Chapter 9 Transcribriform Approach
Introduction
The transcribriform endonasal approach has become an important surgical tool for the treatment of sinonasal and anterior skull base malignancies. Although rare, the most common tumors of the anterior cranial base in the region of the cribriform plate include adenocarcinoma, squamous cell carcinoma, esthesioneuroblastoma, olfactory nerve schwannomas, and olfactory groove meningiomas. The cribriform plate encompasses the areas of the olfactory cleft, the skull base above the ethmoid plate (septum), and the floor of the olfactory bulbs. Experience with endoscopic techniques in sinus surgery has demonstrated that anterior skull base is accessible in its full anteroposterior extent (posterior wall of frontal sinus to planum sphenoidale) and lateral extent (lamina papyracea). Recent reports of the endonasal transcribriform approach have demonstrated good oncologic and functional results in the treatment of anterior skull base tumors.
9.1 Indications
Cerebrospinal fluid (CSF) leaks of the anterior skull base.
Mucoceles.
Large osteomas.
Encephaloceles.
Meningoceles.
Olfactory groove meningiomas.
Olfactory neuroblastoma.
Sinonasal malignancy with anterior skull base invasion.
9.2 Surgical Steps
The transcribriform approach encompasses the medial anterior skull base and olfactory groove. Intracranially, it corresponds to the gyri recti, orbitofrontal gyri, olfactory nerves, and interhemispheric fissure. The most important vital structures related to this approach are the orbits laterally and the anterior cerebral arteries (second segment of the anterior cerebral artery, A2) and their branches (fronto-orbital, frontopolar) ( Fig. 9.1 ). Depending on the localization and/or size of the lesion, the approach can be tailored to prevent unnecessary opening of the skull base. Its anterior limits are the frontoethmoidal recess, but could be extended up to the anterior plate of the frontal bone. Posteriorly, the limit is the planum sphenoidale and/or the posterior ethmoidal arteries (PEAs). In a sagittal cut of a cadaveric specimen, we can observe the measures of the skull base structures and sagittal limits of the approach ( Fig. 9.2 ). Laterally, it is bound by the roof of the ethmoid sinus (fovea ethmoidalis) and the medial orbital wall (lamina papyracea). In general terms, this approach is performed bilaterally; however, in some localized paramedian tumors, a unilateral approach with intention to preserve contralateral olfaction is an option.
The initial step begins with anterior and posterior ethmoidectomy with a wide sphenoidotomy to have a complete access to the skull base ( Fig. 9.3 ); the latter is discussed in previous chapters. Usually, all expanded approaches begin with accessing the sphenoid sinus, which sometimes may be difficult as the tumor precludes a clear access or the bony structures are destroyed because of the tumor infiltration. If this is the case, our philosophy is to first access the bony landmarks and, if not, recognize the vascular landmarks such as the anterior ethmoidal arteries (AEAs) and PEAs.
It is important to enable a bimanual dissection. The ability to use both hands (traditionally with suction in the nondominant hand and a drill, dissector, or elevator in the dominant hand) allows a clear operative field for a safe dissection. We usually use angulated endoscopes (30 or 45 degrees) during the approach depending on the surgeon preferences.
On behalf of an easier explanation, the approach will be described step-by-step starting from the sphenoid up to the frontal sinus.
9.2.1 First Step
Bilateral sphenoidotomies are performed, and the planum sphenoidale is exposed. It is convenient to extend laterally to the level of the medial pterygoid plates to ensure adequate exposure, not only for visualization but also to warrant unobstructed access in the event of critical bleeding. The posterior septal artery should be preserved in case a pedicled nasoseptal flap is required for skull base reconstruction. A posterior septectomy is performed to facilitate the use of multiple instruments simultaneously.
9.2.2 Second Step
Complete ethmoidectomies are performed bilaterally. Identification of the AEA and PEA on both sides (see Chapter 7) and their cauterization or clipping under direct visualization prevent uncontrolled bleeding and contribute to early tumor devascularization.3,4 The AEA runs in an anteromedial direction through the anterior ethmoidal canal, which is usually located between the second and third lamella, posterior to the frontal recess. The PEA courses horizontally in the posterior ethmoidal canal, usually located 10 to 12 mm from the AEA, 7 to 8 mm from the optic foramen, and 18 mm from the tuberculum sella.5,6 At this point, one should have completed the exposure to the anterior cranial base from the posterior aspect of the frontal sinus to the planum sphenoidale and laterally to the lamina papyracea bilaterally. If additional anterior exposure is needed, it can be obtained by a frontal recess approach, with medial widening of the frontal recesses and removal of the frontal nasal septum and inferior frontal sinus septum (in the sense of a Draf type III).
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