The orbital apex is a small, cone-shaped region located between the posterior ethmoidal foramen anteriorly and the openings of the optic canal and superior orbital fissure posteriorly. It contains many critical neurovascular structures, including the optic, oculomotor, and abducens nerves, as well as the ophthalmic branch of the trigeminal nerve. Also nearby are the cavernous sinus, carotid artery, and periarterial sympathetic plexus. At this level, the extraocular muscles attach to the annulus of Zinn, a fibrous ring that surrounds the optic canal and the inferior part of the superior orbital fissure.
Lesions in the orbital apex are rare and typically produce symptoms such as visual acuity reduction, extraocular muscle impairment with diplopia, pain, and exophthalmos. The differential diagnosis is broad and includes inflammatory, infectious, traumatic, vascular, and neoplastic causes.
External surgical approaches to the orbit are well established. External orbitotomies can be performed with or without osteotomy and, in cases of more extensive tumors, the orbitozygomatic craniotomy offers a wide exposure of the orbital contents. However, medial and inferior orbital lesions are the most difficult to reach and are usually addressed via a transcutaneous or transconjunctival medial orbitotomy. However, such approaches are challenging in the cases of posterior tumors, as the cone-shaped surgical field is narrow and damage to neural, muscular, or vascular structures of the orbit can have serious consequences. For intraconal lesions, a temporary section of the medial rectus muscle and retraction of the globe is sometimes necessary.
Many reports of endoscopic transnasal approaches to the orbit have been published during the past several years, and as such, endoscopic orbital surgery is now an alternative option to traditional external approaches in the armamentarium of the surgeon for management of selected orbital lesions.
A complete ophthalmological evaluation is mandatory, including visual acuity, visual field, ocular mobility, and exophthalmometry. With regard to imaging, patients should have both contrast-enhanced magnetic resonance imaging and computed tomography scanning performed preoperatively. In some cases, angiography can still help to solve some clinical dilemmas. Intraoperative neuronavigation should always be available in difficult cases.
The position of the tumor is of crucial importance to determine whether or not endoscopic transnasal resection is a viable option for resection. Axial, coronal and sagittal scans must be carefully studied to determine the position of the tumor in relation to the optic nerve and other important neurovascular structures. Recently three-dimensional reconstruction has been reported as a useful tool to aid in the understanding of tumor morphology. Generally, tumors lateral to the optic nerve, but inferior to a two-dimensional plane passing from the contralateral naris and the long axis of the optic nerve, have been considered amenable to transnasal endoscopic resection. Surgeons need to remember that at the orbital apex level, the possibility of manipulating and displacing structures is reduced, making this zone the most technically challenging. Although the presence of a tumor in this location may pathologically expand this zone, normally there may be less than a millimeter between the lateral border of the medial rectus muscle and the optic nerve in its greatest dimension. Additionally, the insertion of the medial rectus in the annulus of Zinn drastically limits the ability to retract the muscle medially. Therefore to simplify, lesions occupying the superolateral quadrant of the orbital apex are not amenable to transnasal endoscopic resection ( Fig. 26.1 ) and other surgical options must be considered.
Endoscopic Transnasal Approach to the Orbital Apex
After standard preparation and infiltration of the nasal cavity and lateral nasal wall, an uncinectomy is performed. The natural ostium of the maxillary sinus is identified and enlarged posteriorly to the area of the posterior fontanelle with straight-cutting forceps and the microdebrider. A large antrostomy is essential to properly visualize the posterior orbital floor and to avoid obstruction of the ostium if significant prolapse of the orbital fat occurs postoperatively.
A total sphenoethmoidectomy is performed and the sphenoid anterior wall is removed, thereby allowing a wide entry into the sphenoid sinus through the posterior ethmoid. The skull base is identified and cleared and the lamina papiracea is fully exposed. The lamina papiracea can typically be fractured with a Freer elevator (Karl Storz, Tuttlingen, Germany) and flaked off. The hard palatine bone forming the posterior inferomedial orbital angle can be thinned with a small diamond burr and subsequently removed safely. During the removal of the lamina papyracea it is of the utmost importance to preserve the integrity of the orbital periosteum, because herniation of fat in the surgical field can obscure the remaining bone and make its removal difficult. If the pterygopalatine fossa must be entered, at this stage the posterior wall of the maxillary sinus must be removed with a Kerrison bone punch (Karl Storz, Tuttlingen, Germany) and the contents of the pterygopalatine fossa can be bluntly dissected up to the inferior orbital fissure ( Fig. 26.2 ). Inferiorly to the optic canal, the inferomedial part of the superior orbital fissure can be skeletonized. Once the bony layer has been carefully removed, the connective tissues appear underneath. The periorbital layer presents as a continuum with the dura of the lateral sellar compartment and the fascial system covering the inferior orbital fissure and the pterygopalatine fossa ( Fig. 26.3 ). It is important to prepare an adequate bony window before proceeding with the periorbital incision. The inferomedial orbit should be fully exposed and then entered. At this point, the pterygopalatine fossa can also be addressed as necessary if involved by the tumor or to enhance the posterior exposition of the inferomedial orbit.
When a three- or four-handed approach is planned, a posterior septectomy must be performed, wide enough to allow a second corridor for instruments from the contralateral nostril. The periorbital incision is created with a sickle knife, according to the position of the pathology.
In the case of decompressive surgery or when dealing with extraconal disease, a relatively safe blunt dissection between extraconal fat lobules is possible ( Fig. 26.4 ). Fat lobules of the extraconal space can be carefully shrunk by bipolar electrocautery to improve visualization. In cases of tumor removal, manipulation of the diseased material can be performed with relative ease because there are no critical structures in the extraconal space ( Fig. 26.5 ). In cadaver dissection studies, it was shown that in 83% of cases, a medial extraconal vein has been reported deep to the periorbita and is known as the medial ophthalmic vein.
The intraconal compartment is bounded medially by the muscular wall ( Fig. 26.6 ), composed mainly of the medial rectus muscle and, to a lesser extent, the inferior rectus muscle inferiorly and the superior oblique muscle superiorly. The dissection is preferably performed between the medial and inferior rectus muscles. At this point, it is necessary to retract medially or displace superiorly the medial rectus muscle. Different methods to achieve this retraction have been reported in the literature, such as double ball probe retraction, transseptal or transchoanal retraction with vessel loops, blunt dissection, or temporary detachment via a transconjunctival approach. Transseptal retraction, both with suture or with a double ball probe (made by the second surgeon from the contralateral nostril), showed an excellent medial displacement of the medial rectus muscle. In addition, the use of the four-handed approach may offer an advantage with respect to dynamic adjustments in retraction during the case and enhanced protection of the neurovascular inputs of the medial rectus muscle. Our preference is to retract or displace the medial rectus with blunt instruments using a three- or four-handed approach if necessary.