Transcranial Approaches to the Optic Apparatus

Indications and Choice of Approach

Intra-axial and extra-axial tumors of the anterior cranial fossa, anterior aspect of the middle fossa, and frontal lobe can be accessed through the supraorbital and minipterional keyhole approaches. As detailed in Table 35.1 , the most common tumors affecting the optic apparatus (including the intracranial optic nerves, chiasm, and optic tracts) and that are appropriate for the supraorbital or minipterional craniotomy are parasellar meningiomas, craniopharyngiomas, some gliomas, pituitary adenomas, and metastatic brain tumors.

Meningiomas arising from the tuberculum sellae and posterior planum region, anterior clinoid process, and in some instances, the medial sphenoid wing are ideally approached from the supraorbital route. Some invasive parasellar meningiomas that have extensive sellar, cavernous sinus, and/or Meckel cave growth may need to be approached from the supraorbital route for optic apparatus decompression; however, many if not most of these can also be effectively debulked from the endonasal route. Regarding tuberculum sellae meningiomas, for which both endonasal and transcranial approaches are feasible, the three key factors in choosing the approach are tumor size, sellar depth, and lateral tumor extension. As we have previously described, ideal meningiomas for the endonasal route include those less than 3 cm without lateral extension beyond the optic nerves and supraclinoid carotid arteries and a deepened sella. The remainder of tuberculum sella and anterior clinoidal meningiomas, including those that in part involve the medial sphenoid wing, can often be removed by a supraorbital or conventional frontotemporal craniotomy.

The supraorbital approach, which places the ipsilateral optic nerve directly in line with the route, creates a relative surgical blind spot along the undersurface of the ipsilateral optic nerve ( Fig. 35.1 ). Meningiomas of the tuberculum sella elevate the nerve and chiasm and can often grow into the optic canal. Compression of the chiasm can cause a bitemporal hemianopsia, and stenosis of the optic canal by tumor can cause loss of visual acuity as well as variable visual field loss. This poses a challenge to the surgeon, as decompression of the optic canal may be required. Although the supraorbital approach does allow for bony decompression of the optic canal roof, the area inferior to the optic nerve ipsilateral to the approach is not visible in the line of sight of the microscope. Leaving this tumor behind risks continued visual disturbance and increases the likelihood of recurrence. Angled endoscopes and instruments allow the surgeon to visualize and reach under the ipsilateral nerve and remove tumors in this location.

Craniopharyngiomas pose a particular challenge as they can have a variable relationship with the optic nerves and chiasm. Given that most craniopharyngiomas are retrochiasmal in location, resulting in a prefixed chiasm ( Fig. 35.2 ), they are best approached from an endonasal route to minimize manipulation of the optic apparatus. This approach also typically puts the surgical trajectory along the long-axis of the tumor. Craniopharyngiomas that arise anterior to the optic chiasm (postfixed chiasm), or elevate it superiorly ( Fig. 35.3 ), or which have suprachiasmatic anterolateral extensions into the frontal and middle fossae, can often be approached transcranially (in most cases via the supraorbital approach rather than the minipterional approach). Pituitary adenomas with exophytic supradiaphragmatic extensions can similarly be removed from the supraorbital approach, whereas the minipterional route is rarely needed.

Gliomas, lymphomas, germinomas, and metastatic lesions affecting or intrinsic to the optic apparatus are often ideally approached via the supraorbital or minipterional route given that these tumor types are typically above the plane of the optic apparatus (and in the case of gliomas and metastases are intra-axial). As shown in the case example later in this chapter, germinomas often extend up along the infundibulum and may invade the lamina terminalis; as such, the supraorbital route with endoscopic assistance is ideal for accessing this area. In our practice, the supraorbital route, relative to the minipterional route, is favored for accessing anterior cranial fossa and parasellar lesions, as well those extending into the proximal sylvian fissure and medial temporal lobe, and is used almost five times as often as the minipterional route.

The traditional pterional craniotomy is a workhorse approach in neurosurgery, allowing access to the entire circle of Willis, sylvian fissure, optic apparatus, pituitary gland and infundibulum, and the base of the skull in the anterior and middle cranial fossae. The minimally invasive variant of this approach allows similar access to all of these structures, with the only limitation access to the distal sylvian fissure. As detailed in Table 35-2 , the minipterional approach is particularly well suited for lesions that are predominantly in the middle fossa with extension to the ipsilateral optic nerve and chiasm, as well as tumors that extend into the orbit or traverse the superior orbital fissure. Meningiomas are by far the most common tumor approached via the minipterional route, which is particularly effective for sphenoid wing meningiomas that invade the orbital apex and/or optic canal. Bony decompression of the optic canal, orbital apex, and orbit can be accomplished via the minipterional route. Sphenoid wing meningiomas are relatively easy to access with this approach, as the lateral aspect of the wing is drilled and the tumors present themselves close to the surface, as seen in the later example. An anterior clinoidectomy can also be performed extradurally or intradurally. The limitation of the minipterional approach, as with most keyhole approaches, is the difficulty getting light to the deep structures. This is overcome by introducing the endoscope to look up close at the neurovascular structures of the skull base, as well as around corners and into blind spots. This can aid the surgeon in finding small remnants of tumors and improve the likelihood of a complete resection. Fig. 35.4 indicates the overlap and the difference between minipterional and supraorbital approaches. The minipterional approach is better suited for lateral orbital apex and optic canal lesions, whereas the subraorbital approach is ideal for lesions involving the entire optic apparatus.