5.1 Introduction

The translabyrinthine (TL) approach was used for the placement of the first auditory brainstem implant (ABI) and it remains one of the main routes of surgical access to the cerebellopontine angle (CPA). ^{1 , 2 , 3 , 4} This approach traverses the mastoid and the inner ear, providing access to the internal auditory canal (IAC) and the adjacent intracranial area. Its advantages include complete bony resection before opening the dura and that cerebellar retraction is minimized. Fundamental principles of the TL approach include very direct access to the CPA and that resection of bone is preferable to manipulation of brain. The incision for the TL approach is made just behind the ear over the mastoid process, which lies just below the scalp.

The TL approach is ideal for the resection of many neurofibromatosis type 2 (NF2) related acoustic neuromas. Since portions of the inner ear are resected in the course of the procedure, the TL approach cannot be used if hearing preservation is a goal of surgery. Since by definition hearing preservation is not a goal of ABI surgery, this disadvantage is irrelevant in this particular setting.

Use of the TL approach requires understanding of the complex anatomy of the temporal bone, including the course of the facial nerve and the landmarks for the location of the IAC. As such, most centers utilize a team consisting of both a neurotologic surgeon and a neurological surgeon to carry out surgery via this approach. Typically, the bony approach is carried out by the neurotologic surgeon, and intradural tumor resection in the vicinity of the cerebellum and brainstem is carried out by the neurological surgeon.

Some surgeons prefer not to utilize the TL approach for vestibular schwannoma (VS) resection or other surgery of the CPA, such as ABI placement, preferring the retrosigmoid (RS) route. Disadvantages of the TL approach may include increased operative time, a relatively narrow corridor of access, and, arguably, a higher risk of cerebrospinal fluid (CSF) leak. With the TL route, the middle ear is widely opened and there is potentially a direct route for fluid to leak via the Eustachian tube and the nose. Typically, abdominal fat is harvested and used to obliterate the mastoid space, thus forming a barrier to leakage. With an experienced neurotologic surgeon, disadvantages may be minimized.

All operations carried out as part of the initial trials leading to approval of ABI use for patients with NF2 were carried out via the TL approach. ⁵ The TL remains the preferred approach for ABI placement in the setting of NF2 at many, if not most, large centers in the United States. In patients operated for ABI placement for deafness due to causes other than NF2-related tumors, the benefits of the TL approach may be less pronounced. Indeed, this approach may be contraindicated in nontumor patients who have maintained vestibular function despite deafness. The TL approach may also not be appropriate for young pediatric patients, who typically have small, undeveloped mastoids that do not provide sufficient operative exposure.

The TL approach allows for optimal placement of the ABI receiver immediately superior and posterior to the ear. The external ABI hardware thus sits in a natural position on the side of the head, anterior to and more optimal than the placement that can be achieved with the RS approach. The ABI electrode array is placed on the brainstem surface under direct, high power magnification using the operating microscope. The surgeon must have a detailed understanding of brainstem surface anatomy in order to identify the proper site of placement. Anatomic landmarks include the stump of the vestibulocochlear nerve, the glossopharyngeal nerve, and the choroid plexus from the fourth ventricle at the foramen of Luschka. It is also important to recognize that the anatomy of the brainstem can be very distorted by the CPA tumor resected prior to ABI placement.

The cochlear nucleus complex is found deep to the surface of the brain, along the superior-anterior wall of the lateral recess of the fourth ventricle, within the foramen of Luschka. The foramen of Luschka is one of the sites of egress of CSF from the ventricular system of the brain into the cisternal system on the outside of the brain. Under the operating microscope, the ependymal surface of the ventricular system has a distinct appearance, allowing it to be differentiated from the typical pial surface of the brain’s exterior. The electrode array can be simply inserted into this “hole” with its active contacts placed in direct contact with the brain surface in the correct location.

Following insertion of ABI electrode array, it is held in place by packing the recess with Teflon (PTFE), muscle, fat, or other materials. Evoked auditory brainstem response (EABR) testing is then carried out to confirm correct placement. The wound can then be carefully closed after packing the operative cavity with abdominal fat harvested from the patient.

5.2 Anatomic Considerations

The major advantage of the translabyrinthine approach is that it provides direct access to the CPA and to the lateral side of the brainstem without the need for cerebellar retraction. This approach takes advantage of the sizable bony corridor, traversing the pyramidal shape of the temporal bone, including the mastoid sinus and the inner ear. Detailed anatomic understanding is a prerequisite for drilling through these structures. The time required and effort necessary to carefully dissect through the skull base may be seen as a disadvantage by many surgeons. The TL approach is, however, primarily carried out in the extradural space, with nearly all of the exposure done prior to opening the dura. As such, the risk of this approach is in many ways less than that of primarily intradural routes, such as the RS.

The limiting anatomic structures of the translabyrinthine route form a roughly triangular shape. These are the external auditory canal and mastoid portion of the facial nerve anteriorly, the dura of the temporal lobe superiorly, and the sigmoid sinus posteriorly and inferiorly. The sigmoid sinus should be understood as the critical structure influencing all approaches to the CPA. This is essentially a large vein running within the dura, and it carries much of the blood flow from the brain to the jugular vein and then on to the vena cava and heart. It is a bilateral structure with significant anatomic variability. Although usually even in size, it is not unusual for one to be much larger than the other. Small sigmoid sinuses can be sacrificed, but injury to an average-sized one can lead to significant postoperative problems. Injury of a large sinus will usually result in disaster. The TL approach is a presigmoid approach, that is, in front of the sinus, while the RS approach is obviously behind.

With the TL approach, temporal bone is drilled in order to directly expose the dura of the IAC and the adjacent CPA. The normal mastoid consists of a honeycomb of bone and air, allowing for identification of critical structures. Before itself being resected, the distinctly shaped hard bone of the labyrinth is the anatomic structure that acts as a landmark for identification of the other critical structures, most notably the facial nerve traversing the temporal bone on its course to its exit along the inferior skull and then the IAC itself (Fig. 5.1).

As the final step of bony exposure, the facial nerve can be identified lateral to the tumor at Bill’s bar, which is the landmark of its exit from the IAC to the labyrinthine portion of the temporal bone. After identification of the facial nerve, and after all drilling has been completed, the dura of the CPA can be opened directly adjacent to the tumor. The first critical intradural step is release of CSF. CSF release is critical to obtaining brain relaxation and to succeeding in the ongoing surgery. Release of fluid allows the brain to sag, which opens the cisternal spaces and creates enough room for tumor resection and ABI placement. As opposed to the RS approach, there is no need to retract cerebellum or to manipulate the brain to reach tumor or to release CSF.

After tumor resection is completed, hopefully with preservation of all neurologic and vascular structures other than the vestibulocochlear nerve, brainstem landmarks must be identified for ABI placement. The cochlear nucleus complex lies within the brainstem adjacent to the lateral recess of the fourth ventricle. This is inferior to the stump of the vestibulocochlear nerve, which enters the brainstem at the lowest part of the pons. The TL approach provides direct access to the CPA, with these structures near the inferior limit of the exposure. One disadvantage of the TL approach may be somewhat limited or awkward exposure of the site of ABI placement at the very inferior edge of exposure. The degree of difficulty is highly dependent upon individual anatomy. This may also be influenced 1either positively or negatively by distortion of the brainstem caused by large tumors, which is often the case with NF2.

The exact site of placement of the ABI electrode array is within the lateral recess of the fourth ventricle. Access to the lateral recess is via the foramen of Luschka. This is essentially a “hole” at the lateral aspect of the pontomedullary junction. The anterior border of the foramen is upper medulla at the level of the exiting glossopharyngeal nerve. The superior border of the foramen is formed by the pons, or, more properly, the middle cerebellar peduncle. The cerebellar hemisphere forms the foramen’s posterior border. Inferiorly, the telachoroidea, formed by fusion of the surfaces of the brainstem and cerebellum, forms the final border. The enclosed nature of the placement site allows the array to be held in position by packing material such as Teflon felt behind it.

Closure of the TL surgical defect is concerned mostly with prevention of CSF leak. The approach involves opening of the middle ear space, and the middle ear is connected directly to the Eustachian tube. Thus, there is a natural route for fluid leakage into the nasal cavity. Abdominal fat is used to occlude the mastoid cavity. Even with fat packing, the risk of leakage is substantial. This is because intracranial CSF pressure is always elevated after craniotomy, and the ABI cable provides a natural conduit from intra- to extracranial, along which fluid is able to wick.

5.3 Preoperative Evaluation

In our practice, the TL approach for ABI placement is reserved for patients with NF2 undergoing resection of VS or other tumors or, rarely, in patients with NF2 having previously undergone tumor resection. We utilize the RS approach for non tumor patients, both adult and pediatric. Thus, preoperative evaluation of clinical factors is necessary in addition to anatomical ones.

Clinical considerations in these patients revolve primarily around NF2-associated tumors and their resulting intracranial sequelae. The most obvious consideration is that of bilateral VS. The ABI may be placed at the time of first or second-side tumor resection. Generally, the side of worse hearing is operated first, although occasionally the presence of a large and threatening tumor in the ear of better hearing makes this impossible. As TL resection is always hearing destructive and RS resection of a large, NF2-associated tumor provides little hope of this, the decision to operate on a patient’s better-hearing ear is always difficult.

With the possible exception of giant tumors, data does not show that ABI performance is related to tumor size. ^{6 , 7} As such, surgeons should not be overly optimistic when facing small tumors. Giant tumors may be particularly problematic, as they are more difficult to remove from the brainstem atraumatically.

Tumor involving the facial nerve in its labyrinthine portion or at the geniculate should be noted, as this is proof of facial nerve tumor, which may complicate resection and postoperative recovery. Presence of collision tumors in the CPA should also be noted in order to plan accordingly. Furthermore, a large volume of contralateral CPA tumor can be expected to reduce the working space in the operative field as sagging of brain to the opposite side is prevented.

Study of the patient’s temporal bone and skull base anatomy must also be undertaken. If not evident on thin-cut magnetic resonance imaging (MRI), temporal bone computed tomography (CT) may also be performed. Presence of a dominant sigmoid sinus raises both difficulty and risk of surgery. Contracted mastoid, high jugular bulb, and low tegmen should also be noted as these will impede visibility and increase the difficulty of both the approach and the subsequent intradural portion of the procedure. As many NF2 patients will have also undergone prior craniotomy, the effects of this on the proposed procedure should be considered.

Dysfunction of or risk to other cranial nerves is also possible with NF2 patients. Vocal cord function should be confirmed preoperatively via direct laryngoscopy even in the absence of clinical symptoms or obvious tumors on imaging. Failure to recognize pre-existing lower cranial nerve dysfunction, especially if it is on the contralateral side, may result in grave consequences for the patients’ ability to protect their airway after surgery. Bilateral evaluation of trigeminal function as well as vision is also important. Patients without serviceable vision on the contralateral side are likely to suffer disproportionally in the event of facial and/or trigeminal dysfunction on the operative side.

Potential issues outside the CPA should also be considered. Hydrocephalus, venous sinus occlusion from tumors or prior surgery, or large supratentorial tumor burden may exacerbate postoperative CSF pressure elevation and may predispose toward CSF leak or other complication. If a ventriculoperitoneal shunt is to be performed, the side of placement should be considered carefully so as to interfere as little as possible with subsequent treatments. Adjustable shunt valves and ABI receivers cannot coexist on the same side.

Finally, preoperative spinal cord imaging is necessary in order to ensure that the cervical spinal cord especially is not at risk from operative positioning or other manipulation. Spinal cord or peripheral nerve dysfunction may interfere synergistically with vestibular dysfunction to impede mobilization during the postoperative period and thereafter. ⁸

5.4 Operative Technique and Nuances

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