REVIEW

What Are the Limits of Endoscopic Sinus Surgery?:
The Expanded Endonasal Approach to the Skull BaseCarl H. Snyderman,1 Harshita Pant,1 Ricardo L. Carrau,1 Daniel Prevedello,2Paul Gardner2 and Amin B. Kassam3

Department of Otolaryngology, University of Pittsburgh, Pennsylvania, USA
2
Department of Neurosurgery, University of Pittsburgh, Pennsylvania, USA
3
Brain Tumor Center and Neuroscience Institute, Saint John’s Health Center andJohn Wayne Cancer Institute, California, USA
1(Received for publication on March 5, 2009)(Revised for publication on March 30, 2009)(Accepted for publication on April 16, 2009)

Abstract
The advent of endoscopic technologies and techniques has expanded the limits of conventional
endoscopic sinus surgery. The expanded endonasal approach describes a series of surgicalmodules in the sagittal and coronal planes that allow surgical access to the entire ventral skull
base. The sagittal plane extends from the frontal sinus to the second cervical vertebra. The
coronal plane extends from the midline to the roof of the orbit, the floor of the middle cranial
fossa, and the jugular foramen. Key principles of endonasal skull base surgery are choosing asurgical corridor that minimizes the need for neural and vascular manipulation, team surgery,use of the endoscope to enhance visualization, and bimanual tumor dissection under direct visualization. Particular challenges of the expanded endonasal approach are identification of anatomical structures using unfamiliar landmarks, hemostasis, and dural reconstruction. Overthe last decade with more than 1000 completely endonasal skull base surgeries, we have demonstrated that endoscopic endonasal surgery of the skull base can be performed with minimal
morbidity and mortality. The introduction of the septal mucosal flap for dural reconstruction
has decreased the incidence of postoperative cerebrospinal fluid leaks to less than 5%. Early
data suggests that oncological outcomes for malignant sinonasal tumors with skull base involvement are comparable to conventional techniques. Proper training in endonasal surgical
techniques is essential to prevent unnecessary morbidity and achieve good outcomes. (Keio J
Med 58 (3) : 152－160, September 2009)

Keywords: endoscopic sinus surgery, skull base, cranial base, sinonasal neoplasms

Introduction
Endoscopic surgical techniques have transformed allof the surgical specialties. In otolaryngology, endoscopic sinus surgery is well established for the treatment ofinflammatory sinus disease as well as benign and selected malignant sinonasal tumors. Endoscopic techniques
have also been applied to the treatment of cranial basepathology, most commonly cerebrospinal fluid leaks andpituitary tumors.1 Over the last decade, we have demonstrated that the entire ventral skull base is accessible using an endonasal approach. This is termed the “expanded
endonasal approach (EEA)” and provides access to theanterior, middle, and posterior cranial fossae.2-4
The limitations of the EEA have yet to be established.
Initially, the development of endonasal skull base surgery was limited by available technology and anatomical
knowledge of the skull base from an endonasal perspec-

Presented at the 1599th Meeting of The Keio Medical Society in Tokyo, November 21, 2008.
Reprint requests to: Carl H. Snyderman, MD, Department of Otolaryngology, University of Pittsburgh, Suite 500, Eye & Ear Institute, 200 Lothrop St, Pittsburgh, PA15213, USA, Tel: +1-412-647-8186, Fax: +1- 412-647-2080, Email: snydermanch@upmc.edu
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Fig. 1 Schematic depiction of the sagittal plane module on a computed-tomography (CT) scan of the skull base. The various endonasal
surgical approaches are numbered as follows; (1) transfrontal (2) transcribriform, (3) transplanum, (4) transsphenoid, (5) transclivaland (6) transodontoid.

tive. Necessary technology included instruments adapted for intranasal use such as dissecting instruments andpowered instrumentation (drill, ultrasonic aspirator, andmicrodebrider), intraoperative navigational devices, hemostatic materials and devices (bipolar forceps), andmaterials for dural reconstruction. Although much was
known about skull base anatomy, there was little knowledge about key relationships of neurovascular structuresfrom the endonasal perspective, in particular landmarks
for the different segments of the internal carotid artery(ICA). Other potential limitations of endoscopic skull
base surgery include the nature of the pathology (location, diagnosis, vascularity), the patient (age, medical comorbidities), the surgeon (training or expertise), and institution (resources).
Location and Surgical Access
The primary factor limiting EEA is the location of thepathology relative to important neural and vascular
structures. The key principle of skull base surgery ischoosing the best pathway to the pathology. For manytumors, the endonasal corridor provides the most directaccess with the least manipulation of neural and vascular
structures. For example, the endonasal corridor provides
a direct pathway to an olfactory groove meningioma
without the need for brain retraction. Conversely, a tumor lateral to the optic nerve is best approached via another approach. Large tumors may require a combination of external and endonasal approaches.
The EEA consists of multiple modules that are oriented

in sagittal and coronal planes.2, 3 Sagittal plane modules
extend from the frontal sinus to the second cervical vertebra (Fig. 1).1 The transfrontal module provides accessto the floor and posterior wall of the frontal sinus. Examples of lesions found in this area include inflammatory sinus disease, erosive mucocoeles, frontal osteomas,
and nasal dermoids. The transfrontal approach is the anterior limit of an endonasal “craniofacial resection” formalignant sinonasal neoplasms.2 The transcribriform
module extends from the crista galli to the planum sphenoidale and across the roof of the ethmoid sinuses (fovea
ethmoidalis) to the orbital roof. This module is most often used for sinonasal malignancies and olfactory groove
meningiomas.3 The transplanum module provides accessto suprasellar lesions such as pituitary tumors with extrasellar extension and craniopharyngiomas. It is limited
posterolaterally by the optic canals.4 An example of alesion (olfactory groove schwannoma) requiring a transcribriform and transplanum approach is depicted in
Figure 2. The transsellar approach is the standard approach for pituitary pathology but may be combined withother approaches for tumors with extrasellar extension.
It is limited laterally by the cavernous ICA.5 The transclival approach spans the sphenoid bone and clivus
from the posterior clinoids to the foramen magnum andcan be subdivided into superior, middle, and inferiortransclival approaches (Fig. 3). The superior transclival
approach requires a transposition of the pituitary glandsuperiorly with preservation of the pituitary stalk. Typical clival lesions include chordomas and meningiomas.6, 7 The transodontoid approach provides access to154

Snyderman CH, et al: Limits of Endoscopic Surgery

Fig. 2 Magnetic resonance imaging (MRI) of an olfactory groove schwannoma in a teenage male. An endoscopic completely
endonasal transcribriform and tranplanum approach was utilized to remove this tumor. Figures A and B represent preoperative scans
and figures C and D postoperative scans.

Fig. 3 Preoperative (A) and postoperative (B) MRI of a patient with clival meningioma.

the upper cervical spine (C1 and C2) and foramen magnum.5 Rheumatoid degeneration of the upper cervicalspine with pannus formation is associated with basilar
invagination of the spine and compression of the brainstem (Fig. 4). Removal of the ring of C1 and odontoid
decompresses the brainstem anteriorly but still requires
stabilization of the spine posteriorly. A transodontoid
approach is also used for access to spinal tumors at thelevel of the foramen magnum. This approach is limited

laterally by the parapharyngeal ICA and the vertebral arteries. The inferior limit is defined by the nasopalatine
line, a line that is tangential to the inferior edge of thenasal bones and the posterior edge of the hard palate.6, 8
The maximal amount of bone that can be removed transnasally is delimited by where a projection of the line
crosses the cervical spine.
EEA approaches in the coronal plane are divided into
anterior, middle, and posterior coronal planes corre-

Keio J Med 2009; 58 (3): 152－160

155

Fig. 4 MRI of the cervical spine in the sagittal plane showing compression of the lower brainstem and cervical spine from arheumatoid arthritis pannus (white arrows). The pannus was removed via an endoscopic transodontoid approach. Preoperative (A) andpost-operative (B) scans.

Fig. 5 An axial CT scan showing an expansile cholesterol
granuloma (margins marked with white arrows) situated medial
to the petrous internal carotid artery (*). This was drained via
an endoscopic transpterygoid and transphenoid approach.

sponding to the corresponding cranial fossae.4 The anterior coronal plane consists of supraorbital and transorbital approaches. The supraorbital approach requires removal of the medial wall of the orbit and displacementof the orbital tissues to visualize the orbital roof. Thisapproach is used for benign fibro-osseous lesions involving orbital walls and to obtain lateral margins with atranscribriform or transplanum approach. The mid-planeof the orbital roof can be reliably reached with this approach. The transorbital approach is used for intraconal
lesions that are inferior and medial to the optic nerve

such as hemangiomas and schwannomas. Access is between the inferior and medial rectus muscles with preservation of extraocular muscle function.
The middle coronal plane extends from the sphenoidsinus across the floor of the middle cranial fossa superior
to the petrous ICA.9 An important landmark for localization of the petrous ICA is the pterygoid or vidian canal.6 The vidian artery can be followed to the second
genu of the ICA.10 The medial petrous apex can be approached at the level of the clival recess between the
mid-clival bone and the paraclival segment of the ICA.11
Expansile lesions of the petrous apex such as cholesterol
granulomas are well-suited to this approach (Fig. 5).
Meckel’s cave and the cavernous sinus can be approached lateral to the paraclival segment and superior
to the petrous segment of the ICA between the vidian
nerve and second division of the trigeminal nerve, V2,
leading to the foramen rotundum. This requires atranspterygoid approach with full access to the lateral recess of the sphenoid sinus. Meningiomas and schwannomas may require this approach as well as malignant tumors with perineural extension along V2 (squamous cell
carcinoma, adenoid cystic carcinoma). Further laterally,tumors of the infratemporal fossa (mandibular nerve(V3) schwannoma) may be approached. This requires amedial maxillectomy with Denker’s maxillotomy (anteromedial maxillectomy) for full access to the posterolateral wall of the maxillary sinus. The lateral pterygoid
plate is a useful landmark for locating foramen ovale,
where V3 tumors are located.
The posterior coronal plane extends from the foramen
magnum across the occipital condyle and hypoglossal
canal to the jugular foramen.3 Chondrosarcomas, meningiomas, chordomas, are the most common tumors in thislocation. Resection of the Eustachian tube is necessary
to gain full access to the infrapetrous area and is an im-

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Fig. 6 An endoscopic view of the surgical dural defect (margins delineated with black arrows) after an endoscopic transcribriform
approach for excision of an esthesioneuroblastoma. The ventral surface of the frontal lobes is evident beyond the defect.

portant landmark for the parapharyngeal ICA. An infrapetrous approach can also be used to access pathology ofthe petrous apex that cannot be reached using a medialapproach.9
Oncological Resection
A large variety of malignant sinonasal tumors can involve the skull base. For those tumors considered operable, a craniofacial resection with postoperative radiation+/_ chemotherapy is considered the gold standard. Thishas been well established, in particular, for esthesioneuroblastomas where skull base erosion and dural invasion
are typical features. One criticism of endonasal surgery
for sinonasal malignancies is that it does not respect oncological principles. When properly performed, oncological principles do not need to be sacrificed to achieve
a complete resection with clear margins.1 2 Our collective experience with other head and neck neoplasms
demonstrates that piecemeal resection of the tumor does
not compromise results as long as the final margins areclear (endoscopic resection of inverting papilloma, transoral supraglottic laryngectomy, Moh’s technique forskin cancer). Although a sinonasal neoplasm may fill
the nasal cavity, the area of tumor invasion may be quite
limited. In the case of esthesioneuroblastoma, the intranasal component of the tumor is debulked and the bonearound the periphery of the tumor is exposed and removed (floor of the frontal sinuses and crista galli anteriorly, medial orbit and fovea ethmoidalis laterally, andplanum sphenoidale posteriorly).2 The dura is then incised around the periphery of the tumor and dissected
free from the surface of the brain with inclusion of theolfactory bulbs and tracts. Thus, the area of skull baseinvolvement is removed en bloc and clear margins are
confirmed with frozen section analysis (Fig. 6).
Although the literature is quite sparse and follow up islimited, the early results of completely endoscopic endonasal resection of olfactory neuroblastomas are very encouraging.1 3 Until larger series are published with extended follow up (>5 years), a complete oncological resection as outlined here should be performed.
Prevention of Complications
The most important innovation in skull base surgery
for the prevention of complications has been the introduction of true team surgery. Unlike other team surgeries that employ sequential teams of surgeons, endonasal
skull base surgery requires the participation of both surgical specialties throughout the surgery. The benefits ofteam surgery are myriad and include improved visualization, increased operative efficiency, improved decision
making and problem solving, and modulation of individual enthusiasm. Endoscopic visualization provided by
an experienced surgeon is a very dynamic process inwhich constant adjustments are made to provide room
for instrumentation and provide the best view. This assumes critical importance when there is a vascular emergency.
There are a number of technological challenges in performing endonasal skull base surgery. Foremost among
these are hemostasis and dural repair.6, 1 2, 1 4 Strategies
for minimizing intraoperative blood loss include preoperative embolization when feasible, surgical ligation ofvessels at the periphery of the tumor (anterior and posterior ethmoidal arteries), and removal of the bone and dural base of the tumor. The use of a diamond drill forKeio J Med 2009; 58 (3): 152－160

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Fig. 7 Postoperative MRI of the skull base in the sagittal (A) and coronal plane (B) of a patient that underwent endoscopictranscribriform resection of an esthesioneuroblastoma. The extent of skull base defect repaired using a Hadad-Bassagasteguy
nasoseptal flap is demonstrated. A foley catheter balloon can be seen in place that is used to provide support for the flap in theimmediate post-operative period.

bone removal provides a hemostatic effect. The mostimportant instrument is the endoscopic bipolar electrocautery device with exchangeable tips for different
bleeding situatons. A variety of hemostatic materials areavailable for direct application to bleeding sites. Hot
water irrigation (40°C) with saline is a useful adjunct for
diffuse oozing from sinus mucosa as well as intracranially.
Reconstruction of large dural defects has been one of
the greatest challenges of endonasal skull base surgery.
Our initial experience was plagued by an unacceptablelevel of postoperative cerebrospinal fluid (CSF) leaks.
This has been solved with a combination of techniques
including inlay and onlay fascial grafts, direct suturingof dural grafts, fat grafts, the introduction of vascularized
pedicled flaps, and intranasal support with a balloon
catheter.15, 16-20 Our preferred technique is a septal mucosal flap pedicled on the posterior septal artery from thesphenopalatine foramen. This provides complete coverage of a craniofacial defect from the frontal sinuses tothe planum sphenoidale and from orbit to orbit (Fig. 7).
When this tissue is not available, a pericranial scalp flap
(without a craniotomy) or a temporoparietal fascial flap
can be used.14 In a series of 150 consecutive cases withintraoperative CSF leak, the incidence of postoperative
leaks was 4% (unpublished data).
Dealing with Complications
A frequent concern is whether endoscopic endonasal
techniques are sufficient to deal with a major vascular
injury such as injury to the ICA. An experienced teamof surgeons can maintain visualization and gain control
with bipolar electrocautery, application of hemostatic
materials, and focal packing. Indiscriminate packing is
to be avoided since blood may track intracranially. If
necessary, aneurysm clamps can be placed across an in-

jured vessel. Once intraoperative control is achieved,
angiography is performed with possible sacrifice of thevessel.
Infectious complications are surprisingly rare with endonasal skull base surgery, with an incidence of bacterial
meningitis of 1-2%. This is attributed to minimal bacterial colonization of the nasal cavity and sinuses (compared to the oral cavity), prophylactic perioperative antibiotics, frequent intraoperative irrigation, skull base reconstruction with vascularized flaps, and aggressive
management of postoperative CSF leaks. Non-vascularized materials such as bone cement or titanium plates
should be avoided due to the risk of infection.
Postoperative CSF leaks are managed aggressively
with endoscopic surgical repair. In almost all cases, therepair can be augmented with a small fascial or fat graftor with repositioning of the mucosal flap. A lumbar spinal drain is reserved for high-risk repairs (large dural defects, high-flow leaks, prior surgery or radiation therapy)
or recurrent CSF leaks.
Staging of Surgery
Although endoscopic techniques have many potential
advantages, duration of surgery is not always one ofthem. In some cases, it may be more expedient to perform an open approach when outcomes are comparable.
Long surgeries increase risks for the patient and are difficult for the surgeon. Staging of surgery should be considered when extensive bone removal is required to gain
access to the tumor, especially if there is significant
blood loss. Removal of the bone and cauterization of thedural base decreases intraoperative blood loss during thesecond stage. Excessive blood loss can create a coagulopathic state that is more problematic with intradural dissection. Large juvenile nasopharyngeal angiofibromaswith skull base erosion and intracranial extension may

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Fig. 8 MRI showing preoperative (A, B) and postoperative (C, D) scans of a patient with a giant juvenile nasopharyngeal
angiofibroma. This tumor was removed using an entirely endonasal endoscopic approach and required 3 operations for complete
excision.

receive significant blood supply from the ICA and remain highly vascularized despite preoperative embolization of the external carotid arteries. Other factors to beconsidered in staging surgery include the duration of surgery where late in the day, surgeon fatigue and lack ofavailability of experienced operating room personnel
may compromise outcomes. In rare cases, the entire tumor may not be accessed and staging the procedure
could enable complete resection due to movement of theresidual tumor into the surgical field. With large intracranial tumors, determination of tumor margins and assessment of the extent of resection can be difficult due toshifting of the tumor. Image-guided navigation can be
updated at a second stage or with the use of intraoperative imaging.
Special Patient Populations: Pediatric Patients
Decreased morbidity, faster recovery, and greater patient acceptance make the EEA especially suitable forpediatric patients.2 1-2 3 Despite smaller nares and nasal
cavities, most patients older than 4 years have sufficientroom for endoscopic surgery. If not, a transoral approach (midfacial degloving) can be used to access thenasal cavity. Decreased sinus pneumatization in the pediatric population can make identification of normal anatomical structures more difficult. Applications of EEA
in the pediatric population include biopsy for diagnosis,

definitive treatment of benign neoplasms (including pituitary adenoma, juvenile nasopharyngioma and craniopharyngioma) and down-staging of high-grade malignancies such as rhabdomyosarcoma.
Juvenile nasopharyngeal angiofibroma is unique to thepediatric population and poses a particular challenge because of their increased vascularity. Even large tumors
with skull base involvement can be successfully managed using endoscopic techniques and tumors medial
and lateral to the ICA can be managed using one approach. Staging of the surgery is often necessary due toincreased blood loss (Fig. 8). Surgical removal of large
tumors is performed by dividing the tumor into several
segments corresponding to its blood supply. Each vascular segment is excised sequentially and the intracranial
extension is excised last. The endoscopic approach inpediatric patients minimizes morbidity and potentiallyavoids craniofacial growth abnormalities due to the disruption of growth centers.
Training
Proper training in endonasal surgical techniques is essential to achieve optimum surgical and oncological outcomes and to avoid complications.24 Ideally, skull base
surgeons should be trained in both, open and endoscopic,
techniques so that the best surgical option can be offeredto the patient for any given pathology. We have devisedKeio J Med 2009; 58 (3): 152－160

Table 1 Training levels for endonasal skull base surgery
Level I
Level II

Level III

Level IV

Level V

Sinus Surgery
Advanced sinus surgery
Cerebrospinal fluid leak
Intrasellar – sella, pituitary
Extrasellar – sella, pituitary
Optic nerve decompression
Intraorbital surgery
Extradural skull base surgery
Intradural skull base surgery
A. With cortical cuff
Transplanum
Transcribriform
Type I Craniopharyngioma
Intradural skull base surgery
B. Without cortical cuff
Type II/III Craniopharyngioma
Transclival, intradural
Coronal plane, carotid dissection
Vascular surgery

a 5-level training program that is based on our own
learning experience (Table 1).2 5, 2 6 These levels incorporate anatomical complexity, technical difficulty, potential risk of neural and vascular injury, extent of intradural
dissection and type of pathology. Thus, mastering a level before proceding to the next is recommended. This
training program applies to all endonasal surgeons, irrespective of their specialty.
Level I consists of basic endoscopic sinus surgery.
Level II incorporates advanced endoscopic sinus surgery
and the basic skull base surgery including managementof CSF leaks and small pituitary adenomas. This iswhere otolaryngologists and neurosurgeons may work
together to build their team skills and master hemostatic
and reconstructive techniques. Level III includes management of extradural skull base tumors and pituitaryadenomas with extrasellar extension. Level IV procedures require intradural dissection and are among themost challenging cases. “Craniofacial resection” for sinonasal malignancies is included in Level IV due to theimportance of adhering to oncological principles. If
there is no cuff of brain tissue separating the margin of
the tumor from cerebral vessels, an additional level ofdifficulty is assigned (Level IVb). Level V surgeries areless common in routine practice and involve dissection
around the ICA in the coronal plane (infrapetrous approach). Management of vascular tumors and aneurysms
are included in Level V.
Resources
Necessary resources include endonasal instrumentation, the operating theater, operating room personnel and159

institutional resources. Neurophysiologic monitoring of
brain and nerve function is routinely performed to detect
cerebral ischemia and prevent cranial nerve injury. Image guidance using fused CT and MRI scans is essential
to navigate the “black box” of the skull base. Newer
operating theaters incorporate a CT scanner so that image guidance can be updated intraoperatively to accountfor tumor shift and to assess completeness of resection.
The operating theater is designed for endonasal endoscopic surgery and has multiple viewing monitors for individual surgeons and operating room staff so that all areaware of the status of the surgery. A telestration monitor
facilitates communication between surgeons and is a
useful teaching tool.
There are a number of obstacles to the development ofendoscopic endonasal cranial base surgery. The biggest
challenge is assembling a team of surgeons that are interested in working collaboratively and obtaining the necessary training. The surgical team should work together
consistently to learn how to function as a team and develop expertise with hemostatic and dural reconstruction
techniques. They should have a sufficient surgical volume to maintain their expertise. Institutional resources
and billing issues may also limit development.
Conclusions
The entire ventral skull base can be accessed endonasally using completely endoscopic techniques. Surgicalapproaches begin in the sphenoid sinus as a reference
and is classified into modules based on their orientationin the sagittal and coronal planes. Some of the major
obstacles to endonasal brain surgery, including hemostasis and dural defect reconstruction, are being managed
effectively, however, the techniques and materials usedwill continue to evolve. The limits of endonasal cranialbase surgery have not been realized. The choice of asurgical approach depends on patient co-morbidities, tumor characteristics, and the skill and comfort level of thesurgeons. If nerves or vessels need to be mobilized toreach the tumor, them an alternative approach should beconsidered. Relative contraindications to an endonasal
approach include tumor involvement of superficial tissues, the need for vascular reconstruction and the duration of surgery. If sinusitis is identified, an intradural approach should not be undertaken until the infection has
cleared.
Potential advantages of endoscopic endonasal cranialbase surgery include the lack of external incision, decreased trauma to normal soft tissue and bone, improved
visualization, increased access, improved outcomes,
fewer complications, more rapid recovery, and decreased
hospitalization and cost. Disadvantages include unfamiliar anatomy, a long learning curve, and technologicallimitations. Surgeons should be adequately trained inthese procedures and have adequate experience, surgicalSnyderman CH, et al: Limits of Endoscopic Surgery

160

volume, and resources before undertaking more challenging surgeries. The endonasal corridor is one more
option for our patients with diseases of the cranial baseand should be part of the surgical armamentarium of every cranial base surgeon.
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