**4.1. Cortical surgery**

## *4.1.1. Anatomical and surgical considerations*

IONM of cortical tumours is indicated when the lesion is in or near an eloquent area (areas responsible for carrying out basic neurological functions) such as the sensorimotor cortex or the language cortex. IONM enables clinicians to access the function of the motor and senso‐ ry systems of the patient during surgery to preserve neurological function, and it increases the success of radical tumour resection [6].

Functional mapping is initially performed to identify functional areas and their relationship with the tumour. This procedure might help the surgeon to determine the site at which to initiate resection of the tumour. Mapping is then followed by monitoring of the structures requiring continuous vigilance.

#### *4.1.2. Particularities of IONM*

As mentioned previously, ECoG is used to define functional areas. Moreover, electrical stimulation can elicit epileptiform discharges before, after and during mapping and IONM, considering that seizures in brain tumour patients are a common phenomenon accounting for 20–40 percent of cases [49].

Following the identification of functional areas via ECoG (**Figure 1B**), the location of the CS is determined by cSSEP phase reversal (**Figure 1C**). Accurate identification of the CS is extreme‐ ly important because it permits identification of the PMC (**Figure 1C**) [5,6]. There is evidence that cSSEP phase reversal increases both the efficiency and the safety of PMC identification

[50]. Thereafter, DCS for the identification of PMC is performed. Of course, the motor map reference is always the Penfield homunculus [51]. However, when performing motor map‐ ping, we must not forget that it does not conform strictly to reality [52]. It is a scheme upon which there are local movements in different directions [53].

**Figure 1.** Cortical surgery in a patient with a left frontal glioma. (A) Neuronavigator showing coronal, frontal and sagittal views of left cortical tumour. (B) DSA of electrocorticography. For each electrode, there is a colour-coded plot showing the power spectral density for every frequency in the abscissa axis along the time shown in the ordinate axis. For electrodes 3–5, the power of all frequencies is lower than that for the rest of the grid. (C) Placement of the grid over the cortex with sensory and motor mapping. The coloured areas show the motor regions of the abdomen, arm, forearm and hand, and they correspond to the MEPs with the same colour. The dashed line in blue shows the area of the cen‐ tral sulcus, with the phase reversal illustrated in SSEPs in green.

### *4.1.3. Cortical language mapping*

**3.2. Intraoperative monitoring**

222 Neurooncology - Newer Developments

**4. Topographical approach to IONM**

*4.1.1. Anatomical and surgical considerations*

success of radical tumour resection [6].

requiring continuous vigilance.

*4.1.2. Particularities of IONM*

for 20–40 percent of cases [49].

neurosurgery.

**4.1. Cortical surgery**

Monitoring consists of the surveillance of the functional state(s)/structure(s) that is(are) being

Therefore, the duration of monitoring is considerably greater than in the case of mapping, and it usually lasts as long as the surgery itself. This condition naturally requires more dramatic

The objectives in this case are not just the functional identification but also the preservation of functional integrity, which can be compromised by many medical/surgical procedures, ranging from problems related to tissue perfusion (for example, by actively inducing hypo‐ tension or bleeding) to inadvertent surgical injuries such as the placement of spatulas.

In general, neurophysiological monitoring is widely employed in oncological or vascular

IONM of cortical tumours is indicated when the lesion is in or near an eloquent area (areas responsible for carrying out basic neurological functions) such as the sensorimotor cortex or the language cortex. IONM enables clinicians to access the function of the motor and senso‐ ry systems of the patient during surgery to preserve neurological function, and it increases the

Functional mapping is initially performed to identify functional areas and their relationship with the tumour. This procedure might help the surgeon to determine the site at which to initiate resection of the tumour. Mapping is then followed by monitoring of the structures

As mentioned previously, ECoG is used to define functional areas. Moreover, electrical stimulation can elicit epileptiform discharges before, after and during mapping and IONM, considering that seizures in brain tumour patients are a common phenomenon accounting

Following the identification of functional areas via ECoG (**Figure 1B**), the location of the CS is determined by cSSEP phase reversal (**Figure 1C**). Accurate identification of the CS is extreme‐ ly important because it permits identification of the PMC (**Figure 1C**) [5,6]. There is evidence that cSSEP phase reversal increases both the efficiency and the safety of PMC identification

monitored for all or most of the surgical action, given the risk of iatrogenic injury [9].

changes than those used for mapping, beyond the modification of anaesthesia.

Locating the functional cortical regions related to language is the goal of intraoperative language mapping stimulation, which is performed during awake surgery. DCS is the technique of choice, as has been widely discussed in the previous section. Concerning this area of the cortex, we have provided additional information about the procedure employed forthis type of surgery. The cortex is mapped every 5–10 mm, and positive stimulation sites respon‐ sible for language impairment are marked; the same technique is utilized during resection of the tumour via subcortical stimulation. A series of language tasks is conducted by a trained neuropsychologist throughout the duration of the tumour resection.

Although the risk of intraoperative seizures related to DCS is low, it is a real possibility. Additionally, considering that the patients are awake, this complication seems to be a particularly undesirable effect, not only from the perspective of patient discomfort but also because it would interfere with mapping by post-ictal hyperpolarisation. Using ECoG in this type of surgery could prevent, by monitoring and identifying discharges, the occurrence of seizures (**Figure 2**).

The identification of language areas and their fibres is not as successful as the localization of the cortical white matterin the PMC. Sanai and Berger [54] successfully identified the language areas in 145 (58%) of 250 patients with gliomas. Regarding neurological outcome, temporary language deficits were observed in 22% of patients, whereas permanent language deficits were observed in only 1.6% of patients.

**Figure 2.** Mapping language in a patient with a cavernoma in Wernicke's area. (A) DSA showing a relative loss of cort‐ ical fast rhythms in electrodes 12 and 17. (B) Image showing the cortical mapping results. Red: Wernicke's area to 13 mA. Orange: region with negative results above 12 mA. The dotted line indicates the cortical incision for approaching. (C) Recording showing long-term after-discharges following stimulation by electrodes 18/1 –9 mA. The widespread artefact (arrowhead) corresponds to the moment at which cool serum was administered. The channels affected by af‐ ter-discharges are shown in red.
