Paolo Cappabianca, Luigi Maria Cavallo, Oreste de Divitiis and Felice Esposito (eds.)Midline Skull Base Surgery10.1007/978-3-319-21533-4_19

19. The Expanded Endonasal Approach to Skull Base Meningiomas

Amin Kassam¹, Martin Corsten² and Richard Rovin¹

(1)

Department of Neurosurgery, Aurora Health Care, Milwaukee, WI, USA

(2)

Department of Otolaryngology, Aurora Health Care, Milwaukee, WI, USA

Amin Kassam (Corresponding author)

Email: kassamab@gmail.com

Martin Corsten

Email: martin.corsten@aurora.org

Richard Rovin

Email: richard.rovin@aurora.org

19.1 Introduction

Classic approaches to the skull base have involved either anterior, middle, or posterior fossa craniotomies, with retraction of the brain to expose the relevant intracranial anatomy, often with separate approaches from the midface or temporal bone to expose the relevant extracranial anatomy. The expanded endonasal approach (EEA) to skull base tumors was introduced as an alternative minimally invasive strategy for selected tumors in the late 1990s/early 2000s [3, 11, 12]. It offered some significant advantages in selected tumors, including the elimination of the need for skin incisions, the elimination of brain retraction, and the addition of excellent visualization of critical structures. In the early part of the experience with EEA, there was significant controversy about its safety, with critics of the approach citing the risk of infection and the possibility of cerebrospinal fluid leakage as a consequence of the procedure. However, early experience showed the safety of the procedure, especially relative to traditional skull base approaches, in selected tumors. We reported our experience in 800 consecutive patients performed by a single neurosurgeon over a decade and demonstrated the safety of the procedure for a broad range of pathologies [15]. Since then, the procedure has been adopted worldwide and has been made safer by advances in technology and reconstructive techniques. Since our original description of a modular anatomical classification system [25], expansion of the anatomic limits of the procedure has continued to increase the number of skull base tumors accessible by this approach. In this chapter, we will describe the use of EEA for skull base meningiomas, focusing on the anatomic limits of the use of EEA for these tumors, the advantages and disadvantages of EEA, and the technical nuances of the use of EEA for meningiomas.

19.2 The Expanded Endonasal Approach: Overview

EEA refers to a minimally invasive surgical approach to the skull base through the nares. Typically, the surgery is performed, as a two-surgeon, four-handed operation carried out by a neurosurgeon and an otolaryngologist – head and neck surgeon. In our classical description of the approach, the right inferior turbinate is lateralized, and the right middle turbinate resected to gain space for the endoscope, which is brought in through the right nostril [3, 11, 12]. Nasoseptal flaps may be raised (see below), and a posterior septectomy is performed. The operating surgeon operates binarially, while his or her partner performs the endoscopy, with high-resolution views afforded to both surgeons from the endoscope displayed on monitors.

Several technological advances over the past 15 years have improved the safety and efficacy of EEA, with more technologies anticipated to improve the procedure even further. The development of high-definition television screens and advanced optics has allowed for views of anatomical structures with excellent levels of resolution and magnification. Image guidance has been an integral part of the development of EEA and its adoption by multiple centers around the world, by affording surgeons increased information about the proximity to critical structures such as the carotid arteries, optic nerves, and brain. Proponents of the procedure have partnered with equipment companies to create task-specific instrumentation designed for operating endoscopically in the ventral skull base. Some centers have been experimenting with 3-D optics platforms and “look-ahead” technologies to further enhance the operators’ awareness of the anatomic environment of the operative field, in an attempt to make the procedure even safer [1, 5, 24].

It is key, however, to realize that image guidance techniques and enhanced endoscopic images are no replacement for surgical experience and a deep understanding of the anatomy. Identification of the location of the carotid artery, either unilaterally or bilaterally, through its six distinct segments (parapharyngeal, petrous, paraclival, parasellar, paraclinoid, and intradural) is usually a critical component of the dissection in EEA. We have recently published a detailed modular classification of EEA approaches using the carotid artery as a road map [16]. Other cranial nerves, including the optic nerve and chiasm, are often critical to the dissection as well.

Another critical advance that made EEA safer was not technological but surgical. In the early experience with EEA, reconstruction of the skull base was generally accomplished using non-vascularized tissue, along with tissue sealants. This was often successful, but high rates of cerebrospinal fluid leak were often observed. This problem was especially acute in high-flow situations such as craniopharyngiomas, with large dural openings, or with such patient risk factors as obesity or sleep apnea [26]. The introduction of the posteriorly based nasoseptal flap, popularized by Carrau [9], markedly reduced the incidence of CSF leaks and other intracranial complications. This flap, based on the posterior septal artery, is raised before the posterior septectomy and left pedicled posteriorly during the tumor resection. It allows reliable coverage of dural defects and encourages rapid remucosalization of the ventral skull base. This was in many ways the innovation that allowed for EEA to become a safe and reproducible surgical strategy. A later modification, described by Carrau, was the anteriorly based reverse flap [2]. This flap hastened the mucosalization of the denuded anterior septum left behind by the elevation of the NSF with an anteriorly based reverse flap, raised from the contralateral side.

There are other options for reconstruction after EEA; not all cases require a vascularized flap, and many centers continue to use non-vascularized reconstructions in simple cases at low risk for CSF leak. Some surgeons use a “rescue flap,” also described by Carrau [23]. In these cases, the incisions for the NSF are created before the posterior septectomy, but the flap is not raised until the end of the procedure, when the decision is made that the case is of sufficiently high risk for CSF leak that a flap is necessary. Non-vascularized reconstruction can therefore be used in cases deemed to be low risk after the resection.

For cases in which vascularized reconstruction is deemed necessary but the NSF is unavailable, a large number of other flap options have been described, including a flap harvested from the lateral nasal wall [10, 22], pedicled pericranial [19, 28], buccinator [21] or temporoparietal flaps [6], turbinate flaps [7, 20], or even free flaps transferred into the skull base. While the details of reconstruction are beyond the scope of this chapter, this evolution of reconstruction options in EEA played a sentinel role in its development.

EEA is not the treatment of choice for all skull base lesions. The most critical issue in assessing whether EEA should be used for a particular lesion is the position of the tumor relative to cranial nerves. Nerves are significantly intolerant to manipulation and dissection, and EEA is largely a median and paramedian approach to the skull base. When skull base tumors are medial to critical nerve structures, EEA can often represent the preferred approach. Tumors that are lateral to cranial nerves are often better treated with lateral skull base approaches. As a central principle, we expressed the concept of avoiding crossing the plane of a cranial nerve. Tumors that cross cranial nerves may have their medial component treated with EEA, while the lateral component is addressed by an open approach, often at a second stage. In some cases, tumor debulking may be performed as part of a multidisciplinary approach utilizing radiation therapy.

19.3 Specific Considerations for the EEA in Meningiomas

Meningiomas represent the single largest subset of intracranial tumors, accounting for roughly 30 % of all primary brain tumors. Meningiomas are typically benign and cause symptoms through progressive growth and mass effect. These neurological symptoms can be quite dramatic, but these tumors rarely cause death; in fact, life expectancies may be quite long despite the presence of these meningiomas. The goal of surgical extirpation of meningiomas is to reduce or eliminate the neurological sequelae of this mass effect, ideally through complete resection of the tumor. Sometimes, however, a similar benefit for the patient can be obtained through subtotal tumor resection, especially if total tumor resection would result in the loss of neurological function. A careful assessment of the patient’s life expectancy, current neurological disability, anatomic proximity to critical structures, and individual wishes must be made when weighing the optimal extent of resection of these tumors.

The surgical extirpation of meningiomas using EEA requires several important considerations. The first is the creation of tailored surgical working corridors to gain access to the tumor. Our modular approach to EEA has been described, with descriptions of the relevant working corridors to each area in the skull base [3, 11, 12]. The second consideration is the extent of bony dissection and drilling required for circumferential exposure of the meningioma. This bony exposure and removal should be completed before tumor removal is started. The third consideration is a circumferential devascularization of the tumor. Only after these three steps are performed should complete tumor removal and/or cytoreduction be attempted. Removal of tumor after devascularization can often be accomplished with powered, automated equipment such as the Myriad [17].

Preoperative evaluation of the tumor position, extent, and relationship with critical neurovascular structures is imperative. This evaluation certainly requires high-resolution imaging, usually with MRI. MRI imaging should be scrutinized to evaluate the relationship between the meningioma and cranial nerves, with special attention to whether the tumor is displacing nerves medially/laterally, or superiorly/inferiorly. The evaluation of whether EEA is a viable surgical approach to a meningioma depends entirely on the relationship between the tumor and cranial nerves. In addition to MRI, MRA or CTA is very useful to evaluate possible vascular invasion or encasement. Lastly, image guidance protocols will be necessary if the tumor is to be resected through EEA.

Physical examination and the assessment of preoperative cranial nerve function are likewise necessary. Neuro-ophthalmological exam and the evaluation of preoperative visual fields are imperative for any lesions centered around the optic apparatus. Papilledema should be identified; if present, this is often treated with CSF diversion. Evaluation of extraocular movements will give important information about the function of cranial nerves III, IV, and VI, while the presence or absence of facial pain or numbness will evaluate the function of cranial nerve V. Cranial nerves VII and VIII can be evaluated if necessary by facial nerve exam and audiometric studies. Lastly, lesions in the clivus or foramen magnum should have assessment of the lower cranial nerves, through evaluation of palatal movement, flexible laryngoscopy for assessment of vocal cord mobility, evaluation of shoulder function, and visualization of tongue mobility.

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