11.2.1 Approach Selection

There are numerous endonasal approaches to intradural skull base pathology, and choice of approach will depend largely on the anatomic area of interest, as well as the planned extent of resection of lesions and strategies for dural and skull base repair. These approaches can be categorized into five different endonasal corridors (transsphenoidal, transethmoidal, transmaxillary, transnasal, and transfrontal) to reach specific skull base and brainstem targets, as has been described in Chapter 7.

11.2.2 Neuromonitoring

When considering endoscopic endonasal approach (EEA) for intradural lesions of the skull base and brainstem, the region of interest and the anticipated neurovascular structures to be encountered should determine which neuromonitoring modalities should be utilized. For skull base approaches involving the parasellar region and cavernous sinus, in the vicinity of the internal carotid artery and its branches, global cortical mapping with electroencephalogram (EEG) and somatosensory evoked potentials (SSEPs) should be employed to monitor for global and brainstem ischemia. The same is true for transclival approaches involving the vertebrobasilar junction. ^{1 ,​ 2 ,​ 3 ,​ 4} Both of these modalities have been validated in EEA surgeries to account for both cortical ischemia, which would be captured on EEG, and the subcortical ischemia that can be better monitored with SSEPs. ^{1 ,​ 2} The addition of motor evoked potentials (MEPs) can be useful in certain suprasellar or petroclival lesions, given the false-negative rates of EEG/SSEPs. ⁵

EMG has been utilized successfully in EEA surgery to provide real-time feedback for cranial nerve (CN) irritation and probe the functional integrity of a mixed or motor CN. The oculomotor, trochlear, and abducens nerves can be monitored with needle electrodes placed in the inferior rectus, superior oblique, and lateral rectus muscles, respectively. These CNs are most at risk during intradural EEA procedures, ^{6 ,​ 7 ,​ 8} even in cases where the cavernous sinus is not accessed. The oculomotor nerve, for example, is vulnerable in the interpeduncular cistern via the transsphenoidal and transplanum routes, with vascular compromise possible from injury to the inferolateral trunk of the cavernous carotid or its branches. The trochlear nerve may be exposed at the ambient cisternal segment through the transsellar transtuberculum route, and ischemic injury may occur with injury to the superior cerebellar artery. The abducens nerve, being the longest and most ventrally located CN at the level of the clivus and cavernous sinus, is particularly at risk during approaches to petroclival lesions via the midline transclival, paramedian suprapetrous, and medial petrous apex approaches. In these cases, the risk may be increased by abnormal anatomy (e.g., medial displacement of the nerve by a petroclival tumor or upward displacement by a cisternal mass). Like the oculomotor nerve, the abducens nerve may also suffer vascular compromise by injury to the inferolateral trunk from the cavernous segment of the internal carotid artery. The trigeminal nerve may be violated in Meckel’s cave via the transpterygoid corridor. As EEAs are extended to the inferior clivus, as well as through the transcondylar and transjugular corridors, attention must be paid to lower CN monitoring, including the glossopharyngeal, vagus, accessory, and hypoglossal nerves ⁵ (▶ Table 11.1).

For endonasal endoscopic removal of tumors involving the brainstem, brainstem auditory evoked potentials (BAEPs) can be beneficial for detecting brainstem ischemia during surgery at or around the vertebrobasilar junction, as is the case for transclival approaches.

As shown in ▶ Table 11.1, multimodal neuromonitoring, tailored to approach and specific intradural pathology, endeavors not only to detect and identify iatrogenic nervous system dysfunction but also to guide the use of surgical interventions and monitor their efficacy.

For an in-depth analysis of how intraoperative neuromonitoring is applicable to pediatric endoscopic endonasal skull base surgery, please refer to Chapter 6.

11.2.3 Image Guidance

Intraoperative image guidance is especially useful in endoscopic surgeries because of the distorted depth perception inherent in 2D endoscopes, which are still more commonly used than newly developed 3D endoscopes. The utility of image guidance has been well described in the literature for adult endonasal approaches but not for pediatric cases. The few studies in the pediatric population suggest that the benefits of having enhanced localization and immediate feedback through image guidance can optimize pediatric endonasal endoscopic skull base surgery as well. ^{9 ,​ 10 ,​ 11} The small corridor, incomplete pneumatization of the sphenoid sinus, and crowding of the neurovascular structures make image guidance even more critical for pediatric patients.

In a traditional setting, image guidance in the operating room relies on the scans taken preoperatively. These images are then utilized for image guidance intraoperatively, but this method has limited ability to offer accurate anatomical information due to changes from patient positioning or the anatomical shifts during surgery. Intraoperative use of live CT or MR eliminates this problem, as the imaging accurately reflects patient positioning at the time of operation. ¹² With live intraoperative imaging, surgeons can guide their resections for optimal safety around critical structures and minimize the risk of incomplete resection and prevent reoperation due to residual pathology. ^{13 ,​ 14}

The additional arrangement needs for an intraoperative scanner and image guidance system lengthens operation time due to setup and registration. Moreover, intraoperative CT may expose pediatric patients to excessive radiation, which should be avoided. Despite these inconveniences, real-time image guidance may still be desirable in certain extended endonasal cases, considering the extra time and cost incurred, as well as the increased morbidity, from a repeat surgery in the case of incomplete resection. For a thorough understanding of the variations in the operating room setup with and without intraoperative imaging, please refer to Chapter 4.

11.2.4 Ventral Safe Entry Zones into Brainstem

Brainstem lesions represent a challenge to both the surgeon and the patient. The dense concentration of critical nuclei and fibers located in a region roughly the size of the human thumb underlies the morbidity and mortality associated with brainstem pathology. To minimize disruption of eloquent tissue in this region, microsurgical approaches in conjunction with safe entry zones have been developed to optimize safe exposure and resection in different portions of the brainstem. ^{15 ,​ 16 ,​ 17 ,​ 18 ,​ 19 ,​ 20} These safe entry zones and trajectories represent areas where eloquent structures and perforators are sparse, thus minimizing possible damage with a neurotomy.

As endoscopic endonasal surgery has continued to evolve beyond straightforward approaches to the sellar and suprasellar regions, extended endoscopic approaches have allowed for access to ventral brainstem lesions with good results. ^{21 ,​ 22 ,​ 23 ,​ 24} In particular, the EEA allows for safe midline, ventral exposure of the clivus down to the craniovertebral junction. ^{25 ,​ 26} This allows for endoscopic access to three classic ventral safe entry zones: anterior mesencephalic zone (AMZ) in the midbrain, the peritrigeminal zone (PTZ) in the pons, and the olivary zone in the medulla. ^{19 ,​ 27}

Anterior Mesencephalic Zone

The AMZ may be accessed via a standard endoscopic endonasal approach to the sella. However, unilateral transposition of the pituitary gland is necessary to obtain more direct access to the midbrain and anterior mesencephalic sulcus. While complete transposition often causes hypopituitarism, ²⁸ unilateral or extradural transposition with removal of the posterior clinoid on one side can be completed with minimal morbidity. ^{29 ,​ 30} Even with pituitary transposition, the exiting third nerve from the brainstem can block access to the AMZ (▶ Fig. 11.1, ▶ Fig. 11.2a).

The AMZ can be accessed safely through the interpeduncular cistern via the limited area on the cerebral peduncle between the oculomotor tract and nerve medially and the corticospinal tract (CST) laterally. This entry point takes advantage of CST fibers in the intermediate three-fifths of the peduncle and the location of the red nucleus and substantia nigra deep and medial to the entry zone. ³¹ The superior and inferior boundaries are the optic nerve and chiasm superiorly and the oculomotor nerve and superior cerebellar artery complex inferiorly (▶ Fig. 11.1, ▶ Fig. 11.2a). Laterally, the limitation to access via the endonasal approach is the cavernous carotid (▶ Fig. 11.3). This intracavernous carotid distance can be quite narrow in many individuals, and a medially coursing carotid can effectively block off access to the AMZ. ³²

Peritrigeminal Zone

The anterolateral surface of the pons, medial to the trigeminal nerve entry zone, lateral to the corticospinal tracks, and anterior to the motor and sensory nuclei of the trigeminal nerve, has been identified as the peritrigeminal safe entry zone (PTZ) within the pons ^{19 ,​ 33 ,​ 34 ,​ 35} (▶ Fig. 11.2a, ▶ Fig. 11.4, ▶ Fig. 11.5). Utilizing an extended endoscopic transclival approach with removal of the anterior wall and floor of the sphenoid sinus allows for exposure of this safe pontine corridor. Sufficient lateral exposure is critical for this exposure, and the inferior petrosal sinuses represent the lateral borders of the exposure along the clivus (▶ Fig. 11.3). The medial border of the safe zone is the pyramidal tract. The inferior border of the PTZ are the roots of the CN VI medially and CNs VII and VIII laterally (▶ Fig. 11.2a, ▶ Fig. 11.5).

The downward trajectory of the abducens (CN 6) and facial nerve (CN 7) fibers through the pons necessitates taking an upward trajectory for any dissection, lateral and superior to the CN VI exit point (▶ Fig. 11.5, ▶ Fig. 11.6).

While a transcranial subtemporal corridor affords a more direct approach to the PTZ, the endonasal endoscopic approach provides a wide midline to lateral view (▶ Fig. 11.7). This can be a difficult corridor to visualize and work through ventrally without the aid of angulated endoscopes, and makes the resection of deep-seated lesions very difficult. An additional consideration at the pontine level is the variance in the tortuosity of the basilar artery, and the lateral extent of basilar artery perforators that further limit access into the pons (▶ Fig. 11.2b).

Thus, the ventral endoscopic route is only safe for resection of small superficial lesions located anterior or lateral to the CSTs, or biopsy or debulking of larger exophytic lesions. While the superficial pontocerebellar fibers run in a transverse direction, incisions along the ventral aspect of the pons should follow a longitudinal course parallel to the CSTs coursing under them (▶ Fig. 11.2, ▶ Fig. 11.5).

In the future, MR tractography using diffusion-weighted imaging may prove helpful in identifying the location of the pyramidal tracts in relation to pontine lesions, ^{36 ,​ 37} and ultimately guide approach.

Olivary Zone

The olivary zone proceeds through the olives on the anterolateral surface of the medulla. This zone is bordered medially by the anterolateral sulcus and the pyramids, and laterally by the posterolateral sulcus (▶ Fig. 11.4). Within the brainstem, the fibers of the hypoglossal nerve separate the olive from the pyramidal tracts medially and the medial lemniscus deeper (▶ Fig. 11.9).

Endoscopically, a wider exposure is attainable at this level as the inferior petrosal sinuses begin to splay laterally caudally along the brainstem. Exposure of the superior medulla is relatively straightforward with an endoscopic transclival approach, but exposure of the inferior medulla can be challenging. The hard palate can limit the entry angle of the endoscope and instruments caudally, and preoperative radiographic evaluation is crucial when planning a caudal medullary exposure. ^{38 ,​ 39 ,​ 40 ,​ 41} The anterior arch of C1 and the odontoid tip (C2) can also be resected, if necessary, without causing significant cervical occipital instability in patients with no prior cervical kyphosis and healthy facet joints bilaterally. ^{42 ,​ 43} The relatively small diameter of the medulla at this caudal extent allows for visualization of the hypoglossal nerve rootlets (CN XII) and olivary bodies (▶ Fig. 11.8). Thus, the olivary zone can be safely entered laterally via this approach, working in between the hypoglossal nerve rootlets (▶ Fig. 11.8, ▶ Fig. 11.9).

The relatively small size of the medulla and the superficiality of the CST tracts limits surgical maneuverability at this level. A midline approach through the caudal anterior median fissure is not recommended because of the decussating pyramidal fibers (CST + corticobulbar tract) in the medulla (▶ Fig. 11.8).

Related posts:

2 Pediatric Rhinologic Considerations 4 Patient Positioning and Operating Room Setup 8 Combined Transcranial and Endonasal Approaches 5 Instrumentation 10 Endonasal versus Supraorbital Eyebrow Approaches: Decision-Making in the Pediatric Population 7 Endonasal Corridors and Approaches

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