Endoscopic endonasal approaches to the skull base are classified by their orientation in the sagittal and coronal planes. The anterior coronal plane corresponds with the anterior cranial fossa and extends laterally from the midline sagittal corridor across the floor of the anterior cranial base and roof of the orbit. Access to the roof of the orbit is limited medially by the medial orbital wall, ethmoidal arteries, and orbital contents. Tumors that involve the roof of the orbit or extend laterally over the orbit can be accessed endonasally by sacrificing the ethmoidal arteries, decompressing the medial orbit, and displacing the orbital contents. The endonasal approach may be combined with other transcranial and transorbital approaches depending on the location and extent of the pathology.
The anterior cranial base is composed of the frontal, ethmoid, and sphenoid bones. The orbital roof is part of the frontal bone and articulates with the sphenoid bone posteriorly and the ethmoid bone medially ( Fig. 24.1 A ). The optic canal and anterior clinoid are part of the sphenoid bone. The medial wall of the orbit consists primarily of the lamina papyracea of the ethmoid bone ( Fig. 24.1 B). This paper-thin bone articulates with the frontal bone along the frontoethmoidal suture.
Bony foramina for the anterior ethmoid artery (AEA) and posterior ethmoidal artery (PEA) are located along the frontoethmoidal suture line. A middle ethmoidal artery is also present in approximately one-third of patients. The AEA is between the second and third lamellae of the ethmoid bone just posterior to the nasofrontal recess and is in a coronal plane tangential to the posterior surface of the globe. Owing to variability in the pneumatization of the ethmoid and sphenoid sinuses, the PEA may be anterior or posterior to the first bony wall anterior to the optic canal. Both vessels course across the roof of the ethmoid sinus and may be prominent or embedded within the bone ( Fig. 24.2 ).
The optic canal is within the sphenoid bone medial and superior to the superior orbital fissure. It is bounded by the anterior clinoid superolaterally. The lateral optic-carotid recess is between the optic canal and superior orbital fissure and represents pneumatization of the optic strut ( Fig. 24.3 ). Extensive pneumatization can extend into the anterior clinoid.
An endonasal approach in the anterior coronal plane provides access to the roof of the orbit posterior to the globe. With sacrifice of the ethmoidal arteries and displacement of the orbital contents, the midsagittal plane of the orbital roof can be reliably reached (see Fig. 24.1 ). The most common indications for this approach include primary tumors of the bone (osteoma, ossifying fibroma, fibrous dysplasia), sinonasal tumors with medial orbital or lateral dural involvement, or intracranial tumors that extend laterally (meningioma) ( Fig. 24.4 ). With meningiomas, there is often a dural tail that extends beyond the margin of the main tumor mass. Complete resection (Simpson grade I or II) requires removal of all involved bone and excision or coagulation of involved dura. Endonasal resection of sinonasal malignancies such as olfactory neuroblastoma can be reliably performed with clear oncologic margins in most cases. Complete resection of involved dura may require extending the skull base defect beyond the medial orbital wall. If clear dural margins cannot be achieved with resection up to the midsagittal plane of the orbit, the endonasal approach can be supplemented with a transorbital or transcranial approach; alternatively, the entire skull base resection can be performed via a transcranial approach.
Tumors may also involve the anterior clinoid, often in the presence of optic nerve compression. When the optic canal is decompressed, drilling of the roof of the optic canal and a small portion of the medial aspect of the anterior clinoid provides a more complete decompression of the optic nerve. Hypertrophy of the anterior clinoid is usually associated with primary bone pathology (osteoma, ossifying fibroma, fibrous dysplasia) or tuberculum meningiomas ( Fig. 24.5 ). The anterior clinoid cannot be completely removed endonasally and, as noted previously, an endonasal approach may be combined with a transorbital or transcranial approach depending on the location of the tumor.
Rarely a cerebrospinal fluid (CSF) leak may result from a pneumatized optic strut and anterior clinoid following transcranial surgery with drilling of the anterior clinoid. An endoscopic endonasal approach allows repair of the CSF leak without resorting to re-exploration of the craniotomy site.
Inflammatory disease of the orbit may present with a subperiosteal orbital abscess. Subperiosteal abscesses located medially in the orbit are ideally suited to endoscopic drainage via an endonasal approach with medial orbital decompression. Subperiosteal abscesses in the superior orbit often arise from the frontal sinus and can often be drained with an endonasal approach to the orbital roof ( Fig. 24.6 ). For collections that are more lateral, an external approach may be necessary.