21 The Extended Endoscopic Endonasal Approach to the Anterior Fossa
Abstract
Endoscopic endonasal approaches (EEA) have revolutionized the treatment of anterior fossa lesions, including craniopharyngiomas, meningiomas, and craniofacial malignancies, among others. In this chapter, the surgical nuances and related anatomy of the endoscopic transplanum transtuberculum and transcribriform approaches is discussed, apart from the anatomical landmarks and a step-by-step description for those approaches. Anatomical dissections and surgical videos are used to illustrate this chapter and facilitate the comprehension of the nuances of such approaches.
Keywords: Keywords: endoscopy, skull base, surgery, anatomy, sphenoid, anterior fossa
21.1 Introduction
The transsphenoidal approach for resection of skull base lesions was initially developed in the early 20th century.1 , 2 However, its widespread use only occurred in the 1960s with the introduction of the surgical microscope by Guiot and Hardy, who focused primarily on the treatment of pituitary adenomas.1 , 2 With the refinement of microsurgical instruments, extended microscopic transsphenoidal approaches for resection of lesions beyond the sella were developed.3 , 4 However, their application remained limited due to the restricted exposure offered by the tunnel view optics of the microscope and higher rates of postoperative cerebrospinal fluid (CSF) leaks.
The introduction of the endoscope, with its improved illumination and wider field of view, has revolutionized transsphenoidal surgery in the last 20 years.5 , 6 , 7 , 8 , 9 , 10 The expansion of the field resulted in a deeper multidisciplinary collaboration between surgical specialties. The combine efforts of neurosurgeons and ear, nose, and throat (ENT)/head and neck surgeons led to a better understanding of the relationship of the paranasal sinuses and skull base anatomy culminating in the development of extended endonasal approaches (EEA) to the skull base.2 , 11 , 12
Apart from sellar/pituitary surgery, the anterior cranial fossa is the most common target for endoscopic skull base approaches. A variety of different pathologies are suitable for EEA in this region, including non-neoplastic lesions (encephalocoele, primary and secondary CSF leaks, fibrous dysplasia), benign tumors (meningioma, schwannoma), low grade malignancies (chondrosarcoma, adenoid cystic adenoma), intermediate malignancies (esthesioneurolastoma, chordoma), and high grade malignancies (squamous cell carcinoma, nasopharyngeal carcinoma, sarcoma).
Suitable case selection is paramount in achieving success using these techniques. Lesions located superolateral to the medial orbital wall, with a large intradural component (> 4 cm) and with extensions anterior to the posterior wall of the frontal sinuses and/or with encasement of large intracranial arteries, may be better addressed via a transcranial approach. Surgical experience and adequate instrumentation are also necessary for the success of the endoscopic approach. In our opinion there is a steep learning curve in the use of extended endoscopic techniques and attempts to perform extended endoscopic resections should only be considered after a significant experience has been obtained with endoscopic pituitary surgery. Ideally, high-definition endoscopes, endoscopic-dedicated instrumentation, neuronavigation, and micro-doppler should be available in all cases.
In the current chapter we will focus on the surgical nuances of the transplanum/transtuberculum and transcribriform approaches for resection of anterior fossa lesions. Complications and techniques for complication avoidance and management are also discussed.
21.2 Surgical Approaches
21.2.1 The Transplanum Transtuberculum approach
Meningiomas centered at the planum and tuberculum sphenoidale, pituitary adenomas with suprasellar and anterior extensions, and craniopharyngiomas are among the lesions that may benefit from a transplanum transtuberculum approach.
As with any approach a clear understanding of the anatomy of the region is critical for adequate surgical planning. The planum sphenoidale is a flat part of the anterior skull base, located over the sphenoid sinus, posterior to the crista galli and anterior to the limbus sphenoidale. The tuberculum region encompasses the so-called pre-chiasmatic sulcus and is located posterior to the limbus sphenoidale and anterior and superior to the sella turcica (see Fig. 21.1). The dural fold at the limbus separates these regions and helps to define the extension of the surgical exposure (see Fig. 21.2). Lesions arising at the tuberculum sella region such as tuberculum sella meningiomas and infrachiasmatic lesions including craniopharyngiomas can be approached after adequate bony and dural exposure. Planum meningiomas, arising more anteriorly and usually displacing the chiasm posteriorly and inferiorly, require a more anterior extension of the skull base exposure. The optic nerves and chiasm, pituitary stalk, paraclinoid internal carotid artery (ICA), anterior communicating artery, and superior hypophyseal artery are closely related with lesions in this area.
Fig. 21.1 Endoscopic endonasal view of the sphenoid sinus exposing the landmarks in its posterior wall with the use of 0-degree endoscope. The planum, limbus, tuberculum (TS), and sella (4) are visualized in the midline after a large sphenoidotomy. The optic canals and clinoid segment of the internal carotid artery (cICA) are closely related; the lateral optico-carotid recess (LOCR), which corresponds to the optic strut, is located inferior and lateral to the optic canal and separates that structure from the superior orbital fissure. The lateral aspect of the tuberculum sellae is related with the medial optic carotid recess (MOCR) and it is named lateral tubercular strut. (Copyright Joao Paulo Almeida, MD.)
Fig. 21.2 Endoscopic visualization (0-degree endoscope) of the dura anatomy of the sellar and suprasellar region. A transsellar transtuberculum approach with partial removal of the planum is demonstrated. The dura of the planum, tuberculum, and sella is visualized. The limbus sphenoidale is a dural fold located between the planum and chiasmatic sulcus or “tuberculum region.” Dural opening superior and anterior to the limbus will provide access to the suprachiasmatic space; dural opening below the limbus will provide access to the infrachiasmatic space. Identification of the superior intercavernous sinus (SIS), located between the sella and tuberculum region, should be achieved for approaches to the suprasellar space. Its early coagulation and cut will avoid venous bleeding and provide communication of the sellar and suprasellar spaces. (Copyright Joao Paulo Almeida, MD.)
Carefully selected cases of tuberculum sella meningiomas may be effectively removed through an endonasal approach. Some of the advantages include the less invasive nature of the approach with no brain retraction, early devascularization of the lesion, and better access/removal of deep-seated intrasellar, infra/retrochiasmal tumor extensions.
Some tumor characteristics that favor an endoscopic endonasal procedure include small-mid size tumor (< 4 cm) without lateral extensions over the supraclinoid ICAs, a clear plane of dissection with the basal frontal lobes, and no encasement of perforating or major (A1/M1/ICA) vessels. For craniopharyngiomas that usually extend superiorly and posteriorly the corridor for dissection is located between the optic chiasm and the pituitary gland.
21.2.2 Surgical Technique – Transplanum Transtuberculum Approach
●Nasal Phase:
After adequate prepping and draping, the nasal mucosa is decongested with 1/100 topical epinephrine. Next, using a 0-degree endoscope careful inspection of the nasal cavity is carried out. In all extended procedures a right side middle turbinectomy is done routinely followed by exposure and resection of the uncinate process and ethmoid bulla. This maneuver will enlarge the surgical corridor, facilitate the maneuverability of surgical instruments in the nasal cavity, and allow exposure of the planum and tuberculum regions in the superoposterior part of the sphenoid sinus. A vascularized nasal septum flap, supplied by the posterior nasal septal branches of the sphenopalatine artery, is harvested in the usual fashion (refer Haddad paper)17. The posterior part of the nasal septum is then resected while preserving the septum mucosa in the contralateral side. This mucosa may be used as a “mini-flap,” as a complement to the original flap, or as a “reverse” flap13 to cover the nasal septum at the donor site. This posterior septostomy allows for a binostril bimanual microsurgical technique. The rostrum sphenoidale is removed and a large sphenoidotomy is performed with use of varied angled bone punches and high-speed drilling. A successful sphenoidotomy should expose the sella, the lateral and medial optic carotid recesses, the prominences of the paraclinoid carotids, optic canals, and clival recess, and the planum and tuberculum regions (see Fig. 21.1).
●Skull Base Phase:
Once the neurovascular structures at the skull base are anatomically identified, neuronavigation guidance is helpful to confirm the location of the lesion and to define the extent of bone removal. This is especially useful for lesions at the planum, to avoid unnecessary large bony openings in the most anterior part of the exposure. In order to try to preserve olfaction, the approach usually is not extended anterior to the posterior ethmoidal arteries (see Fig. 21.3). Most tumors are adequately exposed after removal of the most superior part of the sella, but bone removal can be extended to the sella floor if necessary for manipulation of the pituitary gland. Drilling is done in a cranio-caudal direction, initially in the midline, until the bone is eggshell thin and can be removed with a dissector or Kerrison rongeur. Adequate bone removal in the tuberculum region is paramount for successful resection of tuberculum meningiomas and craniopharyngiomas. In the lateral plane, a key point is the removal of the bone over the medial optic carotid recess (mOCR) (see Fig. 21.4). This is usually the lateral limit of the bone drilling and exposes the area where the medial portions of the clinoid ICA and optic nerve are located. In some tuberculum meningiomas with lateral extensions, further lateral drilling, with exposure of the carotids and removal of part of the optic canal are necessary for adequate exposure. This might be necessary for removal of tumor extensions into the medial optic canal and resection of tumor adhesions to the dural rings surrounding the ICA.
