Purpose of review
Anterior cranial base surgery is undergoing a revolution in minimally invasive techniques. Both open, microscopic and transnasal endoscopic approaches have gained considerable attention in the last few years. The purpose of the current article is to evaluate recent advances in open and endoscopic approaches to the anterior skull base.
Recent data have evaluated the endoscopic anatomy of the skull base, defined surgical corridors, addressed anatomical limitations to endoscopic skull base surgery, and discussed the limitations of skull base reconstruction. Over the same period of time, new surgical transcranial approaches have challenged the traditional operative corridors to the anterior skull base. In this study, the most recently published data pertaining to open and endoscopic approaches to the anterior cranial base are reviewed.
The data provide support for both open and endoscopic approaches to the anterior cranial base. Limitations to the approaches are discussed.
During the 1970s and 1980s several institutions around the world pioneered open procedures to the anterior cranial base. Often, these approaches required large head and facial incisions, osteotomies, the translocation of neurovascular structures, and multiple morbidities with prolonged recovery time. Recently, less invasive and more cosmetic open techniques to anterior cranial base tumors have been utilized.
In the 1980s, endoscopic sinus surgery was introduced in the United States. The use of nasal endoscopy for the removal of intrasellar pituitary tumors was popularized in the late 1990s . Since that time several institutions have utilized transnasal endoscopy to include tumors along the anterior cranial base. In this article, the latest data regarding endoscopic and open anterior cranial base surgery are presented.
Traditional approaches to anterior skull base lesions involve a frontal craniotomy and an incision behind the hair line. The frontal, bifrontal, pterional approaches, and their variations with extension along the skull base including the expanded bifrontal, frontotemporal orbitozygomatic, and transbasal are the most common open transcranial procedures. They all utilize either a gentle long curvilinear or a Suttar bicoronal incision for better cosmetic results. Although widely utilized, they can involve dissection of the temporalis muscle, division of the superior sagittal sinus, creation of an epidural space requiring obliteration to prevent abscess formation, and risk to the frontalis branch of the facial nerve. The major advantage is the ability to reconstruct almost any defect along the skull base utilizing a variety of techniques including vascular pericranial flaps, split thickness bone flaps, dural substitutes, and temporalis muscle flaps.
During the last decade minimally invasive approaches have been described for select lesions in the anterior cranial fossa and the parasellar region [3,4]. The supraorbital craniotomy, through an eyebrow incision, involves a small craniotomy flap cut flush to the orbital roof. Positioning the head in a slightly extended position combined with early release of cerebrospinal fluid (CSF) allow for exposure of the subfrontal area equivalent to larger approaches with little brain retraction.
A variation of the supraorbital approach was recently described through an incision along an eyelid crease allowing for a small transorbital craniotomy with similar exposure, slightly improved superior visualization, and shortened distance to the parasellar area [5•]. The authors reported on eight patients: five with aneurysms (three anterior communicating arteries, one ophthalmic and one posterior communicating artery), two with pituitary adenomas and one with craniopharyngioma. All lesions were treated adequately through this corridor without any unexpected side effects.
Both approaches allow for limited options for reconstructing the skull base should there be entrance into the frontal, ethmoid, or sphenoid sinus. Furthermore, these approaches can place the frontalis branch of the facial nerve at risk for permanent injury. With supraorbital approaches it is important to avoid entrance in the frontal sinus that requires careful preoperative planning and the use of intraoperative frameless stereotactic guidance. The cosmetic advantage of either minimally invasive approach rests on the limited soft-tissue mobilization and the small linear skin incision that in general is not under tension. Incising the frontalis muscle in line with the eyebrow incision allows for optimal reapproximation, a step necessary for normal postoperative forehead movement.
Based on our clinical experience we recommend the use of the minimally invasive transcranial surgical corridors for lesions limited to the anterior cranial fossa or parasellar space without extension lateral to either optic nerve, or through the osseous skull base into the sinuses. Tuberculum and olfactory groove meningiomas, suprasellar arachnoid cysts, pituitary adenomas, and craniopharyngiomas are all typical lesions that can safely be accessed this way. The presence of significant lateral aeration of the frontal sinus past the supraorbital nerve notch requires either lateral translation of the craniotomy or an alternative approach.
The next challenge for endoscopic skull base surgery is to provide surgical outcomes that are comparable to or exceed traditional open approaches. A recent study compared traditional microscopic, sublabial approaches to the sella compared with purely endoscopic approaches. The rate of postoperative CSF leaks, lumbar drain usage, nasal packing, revision surgery, and length of hospital stay were all greatly reduced in the endoscopic cohort.
Two small studies retrospectively reviewed the experience with endoscopic approaches to the suprasellar tumors through transtuberculum and transplanum sphenoidale approaches. In these studies, only one major complication was reported and the length of hospital stay was brief (3–5 days). The incidence of postoperative CSF rhinorrhea was 15–25%. This rate was decreased with the use of vascular flaps instead of mucosal free grafts. Gross total tumor resection was greater than 80%. Resection of tumor was limited by involvement of the pituitary stalk and optic nerve. These initial reports, though not equal to results with traditional open approaches, suggest that, with time and experience, these endoscopic techniques provide a reasonable option in select tumors.
The historical advantage of the subfrontal approach to the anterior cranial base for sinonasal malignancies with extension to and through the cribriform plate was the ability to perform en bloc resection of disease. The concept that en bloc resection is paramount for surgical success has recently been challenged. Nicolai et al. recently presented a large retrospective study of sinonasal malignancies treated by either nasal endoscopy alone or combined with open cranioendoscopic approaches. Tumors were removed in a layered, piecemeal fashion. Using this technique, tumor that was free from mucosal attachment was debulked. Tumor attached to mucosal surfaces was then removed in en bloc fashion with margins. The 5-year disease-free survival was 91%. Indeed, 5-year survival was greater among the nasal endoscopy group. Certainly, this is due to the advanced nature of disease in patients requiring an intracranial approach; however, even a large proportion of patients with advanced stage 4 disease were treated with nasal endoscopy alone. The authors conclude that in patients with minimal dural involvement and tumor not invading the orbit, nasolacrimal canal, anterior face of the maxillary sinus, massive intracranial extension, or dural extension lateral to the orbit are candidates for the transnasal approach.
Although the anatomy of the paranasal sinuses has been well documented for endoscopic sinus surgery, little has been published on the endoscopic anatomy of the anterior skull base. This anatomical information is critical when removing tumors that breach the bony limits of the sinuses as major neurovascular structures lie on the dorsal side of the skull base. De Notaris et al, in 2008, presented a cadaveric study of the ethmoid planum to help define the critical neurovascular structures in this region and the extent of surgical access. They noted that surgical exposure is limited by the sella posteriorly, orbits laterally, and frontal recess at the junction of the crista gali anteriorly. Intracranial access through the ethmoid roof allowed excellent exposure of the gyri recti, interhemispheric fissure, and olfactory nerves without brain retraction. Limitations would include malignancies with massive dural invasion or any invasion of the orbit and frontal sinuses. They noted that future studies would need to evaluate the width and length of exposure to the ethmoid roof and define the variability in the general population. This provides valuable information to the surgeon for the degree of access to this region.
A critical landmark for tumors involving the sella and suprasellar regions is the internal carotid artery (ICA). As described by Herzallah and Casiano, great variability exists in the course of the carotid artery through the cavernous sinus in cadavers. They noted that a posterior cavernous loop of the ICA narrows the parasellar space and limits surgical exposure. They also provided excellent detail of the relationship of the ICA to cranial nerves in the cavernous sinus.
Accurate access to the second genu of the carotid artery between the petrous and parasellar carotid segments is critical for safe, endoscopic access to the clivus and petrous apex. In a cadaveric and retrospective case review, Kassam et al. showed a consistent relationship between the vidian canal and the ICA. The vidian canal is consistently inferior and medial in location to the second genu of the ICA. This anatomical information allows for the safe removal of clival bone with a high-speed drill to access this portion of the ICA.
A few centers have tried to simplify terminology for approaches to the cranial base based on corridors in the paranasal sinuses. This corridor approach to tumors of the cranial base allows surgeons to understand the anatomical limitations and the critical neurovascular structures encountered. One classification system broke down approaches on a purely anatomical basis into four main corridors: transsphenoidal, transnasal, transethmoid, and transmaxillary. Each of these corridors allows access to named bony structures of the skull base. The classification scheme has been criticized as various disorders of the skull base often require approaches that combine more than one of the above corridors. A second classification system breaks down approaches by major bony structures in the sagittal and coronal planes. The sagittal plane includes the transfrontal, transcribriform, transplanum, transsphenoidal, transclival, and transodontoid approaches. The coronal plane includes the transorbital, petrous apex, lateral transcavernous, transpterygoid, transpetrous, and transcondylar. The various approaches were then organized into levels of difficulty based on the center’s surgical experience. The goal was to provide a step-by-step paradigm for teaching and performing endoscopic cranial base surgery based on the difficulty of the approach.
One of the greatest challenges of endoscopic, anterior cranial base surgery is to provide a consistent technique for closure of dural defects avoiding postoperative CSF leaks. A recent technical note described the common closure technique at one of the busiest centers in the United States. The technique calls for the placement of a synthetic dural substitute and an extradural, intracranial fascial graft followed by an extracranial fascial graft. Fat grafts are then placed covered by cellulose material and supported by a Foley catheter for 4–7 days. Despite this reconstructive technique, the incidence of postoperative CSF leaks was 20%; a rate far higher than traditional open approaches. Failure was accredited to several factors including the high CSF flow and protein content of craniopharyngiomas, history of previous endoscopic skull base surgery, prior radiation, large dural defects, opening of the cisterns or ventricles, large resection volume, incomplete tumor resection, obesity, and compliance with postoperative instructions. This high postoperative CSF leak rate is often cited as a barrier to the widespread acceptance of endoscopic anterior cranial base surgery. The current advantage of open techniques is the use of vascular pedicled flaps and suturing techniques providing a low postoperative CSF leak rate (<2%).
Common explanations for CSF fistula formation after endoscopic cranial base surgery is graft displacement and the use of nonvascular flaps. Often, little space is available for the placement and tying of sutures. Two recent articles have described novel endoscopic suturing techniques in the anterior cranial base. The use of specialized instrumentation, curved needles, and self-tying sutures are an initial advancement for providing closure of skull base defects. Further experience with current technology and expanding technological advances with microinstrumentation will likely allow water tight closures in the future. Unfortunately, not all defects provide a suitable tissue edge for suturing.
Common local tissue flaps such as the middle turbinate flap are readily available and dependent for small dural defects in the cribriform plate and ethmoid roof. Larger defects in the anterior cranial base, though, require a vascular flap with a wider and longer reach. Originally described by Hirsch in 1952, the nasal septal flap, based on the posterior nasoseptal artery, allows coverage of any defect created during an endoscopic anterior cranial base procedure. The flap can extend as far anterior as the frontal recess and posterior to the inferior border of the clivus. Its width can expand from lamina to lamina. The use of this flap has been reported to decrease postoperative CSF leak rates from 20% to around 4%. Other large vascular flaps harvested via minimally invasive techniques have been described at recent society meetings and will likely be published soon.