Endoscopic Subtemporal Surgery: Chapter 10


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Endoscopic Skull Base Surgery Chap. 10: The Fully Endoscopic Subtemporal Approach
By Hrayr K. Shahinian, M.D., FACS

Abstract

This chapter discusses the surgical application of the fully endoscopic subtemporal approach. Performed through a small preauricular skin incision and a keyhole craniotomy, the endoscopic subtemporal approach provides a minimally invasive, short, and direct pathway to access the temporal base, the parasellar and retrochiasmatic regions, and the anterolateral petroclival region. In our practice, we have found that the great advantage of this approach is that it provides an excellent route for wide surgical exposure of the operative field from the ipsilateral greater wing of sphenoid anteriorly, including the suprasellar and parasellar regions, to the petroclival region posteriorly with virtually no temporal lobe retraction. The chapter provides a thorough description of the fully endoscopic subtemporal approach, including indications, operating room setup, patient positioning, operative technique, state-of-the-art illustrative cases (with clinical background on the most important pathologies in this area), potential complications, and ways to avoid these complications (in the author's experience).

1. Introduction

The traditional subtemporal approach, originally adopted for trigeminal rhizotomy and later modified with anterior petrosectomy to access the internal auditory canal (IAC) and adjacent structures, represents a short and direct pathway to access the temporal base, the parasellar and retrochiasmatic regions, and the anterolateral petroclival region. Unfortunately, aggressive temporal lobe retraction in order to provide adequate visualization of the operative field, injury to the temporal veins including the inferior anastomotic vein (of Labbé), and even the possibility of venous infarction of the temporal lobe have often outweighed the unique advantages of this approach.

In our practice, we adopted the use of an endoscopic subtemporal keyhole approach; the great advantage of this approach is that it provides an excellent route for wide surgical exposure of the operative field from the ipsilateral greater wing of sphenoid anteriorly, including the suprasellar and parasellar regions, to the petroclival region posteriorly with virtually no temporal lobe retraction.

The ability to approach the middle cranial fossa with a variety of endoscopic approaches (supraorbital, subtemporal) is advantageous as it allows more options to be tailored according to each patient's individual needs. In this respect, the endoscopic subtemporal approach can be augmented by other endoscopic approaches when accessing complex lesions of the skull base. For instance, when the tumor involves Meckel's cave, the tentorium is opened to aid in dissection of the tumor. The addition of a zygomatic osteotomy, drilling along the floor of the middle cranial fossa to access the infratemporal region, or medial petrosectomy are all applied depending on the specifics of each case, providing both intradural and extradural exposure.

2. Indications

The fully endoscopic subtemporal approach provides minimally invasive surgical access to the ipsilateral petroclival, suprasellar and parasellar, cavernous sinus, and medial sphenoid wing regions. Pathologies in these areas may include hypothalamic gliomas; craniopharyngiomas, especially with a prefixed optic chiasm; trigeminal schwannomas or neurofibromas; petroclival, sphenoid wing, and cavernous sinus meningiomas; arachnoid cysts of the middle cranial fossa; pituitary adenomas with major lateral extensions; chordomas of the middle and upper clivus; carcinomas; rhabdomyosarcomas; and other benign and malignant lesions extending to or through the middle skull base, with or without invasion of the cavernous sinus.

3. Instrumentation

The instruments needed to successfully execute this procedure include an endoscopic tower containing a high-definition digital camera, a xenon or halogen light source, 0- and 30-degree rigid endoscopes, two endoscope holding arms, endoscope irrigation sheaths and pumps, and precision microinstruments.

Figure 1: Operating Room Setup

4. Operating Room Setup (Figure 1)

The patient is positioned supine on the operating room table. Following the induction of general anesthesia, the airway circuit is extended with corrugated tubing and the head is rotated 180 degrees away from the anesthesiologist, who stands at the foot of the operating room table. The surgeon stands at the head of the table on the side of the lesion and the endoscopic tower is placed on the opposite side, directly facing the surgeon. Two separate pneumatically-powered holding arms are affixed to the table, one on each side of the patient, and wrapped in sterile drapes. One holding arm is dedicated to holding the endoscope and the other is used to hold other endoscopic instruments or soft silicone spatulas. A second monitor is placed on the ipsilateral side to allow the scrub nurse, who stands facing the surgeon, to monitor the operation.

5. Patient Positioning (Figure 2)

Figure 2: Patient Positioning

The patient is positioned supine on the operating room table, and the head of the bed is slightly raised to improve venous drainage. The final position of the head is tailored according to the exact location of the lesion by rotating and tilting it up to 15 degrees toward the contralateral side. This position facilitates gravitational temporal lobe retraction and provides wider access to the skull base anteriorly. The head is fixed in position using a three-pin Mayfield clamp.

Figures 3 (A): Marking, Draping and Skin Incision

Figures 3 (B): Marking, Draping and Skin Incision

Figures 3 (C): Marking, Draping and Skin Incision

6. Operative Technique

Figures 3 (D): Marking, Draping and Skin Incision

Following general anesthesia, intraoperative monitoring leads are placed and the preauricular temporal region is cleansed with an aqueous antiseptic solution and then draped. A standard 3 cm skin incision begins in the preauricular skin crease from the inferior rim of the zygomatic arch just anterior to the tragus and extends upwards curving first anteriorly then posteriorly within the natural hairline; curving the incision allows the surgeon to spread soft tissue widely to gain more access anteriorly. The deep temporalis fascia and muscle are then incised in line with the skin incision and dissected down to the squamous temporal bone in a subperiosteal plane. Small hooks are used for bilateral retraction of the skin and underlying muscle. The superficial and deep layers of the deep temporal fascia, attached to the lateral and medial surfaces of the zygomatic arch, respectively, are incised and dissected subperiosteally to expose the upper part of the temporal zygomatic root (Figures 3 (A) - (D)).

Using a microdrill, a 2 cm keyhole craniotomy is then performed in the squamous temporal bone just above the temporal zygomatic root with its base level with the zygomatic arch and extending anteriorly. The superior surface of the zygomatic arch is drilled flat and any residual bone at the temporal base is removed or drilled extradurally to facilitate a more basal subtemporal view (Figures 4 (A) - (D)).

Figures 4 (A): Keyhole Craniotomy

Keyhole Bone Flap

Figures 4 (C): Keyhole Craniotomy

Temporal Dura
Lateral Skull Base (Middle Cranial Fossa Floor)

Figures 4 (B): Keyhole Craniotomy

Figures 4 (D): Keyhole Craniotomy

A semicircular incision is then made in the temporal dura, which is reflected basally (Figures 5 (A), (B), and (C)). A combination of positioning, mild hyperventilation, mannitol, and slow cerebrospinal fluid (CSF) drainage relaxes the temporal lobe and enhances a subtemporal trajectory. Following adequate relaxation of the temporal lobe, a 0-degree endoscope is introduced and gradually advanced along the floor of the middle cranial fossa, displaying a panoramic view of the entire area from the sphenoid wing anteriorly to the petroclival region posteriorly. The procedure will vary from this point according to the pathological condition being addressed.

Figures 5 (A): Extradural anatomy and dural opening

Mandibular (V3) Division of Trigeminal Nerve (V)

Figures 5 (B): Extradural anatomy and dural opening

Mandibular (V3) division of Trigeminal Nerve (V) at Foramen Ovale
Cauterized Middle Meningeal Artery at Foramen Spinosum
Temporal Dura
Lateral Skull Base

Figures 5 (C): Extradural anatomy and dural opening

Reflected Dura
Lower Aspect of Temporal Lobe
Endoscope Trajectory

Once the intracranial portion is completed, the whole area is copiously irrigated and hemostasis is secured. The dura is then closed watertight and covered with a layer of collagen dural substitute membrane and a dural sealant to prevent any CSF leak (Figures 6 (A) and (B)). The keyhole bone flap is then repositioned and fixed in place using resorbable microplates and screws; a hydroxyapatite bone substitute is applied to cement the defect around the keyhole bone flap, restoring integrity of the squamous temporal bone (Figures 7 (A) and (B)). The skin, subcutaneous tissues, and musculature are then sutured in layers without the use of any drains and with careful attention to the aesthetic repair. A small adhesive bandage is then applied to the preauricular suture line (Figures 8 (A) - (D)). The majority of patients undergoing this procedure are monitored in either the intensive care unit (ICU) or a step-down unit overnight and then transferred to the ward until discharged home, typically 48 hours after the operation.

7. Illustrative Cases

7.1. Middle Cranial Fossa and Cavernous Sinus Region Tumors

7.1.1. Background

Traditionally, surgical approaches to the cavernous sinus included the pterional, cranioorbitozygomatic, and extended middle cranial fossa approaches. Currently, advances in endoscopic skull base surgery, neuroanesthesia, neuroimaging, and nerve monitoring have helped tremendously in operating around the cavernous sinus, parasellar, infrachiasmatic, and posterior clinoid regions, and gross-total resection of previously unresectable tumors using endoscopic approaches has been possible in a wide range of patients with excellent functional and cosmetic outcomes.

Craniopharyngiomas, especially with a prefixed optic chiasm (because the endoscopic subtemporal approach provides excellent surgical access from above and beneath the optic chiasm and tracts), trigeminal schwannomas, cavernous sinus meningiomas (including those that arise from the dural reflection of the sinus or grow into it as part of a larger tumor involving the sellar/parasellar dural surfaces, medial sphenoid wing, orbit, clivus, petrous bone, or other areas of the skull base), arachnoid cysts, and others can be completely resected without the need for major exposures. Some tumors, however, might involve or even completely encase major neurovascular structures, such as the internal carotid artery (ICA) or the cavernous sinus, posing major impediments to such a complete removal. The extent of the tumor and the involvement of major vessels are defined by MRI and MRA. The decision regarding the approach is dependent on the anatomical location of the lesion as well as the potential for some of these tumors, such as meningiomas and schwannomas, to extend into two cranial compartments, presenting further difficulties regarding the choice of approach. At the present time, we routinely use the endoscopic subtemporal (or supraorbital) approach to access lesions of the cavernous sinus region according to their location, extent, and site of pathology.

7.1.2. Approach

Under general anesthesia, after positioning, prepping, draping, and skin incision, a 2 cm temporal keyhole craniotomy is performed; the dura is then incised open and CSF is slowly drained. After adequate relaxation of the temporal lobe, a 0-degree endoscope is introduced and gradually advanced medially along the floor of the middle cranial fossa.

For cavernous sinus region tumors, including meningiomas, schwannomas, and others, the tumor is exposed and a small area of its lateral surface is electrocoagulated and biopsied for intraoperative frozen section confirmation of the pathology. The major lateral part of the tumor is then internally decompressed under direct endoscopic visualization using a combination of a microCUSA, bipolar electrocoagulation, microinstruments, and microdissecting techniques. Following internal debulking, the lateral portion of the tumor's capsule is gradually and circumferentially dissected and resected in a piecemeal fashion.

At this point, attention is shifted to the medial portion of the tumor, which is often attached to or extends within the cavernous sinus wall; straight and angled endoscopes are used to obtain different angles of view. Dissection within the cavernous sinus is continued only sharply using microdissectors and microscissors to prevent any traction on the ICA and cranial nerves. Once the cavernous sinus is entered, venous bleeding is controlled by applying gentle pressure to the sinus with hemostatic agents. Following tumor removal, the whole area is inspected with angled endoscopes to identify and remove any tumor remnants. Any bony hyperostoses, such as in meningiomas, is additionally removed.

The same principles apply to the resection of retrochiasmatic craniopharyngiomas and sellar tumors with significant lateral and retrochiasmatic extensions. These tumors are typically attached to the undersurface of the optic tracts and chiasm. After the tumor is identified, a biopsy is obtained and the tumor is internally decompressed and gradually resected in the usual manner; the last portion of the tumor, adherent to the optic apparatus, is always sharply dissected.

After tumor resection is achieved, the 0-degree endoscope is slowly withdrawn and a 30- or 70-degree endoscope is used to conduct a second survey of the entire region; any remaining tumor is further dissected and removed. Following that, hemostasis is secured and the entire region is copiously irrigated, followed by dural closure, cranioplasty, and layered suturing of the subcutaneous tissues and skin.

7.1.3. Cases

7.1.3.1 Trigeminal schwannoma (right sided): Figures 9 (A) - (S)

7.1.3.2 Middle fossa tumor: Figures 10 (A) - (S)

8. Potential Complications

Potential complications of the fully endoscopic subtemporal approach include the following:

Bleeding, infection, meningitis, cerebrovascular accident, and death, as well as the potential risk for endocrinological morbidity, vascular complications, neuropsychological and behavioral disorders, and neurocognitive disorders (these are also potential complications associated with open approaches with more extensive dissection)
Injuries during skin incision include transient or permanent injury to the frontotemporal branch of the facial nerve or less commonly the auriculotemporal branch of the mandibular nerve
Direct injuries can occur to cranial nerves, carotid artery, optic apparatus, or the pituitary gland/stalk
CSF leakage
Temporal lobe cortical injury can cause short memory disturbance, aphasia, seizures, or hemiplegia
Hematoma (intracerebral/epidural/subdural), edema, or lateral herniation of the temporal lobe
Other complications with combined approaches (e.g., transtentorial, transpetrosal, subtemporal-infratemporal)

9. Avoiding Complications in Author's Experience

During the skin incision, the frontotemporal branch of the facial nerve is the most likely motor branch to be injured because it is vulnerable in its superficial path, as it penetrates just below the zygomatic arch close to the temporomandibular joint and traverses an oblique course superiorly over and above the zygomatic arch. This is avoided by carefully elevating the lateral periosteum of the arch to protect the nerve and not extending the skin incision below the lower border of the zygomatic root. The course of the nerve over the zygomatic arch can be estimated by a line connecting a point 0.5 cm inferior to the tragus to a point 1.5 cm lateral to the superior brow.

The auriculotemporal nerve, a sensory branch of the mandibular division of the trigeminal nerve, lies posterior to the superficial temporal artery within the temporoparietal fascia (the surgeon may often encounter an anterior branch of the artery during the skin incision), and therefore elevation anterior to the frontal branch of the superficial temporal artery should proceed with caution to avoid injuring this nerve. Whenever possible, the anterior branches of the superficial temporal artery are preserved to maximize blood supply to the skin; if not, they can be ligated with nonabsorbable suture.

Cranial nerve morbidity caused by intimate involvement of the affected nerves with the lesion is a critical issue in the resection of tumors involving the cavernous sinus. This complication sometimes cannot be avoided regardless of the type of surgery; however, endoscopes with their superior visibility, precision microsurgical instruments, and intraoperative nerve monitoring allow many unnecessary injuries to be avoided. In general, existing preoperative cranial nerve deficits infrequently improve or resolve following resection, while the majority of new and often partial postoperative deficits resolve over time.

Closure after performing this approach requires meticulous attention to preventing CSF leakage; the dura may be thin, especially in old age. Also, the squamous temporal bone is thin and can often be easily fractured; therefore, it is important to avoid lacerating the dura during the bone flap removal. If watertight dural closure could not be obtained, a second layer (e.g., Duragen, muscle) should be placed over the dura and secured in place with 2 cc of fibrin glue.

Bleeding from small vessels at the operative bed usually subsides with copious irrigation. An epidural hematoma is an uncommon postoperative occurrence and is the result of inadequate hemostasis. Of note, bone bleeders, as well as bleeding on the dural surface from branches of the middle meningeal artery, may be encountered and controlled by bone wax application and bipolar electrocoagulation.

The traditional middle fossa subtemporal approach requires long-standing placement of retractors on the temporal lobe; therefore, potential injury to the temporal lobe can occur (e.g., hematoma and edema resulting in aphasia, hemiparesis, or seizures). This concern should not be a problem with the described approach because temporal lobe retractors are not used. The floor of the middle cranial fossa is at the level of the upper edge of the zygomatic arch and the craniotomy should always be kept flush with the floor and its edges trimmed and kept parallel; this, along with correct positioning and other measures, facilitate a subtemporal trajectory for the endoscope without requiring any temporal retraction.

Finally, it is necessary to replace the keyhole bone flap at the end of the operation, as this improves both functional and cosmetic outcomes and avoids transmission of brain pulsations to the skin, which is cosmetically undesirable and may even cause lateral herniation of the temporal lobe.

Legends

Figure 1: Operating Room Setup
Figure 2: Patient Positioning
Figures 3 (A) - (D): Marking, Draping and Skin Incision
Figures 4 (A) - (D): Keyhole Craniotomy

Lower Case Figure 4 (A):
1. Keyhole Bone Flap
Lower Case Figure 4 (C):
1. Temporal Dura
2. Lateral Skull Base (Middle Cranial Fossa Floor)

Figures 5 (A), (B), and (C): Extradural anatomy and dural opening

Lower Case Figure 5 (A):
1. Mandibular (V3) Division of Trigeminal Nerve (V)
Lower Case Figure 5 (B):
1. Mandibular (V3) division of Trigeminal Nerve (V) at Foramen Ovale
2. Cauterized Middle Meningeal Artery at Foramen Spinosum
3. Temporal Dura d. Lateral Skull Base
Lower Case Figure 5 (C):
1. Reflected Dura
2. Lower Aspect of Temporal Lobe
3. Endoscope Trajectory

Figures 6 (A) and (B): Dural Closure

Lower Case Figure 6 (A):
1. Collagen Dural Substitute (Onlay Graft)
2. Lateral Skull Base
Lower Case Figure 6 (B):
1. Dural Sealant

Figures 7 (A) and (B): Cranioplasty

Lower Case Figure 7 (A):
1. Microplate and Screws
Lower Case Figure 7 (B):
1. Hydroxyapatite Bone Substitute

Figure 8 (A) - (D): Skin Closure and Dressing

Lower Case Figure 8 (A):
1. Temporal Muscle and Fascia Closed

Figures 9:

(A) Contrast-enhanced axial and saggital MRIs showing a trigeminal schwannoma
(B) Contrast-enhanced coronal MRIs showing a trigeminal schwannoma
(C) - (E) Intraoperative endoscopic view showing the initial exposure of trigeminal schwannoma
Lower Case Figure 9 (C):
1. Trigeminal Schwannoma
2. Lateral Skull Base
3. Lower Aspect of Temporal Lobe
Lower Case Figure 9 (E):
1. Close up View of Trigeminal Schwannoma
(F) - (O) Intraoperative endoscopic view showing gradual resection of trigeminal schwannoma
Lower Case Figure 9 (F):
1. Tumor Surface Cauterized
Lower Case Figure 9 (I):
1. Trigeminal Schwannoma Partly Debulked
Lower Case Figure 9 (J):
1. Capsule of Trigeminal Schwannoma
Lower Case Figure 9 (K):
1. Sharp Dissection of Posterior Capsule of Trigeminal Schwannoma
Lower Case Figure 9 (M):
1. Maxillary (V2) Division of Trigeminal Nerve (V)
  Mandibular (V3) Division of Trigeminal Nerve (V)
Lower Case Figure 9 (N):
1. Maxillary (V2) Division of Trigeminal Nerve (V)
  Mandibular (V3) Division of Trigeminal Nerve (V)
2. Final Portion of Trigeminal Schwannoma
Lower Case Figure 9 (O):
1. Sharp Dissection of Final Portion of Trigeminal Schwannoma
(P) and (Q) Intraoperative endoscopic view Following complete tumor removal
Lower Case Figure 9 (P):
1. Maxillary (V2) Division of Trigeminal Nerve (V)
2. Mandibular (V3) Division of Trigeminal Nerve (V)
3. Lower Aspect of Temporal Lobe
Lower Case Figure 9 (Q):
1. Gelfoam in Tumor Cavity
2. Lower Aspect of Temporal Lobe
3. Basal Temporal Dura
(R) Postoperative contrast-enhanced axial and saggital MRIs
(S) Postoperative contrast-enhanced coronal MRIs

Figures 10:

(A) Contrast-enhanced Saggital MRI showing a Middle Fossa Tumor
(B) Contrast-enhanced coronal MRI showing a Middle Fossa Tumor
(C) - (E) Intraoperative endoscopic view showing the initial exposure of Middle Fossa Tumor
Lower Case Figure 10 (C):
1. Middle Fossa Tumor
2. Lateral Skull Base
3. Lower Aspect of Temporal Lobe
Lower Case Figure 10 (E):
1. Close up View of Middle Fossa Tumor
(F) - (O) Intraoperative endoscopic view showing gradual resection of trigeminal schwannoma
Lower Case Figure 10 (F):
1. Tumor Surface Cauterized
Lower Case Figure 10 (I):
1. Middle Fossa Tumor Partly Debulked
Lower Case Figure 10 (J):
1. Posterior Capsule of Middle Fossa Tumor
Lower Case Figure 10 (K):
1. Sharp Dissection of Posterior Capsule of Middle Fossa Tumor
Lower Case Figure 10 (N):
1. Final Portion of Middle Fossa Tumor
Lower Case Figure 10 (O):
1. Sharp Dissection of Final Portion of Middle Fossa Tumor
(P) and (Q) Intraoperative endoscopic view Following complete tumor removal
Lower Case Figure 10 (Q):
1. Gelfoam in Tumor Cavity
2. Lower Aspect of Temporal Lobe
3. Basal Temporal Dura
(R) Postoperative contrast-enhanced Saggital MRI
(S) Postoperative contrast-enhanced coronal MRI