possible to stimulate subcortically in the white matter.

Among the most important limitations is the little time available to carry out a cognitive task. For example, it is ideal to explore the denomination since the electrical stimulus can be administered immediately after asking the patient for the name of an object represented in a slide (several seconds), however it would not be possible to administer an electrical stimulus during the elaboration of a narration (**Figures 1**–**3**).

*The supplementary motor cortex (SMA)* is responsible for preparation, initiation, and monitoring movement, and it is located anterior to the cortical representation of the lower limb of precentral circumvolution. In the dominant hemisphere it exerts a function in the articulation of language, so its stimulation can cause alterations in the fluency of language when asking to name pictures. [22, 23].

*The frontal premotor cortex (PMC)* has a ventrolateral division responsible for the articulation of language, which when stimulated can cause anarthria; while the dorsolateral division is involved in the naming network, causing anomia when stimulated, this mainly in the dominant hemisphere. This location is where the intensity values of the stimulation are determined to obtain responses in the rest of the areas to explore [22, 23].

The cortex of the inferior and middle frontal gyri, when stimulated, causes impairment in writing.

**Figure 1.** *Cortical stimulation with bipolar.*

**Figure 2.** *Use of cortical stimulation guided by navigation.*

#### **Figure 3.**

*Cortical stimulation using various contacts.*

Regarding the frontal subcortical mapping, these fasciculi are evaluated by counting, naming, and reading tasks, the main tracts to evaluate in this area are:

a.*Superior Longitudinal Fasciculus (SLF):* it has the cortical projections of the frontal, temporal and inferior parietal lobe. SLF stimulation through the arcuate fasciculus (AF), can cause problems in the production of language, memory alterations and phonemic paraphasia.


Executive functions, working memory, attention, control, judgment and decision-making, functions related to perisylvian and prefrontal areas are also evaluated.

### *1.2.2 Parietal lobe*

The primary somatosensory cortex is located posterior to the primary motor cortex and, if necessary, it is possible to resect it without significant alterations in the sensory function, since other association areas can supply its function. However, the thalamocortical radiation must be preserved, representing the anterior limit of resection of parietal lesions [22–24].

The cortical region of the inferior parietal lobe, the supramarginal and angular gyrus, affect language in the dominant hemisphere and spatial awareness in the non-dominant hemisphere. Writing and calculation tests should be done to avoid iatrogenic Gertsman's syndrome. In the subcortical region of the dominant inferior parietal lobe (Geschwind territory), there are continuity of the pathways that communicate Broca's area (inferior frontal cortex) with Wernicke's area (posterior temporal cortex), the AF and SLF, therefore the stimulation of these areas can cause paraphasia and alteration in the production of language [22, 25].

#### *1.2.3 Temporal lobe*

In the temporal cortex, the main function to identify is language, especially in lesions of the dominant hemisphere. The posterior limit of a temporal pole resection is the arcuate fasciculus, which when stimulated causes paraphasia. Other temporal cortical and subcortical functions are visual recognition and dependent language, which is assessed with picture recognition. Likewise, temporary optic radiation should be evaluated in periventricular lesions in this region, in order to avoid postsurgical hemianopia [23].

#### *1.2.4 Occipital lobe*

The primary visual cortex is the main area to be explored, which when stimulated can produce phosphenes, blurred vision, visual hallucinations, and scotomas. Regarding the subcortical mapping, the final portion of the IFOF can produce alteration in the recognition and conceptualization of objects, so semantic paraphasia can be found [22, 23].

#### *1.2.5 Insular lobe*

The insular cortex and its corresponding subcortical tracts are considered unresectable, since they represent an important anatomical seat of essential functions of sensory, motor, limbic, vestibular and language integration. It is explored using a picture naming test [22, 26].

In a report, Ius et al., were able to identify sites considered unresectable after cortical mapping and resection of the lesion; in the dominant hemisphere the primary sensory and motor areas for the upper and lower extremities, the ventral premotor cortex, Wernicke's area in the posterior part of the superior temporal gyrus, and the supramarginal and angular gyrus; while in the non-dominant hemisphere the primary motor and sensory cortex and the angular gyrus. In certain cases it was possible to excise the rest of the association areas under the principle of maximum resection without greatly affecting the function [20–22]. Likewise, regarding the tracts, the following were considered unresectable: the cortico-spinal tract, posterior limit in patients with frontal lesion; thalamic-cortical radiations, anterior limit in patients with parietal lesions; the stratum sagittale, medial border of temporo-parietal lesions; anterior part of IFOF and perisylvian network [21, 22].

### **1.3 Brain lesions affecting the eloquent areas and surgical criteria**

In general, neoplastic intracranial lesions can displace or invade brain structures. The first group of lesions are not usually candidates for awake resection, since the symptoms are produced by the effect of mass on the cortex and tracts, but their resection does not involve functional areas. Unlike the second group of neoplasms that can infiltrate or even originate in functional areas, and whose resection without the appropriate quality of life approach, can have unacceptable consequences for the functioning of patients. Also, since the patient should be comfortable as much as possible for resection of the lesions, the awake and cortical mapping approaches usually involve convexity or superficial, intra-axial, supratentorial lesions [1, 27].

In a review of several reported series of awake cortical mapping, gliomas are the neoplasms that are most frequently approached by this technique, up to 60%. Highgrade astrocytomas such as OMS GIV glioblastoma is the most frequent glioma reported, followed by oligodendrogliomas, oligoastrocytomas, and low-grade astrocytomas [2, 28, 29]. The second group in frequency are brain metastases, mainly pulmonary and mammary origin. Finally, non-neoplastic lesions such as cavernous angiomas are usually reported as accessible lesions using this technique [30].

Although it is true that all the lesions described in the previous paragraph benefit from a wide resection, at present special emphasis has been placed in low-grade gliomas since these are lesions that usually occur in young adults, and it migrates through white matter tracts at an average rate of 4 mm/year [23, 31]. This raises new paradigms in which a supramarginal resection has been proposed even at the cost of function, hoping that brain plasticity in young patients improves the prognosis and quality of life in the long term [32].

#### *1.3.1 Surgical criteria*

The main criteria that are considered in neurosurgery to determine that a patient is considered for this type of procedure can be consulted in **Figure 4** in the form of a flow chart. Some of the most important criteria will be mentioned below according to the purpose of this chapter:


*Management of Brain Tumors in Eloquent Areas with Awake Patient DOI: http://dx.doi.org/10.5772/intechopen.95584*

#### **Figure 4.**

*Diagram showing the flow of neurosurgical conditions that must be considered to determine that a patient is a candidate for awake surgery (red line). Patient admitted to neurosurgery, from the outpatient or emergency department with an intra-axial lesion, located in a highly specialized area. Candidate for structural, functional and tractography MRI. Neuropsychological assessment confirmed the possibility of surgery with a conscious patient. Surgery is scheduled with a navigation method for resection of the lesion.*


#### **1.4 Pre-operative evaluation**

Once the patient has been selected for resection of the lesion with cortical mapping, extension studies should be carried out to bring us more evidence regarding the patient and their environment through neuropsychological and neuro-anesthesiology assessment. In addition, to plan the intraoperative mapping, it is advisable to perform:

