*4.1.4. Continuous IONM techniques*

the tumour via subcortical stimulation. A series of language tasks is conducted by a trained

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

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

**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‐

neuropsychologist throughout the duration of the tumour resection.

seizures (**Figure 2**).

224 Neurooncology - Newer Developments

observed in only 1.6% of patients.

ter-discharges are shown in red.

Following the determination of the relationship between the tumour and the PMC, continu‐ ous monitoring is performed via DCS using pairs of grid electrodes and employing highfrequency stimulation (**Figure 2B**).

It is accepted that the primary criterion for monitoring MEP in the setting of supratentorial surgery is an amplitude reduction of >50%. Temporary motor deficits have been linked to reversible declines in MEP amplitude of >50%, whereas irreversible declines and losses in MEP are predictors of permanent motor deficits [5]. Pastor et al. [5] described a study involving 34 patients who underwent surgical resection of glioma that was guided using 5-aminolevulin‐ ic acid (5-ALA) with no false-negative results. Nevertheless, postoperative neurological deficits without alterations in MEP are possible. This scenario, may be explained by secon‐ dary events such as postoperative oedema, haemorrhage and tumour resection from the supplementary motor area (SMA) [55].

An interesting issue for discussion is whether patients who harbour tumours in a region of the eloquent cortex other than the language area must be operated under general anaesthesia. As we have highlighted, there has recently been a renewed use of awake craniotomies [40,41]. In our experience, no new neurological deficits are observed in anaesthetized patients [5,20]. Accordingly, surgery near motor and somato-sensory cortical areas can be performed safely with the concomitant use of intensive neurophysiological mapping and monitoring [5]. Moreover, this combined approach is much more comfortable for both the patient and the surgical team.

When sensory function monitoring is also required, cSSEP are directly recorded from the grid. Monitoring the electrode entails the selection of higher amplitude responses for N1/P1/N2 potentials.

#### **4.2. Semi-oval centre surgery**

#### *4.2.1. Anatomical and surgical considerations*

The white matter in each hemisphere located between the cerebral cortex and deep nuclei together has a semi-oval shape. It consists of an association, a commissural and projecting cortical fibres. It contains, among others, the corticospinal, thalamo-cortical (containing somato-sensory and visual projections) or corticobulbar tracts.

In these patients, surgical removal of the tumour is performed far away from the motor cortex but near the subcortical structures such as the basal ganglia and the IC.

#### *4.2.2. Particularities of IONM*

As stated, DCS in these cases is usually precluded, and TES is the technique of choice. Because there is a risk of stimulation beneath the lesion, hemispheric TES must be performed. The somato-sensory pathway shouldalso be monitoredthrough the SSEP and, in a number of cases, even the VEP.

An interesting point is that warning criteria for the motor response may occur segmentally in isolated muscles. As previously described, these segmented changes are the most commonly observed changes [5] (**Figure 3**).

**Figure 3.** Monitoring of a patient with a tumour in the right semi-oval centre. (A) Frontal MRI before surgery, and an image indicating the placement of the electrodes for TES (B) MEPs monitoring of the upper limb at the moment a reversible segmental alteration occurred in the MEPs of the hand is shown in red.

Subcortical electrical stimulation is an important technique for the identification of the corticospinal tract [35]. The most accepted technique to date, in terms of effectiveness with respect to subcortical activation with a lower motor threshold (MT), is monopolar cathodal stimulation [39].

In general, a linear relationship is considered to be present among the five monopolar 0.2–0.5 ms pulses and the 3–4 ms ISI, as well as a threshold of 1 mA of stimulation, which is equiva‐ lent to approximately 1 mm of distance from the CST [56]. Some studies have attempted to determine the lowest intensity of stimulation allowed before resection should be terminated to prevent injury to the CST. This safety margin has not been standardized and has been defined as 6 mA in some studies, whereas other studies have suggested that both significant signal changes in MEP and permanent motor deficits do not occur below a threshold of 1–3 mA [57]. Regardless, it seems reasonable to follow these motor threshold safety margins provided that no alterations in MEP are observed during continuous motor monitoring; otherwise, the resection must be stopped immediately.

VEP are needed when the tumour is located in the occipito-parietal region due to the risk of injury to the thalamo-cortical projections to the visual cortex. In fact, we noted that up to 50% of patients monitored by VEP during semi-oval centre surgery displayed warning criteria [5].

#### **4.3. Cranial base and anterior fossa surgery**

somato-sensory pathway shouldalso be monitoredthrough the SSEP and, in a number of cases,

An interesting point is that warning criteria for the motor response may occur segmentally in isolated muscles. As previously described, these segmented changes are the most commonly

**Figure 3.** Monitoring of a patient with a tumour in the right semi-oval centre. (A) Frontal MRI before surgery, and an image indicating the placement of the electrodes for TES (B) MEPs monitoring of the upper limb at the moment a

Subcortical electrical stimulation is an important technique for the identification of the corticospinal tract [35]. The most accepted technique to date, in terms of effectiveness with

reversible segmental alteration occurred in the MEPs of the hand is shown in red.

even the VEP.

226 Neurooncology - Newer Developments

observed changes [5] (**Figure 3**).
