Anatomical Step-by-Step Dissection of Complex Skull Base Approaches for Trainees: Surgical Anatomy of the Endoscopic Endonasal Approach to the Sellar and Parasellar Regions

Abstract

Introduction  Surgery of the sellar and parasellar regions can be challenging due to the complexity of neurovascular relationships. The main goal of this study is to develop an educational resource to help trainees understand the pertinent anatomy and procedural steps of the endoscopic endonasal approaches (EEAs) to the sellar and parasellar regions.

Methods  Ten formalin-fixed latex-injected specimens were dissected. Endoscopic endonasal transsphenoidal transsellar, transtuberculum-transplanum, and transcavernous approaches were performed by a neurosurgery trainee, under supervision from the senior authors and a PhD in anatomy with advanced neuroanatomy experience. Dissections were supplemented with representative case applications.

Results  Endoscopic endonasal transsphenoidal approaches afford excellent direct access to sellar and parasellar regions. After a wide sphenoidotomy, a limited sellar osteotomy opens the space to sellar region and medial portion of the cavernous sinus. To reach the suprasellar space (infrachiasmatic and suprachiasmatic corridors), a transplanum-prechiasmatic sulcus-transtuberculum adjunct is needed. The transcavernous approach gains access to the contents of the cavernous sinus and both medial (posterior clinoid and interpeduncular cistern) and lateral structures of the retrosellar region.

Conclusion  The anatomical understanding and technical skills required to confidently remove skull base lesions with EEAs are traditionally gained after years of specialized training. We comprehensively describe EEAs to sellar and parasellar regions for trainees to build knowledge and improve familiarity with these approaches and facilitate comprehension and learning in both the surgical anatomy laboratory and the operating room.

Introduction

The complex neurovascular anatomy of the sellar and parasellar regions has traditionally made operating in these areas very challenging. ^{1 2} Knowledge of the anatomical relationships and the surgical implications is essential for safe and effective surgery. ^{3 4 5}

Since the first attempt of resecting a sellar tumor in the early 1900s, stepwise advances in technique and technology have paved the way for safer and more effective access to the sellar and parasellar spaces. Advancements in endoscopic image quality, surgical navigation, instrumentation, skull base closure techniques, and anatomical understanding have led endoscopic endonasal approaches (EEAs) to be increasingly utilized for the surgical treatment of sellar and parasellar lesions. The endoscopic technique demands different surgical skills and a new anatomical perspective that differ from the traditional microscopic training in skull base surgery. ⁶

Although several publications describing the EEAs to these regions are available, ^{2 4 5 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25} most of them offer a general anatomical overview of the approach, without providing a modular, detailed, and step-by-step description as the images available may portray non-operative exposures and perspectives. Consequently, the main goal of this study is to develop an educational resource to help trainees understand the pertinent anatomy and procedural steps of the EEAs to the sellar and parasellar regions.

Materials and Methods

All aspects of this study were approved by our Institutional Review Board and Biospecimens Committee, as required by standard protocols (17–005898).

Ten cadaveric head specimens were dissected at the Surgical Anatomy Laboratory at our institution. All specimens were formalin-fixed and latex-injected using a six-vessel technique.

A 0-degree and 30-degree endoscope (4 mm, 18 cm, Hopkins II, Karl Storz, Tüttlingen, Germany), attached to a high-definition camera and a digital video recorder system, was used together with a complete set of instruments for endoscopic endonasal skull base surgery. Following our dissections, the specimens were 3D photo-documented with macroscopic and endoscopic techniques as previously described by our team. ²⁶ Post processing of images was acquired with Photomatix Pro 6.1.2 and Photoshop software.

Dissections were performed by one dissecting author (E.A.). Supervision was provided by the senior author (M.P.C.) and a PhD in skull-base anatomy with advanced neuroanatomy experience (L.C.P.C.L.). Each dissection was carried forward or repeated until the expected quality level was achieved so that each critical step was clearly documented.

Following successful dissection, representative cases were reviewed to emphasize basic principles of approach selection and planning.

Results

Step-by-Step Surgical Approaches

Patient Positioning and Nasal Cavity Inspection

The head was positioned approximately 10 degrees extended and turned 10 degrees toward the surgeon.

Moving the scope through the middle nasal corridor, the sphenoid ostium was identified between the superior turbinate and the nasal septum, in the posteromedial portion of the sphenoethmoidal recess ( Fig. 1A to C ).

Fig. 1.

Stepwise dissection of the endoscopic endonasal transsphenoidal transsellar approach. ( A ) The 0-degree endoscope is inserted in the nasal cavities along the middle nasal corridor. The middle nasal corridor is bounded by bulbous portion of the middle turbinate laterally and the nasal septum medially. ( B ) The middle turbinate is gently lateralized, and, proceeding posteriorly through middle nasal corridor, the superior turbinate is identified. The sphenoethmoidal recess is the space between the superior turbinate and the anterior wall of the sphenoid sinus, whose size varies according to the degree of pneumatization of the bulbous part of the superior turbinate. Superomedially to the superior turbinate, the supreme turbinate, if present, can be identified protruding into the sphenoethmoidal recess. The sphenoid ostium is localized between the superior turbinate and the nasal septum. ( C ) The posterior septal branch of the sphenopalatine artery is localized in the sphenoethmoid recess. It travels from lateral to medial between the sphenoid ostium superiorly and choana inferiorly, ultimately vascularizing the nasal septum. ( D ) The superior and supreme turbinates are removed. The rescue flap is harvested by a horizontal incision that starts in the inferior margin of the sphenoid ostium and proceeds anteriorly along the nasal septum. ( E ) The rescue flap is completed by detaching and flipping down the mucosa from the nasal septum and anterior wall of the sphenoid sinus. ( F ) The anterior wall of the sphenoid sinus up to the sphenoid floor caudally, the planum sphenoidale and the laminar portion of the superior turbinate cranially, the sphenoid rostrum medially, and the posterior ethmoid laterally. ( G ) The anterior wall of the sphenoid is removed on the other side and a selective posterior septectomy is then performed between the sphenoid rostrum and perpendicular plate of the ethmoid bone, providing communication between two nasal cavities. ( H ) The sphenoidal septa are drilled flush with the posterior wall of the sphenoid sinus. ( I ) The bony landmarks of the posterior wall of the sphenoid sinus are exposed, including the sella, LOCR, MOCR, carotid protuberance, tuberculum sella, prechiasmatic sulcus, limbus sphenoidalis, planum sphenoidalis, and optic canals. ( J ) The bone covering the pituitary gland is fractured and removed from the tuberculum sellae superiorly to the sellar floor inferiorly. The bony removal is taken bilaterally until the medial portion of the anterior wall of the cavernous sinus is exposed. A C -shaped periosteal incision based superiorly is initiated with an inferior horizontal cut, followed by bilateral vertical cuts. ( K ) The periosteum covering the pituitary gland is removed and the anterior lobe of the pituitary gland is exposed. ( L ) By dissecting the corridor between the inferior margin of the pituitary gland and sellar floor, a limited exposure of the dorsum sellae is provided. The posterior lobe of the pituitary gland is visualized just anterior to the dorsum sellae. br., branch; ICA, internal carotid artery; LOCR, lateral optic-carotid recess; MOCR, medial optic-carotid recess; post., posterior; SPA, sphenopalatine artery.

Transsphenoidal Approach

The supreme turbinate when present, and the bulbar portion of the superior turbinate were removed fully exposing the sphenoid ostium in the sphenoethmoidal recess. The anterior wall of the sphenoid sinus was opened bilaterally. Subsequently, bilateral rescue flap incisions were made in the septal mucosa, just inferior to the sphenoid ostium proceeding anteriorly, preserving the posterior septal branch of the sphenopalatine artery ( Fig. 1D–G ). The sphenoidotomy was completed by removing the sphenoid rostrum, and a limited posterior septectomy was then performed ( Fig. 1H ). The intra-sphenoidal septations were carefully drilled flush with the floor of the sella, and the bony landmarks of the posterior wall of the sphenoid sinus were identified ( Fig. 1I ).

Transsellar Approach

The sellar bone was drilled with a diamond burr and removed from the tuberculum sellae superiorly to the sellar floor inferiorly with a Kerrison rongeur, exposing the medial border of both cavernous sinuses without the need to expose the ICA. The dura of the pituitary gland was opened in a C -shaped fashion based superiorly ( Fig. 1J,K ). The dissection proceeded along the infra-hypophyseal corridor, by detaching the meningeal layer of the dura until the dorsum sellae was reached. Subsequently, the anterior lobe of the pituitary gland was gently elevated with the aid of a dissector and the anteroinferior portion of the posterior lobe came into view ( Fig. 1L ).

Prechiasmatic Sulcus Selective Removal

The prechiasmatic sulcus was identified between both optic nerve prominences. When this area of bone was removed and the underlying dura was opened, this created a window to the suprachiasmatic and infrachiasmatic corridors ( Fig. 2A–F ).

Fig. 2.

Endoscopic endonasal view of the suprachiasmatic and infrachiasmatic corridors from a selective removal of the prechiasmatic sulcus. ( A ) After the transsphenoidal approach has been finalized, with a 0-degree optic the prechiasmatic sulcus is identified on the posterior wall of the sphenoid sinus between the tuberculum sellae inferiorly and the planum sphenoidale superiorly. ( B ) A targeted osteotomy of the prechiasmatic sulcus is performed and the underlying dura is exposed. ( C ) The dura is incised and removed, exposing the chiasmatic cistern. ( D ) Within the suprasellar cistern, chiasmatic branches of the superior hypophyseal artery travel along the inferior surface of the optic chiasm and continue posteriorly and anteriorly, providing much of the vascularization of the optic chiasm and intracranial portion on the optic nerves. ( E ) View inferior to the sellar diaphragm. ( F ) The infrachiasmatic corridor is accessed and the anatomical details of the arachnoidal membranes can be observed. The basal arachnoid membrane lies on the diaphragma sellae. It limits inferiorly the suprasellar cistern and wraps the base of the pituitary stalk. The pituitary stalk and superior hypophyseal arteries can be seen. ( G ) A 30-degree optic is positioned in the preinfundibular space and pointed superiorly, elucidating the superior hypophyseal arteries as they vascularize the inferior portion of the optic chiasm. ( H ) After dissecting the cisternal arachnoid trabeculae of the suprasellar cistern posteriorly, access to the interpeduncular cistern is gained. The interpeduncular cistern is limited by the dorsum sella and mesencephalic leaf of the Liliequist membrane inferiorly, the mammillary bodies and tuber cinereum posteriorly, posterior communicating arteries and oculomotor nerves laterally, and the diencephalic leaf of the Liliequist membrane superiorly. ( I ) After the mesencephalic leaf of the Liliequist membrane and the prepontine membrane has been removed, a 45-degree optic is pointed inferiorly. This maneuver allows to see vascular structures including the distal portion of the basilar artery and basilar tip, pontine arteries, and anterior pontomesencephalic and lateral pontomesencephalic segments of the superior cerebellar arteries. ( J ) Moving along the suprachiasmatic corridor, the cistern of the lamina terminalis is accessed. ( K ) Pointing a 30-degree optic superiorly, the fronto-orbital arteries, pre-communicating tract of the anterior cerebral arteries, anterior communicating artery, post-communicating tract of the anterior cerebral arteries, and recurrent arteries of Huebner are progressively identified. ( L ) View of the third ventricle after fenestration of the lamina terminalis. A., artery; A1, pre-communicating segment of the anterior cerebral artery; A2, post-communicating segment of the anterior cerebral artery; AComm, anterior communicating artery; brs., branches; FOA, fronto-orbital artery; HeA, recurrent artery of Heubner; LOCR, lateral optic-carotid recess; MOCR, medial optic-carotid recess; N., nerve; OC, optic canal; OCh, optic chiasm; P1, pre-communicating segment of the posterior cerebral artery; P2, post-communicating segment of the posterior cerebral artery; PComm, posterior communicating artery; PSt, pituitary stalk; SCA, superior cerebral artery; SHA, superior hypophyseal artery.

Along the infrachiasmatic corridor, the suprasellar and interpeduncular cisterns were modularly accessed. The mesencephalic and diencephalic leaves of the Liliequist membrane, the tuber cinereum, and the mammillary bodies were progressively exposed ( Fig. 2G–I ).

The suprachiasmatic corridor was accessed by incising the anterior arachnoid membrane of the cistern of lamina terminalis. Proceeding between the superior surface of the optic chiasm and inferior surface of the anterior communicating artery, the lamina terminalis was opened, providing access to the third ventricle ( Fig. 2J–L ).

Transtuberculum-Prechiasmatic Sulcus-Transplanum Approach

The tuberculum sellae, prechiasmatic sulcus, and the planum sphenoidale were progressively drilled, opening a bony window limited anteriorly by the cribriform plate, posteriorly by the sella, and bilaterally the optic canal and lamina papyracea ( Fig. 3A–C ). The dura was opened, exposing the bilateral gyri recti, anterior cerebral artery complex, supra and infrachiasmatic corridors ( Fig. 3D–H ).

Fig. 3.

Endoscopic endonasal stepwise dissection of the transtuberculum transplanum approach. ( A ) The tuberculum sellae is removed. ( B-C ) Subsequently, with the aid of a 30-degree scope, the osteotomy is extended to the planum sphenoidale, exposing the underlying dura. ( D-E ) The dura is gently incised and removed, and the arachnoid membranes of the chiasmatic and suprasellar cisterns are exposed. ( F ) The dissection of the suprasellar cistern proceeds along the infrachiasmatic corridor. ( G ) Branches of the superior hypophyseal artery supplying the optic chiasm and pituitary stalk are gently dissected from the arachnoidal trabeculae and isolated. ( H ) By turning a 30-degree optic superiorly, the intracranial vascular structures exposed by the transplanum approach can be visualized, including the proximal and distal course of the fronto-orbital arteries and the anterior cerebral artery complex. A., artery; A1, pre-communicating tract of the anterior cerebral artery; A2, post-communicating tract of the anterior cerebral artery; ACF, anterior cranial fossa; AComm, anterior communicating artery; brs., branches; DSe, diaphragm sellae; FOA, fronto-orbital artery; HeA, recurrent artery of Heubner; N., nerve; OC, optic canal; OCh, optic chiasm; oICA, ophthalmic segment of the internal carotid artery; PSt, pituitary stalk; SHA, superior hypophyseal artery.

Transcavernous Approach

Medial Compartment Access and Posterior Clinoidectomy

The sellar craniectomy was laterally extended with a diamond drill and Kerrison rongeurs toward the anterior portion of the carotid prominence to expose the anterior wall of the cavernous sinus. The inferior and superior intercavernous sinus were also identified in the superior and inferior limits of the sellar prominence. To gain access to the cavernous sinus, a midline interdural incision was made at the level of the inferior intercavernous sinus. Using a 90-degree feather knife (Mizuho) - which has a sharp cutting edge toward the surgeon and a gentle non-sharp tip—the incision was extended laterally toward the medial wall of the cavernous internal carotid artery (ICA) and superiorly toward the roof of the cavernous sinus. The periosteal layer of the dura forming the anterior wall of the cavernous sinus was retracted ( Fig. 4A,B ). As the dissection proceeded posteriorly, the inferior parasellar ligaments – which can be seen traveling from the medial wall of the cavernous ICA to the capsule surrounding the pituitary gland – and the meningohypophyseal trunk were identified ( Fig. 4C ; Fig. 5A–F ). The inferior hypophyseal artery was divided. The cavernous sinus dissection proceeded, leaving the medial wall of the cavernous sinus attached to the pituitary gland (meningeal layer). The pituitary gland was superomedially transposed with an interdural technique, providing access to the dorsum sellae and ipsilateral posterior clinoid process, just medial to the nerves lying on the lateral wall of the cavernous sinus ( Fig. 4D ).

Fig. 4.

Endoscopic endonasal stepwise dissection of the transcavernous approach: medial compartment and posterior clinoidectomy. ( A ) After a sellar osteotomy has been performed, the craniectomy is extended laterally toward the carotid prominence, exposing the anterior wall of the cavernous sinus. The osteotomy is expanded inferiorly to the most inferior aspect of the anterior wall of the cavernous sinus, exposing the point where the lacerum ICA enters the cavernous sinus to become the posterior vertical segment of the cavernous ICA. ( B ) The periosteal layer of the inferior intercavernous sinus is incised from the midline moving laterally to the anterior wall of the cavernous sinus. In this way the cavernous sinus is indirectly reached and exposed, reducing the risk of a direct damage to the cavernous ICA and its intracavernous branches. ( C ) The dissection proceeds along the medial compartment of the cavernous sinus, leaving the medial wall of the cavernous sinus attached to the lateral margin of the pituitary gland. The pituitary gland is gently transposed superomedially with an interdural technique, showing the anterior McConnell's capsular artery and inferolateral trunk. The anterior McConnell's capsular artery and inferior hypophyseal artery are sacrificed, allowing for better transposition of the pituitary. ( D ) The posterior clinoid and dorsum sellae are identified at the posterior limit of the medial compartment of the cavernous sinus. ( E ) The interclinoid ligament is identified, attaching on the anterior surface of the posterior clinoid. The interclinoid ligament connects the anterior and posterior clinoid processes and delimits superolaterally the cavernous sinus. It forms the anteromedial limit of the oculomotor triangle and is a reliable landmark to identify the course of the oculomotor nerve in the roof of the cavernous sinus. ( F ) The base of the posterior clinoid is drilled out, showing the clival dura. ( G ) The posterior petroclinoid ligament extends between the posterior clinoid process and petrous apex, anchoring the posterior clinoid process to the dura of the posterior cranial fossa, delimiting the cavernous sinus posteriorly. ( H ) After the superomedial transposition of the pituitary gland is maximized, with the aid of a dissector the posterior clinoid is fractured from the dorsum sellae and detached from the clival dura. ( I ) The dura of the posterior compartment of the cavernous sinus and of the space behind the posterior genu of the ICA are completely exposed. ( J ) The dura is removed, exposing the lateral recess of the interpeduncular cistern. The oculomotor and trochlear nerves are identified before entering the roof and the posterior wall of the cavernous sinus, respectively. The posterior communicating artery travels with oculomotor nerve in the lateral recess of the interpeduncular cistern. A., artery; ant., anterior; caps., capsular; cICA, cavernous segment of the internal carotid artery; comp., compartment; CS, cavernous sinus; DSe, diaphragm sellae; ICS, intercavernous sinus; IHA, inferior hypophyseal artery; horiz., horizontal; lig., ligament; ligs., ligaments; LOCR, lateral optic-carotid recess; N., nerve; oICA, ophthalmic segment of the internal carotid artery; PComm, posterior communicating artery; post., posterior; posterosup., posterosuperior; sup., superior; seg., segment; vert., vertical.

Fig. 5.

Anatomical overview of the endoscopic endonasal anatomy of the transsellar approach and transcavernous approach to the medial compartment of the cavernous sinus. ( A ) Transsellar approach with lateral extension of the osteotomy and exposure of the medial compartment of the cavernous sinus. ( B ) Medial transcavernous corridor, with a general view of the inferior parasellar ligaments and inferior hypophyseal artery. ( C ) Removal of the carotid protuberance and exposure of the medial compartment of the cavernous sinus. ( D ) Anatomy of the meningohypophyseal trunk. The origin of the meningohypophyseal trunk is localized at the medial aspect of the posterior genu of the ICA. The meningohypophyseal trunk typically gives rise to three branches: the inferior hypophyseal artery, which runs medially to the posterior lobe of the pituitary gland; the marginal tentorial artery (or artery of Bernasconi-Cassinari), which travels laterally toward the tentorium; and the dorsal meningeal artery, which goes posteriorly through the cavernous sinus. In this case an additional clival branch descending along the dorsum sellae arises from the meningodorsal trunk. A., artery; ant., anterior; caps., capsular; cICA, cavernous segment of the internal carotid artery; CS, cavernous sinus; DSe, diaphragm sellae; horiz., horizontal; IHA, inferior hypophyseal artery; inf., inferior; ligs., ligaments; MHT, meningohypophyseal trunk; post., posterior; seg., segment.

The dural attachments to the posterior clinoid, namely the interclinoid and posterior petroclinoid ligaments, were identified ( Fig. 4E–G ). The base and medial limits of the posterior clinoid were drilled out, disconnecting it from the dorsum sellae. With the aid of a blunt dissector, the posterior clinoid was detached from its posterior dural attachments and ligaments (posterior petroclinoid, and interclinoid ligaments). The posterior clinoid was finally removed resulting in a complete unilateral posterior clinoidectomy ( Fig. 4H,I ). The dura covered by the posterior clinoid was opened, providing access to the lateral recess of the interpeduncular cistern. In this case, the posterior communicating artery was identified at its origin from the ICA, and the oculomotor and trochlear nerves were visualized as they enter the cavernous sinus ( Fig. 4J ).

Lateral Compartment

Once the medial aspect of the cavernous sinus was exposed reaching the area lateral to the ICA, the lateral wall of the sphenoid sinus lateral to the ICA and the medial aspect of the superior orbital fissure were removed with a diamond drill and Kerrison rongeurs. Using a feather knife, the incision along the anterior wall of the cavernous sinus was extended superolaterally toward the roof of the cavernous sinus and inferolaterally up to the inferior limit of the superior orbital fissure. The periosteal layer of the dura forming the anterior wall of the cavernous sinus was retracted and removed laterally to the cavernous ICA ( Fig. 6A,B ).

Fig. 6.

Endoscopic endonasal stepwise dissection of the transcavernous approach: lateral compartment and petrous apex approach. ( A ) The bone covering the carotid prominence and the lateral wall of the sphenoid sinus are removed and the anterior wall of the cavernous sinus and superior orbital fissure come into view. The osteotomy is extended inferomedially exposing the ipsilateral half of the mid clival dura. The incision of the periosteal layer starts at the level of the ipsilateral half of the inferior intercavernous sinus and is extended laterally, superiorly, and inferiorly. ( B ) The anterior wall of the cavernous sinus is removed, exposing the lateral wall of the cavernous sinus and the cavernous segment of the ICA. The sympathetic branch travels from the wall of the posterior vertical segment of the cavernous ICA and reaches the abducens nerve. The lingual process of the sphenoid bone develops laterally to the ICA and limits the view of the lateral wall of the cavernous sinus. The abducens nerve is localized while reaching the superior orbital fissure. ( C ) Gardner's triangle (green dashed triangle) is bounded superiorly by the abducens nerve, anteriorly by the paraclival ICA, and inferiorly by the petroclival synchondrosis. This triangle provides access to the petrous apex. With the aid of a dissector, the posterior vertical segment of the paraclival ICA is gently lateralized, exposing the petrosal process of the sphenoid bone. The petrosal process of the sphenoid bone is a bone protrusion of the basisphenoid that articulates the petrous apex at the level of the petroclival junction. ( D ) The petrosal process of the sphenoid bone is drilled out. With the aid of a 30-degree scope, the point where the abducens nerve pierces the dura mater of the mid clivus is localized. The abducens nerve is directed anteriorly, superiorly, and laterally, and it crosses the posterior vertical segment of the cavernous ICA in the cavernous sinus. The tip of the lingual process is removed with a drill, enhancing the view of the inferolateral trunk. ( E ) The dissector is placed in between the cavernous ICA and abducens nerve, and the cavernous ICA is medialized. In this way, the ophthalmic and maxillary nerves come into view. The origin of the inferolateral trunk is identified in the lateral aspect of the horizontal segment of the cavernous ICA. The inferolateral trunk runs above the abducens nerve and turns inferiorly passing laterally to the oculomotor nerve and medially to the ophthalmic nerve. ( F ) Moving the 30-degree scope superolaterally, the relationship between the oculomotor, trochlear, and ophthalmic nerves is better analyzed. ( G ) The four branches of the inferolateral trunk are visualized: the superior branch, which supplies the roof of the cavernous sinus, the oculomotor and trochlear nerves; the anterior branch, which supplies the abducens nerve in its intracavernous course and passes toward the superior orbital fissure vascularizing the cranial nerves along their intraorbital course, including the oculomotor, trochlear, ophthalmic, and abducens nerves; the posterior branch, generally dividing in a posteromedial branch, that passes toward the foramen rotundum and supplies the maxillary nerve, and a posterolateral branch, that passes toward the foramen ovale and supplies the mandibular nerve. ( H ) With the aid of a dissector, the maxillary nerve is carefully lateralized. The dissection proceeds along the inferior portion of the lateral wall of the cavernous sinus. ( I ) The posterior vertical segment of the cavernous ICA is medialized and the entry points of the abducens nerve through the dura of the mid clivus and the posterior wall of the cavernous sinus are elucidated. The petrolingual ligament, running laterally to the lacerum ICA, connects the lingual process of the sphenoid bone anteriorly to the petrous apex posteriorly. ( J ) The remaining portion of the petrous apex is removed, exposing the trigeminal nerve within Meckel's cave and porus trigeminus ( green dashed circle ). Ant., anterior; br, branch; cICA, cavernous segment of the internal carotid artery; CS, cavernous sinus; ILT, inferolateral trunk; inf., inferior; N., nerve; OC, optic canal; post., posterior; posterolat., posterolateral; posteromed., posteromedial; sup., superior.

To extend the approach to the inferior aspect of the cavernous sinus, the floor of the sphenoid sinus and clival recess were drilled, until the lacerum ICA came into view. Periosteum of the mid clivus was exposed and removed, revealing the underlying dura. The point where the abducens nerve enters Dorello's canal was visible after removing the periosteal layer of the clival dura. The oculomotor nerve, trochlear nerve, and ophthalmic nerve, were progressively identified along the lateral wall of the cavernous sinus. The sympathetic branch arising from ICA was seen just inferior to the abducens nerve and the abducens nerve was located in the cavernous sinus at the inferior level of the anterior genu of the cavernous ICA ( Fig. 7 ). The inferolateral trunk, arising from the lateral surface of the cavernous ICA, came into view ( Fig. 6B ).

Fig. 7.

Anatomical overview of the lateral wall of the cavernous sinus. ( A–C ) View after the endoscopic endonasal transsellar and transcavernous approaches. ( D–F ) Macroscopic view of the compartments of the cavernous sinus. ( D ) The anteroinferior compartment of the cavernous sinus, as defined by the area surrounding the horizontal portion of the cavernous ICA and anterior genu, is crossed by the sympathetic branch and distal portion of the abducens nerve. ( E ) By removing the pituitary gland, the medial compartment of the cavernous sinus is exposed. ( F ) The cavernous ICA is removed. The posterosuperior compartment of the cavernous sinus is located posterior to the posterior genu and vertical segment of the cavernous ICA, and superiorly to the horizontal segment of the cavernous ICA. This contains the abducens nerve passing through Dorello's canal, and the meningohypophyseal trunk. The relationship between the cranial nerves and the inferolateral trunk with its terminal branches is visualized. 1: optic nerve, 2: ophthalmic artery, 3: ophthalmic segment of the internal carotid artery, 4: pituitary gland, 5: oculomotor nerve, 6: abducens nerve, 7: distal vertical portion of the cavernous segment of internal carotid artery, 8: anterior bend of the cavernous segment of internal carotid artery, 9: horizontal portion of the cavernous segment of internal carotid artery, 10: ophthalmic nerve, 11: proximal vertical portion of the cavernous segment of internal carotid artery, 12: posteromedial branch of the inferolateral trunk, 13: posterolateral branch of the inferolateral trunk, 14: sympathetic branch, 15: lacerum segment of the internal carotid artery, 16: posterior bend of the cavernous segment of internal carotid artery, 17: pituitary stalk, 18: meningohypophyseal trunk, 19: anterior branch of the inferolateral trunk, 20: superior branch of the inferolateral trunk, 21: marginal tentorial artery, 22: dorsal meningeal artery. A., artery; br., branch; cICA, cavernous segment of the internal carotid artery; horiz., horizontal; IHA, inferior hypophyseal artery; ILT, inferolateral trunk; lICA, lacerum segment of the internal carotid artery; N., nerve; post., posterior; SHA, superior hypophyseal artery seg., segment.

The proximal vertical segment of the cavernous ICA was gently lateralized exposing the petrosal process of the sphenoid bone, which was drilled out to better visualize the abducens nerve ( Fig. 6C ).

The corridor between the cavernous ICA and the abducens nerve was gently exposed by retracting the ICA medially, and the origin of the inferolateral trunk and its main branches were identified ( Fig. 6D,E ).

A corridor through the posterosuperior portion of the cavernous sinus was outlined by passing above the inferolateral trunk and the abducens nerve. The interclinoid ligament, which represents the medial border of the oculomotor triangle, came into view. The trochlear nerve was identified above the ophthalmic nerve, inferior to the oculomotor nerve, and superolateral to the abducens nerve ( Figs. 6F and 7 ).

The triangle formed by the maxillary nerve inferiorly, the ophthalmic nerve and abducens nerve superiorly, and the inferomedial wall of Meckel's cave laterally was identified and accessed ( Fig. 6G ). The inferomedial wall of Meckel's cave was gently lateralized and the corridor through the posteroinferior cavernous sinus was accessed ( Fig. 6H ). By gently medializing the proximal vertical segment of the cavernous ICA the lingual process of the sphenoid and a portion of the petrous apex were exposed and drilled out. The abducens nerve as it emerges from the dura mater of the mid clivus, the inferior petrosal sinus, and the trigeminal nerve were identified ( Fig. 6I ). The three divisions of the trigeminal nerve were visualized ( Fig. 6J ).

Representative Case Review

Case 1: Pituitary Adenoma with Suprasellar Extension

A 27-year-old male presented after collapsing at work. His symptoms included headaches and blurred vision. A computed tomography (CT) and magnetic resonance imaging (MRI) showed a large sellar and suprasellar mass causing hydrocephalus ( Fig. 8A ). He was brought to the operating room (OR) for an external ventricular drain placement with septostomy to communicate both ventricles. Preoperative laboratories showed panhypopituitarism with mildly elevated prolactin. He was brought to the OR the following day for endoscopic endonasal resection of tumor. A transsellar approach with a transtuberculum and suprachiasmatic bony extension was utilized. The increased bony opening allowed for greater maneuverability. Reconstruction was performed with an inlay collagen graft and free mucosal graft. Gross total resection of the lesion was achieved ( Fig. 8B,C ). Histopathological examination revealed non-functioning pituitary adenoma. The external ventricular drain was progressively weaned and the patient was discharged home on postoperative day 7. His vision improved and he remained on pituitary hormone replacement.

Fig. 8.

Illustrative cases. ( A – case 1) Preoperative contrast-enhanced T1-weighted MRI in the sagittal plane demonstrates a large, heterogeneous, vividly enhancing sellar and suprasellar mass, most consistent with pituitary adenoma. ( B – case 1) Comparable postoperative contrast-enhanced T1-weighted MRI in the sagittal plane confirm gross total resection of the lesion. ( C – case 1) Intraoperative view of the endoscopic endonasal transsellar approach with the suprachiasmatic extension before dural opening. The bone of the medial optic canals, prechiasmatic sulcus, and tuberculum was removed for better maneuverability although the dural opening was purely sellar. ( D – case 2) Preoperative T1-weighted MRI in the sagittal plane shows a lesion in the left cavernous sinus extending posteriorly into the Meckel's cave. ( E,F – case 2) Intraoperative view of the endoscopic endonasal transsellar transcavernous approach with progressive exposure of the lesion. ( G – case 3) Preoperative contrast-enhanced T1-weighted MRI in the sagittal plane shows a lesion of the planum sphenoidale and tuberculum sellae, radiologically consistent with a meningioma. ( H – case 3) Comparable postoperative contrast-enhanced T1-weighted MRI in the sagittal plane confirms gross total resection of the lesion. ( I – case 3) Intraoperative view of the endoscopic endonasal transtuberculum transplanum approach. ( J – case 4) Preoperative T1-weighted coronal MRI identifies a cystic lesion of the suprasellar space compressing the optic chiasm. ( K – case 4) Postoperative T1-weighted axial MRI confirmed gross total resection of the lesion. ( L – case 4) Intraoperative view of the endoscopic endonasal transtuberculum approach showing the cystic tumor located in the infrachiasmatic corridor before resection. ACA, anterior cerebral artery; cICA, cavernous segment of the internal carotid artery; CS, cavernous sinus; OC, optic canal; PSt, pituitary stalk.

Case 2: Cavernous Sinus Meningioma Extending into Meckel's Cave

A 77-year-old man with a past medical history of follicular lymphoma presented with short-term memory deficits and intermittent double vision lasting for 2 months. Neurological exam was unremarkable. MRI of the brain revealed a lesion in the left cavernous sinus extending posteriorly into the Meckel's cave ( Fig. 8D ). Given the imaging findings and the patient's history of lymphoma, leading differential diagnosis included meningioma and lymphoma. A biopsy was indicated to better direct treatment. After initial incision of the inferior intercavernous sinus in the midline, the incision was extended laterally to the anterior wall of the cavernous sinus using a feather knife ( Fig. 8E,F ). The lesion was exposed, successfully biopsied, and histopathological exam was consistent with a grade 1 meningioma. The postoperative course was uneventful and radiation therapy was administered.

Case 3: Tuberculum and Planum Meningioma

A 39-year-old female was found to have a progressively enlarging lesion of the planum and tuberculum that was radiologically consistent with a meningioma ( Fig. 8G ). She underwent gross total resection of her lesion with an endoscopic endonasal transtuberculum transplanum approach. Intraoperative imaging after gross total resection showed the resection cavity with the fronto-orbital arteries, anterior cerebral arteries, and anterior communicating artery in the suprachiasmatic corridor ( Fig. 8H, I ).

Case 4: Infrachiasmatic Craniopharyngioma

A 68-year-old woman presented with progressive visual loss. Examination showed a cystic lesion compressing the optic chiasm ( Fig. 8J ). Visual field tests showed dense bitemporal hemianopsia and hormone levels were all within normal range. She underwent an endoscopic endonasal transtuberculum approach with prechiasmatic sulcus extension resulting in a subtotal resection with preservation of the pituitary stalk ( Fig. 8K ). The pathology results were diagnostic of craniopharyngioma. After dural opening, a cystic tumor located in the infrachiasmatic corridor was readily visible ( Fig. 8L ). The patient's vision progressively improved and pituitary function was preserved.

Discussion

The advent of EEAs has revolutionized surgery of the sellar and parasellar skull base. When compared with transcranial approaches, EEAs offer a direct medial and inferior perspective to most tumors. ^{27 28} Additionally, as surgical techniques and technologies in EEAs continue to improve, a wider array of lesions, can be surgically addressed. ^{6 29} In our study, we accessed all three cranial fossae as we modularly removed bone around the sphenoid, sella, and parasellar spaces. Versatility of EEAs, surgical corridors unimpeded by major neurovascular structures, obviation of brain retraction, and unparalleled illumination of the surgical field, make them a paramount component of any contemporary skull-base surgeon's armamentarium.

Bony landmarks of the posterior wall of the sphenoid, namely the medial optic-carotid recess (MOCR), lateral optic-carotid recess (LOCR), sella, carotid protuberance, prechiasmatic sulcus, and clival recess, define the selective osteotomies required to address lesions in the sellar and parasellar spaces. ^{30 31 32 33 34 35} Additionally, the relationships between the endoscopic anatomy and transcranial anatomy of these landmarks must be understood. ³⁶ The MOCR's intracranial correlate is the lateral margin of the tuberculum sellae. Understanding how to remove and manipulate the bone of tuberculum sellae is the key to tackle tuberculum sellae meningiomas and other lesions in this region. ^{31 32} Further, LOCR marks the superomedial boundary of the superior orbital fissure, a useful landmark when operating on lesions that involve both sella and the orbit.

The first application of endoscopic endonasal transsphenoidal surgery was to remove a purely sellar pituitary adenoma in 1992. ³⁷ Still, the main utility of transsphenoidal EEA is for pathology involving the sella. ^{37 38} To address sellar lesions, mainly pituitary adenomas and Rathke's cleft cysts, the transsellar approach should be utilized. ³⁹ However, through a lateral extension of the sellar osteotomy, the transsellar approach can also open a direct surgical corridor to the medial portion of the cavernous sinus, useful for the surgical treatment of lesions extending laterally and posteriorly to the cavernous ICA. ^{39 40} We usually open the dura of the sella starting in the inferior half of the sella, we believe this has a higher chance to preserve the arachnoid diaphragm as it many times folds inferiorly over the sellar area, decreasing the risk of postoperative cerebrospinal fluid leakage. For many lesions, however, a purely transsellar approach is not feasible as they invade other surrounding anatomical areas.

To effectively address lesions involving anatomy superior and anterior to the sella, adjuncts including the transtuberculum and transplanum approaches are used. ^{41 42} The limited transtuberculum approach is essentially a superior extension of the transsellar approach and should be considered for lesions with a limited infrachiasmatic extension, such as pituitary adenomas extending into the suprasellar cistern (case 1). ^{41 43} If lesions have a marked suprasellar spread – including large pituitary macroadenomas, craniopharyngiomas, and tuberculum-planum meningiomas (Cases 3, 4) – extension through the prechiasmatic sulcus is paramount, with consideration of a transplanum adjunct. ⁴⁴ A key aspect in affording adequate visualization of neurovascular structures in any of the aforementioned suprasellar extensions is the removal of prechiasmatic sulcus, whose description has been often neglected in the anatomical description of approaches, and often included in the description of transtuberculum or transplanum approaches. The prechiasmatic sulcus exposure constitutes the main window to the suprasellar extensions. It must be noted that the relationship of the optic chiasm to the prechiasmatic sulcus affects the suprasellar window gained through an EEA. A prefixed chiasm limits the access to suprachiasmatic corridor and a postfixed chiasm may restrict access to retrochiasmatic area. ^{45 46}

EEA access to the cavernous sinus encounters the ICA medially as opposed to their intracranial counterpart approaches. Anteriorly, the cavernous sinuses extend to the SOF and anterior clinoid process, and posteroinferiorly they extend toward the petrous apex. ⁴⁷ Critical neurovascular structures fully located inside the cavernous sinus include the cavernous ICA, its branches (the meningohypophyseal trunk, inferolateral trunk, and McConnell's capsular arteries), and the abducens nerve. From superior to inferior the oculomotor nerve, trochlear nerve, and the ophthalmic nerve are in the lateral wall of the cavernous sinus, while the maxillary nerve is located inferiorly, separated from the superior orbital fissure by the maxillary strut ( Fig. 8A–C ). ¹⁰ It is critical to realize that patients anatomy may differ, and by performing an intradural pituitary transposition one may not simply encounter the inferior hypophyseal artery; other arteries must be managed by bipolar cauterization and sharp bisection, such as the McConnell's capsular arteries.

There are two main classifications of the anatomical compartments of the cavernous sinus, both based on the course of cavernous ICA. One by Harris and Rhoton in 1976, ⁴⁸ describing three cavernous sinus compartments (posterosuperior, anteroinferior, and medial), and one by Fernandez-Miranda et al in 2018, ¹⁰ created using observations made in pituitary surgery, that divides the cavernous sinus into four portions (superior, posterior, inferior, and lateral). Harris and Rhoton did not include a lateral cavernous sinus compartment as they suggested “the lateral space” (of the cavernous sinus) is so narrow that the sixth nerve which passes through it is adherent to the ICA on its medial side and to the sinus wall on its lateral side.” ⁴⁸ As observed during our dissections, the lateral compartment is a virtual space, but it is often affected and enlarged by pathologies in the cavernous sinus, for instance, pituitary macroadenomas and meningiomas. When the lateral aspect of the cavernous sinus is exposed during surgery, extreme attention must be paid to the medial retraction to the ICA to avoid inadvertent avulsion of the inferolateral trunk and ICA injury. In between the medial wall of the cavernous ICA and the pituitary gland, a venous complex of variable width houses the inferior parasellar ligaments that stabilize the pituitary gland as well as the inferior hypophyseal artery, whose inadvertent avulsion can cause an ICA injury. This is the reason why the interdural transposition of the pituitary gland to access the posterior clinoids involves coagulation and division of the inferior hypophyseal artery ( Fig. 8D–F ). ⁴⁸

The anterior and inferior intercavernous sinuses represent a direct connection between the bilateral cavernous sinuses. As recently described, ⁴⁹ the inferior intercavernous sinus can be used as an initial entry point to gain access to the cavernous sinus. The incision of the periosteal layer of the inferior intercavernous sinus from the midline is extended laterally to the anterior wall of the cavernous sinus with feather knife, which is a blunt-tip 90-degree blade (Mizuho). This technique provides a theoretically safer access to the cavernous sinus avoiding an initial sharp incision medial to the cavernous ICA ( Fig. 4A,B ).

The endonasal access has become an increasingly popular route to reach deep seated lesions of the retrosellar space and the interpeduncular cistern. Regarding EEAs, the transcavernous approach has been coupled with techniques such as pituitary transposition and selective removal of the superior clivus to reach lesions in the aforementioned areas. ²³

There are two main techniques for hypophyseal transposition with inferior hypophyseal artery sacrifice: an intradural method that follows a plane between the gland and the medial wall of the cavernous sinus, and an interdural method, in which the medial wall of the cavernous sinus is left attached to the pituitary gland. The interdural transposition technique reduces the risk of damage to the pituitary and offers a wide exposure of the posterior region of the cavernous sinus ²³ ( Fig. 4E,F ). ²³ Additionally, the interdural approach offers a wider opening of the retrosellar regions and should be considered for superiorly extending lesions of the clivus. ²³ In contrast, the purely intradural transposition as described by Kassam et al ⁵⁰ offers a more direct route to the retroinfundibular region, but a smaller opening toward the lateral portion of retrosellar space, which is why it is especially useful for retroinfundibular craniopharyngiomas and pituicytomas.

Understanding the complexity of anatomy in the sellar and parasellar space, along with endoscopic technique training is essential to successfully perform surgeries in this area. The step-by-step description of the EEAs to the sellar and parasellar regions, provided in a didactic way, intends to guide and facilitate both learning and understanding of the indications, nuances, and the most relevant anatomical structures involved in these approaches.

Conclusion

Despite the wide utility of EEAs for surgery of lesions involving the cranial base, the anatomical understanding and technical skills required to confidently tackle these lesions are traditionally gained after years of specialized training. To facilitate the understanding and applications of EEAs for trainees, we provide a comprehensive guide to the basic concepts and nuances of EEAs to the sellar and parasellar regions.