33 Translabyrinthine Approach for Vestibular Schwannoma Resection

10.1055/b-0039-169187

33 Translabyrinthine Approach for Vestibular Schwannoma Resection

Brian S. Chen and Derald E. Brackmann

33.1 History

Descriptions of the “transtemporal approach” for vestibular schwannoma (VS) removal, which requires removal of the labyrinth for access to the cerebellopontine angle (CPA), date back to 1904 by Rudolf Panse.s. Literatur This approach quickly fell out of favor due to the high rate of surgical mortality and the idea of spreading “dormant infections” from the unsterile mastoid air cells, “only to nibble at [the] great tumors,” as described by Franciscus Quix in 1911.s. Literatur With the introduction of the dental drill and operating microscope by William House, along with medical advances in antibiotics and anesthesia, House described his first 41 cases of VS removal through the translabyrinthine approach (TLA) in a 1964 monograph, which repopularized this approach.s. Literatur The history of VS surgery is further expounded upon in Chapter 1.

33.2 Patient Selection

Since the reintroduction of the TLA by House, this approach has become the authors’ primary approach for VS removal and is utilized in approximately 50% of our skull base cases. It is our feeling that any tumor in the internal auditory canal (IAC) and CPA, big or small, can be removed through the TLA. This approach provides the most direct route to the IAC and CPA without retraction of the cerebellum or temporal lobe by maximum bone removal—a primary tenet of skull base surgery. Furthermore, the IAC can be dissected fully in a 270-degree fashion from the porus acusticus to fundus safely without damaging the facial nerve. There are a few situations where we believe the TLA is particularly beneficial. First, we prefer any giant VS (>3.5 cm) be removed via a TLA (Fig. 33‑1 ). Giant VSs cause brainstem and cerebellar compression, and wide exposure is required for safe dissection of these critical areas. The TLA provides direct access for tumor dissection without unnecessary retraction of the brain. Others prefer a retrosigmoid approach for very large tumors, illustrating the role for surgeon preference. We also prefer the TLA for patients who have had prior radiation or microsurgery. The exposure of the IAC in the TLA allows for wide exposure necessary for identification and dissection of the facial nerve. This is critical because the facial nerve is often encased in arachnoid scar and more adherent to the tumor in revision surgery. Finally, patients with NF2 who require tumor removal and auditory brainstem implants are also good candidates for the TLA.

**Fig. 33.1** This patient has a sporadic, 4.5-cm, large VS with brainstem and cerebellar compression. This was partially resected via a retrosigmoid craniotomy previously, but regrew. Revision surgery was conducted through a TLA approach.

33.3 Limitations of the Approach

The principal consideration for not choosing this approach is the presence of serviceable hearing. If serviceable hearing is present (pure tone average < 50 dB, speech discrimination > 50%), and the tumor is small (<2.0 cm) and/or in a favorable position, other hearing preservation procedures are considered. Specific anatomical circumstances, most commonly related to the sigmoid sinus and jugular bulb, can make the approach less desirable. A very anterior sigmoid sinus or excessively high-riding jugular bulb can preclude good access to the tumor, unless excessive decompression is performed. If there is an absent, or very small, contralateral venous sinus system, the risk of a vascular complication rises and can be lessened by using a retrosigmoid approach.

33.4 Procedure

In most cases, surgery is performed by a six-person team: the neurotologic surgeon, neurosurgeon, anesthesiologist, circulating nurse, operating room technician, and a neural monitoring technician. After general anesthesia is administered, the patient is turned 180 degrees and prepped for surgery (Fig. 33‑2 ).

**Fig. 33.2** Operating room setup for a TLA.

33.4.1 Monitoring

Arterial lines and a Foley catheter are placed for hemodynamic monitoring. At the start of the case, mannitol and furosemide are given to diurese the patient and lower the intracranial pressure. Facial nerve monitoring electrodes are inserted into muscles of the facial nerve in four branches (frontalis, orbicularis oculi, orbicularis oris, and mentalis) and connected to the neuromonitoring console (NIM-Response 2.0 or 3.0, Medtronic, Jacksonville, FL). Trigeminal nerve, vagus nerve, accessory nerve, and somatosensory evoked potential monitoring can also be utilized if necessary. Finally, only short-term paralytics are used for the intubation to facilitate accurate and reliable neural monitoring.

33.4.2 Positioning

Patients remain supine for TLA surgery with their bodies positioned similar to chronic ear surgery and a gel pad is placed under the head. No pins are necessary for this procedure. The left lower quadrant of the abdomen is prepped for fat harvest.

33.4.3 Soft-Tissue Exposure

A generous shave is given behind and above the ear. Keep in mind the position of the tegmen and sigmoid sinus, which is usually inline with the zygomatic root and follows the posterior contour of the mastoid bone, respectively. The shave should be wide enough for decompression of the sigmoid sinus and postsigmoid dura, and for decompression of the middle fossa dura superiorly. As a general rule, the skin incision should be at least two to three fingerbreadths behind the postauricular sulcus (Fig. 33‑3 ). The skin incision is made down to the level of the temporalis fascia superiorly and the mastoid periosteum inferiorly. Once the temporalis fascia is identified, the mastoid periosteum is dissected toward the ear canal. It is imperative to keep this deep layer robust, as it is used to close over the titanium plate at the end of the case. In this step, the key is to keep the periosteal incision offset from the skin incision to facilitate closure. By offsetting the incision, the risk of plate exposure and a cerebral spinal fluid (CSF) leak through the incision is also reduced.

**Fig. 33.3** Skin incision for a TLA. Note the position of the temporalis muscle, mastoid tip, and ear canal.

The periosteal flaps are raised to expose the posterior edge of the ear canal anteriorly, beyond the gentle curvature of the sigmoid sinus posteriorly, above the linea temporalis superiorly, and to the digastric muscle inferiorly. Electrocautery is helpful to lift the periosteum posteriorly and superiorly. Finally, the anterior flap is retracted with silk sutures and two large self-retaining retractors are placed.

33.4.4 Bony Dissection

The bony dissection begins with identification of the tegmen mastoideum superiorly. The antrum is then identified anteriorly and the posterior canal wall is thinned and followed inferiorly to the mastoid tip. The sigmoid sinus is identified posteriorly and followed superiorly to the sinodural angle. Wide saucerization is performed along the middle fossa dura and behind the sigmoid sinus until the dura is completely uncovered and soft. If a large mastoid emissary vein is present, this should be ligated to improve sigmoid sinus mobility. It is beneficial to bipolar coagulate any emissary veins away from the parent sigmoid sinus to reduce the risk of bleeding. As dissection is continued medially, the middle fossa and presigmoid dura (posterior fossa dura) are decompressed. A thin layer of bone can be left over the sigmoid sinus in the form of a single island of bone (Bill’s island) or in multiple “eggshell” pieces to protect the sinus from instrumentation or drying out as the medial temporal bone is removed (Fig. 33‑4 ).

**Fig. 33.4** After the mastoidectomy is complete, the facial nerve is identified. The middle fossa dura and posterior fossa dura are also decompressed and an island of bone remains over the sigmoid sinus.

Working anterosuperiorly, the mastoid antrum is identified where the lateral semicircular canal and incus are seen. The location of the facial nerve can then be approximated and identified in the vertical segment. The air cells overlying the facial nerve are removed and the facial nerve is traced to the mastoid tip, leaving a thin layer of bone on the facial nerve.

Once the location of the facial nerve is identified, a labyrinthectomy is conducted while decompressing the dura superior and posterior to the labyrinth. As the semicircular canals are removed, the facial nerve is at risk for injury at the second genu and vertical segment while removing the lateral canal and ampullae of the posterior canal, respectively (Fig. 33‑5 ). In particular, any undercutting of the facial nerve at the second genu, while exposing the vestibule, may result in inadvertent injury. In general, drilling superior to the lateral canal and posterior to the posterior canal can be done safely with a cutting burr. A diamond drill is then recommended to remove the remaining labyrinth. During labyrinthectomy, the arcuate artery is invariably encountered as it passes through the subarcuate tract of the superior semicircular canal. This can be easily controlled with a diamond drill and bone wax is rarely needed. As the posterior fossa is decompressed, the endolymphatic sac is seen and should be separated from the duct, as it travels through the petrous face into the vestibule. Dividing the endolymphatic duct often allows the posterior fossa dura to fall away from the petrous bone, which facilitates exposure. As the final portions of the labyrinth are removed, keep in mind the location of the superior and posterior canal ampullae, as these are good markers for the superior and inferior borders of the IAC.

**Fig. 33.5** View of the labyrinth after a partial labyrinthectomy. Note the proximity of the superior canal ampullae, lateral canal, and posterior canal ampullae to the facial nerve.

Next, the sigmoid sinus is decompressed inferiorly and the jugular bulb is identified. Once the jugular bulb is identified, the dura is decompressed superior and anterior to the bulb in order to identify the cochlear aqueduct. Once identified, a Rosen needle is used to disrupt the arachnoid in the cochlear aqueduct and CSF is released. In larger tumors, the cochlear aqueduct may be obstructed at the medial orifice in the CPA. At this juncture, drilling should not continue inferior to the cochlear aqueduct because the lower cranial nerves are in this location as they enter the neural compartment of the jugular foramen.

As the bone superior to the jugular bulb is removed, the inferior trough of the IAC is developed and the inferior edge of the IAC is identified at the porus where the bone is thickest. Once identified, the inferior border of the IAC is skeletonized laterally. As the bone is removed from the IAC laterally, the anterior face of the canal can be palpated to identify the limit of the anterior dissection. Once the inferior trough is complete, the IAC is skeletonized posteriorly and the superior trough is drilled (Fig. 33‑6 ). The superior trough is usually drilled after the inferior trough in order to protect the facial nerve, which most commonly lies in the anterior superior portion of the IAC. If the space between the tegmen and the IAC is narrow, great care must be exercised not to violate the IAC. In these situations, adequate exposure of the inferior trough will allow for tumor mobilization and facial nerve dissection. Bipolar electrocautery can also be used to contract the dura after more extensive decompression and improve surgical exposure. Some surgeons advocate reversing the drill when skeletonizing the inferior trough of a right IAC or superior trough of a left IAC to prevent the burr from “skipping” into the IAC. The final goal is to have the IAC exposed 270 degrees from the porus to the fundus. More circumferential decompression of the IAC allows for more room to dissect the tumor free from the facial nerve, but also improves visualization of the region of the porus acusticus, which is particularly important for large tumors where the risk of facial nerve splay and adherence is greater. As a general rule, all bone work should be completed before opening the dura in order to reduce the risk of facial nerve injury with the drill and to reduce intracranial bone dust dissemination.

**Fig. 33.6** Skeletonizing the IAC. The inferior trough should be dissected first, followed by the superior trough.

33.4.5 Fundus Dissection

The most lateral dissection of the IAC starts inferiorly with identification of the singular nerve. The bone is carefully removed superiorly to uncover the transverse crest (or falciform crest), which separates the inferior and superior vestibular nerves. Finally, the vertical crest (or Bill’s bar, named after William House) is identified superiorly, which separates the superior vestibular nerve located posteriorly from the facial nerve located anteriorly (Fig. 33‑7 ). Once identified, the most lateral portion of the IAC should be skeletonized to the labyrinthine segment. This dissection can be tedious, but is critical for facial nerve identification. Once identified, a sharp hook is used to palpate Bill’s bar and take down the lateral end of the superior vestibular nerve as it enters the vestibule. While this relationship is an anatomical constant, confirmation of anatomy should still be performed with the stimulating probe prior to superior vestibular nerve avulsion. Another commonly referred to landmark for this part of dissection is Mike’s dot (named after Mike Glasscock), where the terminal end of the superior vestibular nerve enters the vestibule. As the superior vestibular nerve is reflected, the facial nerve can be seen just anterior to Bill’s bar (Fig. 33‑8 ).