Fig. 11.1
(continued) Images showing steps of a retrosigmoid approach for resection of a left small VS. Axial CT brain cuts without contrast (a) and with contrast (b) demonstrating medially located VS. (c) Preoperative BAEP responses demonstrate symmetric hearing. (d) Skin incision. (e) A 2.5-cm-diameter craniectomy is performed exposing the transverse and sigmoid sinuses. (f) Dural opening along the margins of the transverse and sigmoid junction. (g) IAC drilling intradural. (h) Sealing of the opened air cells using bone wax after IAC exposure is completed. (i) Dura is opened over the medial aspect of the tumor. (j) The posterior aspect of the tumor exposure and a window is cut after verifying the facial nerve is not located on the posterior aspect of the tumor. (k) The ultrasonic aspirator is used to debulk the center of the tumor. (l) The tumor capsule is gently elevated from the cerebellum and brainstem, and the facial and vestibulocochlear nerves are located at their brainstem exit and entry, respectively. (m) Continued dissection of the capsule from the facial and vestibulocochlear nerves. We carefully use soft cottonoids to protect the nerves during the dissection. (n) The tumor has been removed and the facial nerve is stimulated at the brainstem to verify continuity. (o) The region of the IAC that was drilled is coated with bone wax to prevent any possibility for CSF leak. (p) Fat graft is placed over the drilled IAC and fibrin glue is applied. (q) Large bur hole cover is used to cover craniectomy defect. (r) Postoperative BAER demonstrates preserved hearing. (s, t) Postoperative CT in the coronal and axial planes demonstrating region of bone drilling and tumor removal
Fig. 11.2
(continued) Images showing steps of a retrosigmoid approach for resection of a left large VS. (a) Axial T1-weighted MRI with gadolinium demonstrating large tumor. (b) Axial T2-weighted MRI demonstrating CSF cleft between tumor, brainstem, and cerebellar peduncle. (c, d) Coronal T1-weighted MRI with gadolinium demonstrating large tumor. (e) Dural opening with flaps of dura adjacent to transverse and sigmoid sinuses. (f) Tumor exposure in the CPA and the petrous surface dural opening is made in a semilunar fashion behind Fig. 11.2 (continued) the posterior lip of the IAC. (g, h) Posterior lip of IAC is drilled. (i) IAC is opened and the tumor is identified and the facial nerve is located. (j) The ultrasonic aspirator is used to debulk the tumor. (k) The vestibular nerve is identified entering the tumor. It is then divided sharply at the brainstem. (l) The facial nerve is identified at its root exit zone from the brainstem. It is verified with the stimulator. (m) Continued dissection of the tumor from the facial nerve at the IAC. (n) The capsule of the tumor is dissected from the facial nerve from the brainstem in a medial-to-lateral direction. (o) The remaining portion of tumor is dissected off the facial nerve near the lip of the porus acusticus. (p) A small MEDPOR cranioplasty is used to cover the craniectomy defect. (q, r) Postoperative axial MRI with gadolinium demonstrating complete resection of the tumor
The incision used is a gently curved incision of one to two fingerbreadths behind the mastoid. The course of the transverse and sigmoid junction and transverse and sigmoid sinuses can be anticipated based on the bony anatomy. A straight line is drawn between the roots of the zygoma to the inion, and the transverse sinus lies along this line. It curves inferiorly into the sigmoid sinus at the region of the asterion. The incision is infiltrated with lidocaine and opened with a sharp blade. After self-retaining retractors are placed, we attempt to identify the region of the mastoid emissary vein, which is a guide to the location of the sigmoid sinus. Bleeding from the mastoid emissary is controlled with bone wax. We perform either a craniectomy or a craniotomy , located just below the transverse sigmoid junction at its most superior extent. We look to identify the inferior aspect of the transverse sinus and the posterior aspect of the sigmoid sinus along the course after the bone flap is removed. This ensures an adequate trajectory and maximal visualization of the CPA . A roughly 2- to 3-cm oval opening is made extending from the transverse sinus down along the posterior aspect of the sigmoid sinus. The dura is usually opened in a cruciate fashion that allows the flap to be reflected superiorly or laterally along the transverse and sigmoid sinuses, respectively. Alternatively, a cuff of dura may be left adjacent, and the opening may follow the sinuses. This allows the cuff to be reflected with sutures for added retraction. After the dura is opened, we bring the operating microscope in the field and identify the superior and lateral aspects of the cerebellum. We then continue our dissection down along the petrous ridge at its junction with the tentorium and identify the petrosal vein. The arachnoid is opened in this location to allow egress of cerebrospinal fluid (CSF) . Immediately inferior to the petrosal sinus, we look for evidence of the VS .
Upon visualization of the tumor, the outer layer of arachnoid is reflected down to expose the posterior aspect of the tumor. We use soft cottonoids to reflect the arachnoid and the vasculature from the tumor and expose the tumor to develop the window for resection. We immediately stimulate the back of the tumor to see if there is an aberrant location of the facial nerve. At this time, we use a dissector and identify the IAC . The cuff of dura over the IAC is opened approximately 1–2 cm, and the lateral aspect of the IAC is drilled to enable visualization of the limits of tumor within the canal. This then enables identification of the location of the facial nerve early in the dissection. If the facial nerve is running along the anterior aspect of the tumor as in the vast majority of cases, a window is opened in the posterior aspect of the tumor, and a specimen is removed for pathological analysis. We then debulk the center of the tumor. The amount of removal of the interior aspect of the tumor is determined by the size of the tumor on the scan, with the objective of leaving a margin of the tumor along the tumor capsule to enable the capsule to be dissected from the surrounding cerebellum, brainstem, and nerves .
Attention is then paid to activity of the facial and vestibulocochlear nerves as measured by the electromyographic activity of the facial nerve and the brainstem auditory evoked responses (BAERs) of the cochlear nerve. If the tumor is large and the hearing is nonserviceable, vestibulocochlear nerve monitoring is not performed. We then start a dissection around the capsule of the tumor to identify the CPA and the location of the vestibulocochlear and facial nerves at the brainstem. The vestibulocochlear nerve is more posteriorly located in the brainstem and is usually found without difficulty on the medial aspect of the tumor. If hearing preservation is not a goal, the vestibulocochlear nerve is stimulated to ensure there is no aberrant course of the facial nerve and then is divided sharply. We then continue to rotate the capsule of the tumor laterally and identify the facial nerve, which is located anterior to the root entry zone of the vestibulocochlear nerve. The root exit zone of the facial nerve is identified visually and verified electrophysiologically with stimulation from the Prass electrode . Once the facial nerve is identified at the brainstem, dissection is then continued with debulking of the tumor, removal of the capsule, and slow dissection of the capsule in a medial-to-lateral direction from the facial and possibly vestibulocochlear nerves if hearing preservation is a goal (Fig. 11.1). The most adherent aspect of the tumor to the facial nerve is usually at the region of the lip of the porus acusticus . The nerve often flares in dimension at this location, but in most cases, judicious dissection from medial to lateral can be performed and the nerve dissected free of the tumor. The remaining tumor is removed from the IAC , and careful dissection is performed to sharply divide the vestibular nerve lateral to the attachment to the nerve. At this point, we verify functional continuity of the facial nerve by stimulating the nerve at the brainstem to assess its function. The BAER is recorded carefully after resection of the tumor .
The drilled posterior aspect of the IAC is inspected carefully to see if any air cells have been entered. If air cells have been entered, bone wax is used to obliterate them, and a small piece of abdominal fat is placed over the drilled area and held in place with fibrin glue. Hemostasis is obtained, and a small piece of Surgicel is placed over the region of the brainstem where the tumor has been dissected free. The dura is closed in a watertight fashion using dural substitute if necessary. AlloDerm is our preferred material. The bone is carefully waxed over the region of the mastoid where any air cells have been entered. We either replace the bone flap at this point or place a small titanium or MEDPOR (Stryker, Kalamazoo, MI) cranioplasty over the defect. The muscle is closed in separate layers, and the skin is closed with a 3-0 nylon suture .
Surgical Risks and Complications
The reported mortality rate for retrosigmoid approach for VS is 0.3% [1]. Morbidities including CSF leak in 10.3%, postoperative headache in 17.5%, postoperative symptomatic hemorrhage in 2.2%, major neurological complications in 1.8%, postoperative hydrocephalus in 2.3%, and meningitis in 3% have been reported [1, 27]. Inspection for air cells while opening and drilling the IAC , careful tumor dissection, and meticulous dural closure may help reduce the risk of these morbidities. A systematic review for this approach found residual tumor on imaging postoperatively in 5.6% of the patients planned for gross-total resection and tumor recurred in 6.2% [1].
Surgical Outcome
Middle Fossa Approach
The MFA was used by otolaryngologists to treat pathologies involving the inner and middle ear long before it was adapted by neurosurgeons for resection of intracanalicular VSs . This approach and its modifications provide an excellent corridor for middle and posterior fossa pathologies. The MFA was first performed in Glasgow by R. H. Parry, a Scotch otolaryngologist, in 1904 to treat a 30-year-old patient with left ear pain, tinnitus, and vertigo. The surgery was carried out to divide the vestibular nerve, but unfortunately, the facial nerve was injured when additional bone was removed over the most proximal portion of the fallopian canal during the approach [23]. For this reason, the MFA did not gain popularity until William House and Theodore Kurze described their microsurgical technique in 1961; they used the MFA in 14 patients for treatment of Ménière’s disease and otosclerosis of the middle ear and later treated 106 patients with intracanalicular VS via the MFA without permanent facial nerve paralysis [12–14]. This approach has undergone few modifications over the years, most of which are related to the methods for localization of the IAC .
Indications
Ménière’s disease , dehiscence of the posterior semicircular canal, and VS are the most common indications for MFA. Other indications include facial nerve decompression and repair in trauma and Bell’s palsy, facial nerve neuroma, cholesteatoma drainage, middle fossa encephalocele, CSF leak through the middle ear, petrous carotid exposure for high-flow intracranial bypass for complex aneurysms, skull base tumors, and fungal infections. For patients with VS, we use the MFA for hearing preservation in relatively young patients with purely intracanalicular tumor, for VS located in the IAC with cisternal extension of less than 1–1.5 cm (Fig. 11.3), and for tumor extending to the fundus of the IAC (lateral VS).
Fig. 11.3
Axial (a) and coronal (b) T1-weighted MRI with gadolinium showing patient who presented with progressive vestibular symptoms and preserved hearing. The cuts demonstrate a small VS located at the right IAC extending from the fundus of the IAC to the prepontine cistern
Relevant Surgical Anatomy
The middle fossa is bordered by the sphenoid ridge anteriorly, squamosal bone laterally, mastoid bone posterolaterally, petrous ridge posteromedially, and cavernous sinus and sella medially. The floor of the middle fossa hosts a number of important structures. Rhoton et al. [33] divided the floor of the middle fossa using a vertical plane through the anterior border of the cochlea into anterior and posterior parts. The anterior part contains, from lateral to medial, respectively, the eustachian tube, tensor tympani, and petrous carotid artery, parallel and deep to the greater superficial petrosal nerve (GSPN) in the space between the V3 divisions of the trigeminal nerve anteriorly and the cochlea posteriorly. The eustachian tube and tensor tympani are almost always covered by the bone, but in 63% of cadaveric specimens, the horizontal segment of the petrous carotid artery is apparent with only a very thin or no bony covering [21, 33]. The posterior part of the middle fossa floor contains the bony labyrinth with its three components: the cochlea, vestibule, and semicircular canals. The posterior part of the middle fossa is more vulnerable to injury during surgery given its intimate relationship with the lateral part of the IAC . The cochlea is located at the cochlear angle between the GSPN and the labyrinthine part of the facial nerve just anterior to the IAC fundus. The semicircular canals open inferomedially into the vestibule, which is a small cavity located in the posterolateral margin of the IAC fundus .
Along the petrous ridge on the floor of the middle fossa, a few bony landmarks can be identified: the petrous apex, trigeminal impression, trigeminal prominence, meatal depression, arcuate eminence (AE), and tegmen from medial to lateral, respectively. The superior semicircular canal (SSC) projects toward the middle fossa floor and makes a bony elevation known as the AE. In the anterior part of the middle fossa, the foramen spinosum and foramen ovale are very important landmarks that must be identified. The foramen spinosum, with the middle meningeal artery (MMA) passing through it, is located just posterior and lateral to the foramen ovale. The GSPN carries the preganglionic parasympathetic fiber of the facial nerve to the lacrimal gland, and some taste fibers from the soft palate originate at the geniculate ganglion. It passes anteriorly in the sphenopetrosal groove, where it runs beneath the V3 division of the trigeminal nerve toward the foramen lacerum, and then joins the lesser petrosal nerve to form the vidian nerve in the vidian canal (Fig. 11.4). The lateral extent of the IAC is divided into four quadrants: Bill’s bar (vertical crest) separates the facial nerve in the superior-anterior quadrant from the superior vestibular nerve in the superior-posterior quadrant; the transverse crest separates the superior vestibular nerve in the superior-posterior quadrant from the inferior vestibular nerve in the inferior-posterior quadrant. The cochlear nerve is located in the inferior-posterior quadrant of the IAC .
Fig. 11.4
(a) Cadaveric dissection (right-sided approach) photograph demonstrating the anatomy of the middle fossa. The temporal lobe is elevated extradurally to expose the floor of the middle fossa. This is performed in a posterior-to-anterior direction to avoid traction injury to the facial nerve by putting stretch on the greater superficial petrosal nerve (GSPN). The horizontal segment of the petrous internal carotid artery (ICA) is visible through a bony dehiscence. The arcuate eminence (AE) initially is identified along the petrous ridge. Extradural elevation then is continued anteromedially to expose the geniculate ganglion (GG) and the GSPN. The middle meningeal artery at the foramen spinosum (FS) is divided to allow further release of the temporal dura from the middle fossa cranial base to expose the posterior cavernous sinus and the V2 and V3 branches of the trigeminal nerve. (b) Cadaveric dissection photograph showing the middle fossa rhomboid (red) is bordered by the V3 anteriorly, the GSPN laterally, the AE posteriorly, and the petrous edge medially. The horizontal segment of the petrous ICA courses parallel to and beneath the GSPN. The internal auditory canal (IAC , blue dotted line) lies approximately in the plane that bisects the angle between the GSPN and AE. The cochlea (C) is situated anteromedial and inferior to the geniculate ganglion. Glasscock’s triangle (blue) is bordered by the posterior rim of the foramen ovale, the foramen spinosum, the posterior border of V3, and the cochlear apex. (Liu et al. [18], by permission of Congress of Neurological Surgeons [18])
IAC Localization Techniques in the Petrous Bone
Four techniques are commonly reported in the literature to localize the IAC during MFA. The House technique follows the GSPN posteriorly to the area of the Gasserian ganglion (GG) and the labyrinthine segment of the facial nerve medially toward porus acusticus in a lateral-to-medial direction [12]. The Fisch technique exposes the blue line of the SSC, and the IAC plane is approximated by drawing an imaginary line angled 60° to the SSC plane directed toward the posterior fossa dura [9]. Both techniques expose the IAC in a lateral-to-medial direction where the facial nerve is most superficial in the petrous bone, increasing the risk for facial nerve, SSC, and cochlear injury. The Garcia-Ibanez technique uses the GSPN and AE to locate the IAC [10]. The IAC is localized using a line bisecting the angle between GSPN and AE, and then the drilling is directed toward the porus acusticus to expose the IAC in a medial-to-lateral direction, decreasing the risk for facial nerve injury; this technique does not require exposure of the SSC. Another less popular technique involves drilling a point away from the tip of the GG about 9.9 mm on a line angled with the GSPN about 96° [17].
Given the variation in the middle fossa anatomy, there is no one technique fit for all patients. We generally prefer the Garcia-Ibanez technique (Fig. 11.4) because of the lower risk for injuring the facial nerve and inner ear structures .
Surgical Technique
Bony Exposure
After general anesthesia and intubation, the patient is positioned in either a completely lateral position with the side of the surgery up and the head fixed in a Mayfield head clamp or a supine position with small shoulder post under the ipsilateral side with head turned 60° to the contralateral side (Fig. 11.5). We tilt the head toward the contralateral shoulder until the ipsilateral sigmoid sinus becomes parallel to the floor. Facial nerve monitoring and auditory BAER are used routinely for all VS cases. A lazy “S” skin incision is marked starting at the zygomatic root just anterior to the tragus and then extending superiorly, curving posteriorly and then anteriorly just above the superior temporal line and behind the coronal suture. Preoperative antibiotics, dexamethasone, and mannitol are administered. The incision line is injected with local anesthetic and a vasoconstrictor, after which the skin incision is made down to the galea. This is followed by opening the temporalis fascia and muscle using a monopolar cautery down to the bone . The skin incision is often extended inferiorly over the zygomatic root to allow for a wider exposure when the retractors are placed. Care must be taken not to injure the frontal branch of the facial nerve below the zygomatic arch. A periosteal elevator is used to dissect the temporalis along with the periosteum from the bone anteriorly and posteriorly, making sure the root of the zygoma is completely exposed and centered at the lower margin of the bony exposure. A self-retaining retractor is placed, and two bur holes are made. One bur hole is placed just above the root of the zygoma and the other one just above the squamosal suture in the same plan. A craniotome is used to make the bony cuts; and a bone flap (~5 × 5 cm) is elevated while maintaining the integrity of the dura, which is then tacked up to the edge of the craniotomy superiorly. The lower edge of the craniotomy is made flush with the middle fossa floor using a cutting drill. Air cells of the temporal bone may be encountered in some cases and must be closed using bone wax to prevent the risk of CSF leak. Under microscopic visualization, the dural elevation is commenced. A small suction and a dural dissector are used to elevate the dural from the middle fossa floor. This process is started by identifying the foramen spinosum and the MMA in the middle fossa floor, which is considered a very important landmark. The foramen spinosum is always located posterolateral to the foramen ovale and V3, so injury of the GSPN at this stage of the operation is unlikely. We then identify the foramen ovale and the V3 anteriorly and medially. The foramen spinosum and ovale with the MMA and V3 mark the anterior limit of the surgical exposure. The MMA is coagulated and divided sharply, which will release the dural tethering and ease the dural elevation to identify the foramen ovale and V3. Bipolar cautery is avoided in this area to prevent thermal injury to the GSPN or GG. Bleeding from small emissary veins from both foramina is not uncommon and is easily controlled by using a hemostatic agent. Once that is done, attention is turned toward the petrous ridge posteriorly, where the dura is elevated from lateral to medial along the ridge, identifying the tegmen, AE, meatal depression, and trigeminal prominence. The trigeminal impression and petrous apex do not need to be exposed. Dissection then is continued in a posterior-to-anterior direction along the floor of the middle fossa toward the foramen ovale to identify the GSPN , which is dissected from the dura and followed to where it enters under V3. A malleable self-retaining retractor is placed as far deep as the superior petrosal sinus to expose the petrous ridge and retract the temporal dura superiorly. The IAC location is now approximated using a line bisecting the angle between the GSPN anteriorly and the AE (SSC) posteriorly. A 3-mm diamond drill is used with a continuous suction-irrigation system to expose the IAC. The drilling starts medially close to the petrous ridge and deepens inferiorly until the medial portion of the IAC dura is identified. The drilling is continued laterally until we identify Bill’s bar, which can serve as an important landmark for the facial nerve at the lateral canal. Drilling continues ~270° around the IAC dura to provide a room for maneuverability after dural opening. Care must be taken not to open the cochlear angle just anteromedial to the junction of the labyrinthine facial segment and the GG or the vestibule, which is located posterolateral to this lateral end of the IAC (Fig. 11.5) .
