8.1 Key Learning Points

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  The utilization of the endoscopic endonasal approach for properly and comprehensively selected aneurysms can minimize the amount of manipulation of neurovascular structures, provide superior proximal vascular control and visualization, and minimize complications in comparison to traditional microscopic approaches.

  
  
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  The established principles of microsurgical and microvascular techniques should be employed during endoscopic endonasal surgery.

  
  
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  Complications can be avoided through interdisciplinary collaboration to overcome the steep learning curve, prepare for possible vessel injury, and provide contingency planning.

  
  
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  Detailed knowledge of the anatomy and practicing endoscopic techniques can mitigate complications, along with the adjuvant utilization of intraoperative endoscopic indocyanine green (ICG) videoangiography/Doppler ultrasound/intraoperative angiography and multilayered, vascularized flap reconstruction that takes clip protrusion into account.

  
  
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  There are two specific locations which have the strongest anatomic basis for endonasal aneurysm treatment: (1) medial/inferior pointing paraclinoidal aneurysms (superior hypophyseal artery/carotid cave); (2) low-lying basilar apex or basilar trunk and branches.

  
  
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  The steep learning curve associated with endoscopic endonasal surgery must be considered when evaluating vascular indications. Only teams with significant experience with both simple and advanced cases should contemplate these Level 5 approaches.

8.2 Introduction

The endoscopic endonasal approach (EEA) has seen an increase in the breadth of its applications in neurosurgery in recent years. Although the EEA may reduce certain components of invasiveness, such as the area of exposed tissue and external incisions, it should not be seen merely as a minimally invasive alternative to transcranial approaches to the ventral skull base. Rather, as part of an overarching approach selection paradigm in surgery, particularly when dealing with regions with a high density of critical structures, the EEA provides a corridor with the potential to minimize the exposure and manipulation of major vessels and neural elements. By providing direct access to lesions which are midline or medial to key structures (e.g., optic nerve), these corridors can reduce the likelihood and severity of complications.

Favorable aneurysms may be selected generally based on (1) the location and (2) the projection of the aneurysm relative to the parent vessel and neighboring nerves. This is a small proportion of aneurysms generally near the central skull base. Additionally, this approach presents a steep surgical learning curve for a team of neurosurgeons and otolaryngologists. However, the approach mitigates a substantial number of risks and challenges in certain cases, and therefore slowly continues to enter discussions of intracranial aneurysm management. Hopefully, this framework will help determine which aneurysms may be best treated with EEA, but each aneurysm is unique and calls for a tailored and thorough consideration of endoscopic, microscopic, and endovascular modalities.

8.3 Vascular Challenge

Although endovascular intervention has revolutionized the management of intracranial aneurysms, there remains a set of aneurysms that are best addressed using clipping based on criteria including morphology, neck anatomy, location, as well as the demographic of the patient. ¹ For younger patients, clipping (open or EEA) provides a strong option that avoids the long-term, dual antiplatelet therapy needed after endovascular management without requiring a craniotomy. Clipping may also be a safer option for patients with a history of bleeding complications or poor response to oral antiplatelet or anticoagulant therapy. Additionally, endovascular intervention may not be indicated for aneurysms with wide necks at major branch points, saccular aneurysms with multiple arterial outputs, multilobed aneurysms, or large aneurysms exerting mass effects on surrounding structures. ² For some of these patients for which clipping is indicated, EEA offers a microsurgical clipping option via a potentially superior corridor compared to “open” craniotomy.

The primary argument against EEA for appropriate aneurysms is one of expertise and experience. Currently, microsurgical clipping is vastly more well-studied and has proven excellent long-term outcomes and cure rates. Beyond the literature, the approach is more comfortable and familiar to a majority of surgeons. EEAs require a steep learning curve to master the techniques. Snyderman et al developed a training program for EEA that stratifies the type of EEA surgery into levels of difficulty; use of the approach for intracranial aneurysms was placed at the highest level of difficulty. ^{3 ,​ 4} Importantly, Lavigne et al found that certain surgical complications, including blood loss, cranial nerve injury, and cerebrospinal fluid (CSF) leaks, rose as the level of EEA difficulty escalated to that of an intracranial aneurysm. This highlights the need for substantial training in this approach by the endonasal team both inside and outside of operating room in order to minimize avoidable adverse outcomes. ⁴ Once sufficiently experienced, an advanced endonasal team can begin to explore aneurysm treatment safely.

Certain aneurysms present a challenge and/or drawback when managed through a traditional craniotomy. In the anterior circulation, transcranial access to inferomedial-pointing paraclinoid aneurysms arising from the internal carotid artery (ICA) provides poor visualization requiring manipulation of the optic nerve. It also poses a significant challenge for proximal ICA control, requiring at least anterior clinoidectomy (which still may be subpar even with division of the dural rings), transcavernous dissection (with cranial nerve manipulation), or cervical ICA exposure (which does not account for intervening collaterals). In addition to the optic nerve exposure and mobilization required to visualize and access the aneurysm neck, the superior hypophyseal artery is also poorly visualized. This has the potential for increased risk of visual loss and even hypopituitarism postoperatively. Similarly, aneurysms arising from the medial aspect of the ophthalmic artery also present anatomical challenges due to their intracranial depth, which may require substantial optic retraction and avoidance of critical and often anatomically variable vessels. ^{5 ,​ 6} In the posterior circulation, the relationship of basilar trunk and distal vertebral aneurysms to cranial nerves, brainstem, and varying perforating vasculature that must be identified and dissected imposes significant challenges to transcranial approaches, which present an inherently lateral to medial corridor. This is especially true for aneurysms that arise close to the midline (for retrosigmoid or far-lateral approaches) or for low-lying basilar apex or trunk aneurysms (for orbitozygomatic or subtemporal approaches). ⁷

The ventral pons and medulla and associated vasculature are not well accessed through traditional, “open” posterolateral or anterolateral approaches. Low-lying basilar, midbasilar/anterior inferior cerebellar artery (AICA), ventromedial posterior inferior cerebellar artery (PICA), and vertebrobasilar junction aneurysms all present challenges with simultaneous proximal and distal control and full visualization of aneurysm neck and associated perforators. An endonasal, transclival approach can provide direct access and visualization of all these regions.

Due to the direct view of the endoscope and somewhat narrow corridor relative to the parasellar contents, utilization of endoscopy-specific tools for aneurysm clipping is necessary in order to prevent obscuration of the target during clipping. An elongated, single-shaft applier, potentially with a malleable portion system, maximizes the ability to maintain en face visibility while placing aneurysm clips. In addition, some clip constructs require more creativity and clip variability given the lack of wide angles for application. Curved or angled clips may provide better visibility than straight clips depending on the location. These technical limitations can often be mitigated and accounted for in ways that will be discussed in the following sections, but they must always be considered when discussing microscopic versus endoscopic aneurysm surgery.

8.4 Injury Avoidance

The basic principle in complication avoidance is a robust understanding of anatomic landmarks from an endonasal perspective (Fig. 8.1). Two-surgeon teams provide numerous advantages including consensus verification of critical neurovascular structures. A more subtle benefit of having a second surgeon driving the endoscope versus an endoscope holder is that dynamic endoscopy helps to provide the ideal view constantly as well as some degree of depth perception despite the two-dimensional video screen. ⁷ Lastly, in the case of a vascular injury or aneurysm rupture, having a mobile endoscope ensures being able to clear the view from torrential bleeding so that the site of injury can be addressed expediently. Frequent repositioning of the endoscope on either side of instruments or even between instruments is necessary in order to continue to maintain a clear view of vessels and the aneurysm during the clipping process. A well-coordinated team learns to move together to ensure this can be performed quickly. This is one of the key reasons that these cases are considered Level 5 for any team to even consider. ^{5 ,​ 6} The surgical team must be very far into their combined learning curve to attempt endonasal clipping. ^{3 ,​ 4}

The most important way to avoid severe complications with endoscopic endonasal surgery is to be lucid about its indications and limitations. This includes strict selection criteria for aneurysms that are optimal for an endoscopic approach, interdisciplinary collaboration between neurosurgeons and otolaryngologists in planning and operation, optimization of distal and proximal control, and contingency planning.

Just as in open approaches, ICG angiography can improve the safety and efficacy of endoscopic endonasal aneurysm surgery. Endoscopic ICG angiography ensures lack of aneurysm filling without jeopardizing the parent artery or perforators (often angiographically occult) and allows for immediate intraoperative assessment of clip placement and adjustment, if needed. Likewise, utilization of intraoperative Doppler can be useful to ensure adequate flow within larger arteries after clipping as well as the absence of pulsations within the aneurysm dome. Finally, the use of intraoperative digital subtraction angiogram can be easily done utilizing the EEAs. With the patient being supine, access to the femoral or radial artery is easily established to assess complete aneurysm obliteration and patency of surrounding vessels.

Due to the EEA’s ability to access the full ventral skull base, there is an inherent risk of injury to neurovascular structures and cranial nerves (CNs) III to XII. To proactively foresee and reduce the likelihood of postoperative neurological deficits, intraoperative neurophysiologic monitoring is commonly used in endoscopic endonasal surgery. ⁸ Somatosensory evoked potentials (SSEPs) allow for intraoperative detection of perfusion changes to the spinal dorsal columns, medial lemniscal pathways, and thalamic projections to the primary sensory cortex. Brainstem auditory evoked potentials (BAEPs) allow for active monitoring of hearing function. The multimodal use of SSEPs and BAEPs intraoperatively allows for real-time injury detection and active intervention that has been shown to provide value in endoscopic endonasal surgery by preventing potentially permanent and disabling neurological defecits. ⁹

8.4.1 Case Selection

The location and direction of projection of an aneurysm are critical when determining the value and utility of an endoscopic approach. A particularly effective application of the EEA is for paraclinoid aneurysms that project toward the midline into the suprasellar space. Current risks of microscopic clipping for paraclinoid aneurysms include optic nerve manipulation and the need to unroof the cavernous sinus, with or without division of the dural rings for subpar proximal vascular control. ¹⁰ For these reasons, management of paraclinoid aneurysms has largely shifted to endovascular coiling and/or flow diversion. However, endovascular options are particularly contraindicated in wide-necked aneurysms at major branch points, saccular aneurysms with multiple arterial outputs, certain multilobed aneurysms, and aneurysms exerting mass effect, such as compression of the optic nerve. ² Additionally, consistent dual antiplatelet therapy is mandatory and flow diversion is a relatively novel technology that requires further study to understand long-term outcomes as well as the mechanisms that lead to its rare risk of subsequent intraparenchymal hemorrhage. ¹¹ In such cases in which clipping is indicated, the endoscopic approach affords minimal manipulation of surrounding structures with good proximal and distal control. The trajectory of the endonasal corridor here provides an excellent view of the vasculature without the need for anterior clinoidectomy or manipulation of the optic nerve. ¹² Additionally, direct proximal control can be established through the medial compartment of the cavernous sinus, which is associated with low rates of postoperative complications when compared to opening the roof of the cavernous sinus. ¹³ The medial cavernous sinus can be opened sharply medial to the carotid and a hemostatic agent gently injected to obtain hemostasis. An upgoing knife such as a feather blade (Mizuho) can then be used to incise the dura laterally over the carotid genu.

The EEA is also used for select anterior communicating artery (AComm) aneurysms, largely depending on the angle of projection. In the case of a medial inferior projection that allows for aneurysm neck exposure from the endoscopic corridor, for instance, an endoscopic approach can be considered. ¹⁴ However, the endoscopic approach is often not favorable for AComm aneurysms because it frequently involves manipulation above the optic chiasm. For AComm aneurysms in which a midline corridor is indicated, for instance, an interhemispheric approach can provide similar exposure while avoiding the optic chiasm. Additionally, only select AComm aneurysms allow for sufficient proximal control to be established. Aneurysms in which the optic chiasm is post-fixed (overlying the dorsum sellae) and/or in which the AComm is more anterior relative to the chiasm allow for better proximal control; in most other cases, obtaining adequate proximal control in the event of an intraoperative rupture proves challenging. Although EEA is possible for many AComm aneurysms and has some cosmetic advantage, it is not an ideal choice unless the projection of that specific aneurysm, spatial relation to the optic chiasm, and plan for vascular control have been carefully considered (s. Abb.).

As mentioned, EEA may also be indicated for low-riding basilar apex or basilar trunk aneurysms, along with superior cerebellar artery (SCA) and AICA aneurysms, which require complication-prone posterior clinoidectomy with or without a transcavernous corridor when approached microsurgically, while the subtemporal approach provides poor visibility and vascular control of the contralateral vessels. ¹³ The anterior trajectory of the endoscope provides excellent visualization of the basilar trunk, perforators, and distal arteries in many cases, particularly if the basilar apex arises low in the prepontine cistern. Nonetheless, in large superiorly pointing low-lying basilar tip aneurysms, especially those that are retroflexed, the limitations in visualization of the basilar perforators remain, as these vessels are draped on the back of the aneurysm, and a purely ventral approach may not be ideal. Unlike paraclinoid aneurysms, clipping trajectory is less predictable and may require more creativity; it is most feasible if the basilar aneurysms projects laterally. ⁷

A general rule of thumb for posterior circulation aneurysms is that this approach provides good access to lesions that are ventral to the brainstem and at or below the sella (see Case II below), in which the approach offers more adequate proximal control and visualization when compared to transcranial approaches. ¹⁵

8.4.2 Proximal and Distal Control

Preoperative planning should consider the level of vascular control that can be achieved endonasally. For larger paraclinoid aneurysms which block access to the supraclinoidal ICA distal to the aneurysm neck, this may indicate the addition of a transsylvian microsurgical approach for more adequate distal vascular control. ⁵ In fact, some surgeons may prefer preparing for a pterional craniotomy before the endonasal approach is undertaken for additional control in the event of a rupture. ¹⁶

Both proximal and distal control should always be preplanned before the surgery. Gaining quick control is more critical in the case of an intraoperative rupture. Depending on the location of the aneurysm, a proximal region along the parasellar or paraclival ICA that can be quickly clipped should be identified and adequately exposed prior to aneurysm exposure. These considerations are particularly important in endoscopic surgery, where limited maneuverability within the endoscopic corridor should be accounted for when exposing a site of proximal control. Proximal control for basilar trunk or apex aneurysms is also often superior via an endonasal approach, given the ability for wide access from the vertebrobasilar junction to the basilar apex afforded by a wide clival resection (“far medial approach”) or by pituitary transposition. ^{17 ,​ 18 ,​ 19}

Preoperative planning of distal control is equally important for aneurysms arising from large vessels, which can have significant retrograde bleeding from collateral circulation. After a proximal control point is identified and exposed, a region of the artery “downstream” from the aneurysm should also be adequately visualized and exposed, with equal consideration to clipping angle through the endoscopic corridor. ²⁰ As referenced above, distal control for some paraclinoidal aneurysms may be compromised endonasally. In these situations, a craniotomy or distal endovascular balloon should be considered to ensure distal control. Distal control of the basilar artery is relatively straightforward unless the artery is high riding. Understanding its relationship to the sella is critical in addition to understanding the collateral circulation provided by the posterior communicating arteries (Fig. 8.3).

8.4.4 Reconstruction

Endoscopic skull base surgery has a steep learning curve and requires extensive collaboration between neurosurgery and otolaryngology before, during, and after the surgery. ³ The position and angle of the clip have a significant potential impact on the subsequent reconstruction. If improperly angled, a protruding clip can tent the nasoseptal flap and prevent it from adhering directly to the defect, resulting in a higher likelihood of a CSF leak. In addition, aneurysm clips pulsing in direct contact with the flap can erode through the flap over time (Fig. 8.4). CSF leak rates are high following these surgeries and the team must be prepared for a robust, multilayer reconstruction. ⁵ Although vascularized nasoseptal flaps have significantly decreased rates of CSF leaks in EEA surgeries, protrusion of the clip into the sphenoid sinus requires greater coverage, and we prefer to place an inlay collagen graft around the clip, followed by fascia and/or fat with a nasoseptal flap covering the entire reconstruction. As a result, a full size or even extended flap is recommended even if the primary dural opening is small.

8.5 Related Pathologies

A number of other vascular pathologies can also be effectively approached endonasally. Several reports describe perichiasmatic and brainstem cavernomas that have been successfully removed with an endoscopic endonasal approach. ²⁴ The approach allows for direct visualization of the inferior chiasm, infrachiasmatic space, and anterior circulation while minimizing dissection and manipulation of the neighboring neurovasculature. ^{25 ,​ 26} EEA has also been utilized for arteriovenous malformations (AVMs) along the ventral skull base, as described by Kassam et al in 2007. ²⁷ Interesting cases of its utilization in select cases of anterior dural arteriovenous fistulas supplied from ophthalmic artery branches have also been reported. ²⁸ Although the approach should still be considered novel for these applications and should not be considered as a first-line approach for any cerebrovascular pathology, it demonstrates a growing avenue warranting further study and offers an alternative approach for lesions that are otherwise difficult to access using traditional open approaches.