Anatomical Step-by-Step Dissection of Complex Skull Base Approaches for Trainees: Endoscopic Endonasal Approach to the Orbit

Abstract

Introduction  Although endonasal endoscopic approaches (EEA) to the orbit have been previously reported, a didactic resource for educating neurosurgery and otolaryngology trainees regarding the pertinent anatomy, techniques, and decision-making pearls is lacking.

Methods  Six sides of three formalin-fixed, color latex–injected cadaveric specimens were dissected using 4-mm 0- and 30-degree rigid endoscopes, as well as standard endoscopic equipment, and a high-speed surgical drill. The anatomical dissection was documented in stepwise three-dimensional (3D) endoscopic images. Following dissection, representative case applications were reviewed.

Results  EEA to the orbit provides excellent access to the medial and inferior orbital regions. Key steps include positioning and preoperative considerations, middle turbinate medialization, uncinate process and ethmoid bulla removal, complete ethmoidectomy, sphenoidotomy, maxillary antrostomy, lamina papyracea resection, orbital apex and optic canal decompression, orbital floor resection, periorbita opening, dissection of the extraconal fat, and final exposure of the orbit contents via the medial-inferior recti corridor.

Conclusion  EEA to the orbit is challenging, in particular for trainees unfamiliar with nasal and paranasal sinus anatomy. Operatively oriented neuroanatomy dissections are crucial didactic resources in preparation for practical endonasal applications in the operating room (OR). This approach provides optimal exposure to the inferior and medial orbit to treat a wide variety of pathologies. We describe a comprehensive step-by-step curriculum directed to any audience willing to master this endoscopic skull base approach.

Introduction

Endoscopic assistance has been used to enhance visualization in transcranial approaches to the orbit since at least 1981. ¹ More recent technological advances have resulted in the development of the endoscopic endonasal approach (EEA) to the orbit as a minimally invasive alternative route to medial and inferior orbital lesions. ^{2 3} EEA provides ready access to medial aspects of the orbital wall, apex, optic canal, and floor with advantages including enhanced cosmesis and reduced external scarring, decreased displacement of orbital structures or disinsertion of extraocular muscles, and improved visibility. ⁴ In contrast, key barriers to adopting EEA for orbital surgery include limited depth perception attributable to monocular view, potentially unfamiliar endonasal, maxillary, and ethmoid anatomy, and a the potentially steep learning curve for surgeons not well versed in endoscopic techniques. ⁵ Given the complex and challenging nature of orbital EEA and the lack of specific didactic resources aimed at aiding trainees in overcoming these barriers, there is a need for practical, step-by-step, anatomically-based resources to better inform self-guided and intraoperative learning. In the current study, we use cadaveric specimens and illustrative companion case discussions to provide a comprehensive introductory resource for skull base trainees regarding the key technical and clinical aspects of EEA for orbital surgery.

Materials and Methods

All pertinent aspects of this study were approved by our institutional review board and biospecimens committee. Three cadaveric specimens were formalin-fixed and injected with colored latex (red for arteries and blue for veins) using a standard six-vessel technique, as we have described. ⁶ Six sides were then dissected using 4-mm 0- and 30-degree rigid endoscopic lenses for visualization (Karl Storz and Co, Tuttlingen, Germany), a high-speed surgical drill with endoscopic compatibility (Medtronic Midas Rex electric system; Medtronic USA Inc., Jacksonville, Florida, United States), and standard endoscopic instrumentation for dissection. Anatomical dissections were photographically captured using three dimensional (3D) endoscopic images and described using common surgical vernacular in a stepwise fashion. Following dissection, representative clinical case was reviewed.

Results

Step-by-Step Surgical Approach

Positioning and Preoperative Considerations

Following induction of general endotracheal anesthesia and initiation of appropriate cranial nerve monitoring, the oropharynx and hypopharynx are packed with moist gauze to reduce the risk of aspiration or gastric accumulation of blood products, or alternatively, an orogastric tube is placed and the stomach is aspirated after the procedure. The patient is positioned supine with the head in neutral position. While surgeon positioning varies among centers, we prefer to stand on opposite sides of the patient's bed as a more ergonomic alternative to the classical approach of both surgeons operating on the same side, in which case the head could be rotated toward the surgeons' side. The operating table is then placed in 30 degrees of the reverse Trendelenburg which requires careful padding and securing of the body to the table to prevent cervical spine injury from translation of the body while the head is fixed. Prior to incision, nasal mucosal decongestion is achieved by placing oxymetazoline 0.05%-soaked pledgets bilaterally for at least 5 minutes to achieve optimal vasoconstriction.

Endoscopic unilateral approach provides adequate corridor for the vast majority of tumors. The use of angled endoscopes and instruments allow adequate dissection minimizing the need for a binarial approach. The addition of the contralateral transnasal corridor provides a straighter trajectory to the orbit allowing straight instruments to reach more laterally; however, it requires a large septectomy. A binarial approach also allows easier four-handed manipulation within the nasal cavity and its wider working angle may be advantageous for extensive tumors.

Middle Turbinate Medialization

The middle turbinate is identified with a 0-degree endoscope and gently medialized, from anterior to posterior. This maneuver is typically completed using the back end of a J-curette, or a Freer elevator, and care should be taken to protect the middle turbinate mucosa. Care should be taken during this maneuver to avoid fracture of the vertical portion of the turbinate that can extend superiorly toward the skull base with risk of cerebrospinal fluid (CSF) leak. If possible, we recommend middle turbinate preservation, as it provides a key orienting landmark for the skull base; however, certain cases may require resection of the turbinate if the nasal cavity is narrow ( Fig. 1A ).

Fig. 1.

Step-by-step endonasal endoscopic approach to the medial and inferior walls of the orbit in an anatomical specimen, right side. (A–I: 0-degree endoscope view). ( A ) Overview of the right nasal cavity and its contents before the beginning of the endonasal approach. The anatomical position of the middle turbinate, an essential endoscopic landmark attached anteriorly to the lateral nasal wall by the axilla, is depicted. Note the middle meatus located laterally to the middle turbinate, between the middle turbinate and the lateral nasal wall. ( B ) The middle turbinate has been retracted medially using a Freer Elevator exposing the middle meatus, as well as providing a better view of the uncinate process and the ethmoid bulla. The hiatus semilunaris is located posterior and lateral to the uncinate process and anterior to the ethmoid bulla. ( C ) After an uncinectomy is performed and the ethmoid bulla is removed, the ethmoidal labyrinth is exposed preserving both the middle turbinate axilla and the lacrimal bone, located anterior to the axilla. ( D ) The ethmoidal cells are removed using a Blakesley's forceps showing the medial wall of the orbit covered by the lamina papyracea. At this point, it is possible to observe the Onodi cell, the most posterior and large ethmoidal cell. When removing the ethmoidal air cells is important to maintain the horizontal attachment of the middle turbinate, otherwise the turbinate tends to be destabilized and lateralized after the approach. ( E ) The endoscope is advanced posteriorly reaching the lamina papyracea. In this specimen an Onodi's cell is present which pneumatizes toward the optic canal. ( F ) The Onodi cell has been removed to open the sphenoid sinus. The lamina papyracea has been gently resected using a dissector showing the medial periorbita. The medial wall of the maxillary sinus has been also removed to gain access to the inferior orbital wall. ( G ) The roof of the maxillary sinus medial to the infraorbital nerve has been removed exposing the periorbita of the orbital floor. The resection of the medial orbital wall has been continued posteriorly exposing the medial aspect of the optic canal. ( H ) The medial periorbita has been opened exposing the insertion of the medial and inferior rectus muscles at the orbital apex and the dura covering the medial aspect of the optic canal. ( I ) After removing the inferior aspect of the periorbita, the infraorbital nerve and artery are observed at the lateral aspect of the exposure, conforming an important landmark during the dissection of the maxillary sinus roof. Note how part of the medial and inferior orbital fat has been already removed, showing the optic nerve and inferior ophthalmic vein through the medial endoscopic corridor to the orbit. (J–L: 45-degree endoscopic view). ( J ) The medial rectus muscle is retracted superiorly to enhance the orbital exposure through the medial endonasal corridor, limited by the inferior rectus muscle and medial rectus muscle. The orbital fat has been completely removed. The ophthalmic artery can be visualized above the optic nerve giving the central retinal artery branch. ( K ) Inferior orbital corridor through the orbital floor. Another surgical corridor can be attempted endonasally between the inferior rectus muscle and the infraorbital artery and nerve. ( L ) Improved view through the inferior corridor to the orbit. This corridor allows a better view not only for the central retinal artery but also for the ciliary ganglion and its short ciliary nerves located between the lateral rectus muscle and the lateral aspect of the optic nerve.

Uncinate Process and Ethmoid Bulla Resection

Once the middle turbinate has been medialized, the uncinate process is easier identifiable as a hook-shaped bony projection on the lateral wall of the middle meatus. The uncinate process can be removed with backbiting forceps from posterior to anterior, starting from the free edge until the hard lacrimal bone is encountered anteriorly. Special care is required to protect the lacrimal bone and underlying nasolacrimal duct. The uncinectomy can also be performed from anterior to posterior following an incision along the anterior border of the uncinate process ( Fig. 1B ). After the uncinectomy, the ethmoid bulla is visualized posteriorly and superiorly. The medial aspect of the ethmoid bulla is fractured using a J-curette, starting at the retrobullar space and proceeding in an anterolateral direction. Once the bulla has been fractured, the mucosa and bony fragments can be removed using thru-cut and Blakesley's forceps. Microdebrider can also be used for the uncinectomy and bullectomy ( Fig. 1B ).

The basal lamella of the middle turbinate attaches the turbinate to the orbit and separates the anterior and posterior ethmoid cells. After the resection of the ethmoid bulla, the basal lamella is removed leaving the vertical and horizontal attachments of the middle turbinate intact. The resection of the basal lamella should start at the inferior and medial region away from the skull and orbit, respectively. Care should be taken inferiorly to avoid inadvertent transection of the horizontal portion of the middle turbinate, as this may destabilize it and cause postoperative lateralization. Resection of the uncinate process, the ethmoid bulla, and the basal lamella of middle turbinate provides the principal corridor for accessing the posterior ethmoid labyrinth ( Fig. 1C ).

Complete Ethmoidectomy and Transethmoidal Sphenoidotomy

The dissection is then directed posteriorly, resecting the posterior ethmoidal cells from the opening of the basal lamella until the anterior wall of the sphenoid sinus is reached ( Fig. 1D ). A critical aspect of any procedure involving posterior ethmoidectomy is the identification of a potential Onodi's cell. This eponym describes a posterior ethmoid cell arising superior and posterior to the sphenoid sinus, resulting in a lateral ethmoid wall that is in continuity with the optic canal and therefore high risk for optic nerve injury during ethmoidectomy. Cautious resection of an Onodi's cell floor can safely expand the sphenoid cavity; however, dissection through the posterior wall of an Onodi's cell may result in intracranial or orbital apex injury and should be avoided unless critical to the surgical plan.

Lateralization of the superior turbinate allows direct visualization of the sphenoid ostium, since it is located medial to the superior turbinate in the majority of the cases. The ostium may be enlarged using a sphenoid punch forceps or a Kerrison's rongeurs. The anterior wall of the sphenoid is further removed with high-speed drill or Kerrison's rongeurs, allowing access the orbital apex and the optic and carotid protuberances within the sphenoid sinus lateral wall and roof ( Fig. 1E ). Once the sphenoid sinus is reached, the dissection proceeds from posterior to anterior until all ethmoid septations have been completely removed along the skull base and lamina papyracea ( Fig. 1E ).

Maxillary Antrostomy and Medial Maxillectomy

The medial wall of the maxillary sinus separates the sinus from the nasal cavity and is the site of its natural ostium which is located in the ethmoid infundibulum between the uncinate process and the ethmoid bulla. Maxillary antrostomy typically begins with uncinectomy which was previously completed during ethmoid resection. Correspondingly, the maxillary sinus ostium should be readily identified at the inferior aspect of the ethmoid infundibulum. Although a 0-degree endoscope is sufficient, visualization of the ostium may be improved using a 30-degree endoscope. Thru-cut forceps are used to enlarge the opening posteriorly and inferiorly, removing the membranous posterior fontanelle. Backbiting forceps are used to enlarge the antrostomy anteriorly and remove remnants of the uncinate process. After a wide maxillary antrostomy is completed, the transmaxillary corridor extends from the posterior border of the nasolacrimal duct anteriorly to the posterior wall of the maxillary sinus posteriorly; and from the orbit superiorly to the superior margin of the inferior turbinate inferiorly.

Lamina Papyracea Removal

The lamina papyracea is a very thin and at times dehiscent bony structure separating the orbit from the ethmoid labyrinth. Removal is initiated by gently fracturing the lamina with a small curette or Freer elevator just millimeters posterior to the posterior margin of the lacrimal bone. The periorbita is exposed the fragments of bone that are progressively elevated posteriorly with the Freer and removed with a Blakesley's forceps until the anterior wall of the sphenoid sinus and orbital apex are reached, taking care throughout to protect the periorbita, as the protrusion of orbital fat into the surgical field will markedly impair visualization. Special care must be taken when removing the superior part of the lamina papyracea, and the bone removal should be kept below the ethmoid foramina to avoid damage to the ethmoid arteries which can result in retrobulbar hematoma and risk of vision loss. A frontal sinusotomy is recommended to decrease the risk of obstruction of the frontal sinus outflow from orbital fat prolapse after the intraperiorbital dissection ( Fig. 1E and F ).

Orbital Apex and Optic Canal Decompression

The approach is subsequently carried posteriorly removing the bone covering the medial wall of the orbital apex and optic canal along the lateral wall of the sphenoid. In most patients, the bone covering the orbital apex is approximately 0.5-mm thick and may be in some instances safely fractured and removed, although in some cases, it may require drilling. However, the bone's width at the optic protuberance is variable, ranging from fully dehiscent in some cases to sufficiently thick to require direct drilling with a diamond bit before out-fracturing. Where required, drilling should universally proceed under continuous irrigation to avoid heat injury to the optic nerve. Once the bone of the optic canal is thinned, it can be gently removed with a microdissector or Cottle elevator, directed away from the optic nerve. We recommend starting at the area between both optic nerve prominences in midline (prechiasmatic sulcus), given that the optic nerve has no direct contact with that bony segment. The bony optic canal must be decompressed from the orbital apex to the tuberculum sellae and from the planum to the upper part of the lateral opticocarotid recess, exposing approximately 1 to 1.5 cm of the optic nerve sheath and providing access to at least 180 degrees of the nerve's medial circumference ( Fig. 1G and H ).

Orbital Floor Removal

Using the 30- or 45-degree endoscope and angled instruments, the orbital floor which is also the superior wall of the maxillary sinus is removed by gently out-fracturing and removing the bone in the same manner as the medial orbital wall. The orbital floor removal is performed posterior to the eye globe to prevent hypoglobus. The anterior limit of the bone resection corresponds to the anterior border of the maxillary antrostomy and is carried posteriorly toward the posterior wall of the maxillary sinus. The lateral limit of the orbital floor resection is the infraorbital nerve. This degree of bone removal allows satisfactory instrumentation through the inferior and medial recti muscles corridor with preservation of an adequate strut for the eye globe. Care should be taken with the infraorbital neurovascular bundle ( Fig. 1I ).

For inferiorly located extraconal tumors, additional opening of the posterior wall of the maxillary sinus and pterygopalatine fossa dissection may be beneficial to identify the maxillary branch of the trigeminal nerve and prevent inadvertent damage.

Periorbita Opening and Dissection of the Extraconal Fat

The exposed periorbita is incised with a sickle knife or endoscopic microscissors, creating a surgical corridor that extends fully to the limits of the bony aperture. The first periorbital incision is vertical and placed just anterior to the sphenoid sinus. Then a superior and inferior incisions are made from posterior to anterior. The superior incision is performed next to the skull base and the inferior incision is carried along the superior border of the antrostomy. It is recommended to perform the superior incision first to minimize obstructed visualization from orbital fat prolapse into the field. Once all three cuts are completed, the posterior edge of the vertical periorbital incision is grasped with a Blakesley's forceps and gently reflected anteriorly.

An alternative opening is a horizontal incision at the level between the inferior and medial recti muscles with retraction of the periorbita superiorly and inferiorly. Each cut requires precision and care to prevent any excursion of the instruments deeper than the periorbita which may result in iatrogenic injury to the orbital contents.

We do not open the optic nerve sheath routinely due to the increased risk of CSF leak, injury to the nerve, or ophthalmic artery.

Once exposed, the periorbital fat is dissected and gently pulled away, exposing the medial and inferior recti muscles underneath. Blind dissection should be avoided; however, bipolar cautery may prove useful for fat emulsification or coagulation of any errant vessels encountered in the fat that might ooze and obscure the surgical field. If bipolar cautery of the orbital fat is not sufficient, a cotton-tip applicator can serve as an excellent retractor ( Fig. 1I ).

Final Exposure

Once the extraconal and intraconal fat is adequately reduced or resected via the surgical corridor, the neurovascular anatomy of the orbit is fully exposed ( Figs. 1 and 2 ). The principal operative corridor lies between the medial and inferior recti which can be expanded by gently retracting the muscles. Care is required to prevent disinsertion of the muscles and/or damage to the III nerve branches, as they enter the respective medial aspect of the muscles which would potentially lead to postoperative diplopia. Alternatively, an oculoplastic surgeon can help with external traction of the medial and inferior recti muscles to improve the instrumentation through this corridor. This makes the muscles thinner from the applied tension and consequently increases the space between them. This can be achieved with dissection of the bulbar conjunctiva and identification of the insertion of both muscles. A muscle hook is used to fish each muscle at its insertion in the globe and adequate tension is applied.

Fig. 2.

Macroscopic anatomical representation of the medial wall of the orbit in an anatomical specimen, right side. ( A ) The nasal cavity and its contents, as well as the medial wall of the maxillary sinus, anterior wall of the sphenoid sinus, and lamina papyracea have been removed. The medial and inferior aspects of the periorbita have been exposed from the ethmoidal arteries superiorly and medially to the infraorbital nerve inferiorly and laterally. ( B ) The periorbita has been removed at a level of the orbital apex revealing the underlying orbital fat body. The medial aspect of the optic canal has been exposed. ( C ) Exposure of the medial orbital contents after orbital fat removal. ( D ) The medial rectus muscle is retracted superiorly to enhance the medial orbital corridor through the medial and inferior rectus muscles. The optic nerve and the neurovascular structures surrounding it can be observed.

From this inferomedial perspective, the ophthalmic artery, located in most cases above the optic nerve, may be visualized as the origin point of the central retinal artery and its medial branches: medial palpebral, infratrochlear, supratrochlear, dorsal nasal, infraorbitalis, and anterior and posterior ethmoidal. The superior ophthalmic vein coalesces as several minor tributaries join in the medial orbital corner where the vein typically crosses above the optic nerve. Other key structures and relationships readily observed via EEA to the orbit including the trochlear nerve and its insertion at the superior oblique, the nasal portion of the nasociliary nerve giving rise to the anterior and posterior ethmoidal nerves, and the inferior division of the oculomotor nerve coursing along the orbital floor and innervating the inferior and medial recti. These structures can be easily dissected and seen in an anatomical specimen and rarely individualized during surgery due to the presence of intraorbital fat. Another surgical corridor can be located lateral to the inferior rectus muscle; however, this implies considerable retraction of the inferior rectus muscle superiorly and it is limited by the infraorbital canal inferiorly and laterally ( Figs. 1 and 2 ).

Reconstruction

Reconstruction of the medial orbital wall is not routinely necessary, especially for posterior dissections near the annulus of Zinn and optic canal. However, large orbital defects, particularly along the orbital floor with disruption of the periorbita, may result in significant orbital fat herniation, leading to diplopia and enophthalmos. In such cases, synthetic orbital implants or cartilage grafts are used to provide support. ⁷ The implant may be placed endonasally or a through a transconjunctival approach. If a large defect is anticipated, customized implants using the patient's computed tomography (CT) scan can be obtained for reconstruction. Even though not routinely necessary, mucosal reconstruction may be performed with a free mucosal graft to cover the exposed orbital musculature. Alternatively, a pedicled nasoseptal mucoperichondrial flap can be harvested to reconstruct the orbital wall following extensive orbit dissections. Especially in cases where orbital implants are used, the nasoseptal flap would separate the hardware from the nasal cavity decreasing the risk of infections.

Nasal Packing

At the end of the surgery, the ethmoid region is gently packed only with absorbable hemostatic material to prevent postoperative bleeding. This would help to bring the herniated fat back to the orbit and keep the middle turbinate medialized. Of note, the intranasal packing should not be occlusive, given the risk of inadvertently raising the intraocular pressure in the case of a large orbital defect. Care must be taken during packing to avoid lateralization of the middle turbinate; endoscopic verification of its final position is advised. Maintaining the middle turbinate in a medialized position is essential to prevent postoperative synechiae between the middle turbinate and lateral nasal wall which is in turn critical for long-term success on the operation.

Representative Case Review

Video 1 Surgical video shows the resection of the tumor of the illustrative case. In this case, resection of the middle turbinate was necessary to allow maneuverability as the nasal corridor was very narrow.

Download video file ^{(15.9MB, mp4)}

A 62-year-old female presented with a history of increasing proptosis on the right eye for a few months. Imaging studies showed an intraorbital, intraconal mass on the right side. An EEA was performed with gross-total resection without complications. The final pathology was cavernous hemangioma. The patient's proptosis resolved ( Figs. 3 , 4 ; Video 1 ).

Fig. 3.

Illustrative case. Preoperative magnetic resonance imaging (MRI) ( A–C ) and computerized tomography (CT) ( D–F ) showing a right intraconal enhancing orbital tumor located medial and superior to the optic canal. ( G-I ) Postoperative CT without contrast demonstrating resection of the tumor.

Fig. 4.

Illustrative case, intraoperative images. ( A ) Removal or lamina papyracea. ( B ) Opening of the periorbita. ( C ) Retraction of the medial rectus muscle superiorly. ( D ) Dissection of the tumor from the orbit and gentle retraction. ( E ) Division of the last vascular attachments.

Discussion

Overview of the Approach and Its Development

The proximity of the medial and inferior orbital wall and the orbital apex to the paranasal sinuses highlights the potential for successful application of EEA technique to operations in these anatomic regions. Since the use of the endoscope for orbital biopsy more than 30 years ago, endoscopic approach has been employed to treat a diverse range of diseases ^{8 9 10} with notable described procedures including optic and orbital wall decompression, orbital floor fractures reconstruction, medial and inferior orbital tumor resection, resection of anterior skull base or sinonasal tumors involving the orbit, and traumatic optic neuropathy refractory to steroid treatment. ^{7 11 12 13} EEA is ideally suited to medial, inferior, and orbital apex pathologies. ¹⁴ However, approaching the posterior orbit and orbital apex is challenging, given the very narrow surgical corridor and the close proximity of critical neurovascular structures in a small anatomic region with limited manuverability. ¹⁵ Successful application of EEA to the full range of accessible orbital pathologies depends on both nuanced understanding of the involved neuroanatomy and focused, guided training, ideally via cadaveric dissection. With these considerations in mind, the chief goal of the current study was to provide a practical, step-by-step guideline for trainees to use in the laboratory and in preparation for surgery, as they increase their familiarity and comfort with orbital applications of EEA techniques, as well as general skull base endoscopy, which similarly benefits from extensive rehearsal in the laboratory setting.

EEA to the orbit is a minimally invasive technique, as compared with traditional orbital surgery for endoscopically accessible lesions. ¹⁶ More specifically, transcranial supraorbital craniotomy requires longer anesthesia time and frontal lobe manipulation, both of which are associated with increased risk of postoperative complications including new neurocognitive deficits. ¹⁵ Similarly, the transcaruncular approach requires medial retraction of the globe, with associated risk of inferior rectus disinsertion, a significant disadvantage given that the transcaruncular corridor is quite limited and would be considered inadequate for tumor resection as compared with EEA for most lesions. ¹⁷ Perhaps most importantly, the core goals of any orbital operation are preservation of ocular function coupled to effective treatment outcomes which may be best realized by taking advantage of the enhanced, panoramic visualization afforded by the endoscope, as well as the substantial cosmetic and functional results, as compared with traditional transorbital, transcaruncular, or transcranial approaches. ^{14 18} The risk of CSF leak for intraconal orbital diseases that do not involve the optic sheath is low. ¹⁹ Nasal obstruction and congestion are the most common morbidity associated with the endoscopic approach, and it may require office-based debridement to improve the quality of life after surgery. ²⁰

Clinical Pearls for Endoscopic Endonasal Approach to the Orbit

Case selection, in particular with respect to the limitations of EEA, is the key factor for optimal clinical application in the setting of orbital pathology. The exact location of the optic nerve and the orbital lesions must be considered; as such, high-resolution CT and a complete orbital magnetic resonance imaging (MRI) protocol, including T1-weighted contrast-enhanced and suppressed T2-weighted sequences, especially fat-sat, are important for preoperative planning. Although orbital EEA provides excellent access to the medial and inferior orbital compartments, as well as the orbital apex and the optic canal, it is limited by the optic nerve and ophthalmic vessels, rendering it suboptimal for lesions with lateral compartment involvement. ¹⁶ As such, lesions involving the orbital compartments superior or lateral to the optic nerve require either traditional transorbital/transcranial techniques or a combined approach. ²¹ Superior orbital apex tumors are readily accessed via supraorbital craniotomy which may in turn be combined with endoscopic optic decompression and/or resection, as indicated by the pathology. ²² Lesions extending posteriorly and laterally into the suprasellar cistern or cavernous sinus frequently require an additional approach; however, a combined strategy incorporating an EEA stage substantially reduces the need for frontal lobe retraction, as well as individualization of the surgical plan. Inferiorly, the EEA is limited by the infraorbital nerve, beyond which consideration for a transconjunctival or sublabial transmaxillary approach may be required. Cases that require maxillary nerve resection due to malignant spread would similarly require a plan that can extend beyond the infraorbital foramen, while an inferior orbital mass predominantly arising lateral to the nerve may be best managed via lateral orbitotomy ( Fig. 5 ). ²³ Lesions located anterior to the posterior plane of the globe, as well as those that involve the skin or require orbit exenteration, should be approached by an anterior route or a combined strategy. ²⁴

Fig. 5.

Macroscopic anatomical representation of the orbit neurovascular structures from a lateral perspective in an anatomical specimen, right side. ( A ) The lateral osseous component of the orbit has been removed. ( B ) The lateral rectus muscle is retracted inferiorly and the lacrimal artery and nerve are retracted superiorly to show the lateral and superior contents of the orbit, which are shown with a closer view in image ( C, D ) The lateral rectus muscle is retracted superiorly to show a different perspective of the lateral and inferior contents of the orbit, which are further illustrated with a closer view in image ( E ) Note the ciliary ganglion and its sensory, sympathetic and parasympathetic roots. ( F ) Overview of the orbit contents after disinsertion and lateralization of the lateral rectus muscle. ( G ) Lateral view of the orbit with a dissector placed between lateral to the inferior rectus muscle endonasally, this corridor is very narrow and not usually employed endonasally. ( H ) Lateral view of the orbit with a dissector placed between the inferior and medial rectus muscles endonasally, which is the usual endoscopic approach to intraconal lesions.

With respect to endoscopic optimization, a 0-degree scope provides an excellent view of the medial orbit from the optic canal to the anterior medial wall. Conventional working angles are suitable across the region spanning the nasal vestibule through the posterior ethmoids, rendering the standard technique ideal for posterior medial and medial optic canal lesions. However, approaching the anterior medial wall and inferior orbit requires maxillectomy and is enhanced by the use of angled 30- and 45-degree endoscopes. Where indicated, further expanding EEA techniques via the maxillary sinus maximizes the surgical corridor, potentially facilitating the removal of larger lesions. ¹²

Tips for Prevention and Treatment of Complications

The major complications associated with orbital EEA are diplopia, enophthalmos, visual loss, CSF leak, and internal carotid artery (ICA) injury. ¹⁹ Regular use of stereotactic neuronavigation during surgery is recommended, particularly if a preoperative CT angiogram is available, as intraoperative localization of the ICA is critical to the avoidance of a rare but devastating injury, particularly for more posterior lesions. It is important to keep in mind the inferior slope of the skull base from anterior to posterior is adamant to avoid accidental transgression of the skull base. Also critical is knowledge of pertinent anatomic variations at the posterior and superior sphenoid sinus to minimize the risk of ICA injury. The degree of pneumatization within the sphenoid sinus is highly variable, and carotid or optic canal dehiscence, although rare, should always be considered and excluded prior to dissection or drilling in these regions. ²⁵ Similarly, Onodi's cells are encountered in approximately 33% of patients, resulting in a posterior ethmoid whose bony margins are intimately related to the optic nerve, ICA, and sellar floor. ²⁶ Correspondingly, careful preoperative study and intraoperative dissection is mandatory to identify any such cell, proactively protect the underlying structures, and minimize the use of aggressive osteotomies or rotational forces during posterior ethmoidectomy.

During exposure and dissection, exquisite care is recommended in manipulating the middle turbinate and overlying structures, given the proximity to the paper-thin cribriform plate above which is at risk of inadvertent fracture and associated CSF leak. Careful study of preoperative imaging is recommended, to ensure understanding the anatomic variations of the cribriform plate, with particular attention to the individuals with Keros' type-3 configurations in which the depth of the olfactory fossa is very pronounced, as they are at the highest risk iatrogenic skull base fracture during ethmoidectomy. ²⁷

Improper techniques or instrumentation during orbital EEA also increase the risk of CSF leak, emphasizing the need for deliberate preoperative study and simulation. Key pearls include careful anterior ethmoidectomy which should proceed from medial to lateral to reduce the risk of fovea ethmoidalis fracture. Similarly, twisting or rotational forces should be minimized during any maneuver near the skull base, and monopolar electrocautery along the skull base is strictly avoided. In the event of an inadvertent CSF leak, careful and definitive identification is mandatory, as is adequate treatment during the primary operation. Flow rates classified as low or very low may be amenable to mucosal free grafting; however, moderate or high flow leak most likely would require pedicled nasoseptal flap reconstruction. ²⁸

A constricted surgical space within the nasal cavity can increase morbidities of endoscopic orbital surgery. Concha bullosa, a pneumatized middle turbinate found in 34% of patients, can limit the medialization of the middle turbinate and decrease the endoscopic working space. ²⁹ This can be resolved by resecting the lateral half of the middle turbinate without disrupting the attachment to the cranial base. ³⁰ Another anatomic variant is the Haller cell, an infraorbital ethmoid cell which may extend into the maxillary sinus and block its ostium or the ethmoid infundibulum. Haller's cells can limit the passage of endoscope during an inferior orbital approach. ³¹

Other complications include nasolacrimal duct obstruction. Special care must be taken during the uncinectomy and maxillary antrostomy to avoid injury to the nasolacrimal duct by extending the resection anteriorly to the lacrimal bone. ^{23 32} Diplopia can be prevented by preserving an anterior bony strut between the medial and inferior orbital wall with a posterior length that extends approximately to the level of the natural ostium of the maxillary sinus which will prevent the eye globe from rotating medially, and by respecting the muscles during the dissection to preserve their function. ³³ Lastly, it is important to remember during ethmoidectomy that the anterior ethmoidal artery exits the orbit along the cranial base from posterior to anterior toward the cribriform plate, and it can be totally covered by bone or freely suspended in the ethmoidal labyrinth, placing it at risk of inadvertent injury.

Conclusion

EEA to the orbit represents a challenging set of techniques, particularly for trainees who are unfamiliar with nasal and paranasal sinus anatomy, as well as those who do not do routine endoscopic operations beyond the confines of the sella. Notwithstanding, orbital EEA has a range of important advantages over traditional microscopic approaches and should be considered foundational to the armamentarium of contemporary skull base surgeons. The current study provides a novel and comprehensive resource for developing comfort and competency with orbital EEA for neurosurgeons and trainees at many levels of experience. By carefully reviewing the described techniques and recommendations, in particular via cadaveric dissection, surgeons may minimize the risk of complications and expand their ability to treat orbital disease using EEA techniques. We strongly emphasize the importance of paranasal sinus anatomy, detailed study of preoperative imaging, and careful and individualized preoperative planning. In appropriately trained hands, EEA is a safe, effective, and minimally morbid technique which can be applied successfully as the primary approach for a wide range of medial and inferior orbital pathologies, as well as an important adjunct to many traditional or combined approaches.

Funding Statement

Funding The laboratory received support from a NREF grant awarded to M.P.-C., laboratory grants from Medtronic and Storz.