Comparative analysis of the anterior transpetrosal approach with the endoscopic endonasal approach to the petroclival region

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OBJECTIVE

The endoscopic endonasal approach (EEA) offers direct access to midline skull base lesions, and the anterior transpetrosal approach (ATPA) stands out as a method for granting entry into the upper and middle clival areas. This study evaluated the feasibility of performing EEA for tumors located in the petroclival region in comparison with ATPA.

METHODS

On 8 embalmed cadaver heads, EEA to the petroclival region was performed utilizing a 4-mm endoscope with either 0° or 30° lenses, and an ATPA was performed under microscopic visualization. A comparison was executed based on measurements of 5 heads (10 sides). Case illustrations were utilized to demonstrate the advantages and disadvantages of EEA and ATPA when dealing with petroclival conditions.

RESULTS

Extradurally, EEA allows direct access to the medial petrous apex, which is limited by the petrous and paraclival internal carotid artery (ICA) segments laterally. The ATPA offers direct access to the petrous apex, which is blocked by the petrous ICA and abducens nerve inferiorly. Intradurally, the EEA allows a direct view of the areas medial to the cisternal segment of cranial nerve VI with limited lateral exposure. ATPA offers excellent access to the cistern between cranial nerves III and VIII. The quantitative analysis demonstrated that the EEA corridor could be expanded laterally with an angled drill up to 1.8 times wider than the bone window between both paraclival ICA segments.

CONCLUSIONS

The midline, horizontal line of the petrous ICA segment, paraclival ICA segment, and the abducens nerve are the main landmarks used to decide which approach to the petroclival region to select. The EEA is superior to the ATPA for accessing lesions medial or caudal to the abducens nerve, such as chordomas, chondrosarcomas, and midclival meningiomas. The ATPA is superior to lesions located posterior and/or lateral to the paraclival ICA segment and lesions with extension to the middle fossa and/or infratemporal fossa. The EEA and ATPA are complementary and can be used independently or in combination with each other in order to approach complex petroclival lesions.

ABBREVIATIONSATPA = anterior transpetrosal approach; CN = cranial nerve; EEA = endoscopic endonasal approach; GSPN = greater superficial petrosal nerve; ICA = internal carotid artery; SPS = superior petrosal sinus.

OBJECTIVE

The endoscopic endonasal approach (EEA) offers direct access to midline skull base lesions, and the anterior transpetrosal approach (ATPA) stands out as a method for granting entry into the upper and middle clival areas. This study evaluated the feasibility of performing EEA for tumors located in the petroclival region in comparison with ATPA.

METHODS

On 8 embalmed cadaver heads, EEA to the petroclival region was performed utilizing a 4-mm endoscope with either 0° or 30° lenses, and an ATPA was performed under microscopic visualization. A comparison was executed based on measurements of 5 heads (10 sides). Case illustrations were utilized to demonstrate the advantages and disadvantages of EEA and ATPA when dealing with petroclival conditions.

RESULTS

Extradurally, EEA allows direct access to the medial petrous apex, which is limited by the petrous and paraclival internal carotid artery (ICA) segments laterally. The ATPA offers direct access to the petrous apex, which is blocked by the petrous ICA and abducens nerve inferiorly. Intradurally, the EEA allows a direct view of the areas medial to the cisternal segment of cranial nerve VI with limited lateral exposure. ATPA offers excellent access to the cistern between cranial nerves III and VIII. The quantitative analysis demonstrated that the EEA corridor could be expanded laterally with an angled drill up to 1.8 times wider than the bone window between both paraclival ICA segments.

CONCLUSIONS

The midline, horizontal line of the petrous ICA segment, paraclival ICA segment, and the abducens nerve are the main landmarks used to decide which approach to the petroclival region to select. The EEA is superior to the ATPA for accessing lesions medial or caudal to the abducens nerve, such as chordomas, chondrosarcomas, and midclival meningiomas. The ATPA is superior to lesions located posterior and/or lateral to the paraclival ICA segment and lesions with extension to the middle fossa and/or infratemporal fossa. The EEA and ATPA are complementary and can be used independently or in combination with each other in order to approach complex petroclival lesions.

The petroclival region is one of the most complex skull base territories. The apex of the petrous portion of the temporal bone, the posteroinferior surface of the sphenoid, and the clival process of the occipital bone converge to form the petroclival synchondrosis. Several cranial nerves (CNs) traverse the ventral cisterns on their way to pierce the dura just dorsal to the petroclival synchondrosis. This intricate anatomy and large number of key neurovascular elements in such a small area increase the complexity of approaching any lesions that arise here; these tumors tend to displace the CNs, encase the vessels of the posterior circulation, and exert mass effect on the brainstem and cerebellum. Due to these factors, resection of these lesions poses a high potential for significant postoperative morbidity.

Several methods for accessing the petroclival region have been proposed, including the anterior transpetrosal,8,12,17 retrosigmoid,17 and presigmoid retrolabyrinthine approaches. More recently, the endonasal route under endoscopic visualization has been added to the skull base surgeon's armamentarium.20 To decide which approach to use, the surgeon must consider key facts about the lesion's location and biological properties. Petroclival tumors are those that arise in the petroclival region, either extradurally or intradurally.

The anterior transpetrosal approach (ATPA) was originally designed for the treatment of aneurysms of the basilar complex.10 A temporal craniotomy is performed, followed by extradural drilling of the floor of the middle fossa and more specifically the petrous bone medial and superior to the course of the greater superficial petrosal nerve (GSPN). This creates a window through which the surgeon can access the posterior fossa after opening the dura. Although effective for the petrous apex, this technique is hindered by the lack of visualization toward the midline and inferior to the abducens nerve. The area around the central clival depression is difficult to visualize using the ATPA. Moreover, certain pathologies, such as clival meningiomas, have a tendency to displace the CNs laterally, thus positioning them directly between the surgeon's instrument and the pathology when approaching them with a lateral-to-medial trajectory, as is the case with ATPA.

The endoscopic endonasal approach (EEA) to the posterior fossa utilizes the natural corridors of the nasal cavity and paranasal sinuses to access the central skull base ventrally. By combining surgical modules in both the sagittal (transclival) and coronal (supra- and infrapetrous) planes, the surgeon has ample access to the petroclival junction. However, this technique is limited laterally by the internal carotid artery (ICA) and the abducens nerve.

The purpose of this study is to describe and compare the ATPA with the EEA for accessing the petroclival region, as well as to analyze their respective advantages, disadvantages, and applications. Consequently, the most lateral reach via EEA was measured to calculate the maximal lateral expansion of the EEA corridor in the petroclival area. There is no study in the literature that takes into account such considerations.

Methods

All anatomical dissections were performed at the Anatomical Laboratory for VisuoSpatial Innovations in Otolaryngology and Neurosurgery (ALT-VISION) at The Ohio State University. Eight embalmed human cadaveric heads were injected with red silicone through the common carotid and vertebral arteries and with blue silicone through the jugular veins. The specimens were positioned to simulate the surgical position in the operating room. The endonasal approach was performed under endoscopic visualization utilizing 18-cm-long endoscopes (4 mm diameter) with 0° and 30° lenses (Karl Storz GmbH and Co.). The endoscope was connected to a light source with a fiber optic cable and a video camera with a 3 charge-coupled device sensor (Karl Storz GmbH and Co.) and displayed on a high-definition monitor. Dissections were performed utilizing dedicated endonasal instrumentation (KLS Martin Group) and drills (Stryker-Leibinger Corp./Medtronic). The ATPA was performed under microscopic visualization (Carl Zeiss Co.) with standard craniotomy and microsurgery instruments (KLS Martin Group) and drills (Stryker-Leibinger Corp./Medtronic).

Additionally, we measured the reachable distance to the farthest lateral point from the medial line of the paraclival ICA during EEA. With a 30° angled drill (Medtronic) the petrous apex was drilled out from the EEA perspective, and the reachable distance from the medial line of paraclival ICA to the farthest accessible lateral point behind the ICA was measured. The horizontal line of the petrous ICA segment, the paraclival ICA segment, and the abducens nerves were defined as the main landmarks used in planning the approach to the petroclival region. The distance between these landmarks was measured to evaluate the working spaces. The navigation system (Stryker-Leibinger Corp.) showed the coordinates of the points and was used for measuring the distance. The distance was measured by a simple mathematical method (straight distance between 2 coordinates). Five cadaver heads (10 sides) were used to measure the distance from the medial line of the paraclival ICA to the farthest accessible lateral point after drilling.

Surgical Technique

Anterior Transpetrosal Approach

The petroclival region can be accessed from a lateral-to-medial trajectory using ATPA. This approach has a wide working angle up to 90°, thereby exposing the surgical field superior to CN VI and anterior to CNs VII and VIII. This procedure has been described in detail in previous reports8,10,12 and will be only briefly explained here. The specimen is placed in a lateral supine position and secured with 3 cranial fixation pins in a head holder. An inverted U-shaped incision is placed above the ear, and a temporalis fascia flap is raised for eventual closure. Craniotomy is then performed with its base parallel to the zygomatic arch and the floor of the middle fossa, just above the root of the zygoma and the external auditory meatus. The bone edges at the base of the craniotomy are drilled down flush with the temporal fossa floor to maximize the view. Next, the dura is carefully dissected from the petrous temporal bone following a sequence of steps.

In the first of these steps, the periosteal dural layer at the middle fossa floor is peeled cranially from the skull base. The foramen spinosum is exposed, and the middle meningeal artery is visualized, coagulated, and cut. Next, the foramen ovale is identified anteriorly in the surgical field. Care should be taken to avoid entering the interdural space or disrupting the venous pool that resides in the foramen during surgery. The sphenopetrosal groove, which is a landmark for the petrous portion of the ICA, is then located. This bony depression, along with the GSPN, serves as the anterolateral boundary of the drilling area, which is further bounded by the trigeminal impression anteromedially, the arcuate eminence posteromedially, the carotid canal inferiorly, and the internal auditory canal inferolaterally. Once drilling of the petrous pyramid is concluded, the posterior fossa dura is opened, revealing the lateral aspect of the pons and the root of the trigeminal nerve as observed in Fig. 1A. Initially, a T-shaped cut is placed over the temporal dura and along the superior petrosal sinus, exposing the edge of the tentorium. The tentorium itself is then divided until the tentorial notch is opened. This step is performed with extreme care to avoid injury to the trochlear nerve, as seen in Fig. 1B (Fig. 1C shows a higher magnification view of ATPA). Figure 1D highlights the available surgical field that is accessible with the ATPA as viewed with a 0° endoscope, demonstrating CNs III, IV, V, and VI, as well as the basilar artery. The posteroinferior view obtained with a 30° endoscope shows CNs VII and VIII entering the internal auditory canal, as well as the adjacent anterior inferior cerebellar artery (Fig. 1E). Dividing the tentorium exposes the cisternal trigeminal nerve root, which is then followed into Meckel's cave by extending the dural opening approximately 1 cm anteriorly (arrow in Fig. 1F).

FIG. 1.
FIG. 1.

Identification of anatomical landmarks on ATPA, as visualized with the microscope (A–C) and endoscope (D–F). The approach begins with craniotomy and the epidural approach to the medial middle fossa floor. A: The drilling area is outlined by the trigeminal impression anteriorly, the arcuate eminence posteriorly, the major petrosal groove laterally, the carotid canal inferiorly, and the internal auditory canal inferoposteriorly. B: After incision of the tentorium and opening the dura in the posterior fossa, the trigeminal nerve is seen in front of brainstem. C: Higher magnification is shown. The cisternal segment of the abducens nerve can be seen medial to the trigeminal nerve, passing into Dorello's canal. D: The orifice of Meckel's cave is covered by 2 thick dural folds: the petroclival fold and the petroclinoid fold (tentorial), and CN IV is seen inferior to the medial edge of the tentorium. E: A posterior endoscopic view shows the facial and vestibulocochlear nerves en route to the internal auditory canal. To expose the gasserian ganglion, the tentorium is incised posteriorly to the dural entrance of the trochlear nerve, the petroclinoid fold must be incised 1 cm anteriorly. F: After cutting the tentorium, the gasserian ganglion is exposed, and access is provided to the foramen ovale and the foramen rotundum. AICA = anterior inferior cerebral artery; Ant. = anterior; BA = basilar artery; Cv = clivus; FO = foramen ovale; Inf. = inferior; Lat. = lateral; Med. = medial; Post. = posterior; SPS = superior petrosal sinus; Sup. = superior; TE = tentorium; III = oculomotor nerve; IV = trochlear nerve; V = trigeminal nerve; VI = abducens nerve; VII = facial nerve; VIII = vestibulocochlear nerve. Figure is available in color online only.

Endoscopic Endonasal Approach

Previous reports have already described the transclival and supra- and infrapetrous modules of the endonasal approach to the petroclival region.6 Nonetheless, no detailed depiction of the combination of these endonasal modules, coupled with the skeletonization and lateral displacement of the ICA, is currently available in the literature. This maneuver aims to widen the surgical corridor and address a significant drawback of the EEA: limited access to the area located posterior and lateral to the petrous apex.

The specimen is positioned supine and secured with 3 cranial fixation pins in a head holder. The right middle turbinate is removed to increase maneuverability within the nasal pathway. The sphenoid ostium is enlarged, and a nasoseptal flap pedicled on the sphenopalatine artery contralateral to the lesion is elevated to allow skull base repair with vascularized tissue after performing the anterior transpterygoid approach. A wide sphenoidotomy and ethmoidectomies are performed, and the nasal exposure is completed with a posterior septectomy, thus creating a single, wide posterior nasal cavity. Attention is then turned to the coronal plane. The maxillary antrum ipsilateral to the lesion is entered, and its posterior wall is removed to expose the contents of the pterygopalatine fossa. Starting at the caudal border of the vomer, the basopharyngeal fascia is bluntly dissected from the roof of choana. The palatovaginal vessels and nerve are coagulated and cut, and the vidian nerve and canal are identified. These steps complete the soft-tissue dissection portion of the procedure, and drilling is performed next.

Initially, the floor of the sphenoid sinus is drilled flush with the clivus. The vidian canal is skeletonized, and the vidian nerve is followed posteriorly until the fibrous tissue that shields the anterior face of the foramen lacerum and the anterior genu of the ICA in its transition from the petrous to the paraclival segments is reached. Whenever possible, we advocate preserving the vidian nerve.15 The clivus is then thinned with the drill. Substantial bleeding may be encountered on entry to the cancellous portion of the clivus, which can be controlled with bone wax and Floseal (Baxter Inc.). The inner cortex is carefully removed with a combination of drilling, elevation with a Cottle dissector, and use of Kerrison rongeurs to avoid injury to the dura mater. Drilling the petrous bone inferior to the horizontal ICA should be performed in a caudal-to-rostral direction, with the vidian canal representing the superior limit of this portion of the dissection. The petrous apex is progressively drilled from medial to lateral until sufficient lateral exposure is achieved. This can be confirmed by intraoperative navigation. Finally, the carotid canal is skeletonized, and the bone is carefully elevated to expose the periosteum that surrounds the petrous and paraclival ICA at this level. This maneuver is critical to ensuring optimal ventral exposure of the petroclival junction, thereby enabling the surgeon to lateralize the ICA and further drill the posterior clival and petrous remnants. If additional lateral displacement of the ICA is required, the parasellar bone covering it superiorly must also be removed in similar fashion. In specific situations, especially with tumors that extend superiorly into the cavernous sinus and sellar compartment, bone removal can be expanded to encompass exposure of the para-, supra-, and intrasellar spaces. To gain entry into the interpeduncular cistern, a pituitary transposition with removal of the dorsum sellae may be performed. This technique can be employed either bilaterally or only unilaterally, in which a “hemi-transposition” of the gland grants access to the ipsilateral half of the dorsum sellae. This modification is particularly useful for accessing lesions that expand superiorly from the petroclival synchondrosis, as observed in large chordomas and some chondrosarcomas.

The clival dura is exposed completely between the paraclival segments of the ICA in order to maximize the operative space, and the initial dural incision is made at the midline to avoid injury to the abducens nerve that runs in the interdural space at the level of pons. Careful hemostasis can control bleeding from the venous plexus of the intradural space into the clival dura. The intradural operative space after this dissection is shown in Fig. 2A and under higher magnification in Fig. 2B. Figure 2C depicts the postchiasmatic space using a 30° endoscope from the inferior direction. The mammillary bodies are located on the undersurface of the brain behind the pituitary stalk in the interpeduncular fossa, representing the superior and posterior limits of this approach. The oculomotor nerve is seen from its origin at the lateral surface of the midbrain and passes between the superior cerebellar artery and posterior cerebral artery into the cavernous sinus, which represents the lateral limit of this exposure. Figure 2D demonstrates the left lateral petroclival area as viewed with the 30° endoscope, while Fig. 2F depicts a more magnified view of this region. The petrosal apex is seen posterior to the paraclival segment of the ICA in this dissection, inferior to the cavernous sinus. Figure 2E demonstrates the region visualized via EEA after lower clival dissection; CNs VII–XII are not well visualized without resection of the jugular tubercle.

FIG. 2.
FIG. 2.

Identification of anatomical landmarks on EEA utilizing 0° (A, B, and E) and 30° (C, D, and F) endoscopes. High magnification views of the pre-chiasmal space (B) and postchiasmal spaces (C) are provided. D and F: High magnification views of the left lateral petroclival area. E: Lower clival area. Caud. = caudal; IC = internal carotid artery; Med = medulla; PG = pituitary gland; Rost. = rostral; VA = vertebral artery. Figure is available in color online only.

Results

Comparison of the Anatomical Regions Accessible by EEA and ATPA

Prior to deciding which approaches to employ in order to access the petroclival region, the surgeon must first decide whether the pathology involves the epidural space, intradural space, or both. The areas made available by the EEA and the ATPA are compared in Fig. 3 both extradurally (Fig. 3A–E) and intradurally (Fig. 3F–K). The key landmarks to consider when comparing approaches to the epidural space are 1) the midline, 2) the imaginary line defining a horizontal plane at the level of the petrosal segment of the ICA, and 3) the medial side of the paraclival segment of the ICA. The key landmarks to consider when comparing approaches to the intradural space are 1) the midline, 2) the imaginary line defining a horizontal plane at the level of the petrosal segment of the ICA, 3) the medial side of the paraclival segment of the ICA, and 4) the abducens nerve.

FIG. 3.
FIG. 3.

Comparison of the petroclival regions accessible via EEA and ATPA in the epidural (A–E) and intradural (F–K) spaces. The key landmarks are the midline (i), the horizontal plane defined by the horizontal petrosal segment of the ICA (ii), the medial border of the paraclival ICA (iii), and the abducens nerve (iv). The blue regions are safely accessed by EEA, the green regions are safely accessed by both approaches, and the red regions are safely accessed by ATPA. G: The green region B2 surrounded by the dots is located beyond the abducens nerve in this view. AE = arcuate eminence; A1 = contralateral clivus; A2 = ipsilateral clivus rostral to the abducens nerve; A3 = ipsilateral clivus caudal to the abducens nerve; A4 = ipsilateral clivus rostral to the petrosal segment of the ICA; B1 = petrous apex; B2 = intradural space under the same area of A4 and the ipsilateral intradural space rostral to the petrosal segment of the ICA and medial part of the paraclival segment of the ICA; C1 = cistern posterior to the paraclival segment of the ICA (lateral aspect of brainstem); FO = foramen ovale; FS = foramen spinosum; N = nerve; 6th = abducens nerve. Figure is available in color online only.

The epidural and intradural spaces defined by the aforementioned landmarks define the distinct anatomical regions depicted throughout Fig. 3 and are colored as follows: blue areas may be more safely accessed via an EEA, green areas may be safely accessed with either approach, and red areas may be safely accessed via an ATPA. For epidural dissection, the petrous segment of the ICA defines the inferior limit of dissection, so all structures inferior to this (Fig. 3; shaded regions A2, A3, and a portion of A1) are much more safely approached via EEA. Additionally, the paraclival segment of the ICA lies directly between the surgeon and the clivus, thereby rendering the EEA a safer approach for significant extradural access to the clivus (Fig. 3; shaded region A4). The medial-most portion of the petrous apex just posterior to the paraclival segment of the ICA (Fig. 3; shaded region B1) may be reached by either approach. Intradurally, the midline defines an important limitation of ATPA, as the convexity of the brainstem precludes contralateral visualization unless the tumor has deformed the brainstem (Fig. 3; shaded region A1). In other words, the surgeon cannot “look beyond the horizon.” Besides the fact that the petrous segment of the ICA limits bony resection inferiorly via ATPA, once intradural, working medial to the petrous ICA (deep to it from the surgeon's perspective) creates an angle to “look down” (inferiorly) that is slightly inferior to the petrous ICA plane. This permits access to the cisternal regions between CN III and the CN VII/VIII complex via ATPA.8,9 By doing so, the cisternal portion of the ipsilateral abducens nerve defines the inferior-most region that can be accessed via ATPA, and microsurgery becomes increasingly difficult when approaching this limit. The area inferior to CN VI and over the midline is not well exposed by the ATPA but is clearly accessible through an EEA. Usually the tumor pushes the abducens nerve away laterally or medially and makes a corridor through which surgeons can maneuver instruments.

Quantitative Analysis of the Transclival Approach in EEA to Laterally Expand the Corridor and Access the Lateral Petroclival Lesion Using a 30° Angled Drill

The distance reachable using a 30° angled drill from the medial line of the paraclival ICA to the farthest accessible lateral point behind the ICA was measured by the navigation system (Stryker-Leibinger Corp.) (Fig. 4A). The medial line of the paraclival ICA was decided as the vertical line of the paraclival ICA when possible (Fig. 4B). The distance between the farthest lateral points was 41.2 mm (SD 3.4 mm) (Fig. 4C) (Table 1). The distance between the paraclival ICAs was 22.9 mm (SD 2.4 mm) (Fig. 4C). The distance between the lower floor of the dorsum sellae and the inferior line of the petrous ICA was 18.3 mm (SD 2.1 mm) (Fig. 4D). As the abducens nerve penetrates the meningeal dura, it enters into the interdural space proximal to Dorello's canal, which is located behind the paraclival carotid artery.22 During expansion of the corridor, attention should be paid to avoid injury of the abducens nerve. The distance between both abducens nerves at the dural entry point was 18.9 mm (SD 2.4 mm) (Fig. 4D). To avoid injury during bone work in the epidural space, it is important to know the location of Dorello's canal. When the corridor was expanded and a medial petrous apex approach was performed, the interdural segment of the abducens nerve was exposed. At the gulfar or superior petrosal part of the abducens nerve, it changes its trajectory (Fig. 4E). The abducens nerve originated from the transition between the caudal border of the pons and superior end of the pyramid of the medulla in the cisternal segment (Fig. 4F).

FIG. 4.
FIG. 4.

A: The navigation system used for measuring the distances between coordinates. The intersection between the horizontal and vertical lines in this picture shows the farthest lateral point that a 30° angled drill can access. B: The distance between the medial lines of the paraclival ICAs (white dotted lines) and the farthest point a 30° angled drill can reach (white arrow). C: The method used to measure how far lateral a 30° angled drill expands the corridor for EEA. The farthest distance (a) that a 30° angled drill can reach between lateral points. The distance (b) between the medial lines of the paraclival ICAs (white dotted line). D: Distances on the epidural view in EEA, including the distances between the inferior line of dorsum sellae and the inferior line of petrosal part of ICA (c), in panel C, and the distance between both abducens nerves at the dural entry point (white arrow), as labeled (d). The white dotted line shows the medial line of the paraclival ICA. E: A 70° angled camera shows the right petrosal apex and petroclival region in the epidural space after removing the periosteal dura. The abducens nerve pierces the meningeal dura at the entry point (white dotted circle). F: The opened meningeal dura shows that the abducens in the cisternal part pierces the meningeal dura and enters the interdural space inferior to the petrosal sinus. D = dura; PA = petrosal apex; Vo = vomer bone. Figure is available in color online only.

TABLE 1.

Maximal lateral expansion of the corridor in EEA

Type of Distance*Mean ± SD, mm
(a)41.2 ± 3.4
(b)22.9 ± 2.4
(c)18.3 ± 2.1
(d)18.9 ± 2.4
(a)/(b)1.80 ± 0.12

These distances are defined as a) the farthest distance between lateral points that a 30° angled drill can reach into the petroclival lesion, b) distance between both medial lines of the paraclival ICAs, c) distance between the inferior line of the dorsum sellae and the inferior line of the petrosal part of ICA, and d) distance between both abducens nerves at the dural entry point. (a)/(b) means that drilling by an expert with a 30° angled drill can expand the corridor almost 1.8 times wider than the bone window between both paraclival ICAs.

Case Illustrations

Case 1: ATPA for Resection of a Petroclival Meningioma

A 53-year-old female patient presented with throbbing headache, and left visual acuity deterioration was observed. MRI demonstrated a homogeneously enhancing mass centered on the upper petroclival junction with a wide-based implantation and attachment to the upper clivus and tentorium (Fig. 5A and B). An ATPA was performed. After tentorial incision near the entrance of the trochlear nerve to the tentorium, the trigeminal nerve was seen to shift inferolaterally and become partially encased in the tumor, which was attached to the upper clivus and partially calcified. The abducens nerve was observed to have been displaced inferiorly upon resection of the tumor. Gross-total tumor resection was achieved, as shown on MRI (Fig. 5C and D). The patient suffered partial lateral gaze palsy postoperatively, which completely resolved 1 month later. This tumor originated from the intradural space and was located in the B2 and C1 areas in Fig. 3. The ATPA was a suitable approach in this case.

FIG. 5.
FIG. 5.

Case 1. Contrast-enhanced T1-weighted MR images obtained in a patient harboring a petroclival meningioma treated using an ATPA. Preoperative coronal (A) and axial (B) images showing the petroclival meningioma originating from the petrous apex and attached to the tentorium. The tumor enhanced homogeneously and compressed the brainstem cavernous sinus invasion. Postoperative coronal (C) and axial (D) images demonstrating gross-total tumor resection without brain injury.

Case 2: ATPA for Resection of Clival Chordoma

A 19-year-old female patient presented with diplopia and dysphagia. Physical examination revealed partial dysfunction of right-eye abduction, swallowing impairment, and rightward tongue deviation. A head CT scan and brain MR image revealed a tumor that was mainly located in the middle to lower clivus and extended into the parapharyngeal space and invaded into the petroclival and cerebellopontine angle area intradurally, and a staged operation was planned. A presigmoid infralabyrinthine approach was first performed to resect the portion of the tumor located in the lower clivus, premedullary cistern, and parapharyngeal space. Postoperative MR images showed residual tumor in the petroclival region intradurally and at the petrous apex epidurally (Fig. 6A–C). On physical examination, right-eye abduction palsy remained, as did the swallowing and tongue motor impairments. Seven months after the first surgery, an ATPA was performed. Subtotal resection was achieved because the portion of the petrous bone infiltrated by the tumor caudal to the petrous segment of the ICA could not be visualized, though all of the visible bone was removed. MRI performed 2 months after the ATPA demonstrated subtotal tumor resection with residual tumor located in the epidural portion of the tip of petrous bone (Fig. 6D–F). Adjuvant radiotherapy was performed using the CyberKnife (Accuray), which delivered a 40-Gy dose to the tumor while limiting brainstem exposure to a maximum of 26 Gy. The patient's lower cranial nerve dysfunction has since resolved, but she still complains of diplopia. This tumor originated from the epidural area and invaded into the intradural area located in the A2, A3, A4, B1, B2, and C1 areas in Fig. 3. First, via the presigmoid infralabyrinthine approach, the tumor in A2 and A3, and second, via ATPA, the tumor in B1, B2, and C1 was removed. The residual tumor in the A4 area could not be reached by the ATPA (Fig. 6I).

FIG. 6.
FIG. 6.

Case 2. Contrast-enhanced T1-weighted MR images obtained in a patient harboring a chordoma treated in a staged fashion, first via a presigmoid infralabyrinthine approach and second via the ATPA. A–C: Preoperative images. Axial images obtained at the level of midbrain (A) and at level of the pons (B) and coronal image (C) showing the tumor located in right petroclival region and cerebellopontine angle, causing brainstem compression. D–F: Images obtained after the first stage. Axial images obtained at the level of the pons (D) and at the level of medulla (E) and coronal image (F) showing residual tumor at the petrous apex and middle clivus epidurally and in petroclival lesion intradurally. G–I: Images obtained after the second stage. Axial images at the level of midbrain (G) and at the level of pons (H) and coronal image (I) showing near-total tumor resection with residual tumor remaining in the epidural region of the medial petrous apex.

Case 3: EEA for Resection of Petroclival Meningioma

A 52-year-old female patient presented with diplopia and intermittent headaches. On physical examination, the patient exhibited impaired right-eye abduction. MRI demonstrated a homogeneously enhancing right petroclival junction mass with a wide-based clival attachment that was suggestive of meningioma (Fig. 7A–C). A transclival EEA with partial petrous apicectomy was performed. Postoperative MRI confirmed gross-total resection (Fig. 7D–E). Note the enhancing nasoseptal flap used for reconstruction. The patient presented with a CSF leak at postoperative Day 7. Operative examination revealed that this was caused by displacement of the right superior corner of the nasoseptal flap, which was successfully and permanently corrected by repositioning the flap. The patient recovered well from surgery, and extraocular movements returned to normal. This tumor originated from the intradural space and located in the B2 area (Fig. 3F–K). An ATPA would also be a reasonable approach for this case.

FIG. 7.
FIG. 7.

Case 3. Contrast-enhanced T1-weighted MR images obtained in a patient harboring a petroclival meningioma that was treated via an EEA. Preoperative coronal (A), sagittal (B), and axial (C) images demonstrating the petroclival meningioma originating from the upper clivus. The tumor enhanced homogeneously and compressed the brainstem without cavernous sinus invasion. Postoperative coronal (D), sagittal (E), and axial (F) MR images demonstrating gross-total tumor resection without brain injury. E: The white arrow shows nasoseptal flap enhancement, indicating preservation of its blood supply.

Case 4: EEA for Resection of Clival Chordoma

A 47-year-old female patient presented with minimal movement of the tongue, a frozen left eye with complete ptosis, right-sided complete hearing loss, left-sided hearing impairment, severe dysphagia, and dysarthria. Prior trans-nasal biopsy yielded a diagnosis of chordoma, and she underwent proton beam therapy. MRI demonstrated a heterogeneously enhancing mass in the middle and lower clivus that extended to the middle fossa (Fig. 8A–C). A staged, combined endonasal and open procedure was planned. The first stage, an EEA to the midline portion of the mass, was performed without complications. Postoperative MRI at the level of the pons demonstrated resection of the midline portion of the tumor, but not the lateral-most portion of the middle fossa component, thereby highlighting this limitation of the EEA (Fig. 8D–F). One week later, a left subtemporal approach was performed. Complete resection of the middle fossa component of the mass was demonstrated on MRI; however, it also revealed residual mass in the right condyle. A third stage was performed via an EEA to resect this mass, and the defect was repaired with a pedicled left temporoparietal fascial flap transposed through the pterygopalatine fossa. Somatosensory evoked potentials were monitored during all 3 surgeries and remained stable. During the third surgery, a gastrostomy tube was placed because of persistent severe dysphagia. Postoperative MRI demonstrated no residual tumor (Fig. 8J–L). The patient recovered well and had noted improvement in hearing and left-eye movement. The tracheostomy tube was removed, and she ultimately underwent adjuvant proton beam therapy. This tumor originated from epidural area, invaded into intradural area, and was located in the A1, A2, A3, A4, B1, and B2 areas and the middle fossa in Fig. 3. Via the first and second EEAs, the tumor in the A1, A2, A3, A4, B1, and B2 areas was removed. Second, via the subtemporal approach, the residual tumor situated in the middle fossa was removed. Except for the middle fossa tumor, this tumor extended widely, but all were available by EEA. EEA was suitable approach in this case.

FIG. 8.
FIG. 8.

Case 4. Contrast-enhanced T1-weighted MR images obtained in a patient harboring a chordoma that was treated in a staged fashion, first via EEA, next via a subtemporal approach, and finally with a repeat EEA. Axial images obtained at the level of pons (A) at the level of medulla (B), and coronal image (C) showing that the heterogeneously enhancing tumor located in the middle and lower clivus extended to middle fossa after the initial stage. After performing an open approach to resect the middle fossa residual tumor, axial images obtained at the level of pons (D) and at the level of medulla (E), and coronal image (F) demonstrate resection of the lateral residual tumor that was inaccessible via an EEA. Residual tumor was left in the right condyle due to patient intolerance of the protracted operative time. The left subtemporal approach was then done. Axial images obtained at the level of pons (G) and at the level of medulla (H) and coronal image (I) showing that the tumor in the middle fossa was resected totally. Third, an EEA was performed to resect the tumor in right condyle and posterior fossa. Axial images obtained at the level of the medulla (J) and at the level of medulla (K), and coronal image (L) showing no residual tumor.

Case 5: EEA for Drainage of Petrous Apex Cholesterol Granuloma

A 36-year-old male patient presented with progressive left trigeminal neuralgia. Investigation with brain MRI (Fig. 9A) and head CT (Fig. 9B) demonstrated a lesion located in the left petrous apex, directly adjacent to the ipsilateral Meckel's cave. Axial FIESTA MRI sequencing (not shown) demonstrated no evidence of vascular compression in the cisternal segment of the left trigeminal nerve, so the cholesterol granuloma was presumed to be responsible for his pain. Transclival transpetrous EEA was performed for the planned partial resection, and marsupialization was performed for permanent drainage of the cyst into the sphenoid sinus with the placement of a silastic tube into the cavity to prevent resealing of the cyst wall. Postoperative head CT confirmed drainage of the cyst and communication with the sphenoid sinus via the silastic tube (Fig. 9C and D). After 3 months, the silastic tube was removed in the clinic, and the patient's trigeminal neuralgia had resolved. This tumor originated from the epidural space and was located in the B1 area in Fig. 3. Although EEA and ATPA allow reasonable anatomically exposure in this case, EEA has the advantage of allowing direct marsupialization to the sphenoid sinus, whereas ATPA would require cyst wall removal since marsupialization of the mastoid cells is limited.

FIG. 9.
FIG. 9.

Case 5. Images obtained in a patient harboring a petrous apex cholesterol granuloma that was treated via an EEA. Axial, contrast-enhanced MR image (A) and CT scan (B) showing the heterogeneously enhancing cholesterol granuloma and tumor invasion into the petrous bone. Postoperative bone window CT (C) and soft tissue CT scan (D) demonstrating the emptying of the cyst and the presence of the intracavitary silastic tube communicating the resection cavity with the sphenoid sinus.

Discussion

Several different pathologies can afflict the petroclival region, and the surgical management depends on a relatively complex combination of anatomical and surgical considerations. The ultimate surgical goal depends on the extent of resection required and the avoidance of neural and vascular injury during resection. Perhaps the most important factor to analyze preoperatively is if a pathology's origin is extradural or intradural, even though this distinction is not always possible, as this single factor provides clues to both the pathology's etiology as well as what neural and vascular elements may be involved. Although they may share the petroclival region, each of the pathologies carries its own characteristics, including pattern of growth, vascularity, density, and degree of malignancy, thereby necessitating significantly different treatment strategies.

Extradural Tumors

The main pathologies that affect the petroclival area extradurally include cholesterol granulomas, chordomas, and chondrosarcomas.

Cholesterol Granuloma

Cholesterol granuloma is a benign entity typically located within the petrous temporal bone and may cause regional mass effect. Its surgical treatment is based on drainage with or without capsule resection, which can be performed through the ventral transsphenoidal and transclival routes.

The EEA for drainage of cholesterol granuloma can be approached by partial clivectomy via a transsphenoidal approach with or without ICA lateralization. When there is medial expansion of a cholesterol granuloma into a well-pneumatized sphenoid sinus, the surgical approach is direct with no need for ICA manipulation via the transsphenoidal approach.26 Once a wide opening is performed in the medial aspect of the cholesterol granuloma, its contents drain into the nasopharynx. Angled endoscopes may be used to visualize the most lateral components, and angled suction tips facilitate the removal of all granuloma contents as well as irrigation of the cavity.

We advocate inserting a silastic tube into the cavity, draining it into the sphenoid sinus and nasopharynx, and preventing the healing of the granuloma wall. The tube is subsequently removed at 3 to 6 months postoperatively. Extensive drilling of the petrous apex is typically not required for EEA routes, as marsupialization of this histologically benign lesion may be a more prudent, risk-minimizing treatment strategy to prevent future expansion.

ATPA for the treatment of cholesterol granulomas has the disadvantage of not creating a pathway for permanent drainage by virtue of the approach, as EEA does. Consequently, there is a need for more aggressive capsule resection in order to avoid recurrence, and further drilling of the medial skull base between V1 and V2 may be required to fenestrate the cavity to the sphenoid sinus with or without placement of the aforementioned silastic tube. Furthermore, ATPA permits drilling superior and dorsal to the petrous segment of the ICA, rendering inferiorly located cholesterol granulomas difficult to reach via this approach.

EEA is ideal for lesions that abut the lateral recesses of the sphenoid sinus; the sinus itself provides a suitable chamber into which the cholesterol granuloma contents may permanently drain. Nonetheless, it occasionally requires skeletonization and lateral displacement of the paraclival ICA. Although this may be safely achieved, doing so may unnecessarily risk ICA injury.

Clivus Chordoma and Chondrosarcoma

Chordomas and chondrosarcomas are rare, malignant, extradural-originating neoplasms of connective tissue that can occur throughout the neuraxis. However, relative to skull base surgery, a chordoma typically arises in the clivus, while a chondrosarcoma arises in the petroclival synchondrosis. Though they are histologically distinct entities, they arise in nearly the same area and have a propensity to invade adjacent structures. For chordoma, the median survival is 6.29 years with the 5-year, 10-year, and 20-year survival rates dropping precipitously to 67.6%, 39.9%, and 13.1%, respectively.25 For chondrosarcomas, the 5-year survival rate is 90% for Grade 1 lesions, 60% for Grade 2 lesions, and 30–50% for Grade 3 lesions.24 The gold-standard treatment for both of these tumors is complete excision with wide margins and postoperative radiation therapy.

Chordomas and chondrosarcomas typically arise from the midline or just adjacent to it, thereby offering a significant advantage to performing EEA on these lesions. Chordomas arise in the clivus from the notochord remnants in the spheno-occipital synchondrosis, and they may expand through the clivus to reach the occipital condyles inferolaterally and the dorsum sellae superiorly. They may expand laterally behind the ICAs and reach the petroclival area. EEA can provide access to all of these regions extradurally without crossing any critical neural structures; however, ICA skeletonization is often required for a more lateral extension. In cases in which there is significant lateral bony or middle fossa extension, a combination of approaches may be necessary for complete resection.21 ATPA does not offer adequate clival exposure, particularly in relation to the contralateral ICA, due to the presence of the paraclival segment of the ICA or the inferior portion of the clivus due to the presence of the petrous segment of the ICA. The ATPA therefore should only be used for select chordomas and, in general, in combination with other approaches.

Chondrosarcomas arise from the petroclival synchondrosis13 and can reach the sphenoid sinus, posterior fossa, middle fossa, jugular foramen, and high cervical region, particularly when they are complex and large, and all of these regions are accessible via EEA. The ATPA does not offer safe access to lesions below the internal auditory meatus and is consequently not ideal as a primary treatment modality for most of these lesions. Chondrosarcomas, and especially chordomas, have a tendency to envelop the paraclival ICA and may erode its wall, increasing the risk of vascular injury. A recently published study identified chondroid tumors as the pathology that harbors the highest risk for ICA rupture during EEA.2

Intradural Tumors

The most common pathologies that affect the intradural petroclival space are meningiomas and epidermoid/dermoid tumors.

Meningioma

Petroclival meningiomas can be classified into 2 subtypes: midline origin and lateral origin. This distinction is based on the main site of dural attachment and the pattern of CN deviation. Meningiomas that arise from the meningeal layer of the petroclival region dura easily invade Meckel's cave and the cisterns, often leaving no clear dissection plane between the tumor and CNs. This is especially true for the abducens nerve.

Petroclival meningiomas represent one of the most controversial entities with regard to approach selection, especially when deciding between an open versus endonasal route. When choosing an approach to the intradural portion of the petroclival region, the location of critical neurovascular structures relative to the lesion must be considered, including the abducens and trigeminal nerves.

The position of these CNs in relation to the tumor is key to selecting an approach. Midline-originating, midpetroclival meningiomas tend to displace the trigeminal nerve laterally and the abducens nerve inferiorly, rendering a midline approach (i.e., EEA) safer than a lateral approach, as the trigeminal nerve lies directly between the mass and the surgeon when performing ATPA. Lateral-originating meningiomas displace both the trigeminal and abducens nerves medially, rendering ATPA safer because the surgical corridor allows the surgeon to access the lesion without crossing the CNs.

When a petroclival meningioma encases these nerves, neither approach mitigates the challenge of locating and dissecting the CNs within the mass. In these instances, the use of neuromonitoring, such as using somatosensory evoked potential monitoring and Kartush dissectors (Medtronic ENT), which provide real-time feedback, is especially helpful. There is no perfect sequence of approaches to petroclival meningiomas with significant extension into regions poorly addressed by a single approach, and deciding which approach to use is first based on which approach is most appropriate to address the main component of the tumor. Indeed, the anterolateral approach may also be needed for tumors that expand into the middle fossa and sylvian fissure.

Both approaches offer distinct advantages and disadvantages with regard to the morbidity of the approach itself. The ICA crosses this region and is a source of potential morbidity regardless of the approach. However, the position of the ICA does not affect ATPA, provided the surgical landmarks for injury avoidance, such as the GSPN, are respected. During EEA, however, skeletonization of the paraclival and parasellar segments of the ICA may be required to allow its lateral mobilization and more extensive tumor resection. Although, this maneuver does not add morbidity when performed by experienced hands,7 it certainly increases operative time and complexity. Conversely, EEA does not require retraction or mobilization of the temporal lobe, which carries the inherent risk of venous congestion and later encephalomalacia. Both approaches offer straightforward access to the vascular supply of the meningiomas in this region. ATPA offers access to the tentorial artery of Bernasconi and Cassinari, while EEA provides access to the dorsal meningeal artery and its branches.

Epidermoid and Dermoid Tumors

Epidermoid and dermoid tumors located in the petroclival region lie completely within the intradural space and can be clearly separated from the surrounding meninges. They are usually soft and amenable to resection with suction. Nonetheless, encasement of the CNs and vessels of the posterior circulation is common, and gross-total removal without spillage of the tumor contents is the surgical goal. ATPA offers an effective option for their resection.18 EEA for removal of these lesions should only be attempted in cases where gross-total resection is feasible; otherwise the risk of regrowth or secondary spread of tumor remnants is high. Furthermore, due to the relatively avascular nature of these masses, residual tumor left via EEA may also carry a higher risk of postoperative infection in comparison with the relatively more sterile ATPA. Regardless, the choice of approach depends on the factors that mirror those of approaching a petroclival meningioma: namely, the lesion's position relative to the cisternal portions of CNs V and VI.

Recent Studies

Van Gompel et al. provided a volumetric study of the epidural area of the petrous apex bone by ATPA and EEA.23 They discussed the resectable bone volumes of the petrous apex for EEA and ATPA. They concluded that EEA is suitable for inferior anterior petrosectomy, and ATPA is suitable for superior anterior petrosectomy. However, this information helps to infer the resectable area but only for epidural lesions. Jacquesson et al. concluded that extradural midline tumors (chordoma, chondrosarcoma, and cystic lesion when drainage is essential) are indications for EEA, and lesions in the intradural space are not indications for EEA.4,5 While it is not mentioned in the article by Jacquesson et al.,4 we showed that EEA can address the intradural midline middle clivus area as presented in the illustrative case of the clival meningioma in Fig. 7. The article by Roth et al. showed a thoughtful and balanced general review of the anatomical limitations of EEA and each transcranial approach.16 While discussing access to the prepontine cistern and basilar artery (what they called area “D”), they did not address specific pathologies nor separate them as epidural or intradural when defining the criteria for choosing an approach. For each cranial approach, retrosigmoid approach, orbitozygomatic approach, retrolabyrinthine approach, transcochlear approach, transotic approach, and transcrusal approach, Chang et al.,1 Horgan et al.,3 Siwanuwatn et al.,19 and Little et al.11 have well studied bone resection and the working areas that access petroclival lesions. They did not discuss EEA.

Analogously, most previous studies did not take into consideration the pathologies or locations (e.g., epidural and/or intradural) when planning approaches to the petroclival area. Therefore, we used representative real cases to assist the reader when making a surgical plan concerning which route is most suitable for accessing tumors related to the petroclival region. Additionally, we would like to emphasize the importance of combined approaches by demonstrating the limitation of each single approach in our representative cases.

Lateral Access Limitations of EEA

Regarding access limitations for epidural and intradural lesions, the vertical paraclival ICA is thought to be the limitation in EEA, as shown in Fig. 3. In this figure, the petrous apex behind the vertical paraclival ICA is shown as area B1 in green. Accessing this area is feasible in both approaches.14 Currently, angled scopes and angled instruments are capable of accessing the B1 area. When planning a surgical approach, information about how far lateral EEA can reach behind the ICA is important information for defining if the EEA and ATPA combined approach is necessary.

We quantitatively analyzed how far lateral we could expand the corridor in EEA. When the corridor was expanded and the medial petrous apex approach was performed, the interdural segment of the abducens nerve was exposed. The distance between the farthest lateral points was 41.2 mm (SD 3.4 mm), labeled (a) in Fig. 4C; (see also Table 1). The distance between the paraclival ICAs was 22.9 mm (SD 2.4 mm), labeled (b) in Fig. 4C. As a result, on average, expert drilling with a 30° angled drill can expand the corridor almost 1.8 times wider than the bone window between both paraclival ICAs.

No Comparable Data in the Literature

Knowing the point where the abducens nerve pierces the dura in order to achieve its interdural position is important for avoiding injury during bone work in the epidural space. After passing through this point, the interdural or inferior petrosal part of the abducens nerve runs between the 2 layers of the dura that form the wall of the inferior petrosal sinus connected to the basilar plexus.

In the epidural space, since the abducens nerve cannot be seen, the medial line of the paraclival ICA could generally be considered the lateral limit. The dural entry point was found to be at the level of the superior surface of the petrous ICA as it becomes the lacerum segment of the ICA. The distance between both abducens nerves at the dural entry point was 18.9 mm (SD 2.4 mm), labeled (d) in Fig. 4D. This distance is important when working in intradural spaces. Therefore, we defined the abducens nerve as the critical landmark of lateral limitation.

Advantages and Disadvantages of Each Approach

In summary, an ideal surgical approach permits tumor removal while minimizing the risk of neural or vascular injury. The chief advantage of EEA over ATPA is that EEA avoids brain retraction, which is a fundamental tenet of modern skull base surgery. Additionally, angled lenses allow lateral visualization, i.e., the ability to “look around corners.” It also often enables straightforward access to the tumor's vascular supply, often during the approach, and extensive removal of infiltrated skull base bone and dura, especially for meningiomas, chordomas, and chondrosarcomas. When applying EEA to this region, venous bleeding from the interdural space, especially around the foramen ovale, is avoided, and the superior petrosal sinus does not require ligation. The sphenobasal vein runs beneath the temporal lobe, and there is a risk of injury when retracting the temporal lobe during ATPA, which is avoided with EEA. In cases in which a developed sphenobasal vein drains into the pterygoid plexus and the sylvian vein drains into it, ATPA should be avoided.

EEA harbors significant disadvantages in comparison with ATPA, in addition to the anatomical limitations described. Endonasal skull base surgery requires additional training and equipment, 2 surgeons (both an otolaryngologist and neurosurgeon), and entails a steep learning curve. Even though substantial progress has been made in this field both in terms of its efficacy as well as complication avoidance, CSF leakage can still occur, as shown in the case illustrations. The utilization of vascularized flaps has decreased the frequency of CSF leakage and established the indications of EEA for intradural lesions. Dealing with tumors with vascular encasement makes any surgery more challenging, independent of the approach. However, this issue can be especially daunting during EEA due to the narrow surgical corridor in comparison with ATPA. This problem may be resolved by the development of instruments in the near future.

We do not advocate the indiscriminate use of either approach to the petroclival region, but instead advocate that EEA is a significant advance to modern skull base surgery and should be used when it offers decreased morbidity and superior access in comparison with open approaches. Furthermore, when dealing with complex lesions involving the petroclival region, skull base surgeons have the opportunity to use both EEA and ATPA in a complementary way in order to decrease morbidity and optimize tumor resection and patient quality of life.

Conclusions

ATPA is an established technique for accessing the petroclival region through the middle fossa floor. EEA to the petroclival region is feasible, safe, effective, and avoids brain retraction or CN manipulation for appropriately selected patients in the hands of experienced skull base surgeons. The key landmarks to each approach are the midline, the horizontal plane of the petrous segment of the ICA, the paraclival segment of the ICA, and the abducens nerve.

EEA offers greater access and decreased morbidity for the resection of midline clival lesions, especially lesions medial or caudal to the abducens nerve such as chordomas, chondrosarcomas, and cholesterol granulomas in epidural lesions and midline-originating middle clival meningiomas in intradural lesions. ATPA is superior for lesions located posterior and/or lateral to the paraclival ICA segment and lesions with extension to the middle fossa and/or infratemporal fossa.

As a result of the quantitative analysis regarding how far lateral one could expand the EEA corridor behind the ICA, we found that, on average, using a 30° angled drill, it could be widened up to 1.8 times the bone window between both paraclival ICAs.

We reiterate the growing sentiment in the skull base community that the EEA merely represents an approach that may be more appropriate than traditional open approaches under select conditions, and it must be considered as an option, but not the only option, for addressing skull base lesions. Mastering open and endoscopic endonasal approaches allows the skull base surgical team to better serve their patients and minimize morbidity by offering either approach or a combination of them for complex lesions.

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Disclosures

This study was performed at the Anatomical Laboratory for Visuospatial Innovations in Otolaryngology and Neurosurgery (ALT-VISION) at The Ohio State University. The ALT-VISION lab receives educational support from the following companies: Carl Zeiss Microscopy, Integra Foundation, Intuitive Surgical Corp., KLS Martin Corp., Karl Storz Endoscopy, Leica Microscopy, Medtronic Corp., Stryker Corp., and Vycor Medical. Dr. Muto is currently engaged in a research fellowship at The Ohio State University with funding from the Strategic Young Researcher Overseas Program scholarship and postdoctoral fellowship by the Uehara Memorial foundation. Dr. Ditzel Filho is currently engaged in a clinical fellowship at The Ohio State University that is partially funded by Karl Storz Endoscopy and the KLS Martin Corporation.

Author Contributions

Conception and design: Prevedello, Muto. Acquisition of data: Muto. Analysis and interpretation of data: Muto. Drafting the article: Prevedello, Muto, Ditzel Filho, Tang, Oyama, Kerr, Yoshida. Critically revising the article: all authors. Reviewed submitted version of manuscript: Prevedello, Muto, Otto, Kawase, Carrau. Study supervision: Prevedello, Muto, Kawase, Yoshida.

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Article Information

INCLUDE WHEN CITING Published online February 5, 2016; DOI: 10.3171/2015.8.JNS15302.

Correspondence Daniel M. Prevedello, Department of Neurosurgery, The Ohio State University Wexner Medical Center, N-1049 Doan Hall, 410 W. 10th Ave., Columbus, OH 43210. email: daniel.prevedello@osumc.edu.

© AANS, except where prohibited by US copyright law.

Headings

Figures

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    Identification of anatomical landmarks on ATPA, as visualized with the microscope (A–C) and endoscope (D–F). The approach begins with craniotomy and the epidural approach to the medial middle fossa floor. A: The drilling area is outlined by the trigeminal impression anteriorly, the arcuate eminence posteriorly, the major petrosal groove laterally, the carotid canal inferiorly, and the internal auditory canal inferoposteriorly. B: After incision of the tentorium and opening the dura in the posterior fossa, the trigeminal nerve is seen in front of brainstem. C: Higher magnification is shown. The cisternal segment of the abducens nerve can be seen medial to the trigeminal nerve, passing into Dorello's canal. D: The orifice of Meckel's cave is covered by 2 thick dural folds: the petroclival fold and the petroclinoid fold (tentorial), and CN IV is seen inferior to the medial edge of the tentorium. E: A posterior endoscopic view shows the facial and vestibulocochlear nerves en route to the internal auditory canal. To expose the gasserian ganglion, the tentorium is incised posteriorly to the dural entrance of the trochlear nerve, the petroclinoid fold must be incised 1 cm anteriorly. F: After cutting the tentorium, the gasserian ganglion is exposed, and access is provided to the foramen ovale and the foramen rotundum. AICA = anterior inferior cerebral artery; Ant. = anterior; BA = basilar artery; Cv = clivus; FO = foramen ovale; Inf. = inferior; Lat. = lateral; Med. = medial; Post. = posterior; SPS = superior petrosal sinus; Sup. = superior; TE = tentorium; III = oculomotor nerve; IV = trochlear nerve; V = trigeminal nerve; VI = abducens nerve; VII = facial nerve; VIII = vestibulocochlear nerve. Figure is available in color online only.

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    Identification of anatomical landmarks on EEA utilizing 0° (A, B, and E) and 30° (C, D, and F) endoscopes. High magnification views of the pre-chiasmal space (B) and postchiasmal spaces (C) are provided. D and F: High magnification views of the left lateral petroclival area. E: Lower clival area. Caud. = caudal; IC = internal carotid artery; Med = medulla; PG = pituitary gland; Rost. = rostral; VA = vertebral artery. Figure is available in color online only.

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    Comparison of the petroclival regions accessible via EEA and ATPA in the epidural (A–E) and intradural (F–K) spaces. The key landmarks are the midline (i), the horizontal plane defined by the horizontal petrosal segment of the ICA (ii), the medial border of the paraclival ICA (iii), and the abducens nerve (iv). The blue regions are safely accessed by EEA, the green regions are safely accessed by both approaches, and the red regions are safely accessed by ATPA. G: The green region B2 surrounded by the dots is located beyond the abducens nerve in this view. AE = arcuate eminence; A1 = contralateral clivus; A2 = ipsilateral clivus rostral to the abducens nerve; A3 = ipsilateral clivus caudal to the abducens nerve; A4 = ipsilateral clivus rostral to the petrosal segment of the ICA; B1 = petrous apex; B2 = intradural space under the same area of A4 and the ipsilateral intradural space rostral to the petrosal segment of the ICA and medial part of the paraclival segment of the ICA; C1 = cistern posterior to the paraclival segment of the ICA (lateral aspect of brainstem); FO = foramen ovale; FS = foramen spinosum; N = nerve; 6th = abducens nerve. Figure is available in color online only.

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    A: The navigation system used for measuring the distances between coordinates. The intersection between the horizontal and vertical lines in this picture shows the farthest lateral point that a 30° angled drill can access. B: The distance between the medial lines of the paraclival ICAs (white dotted lines) and the farthest point a 30° angled drill can reach (white arrow). C: The method used to measure how far lateral a 30° angled drill expands the corridor for EEA. The farthest distance (a) that a 30° angled drill can reach between lateral points. The distance (b) between the medial lines of the paraclival ICAs (white dotted line). D: Distances on the epidural view in EEA, including the distances between the inferior line of dorsum sellae and the inferior line of petrosal part of ICA (c), in panel C, and the distance between both abducens nerves at the dural entry point (white arrow), as labeled (d). The white dotted line shows the medial line of the paraclival ICA. E: A 70° angled camera shows the right petrosal apex and petroclival region in the epidural space after removing the periosteal dura. The abducens nerve pierces the meningeal dura at the entry point (white dotted circle). F: The opened meningeal dura shows that the abducens in the cisternal part pierces the meningeal dura and enters the interdural space inferior to the petrosal sinus. D = dura; PA = petrosal apex; Vo = vomer bone. Figure is available in color online only.

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    Case 1. Contrast-enhanced T1-weighted MR images obtained in a patient harboring a petroclival meningioma treated using an ATPA. Preoperative coronal (A) and axial (B) images showing the petroclival meningioma originating from the petrous apex and attached to the tentorium. The tumor enhanced homogeneously and compressed the brainstem cavernous sinus invasion. Postoperative coronal (C) and axial (D) images demonstrating gross-total tumor resection without brain injury.

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    Case 2. Contrast-enhanced T1-weighted MR images obtained in a patient harboring a chordoma treated in a staged fashion, first via a presigmoid infralabyrinthine approach and second via the ATPA. A–C: Preoperative images. Axial images obtained at the level of midbrain (A) and at level of the pons (B) and coronal image (C) showing the tumor located in right petroclival region and cerebellopontine angle, causing brainstem compression. D–F: Images obtained after the first stage. Axial images obtained at the level of the pons (D) and at the level of medulla (E) and coronal image (F) showing residual tumor at the petrous apex and middle clivus epidurally and in petroclival lesion intradurally. G–I: Images obtained after the second stage. Axial images at the level of midbrain (G) and at the level of pons (H) and coronal image (I) showing near-total tumor resection with residual tumor remaining in the epidural region of the medial petrous apex.

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    Case 3. Contrast-enhanced T1-weighted MR images obtained in a patient harboring a petroclival meningioma that was treated via an EEA. Preoperative coronal (A), sagittal (B), and axial (C) images demonstrating the petroclival meningioma originating from the upper clivus. The tumor enhanced homogeneously and compressed the brainstem without cavernous sinus invasion. Postoperative coronal (D), sagittal (E), and axial (F) MR images demonstrating gross-total tumor resection without brain injury. E: The white arrow shows nasoseptal flap enhancement, indicating preservation of its blood supply.

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    Case 4. Contrast-enhanced T1-weighted MR images obtained in a patient harboring a chordoma that was treated in a staged fashion, first via EEA, next via a subtemporal approach, and finally with a repeat EEA. Axial images obtained at the level of pons (A) at the level of medulla (B), and coronal image (C) showing that the heterogeneously enhancing tumor located in the middle and lower clivus extended to middle fossa after the initial stage. After performing an open approach to resect the middle fossa residual tumor, axial images obtained at the level of pons (D) and at the level of medulla (E), and coronal image (F) demonstrate resection of the lateral residual tumor that was inaccessible via an EEA. Residual tumor was left in the right condyle due to patient intolerance of the protracted operative time. The left subtemporal approach was then done. Axial images obtained at the level of pons (G) and at the level of medulla (H) and coronal image (I) showing that the tumor in the middle fossa was resected totally. Third, an EEA was performed to resect the tumor in right condyle and posterior fossa. Axial images obtained at the level of the medulla (J) and at the level of medulla (K), and coronal image (L) showing no residual tumor.

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    Case 5. Images obtained in a patient harboring a petrous apex cholesterol granuloma that was treated via an EEA. Axial, contrast-enhanced MR image (A) and CT scan (B) showing the heterogeneously enhancing cholesterol granuloma and tumor invasion into the petrous bone. Postoperative bone window CT (C) and soft tissue CT scan (D) demonstrating the emptying of the cyst and the presence of the intracavitary silastic tube communicating the resection cavity with the sphenoid sinus.

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