*4.3.2. Particularities of IONM*

According to the anatomy of the anterior and middle fosses, the main structures to monitor are the CNs. Therefore, the standard technique is recording fEMG to identify different discharge patterns related to either irritative or injury activity. Several criteria have been advanced to identify patterns that predict transitory or permanent nerve injury, but these criteria lack uniformity, and the correlation with the postoperative outcome is often unsatis‐ factory [59]. sEMG is a functional technique that is also essential during these types of surgeries.

Various techniques have been used for the placement and recording of the superior rectus muscle for CN III and the lateral rectus muscle for CN VI. These include the manual place‐ ment of bipolar needles, or single-shafted bipolar needles using an orbital ultrasound [60] or image guidance [61].

Corticobulbar tract (CBT) MEP monitoring is a neurophysiological technique that has improved greatly over time and is increasingly used in brainstem tumour surgery. Although the correlation between the intraoperative corticobulbar response and the postoperative outcome may not be completely accurate due to the possibility of false-positive and falsenegative responses [62], experience seems to indicate that the complete disappearance of a corticobulbar MEP usually correlates with a severe, mostly irreversible, postoperative deficit. Conversely, when the corticobulbar MEP remains unchanged at the end of surgery, a transient deficit may not be prevented, but the great majority of these patients have recovered their preoperative status at the time of follow-up [59].

The vestibulocochlear nerve (CN VIII) and, to a greater extent, the auditory pathways as they pass through the brainstem are especially at risk during cerebellopontine angle (CPA), posterior/middle fossa or brainstem surgery. CN VIII can be damaged by several mecha‐ nisms, from vascular compromise to mechanical injury by stretch, compression, dissection and heat injury. Additionally, the cochlea itself can be significantly damaged during temporal bone drilling, by noise, mechanical destruction, or infarction, and due to rupture, occlusion or vasospasm of the internal auditory artery. CN VIII monitoring can be successfully achieved by live recording of the function of one of its parts, the cochlear or auditory nerve (AN), using the BAEPs [63].

The necessity to protect the optic tract is not uncommon in this setting, and thus intraopera‐ tive monitoring of the VEP is mandatory. Although such monitoring techniques were previously not well-established, high rates of feasibility have recently been reported in a number of publications, and stable VEPs have been associated with good postoperative visual function [30].

From a neurophysiological perspective the ICA emits at the supraclinoid segment, an important collateral branch: the posterior communicating. This artery is of paramount importance because in addition to providing branches to the optic chiasm and oculomotor nerve, it is also responsible for irrigating the thalamus. Therefore, in this type of surgery, the MEP and SSEP must be monitored.

#### **4.4. Posterior fossa surgery**

*4.3.2. Particularities of IONM*

228 Neurooncology - Newer Developments

surgeries.

image guidance [61].

the BAEPs [63].

function [30].

preoperative status at the time of follow-up [59].

According to the anatomy of the anterior and middle fosses, the main structures to monitor are the CNs. Therefore, the standard technique is recording fEMG to identify different discharge patterns related to either irritative or injury activity. Several criteria have been advanced to identify patterns that predict transitory or permanent nerve injury, but these criteria lack uniformity, and the correlation with the postoperative outcome is often unsatis‐ factory [59]. sEMG is a functional technique that is also essential during these types of

Various techniques have been used for the placement and recording of the superior rectus muscle for CN III and the lateral rectus muscle for CN VI. These include the manual place‐ ment of bipolar needles, or single-shafted bipolar needles using an orbital ultrasound [60] or

Corticobulbar tract (CBT) MEP monitoring is a neurophysiological technique that has improved greatly over time and is increasingly used in brainstem tumour surgery. Although the correlation between the intraoperative corticobulbar response and the postoperative outcome may not be completely accurate due to the possibility of false-positive and falsenegative responses [62], experience seems to indicate that the complete disappearance of a corticobulbar MEP usually correlates with a severe, mostly irreversible, postoperative deficit. Conversely, when the corticobulbar MEP remains unchanged at the end of surgery, a transient deficit may not be prevented, but the great majority of these patients have recovered their

The vestibulocochlear nerve (CN VIII) and, to a greater extent, the auditory pathways as they pass through the brainstem are especially at risk during cerebellopontine angle (CPA), posterior/middle fossa or brainstem surgery. CN VIII can be damaged by several mecha‐ nisms, from vascular compromise to mechanical injury by stretch, compression, dissection and heat injury. Additionally, the cochlea itself can be significantly damaged during temporal bone drilling, by noise, mechanical destruction, or infarction, and due to rupture, occlusion or vasospasm of the internal auditory artery. CN VIII monitoring can be successfully achieved by live recording of the function of one of its parts, the cochlear or auditory nerve (AN), using

The necessity to protect the optic tract is not uncommon in this setting, and thus intraopera‐ tive monitoring of the VEP is mandatory. Although such monitoring techniques were previously not well-established, high rates of feasibility have recently been reported in a number of publications, and stable VEPs have been associated with good postoperative visual

From a neurophysiological perspective the ICA emits at the supraclinoid segment, an important collateral branch: the posterior communicating. This artery is of paramount importance because in addition to providing branches to the optic chiasm and oculomotor

## *4.4.1. Anatomical and surgical considerations*

The posterior cranial fossa is the deepest and most capacious fossa of the skull base. It contains the cerebellum, pons and medulla oblongata. Recent advances in diagnostic and surgical techniques have increased the accessibility of this region to surgery, providing new and neurologically safer treatments for these patients [64].

Tumours of the skull base show a tendency to be large in size, they are critical brain lesions because they extend inside and around the brainstem, placing not only the CNs and/or their motor nuclei at risk of injury but also the motor and somato-sensory pathways.

The particular approach is determined not simply by the location of the lesion but largely by the path that allows access to the lesion while minimally disturbing critical structures. The surgeon must select a cranial approach that provides line-of-sight access to the lesion while avoiding excessive manipulation; however, this achievement is not simple due to the high density of critical structures in the brainstem, and even a mild manoeuvre can result in the injury to the delicate nuclei, tracts, or nerves.

#### *4.4.2. Particularities of IONM*

There is a critically high potential risk of damage to neural structures during the resection of these tumours, and therefore IONM is an indispensable tool for posterior fossa surgery. The decision to monitor certain structures depends on both the anatomical location of the tu‐ mour and the surgical approach selected by the surgeon.

In general, IONM consists of three procedures (**Figure 4**): (i) brainstem mapping (BSM), (ii) CBT-MEP monitoring and (iii) monitoring of ascending and descending long pathways and transverse auditory radiations. BSM is a neurophysiological technique that is used to locate cranial nerves and their motor nuclei on the floor of the fourth ventricle [65] through sEMG. During resection of brainstem tumours involving incision of the fourth ventricle, the facial colliculus (intramedullary roots of the CN VII around the abducens nucleus) and motor nuclei of the lower CNs nucleus are especially prone to injury because the tumour often grossly distorts the brainstem anatomy, and the normal landmarks on the floor of the fourth ventri‐ cle are missing. If the neurosurgeon cannot identify these landmarks, he will not be able to make a safe incision. Hence, the mapping assists surgeons in locating these important structures.

The recording technique used for targeted muscles is the same as discussed previously, and the standard set of muscles used to record muscle activity are as follows: the orbicularis oris, orbicularis oculi, and mentalis for the VII cranial motor nucleus (CMN), the posterior pharyngeal wall for CMN IX, the cricothyroid or vocalis muscle for X and the intrinsic tongue muscles for CMN XII. Electrical stimulation of the floor of the fourth ventricle is delivered through a monopolar hand-held probe using the same parameters described above for the stimulated EMG technique. It is important to stress that during this procedure, the threshold intensity is essential for proper localization of the CMN.

**Figure 4.** Brain stem mapping in a patient with a tumour locate on the floor of the fourth ventricle. (A) Frontal and sagittal MRI before surgery. (B) Stimulated EMG during identification of the right CN IX (above) and the left CN VII (below). (C) Identification of the left CN IX (above) and the right CN VII (below). (D) Brain stem monitoring compris‐ ing SSEPs (top) and BAEPs (bottom).

This technique is very valuable because although there has been a recent advancement of neuroimaging techniques, BSM remains the only way to reveal the surgical anatomy in the

operative field [65,66]. We must not forget that it is a functional localization technique and can be performed only intermittently. Moreover, discrepancies between the final intraoperative recordings and postoperative function may occur. This situation can be explained by an injury to the afferent fibres during resection, in which the lower motoneuron is still intact or a nuclear injury results from a stimulation-elicited response activating the intramedullary root. Both instances can produce false-negative results [59,65]. A similar situation may occur during monitoring of the IX and X cranial nerve, difficulties in coughing or swallowing can occur postoperatively despite positive and normal CMAP recordings at the end of surgery. This phenomenon may be explained by BSM responses reflecting only the functional preserva‐ tion of the efferent arc of these reflexes, but no information has been provided regarding the integrity of the sensory afferents.

For lesions that compress the brainstem ventrally or laterally, BSM provides a minimal contribution. In these cases, CBT-MEP monitoring is the neurophysiological technique indicated to monitor the entire CMN motor pathway from the cerebral cortex to the targeted muscle [59]. Likewise, when motor CN monitoring is considered, as mentioned previously, recording of fEMG with the identification of different discharge patterns will allow the identification of injury activity.

Electrical stimulation must be performed with extreme care due to the presence of vital structures within a very small space. Therefore, the frequency should be approximately 2.45 Hz, and the pulse width should be maintained as low as possible to provide specific stimula‐ tion andavoidcurrent spreading. However, a compromise between currentintensity andpulse width is essential, and therefore 100 μs and a current lower than 2 mA should be desirable.

Regarding the remaining critical neural structures, the ascending sensory pathways are monitored by BAEPs as well as SSEP, which together can be used to monitor approximately 20% ofthe brainstem [59]. Information concerning the descending motor pathways is provided by the use of MEP.
