Acoustic Neuromas: Translabyrinthine Approaches

Nicholas Saunders and Paul A. Fagan

Skull base surgeons owe a great debt to Dr. William House, who, in the face of significant opposition, popularized the translabyrinthine approach nearly 50 years ago. The term translabyrinthine is something of a misnomer, implying as it does a narrow approach via only the posterior and superior parts of the otic capsule. A more accurate if somewhat ponderous description would be transoccipital, transtemporal, transmastoid, and translabyrinthine, emphasizing the need to remove a significant portion of the temporal and occipital squama. This point is well demonstrated by the size of the bony defect evident in the postoperative computed tomography (CT) scan shown in Fig. 15.1. Provided that these steps are taken, the authors believe that no tumor is too large to be successfully dealt with by this approach.

Although there has been concern in the past that this approach provides inadequate access to the cerebellopontine angle, it has been shown to compare favorably with the retrosigmoid approach in this regard.¹ In addition it gives a very clear view of the facial nerve in the lateral end of the internal auditory canal. Lastly, brain retraction is extradural.

The only significant drawback is that there is no prospect of saving hearing, although the issue of hearing preservation surgery remains contentious.² Very occasionally an anatomic aberration such as a very high or very large dominant jugular bulb or extreme forward position of the sigmoid sinus will make the approach technically difficult. However, this is a rare event if sufficient bone is removed from behind the sigmoid sinus and from around the bulb itself.

With experience, this approach can be performed very rapidly. In the authors’ hands the time taken from the skin incision until the dura is opened is less than 90 minutes.

♦ Patient Selection

In the authors’ unit, the translabyrinthine approach is now the choice for acoustic neuroma surgery, other than in patients with small tumors and good hearing, when formerly the middle fossa route was chosen. In recent years, tumors smaller than 15 mm in the cerebellopontine angle have been managed by the so-called wait-and-scan technique.³ The retrosigmoid approach is reserved for meningiomas and other cerebellopontine angle tumors where hearing preservation is more predictable and where visualization of the lateral end of the internal auditory canal is not required.

♦ Positioning and Preparation

After induction of anesthesia, the area of the incision is shaved and the incision marked and infiltrated generously with a local anesthetic and vasoconstrictor mixture. The patient is positioned supine on the operating table with the head turned away from the side of surgery and the face supported by a soft pad. The use of a head ring is avoided as it flexes the neck, closing the angle between the ear and shoulder and hampering access, as well as limiting head rotation. Rigid head fixation is not required. The patient is strapped to the operating table, as this allows the table to be rolled toward or away from the surgeon as the operation proceeds, changing the angle of the head accurately and predictably, and without disturbing the drapes. Facial nerve monitoring is mandatory, and both passive electromyogram (EMG) monitoring and an active electrode are required. The anesthetist must be made aware of the use of the monitor, as muscle relaxants will abolish EMG potentials. Short-acting relaxants, however, may be used for induction of anesthesia.

Regarding draping, an adhesive clear-plastic drape allows the pinna to be held forward and out of the operative field. A plastic pouch protects the surgeon’s lap from irrigation fluid. The surgeon sits and is positioned as for tympanomastoid surgery, with the microscope base at the head of the table and the scrub nurse on the opposite side of the patient from the surgeon (Fig. 15.2).

Fig. 15.1 Postoperative computed tomography (CT) scan shows the extent of bone removal.

♦ Incision and Exposure

The incision is shown in Fig. 15.3. It is important to make the incision sufficiently posteriorly to allow bone removal behind the sigmoid sinus and sufficiently superiorly to be able to expose at least 3 to 4 cm of bone above the temporal line. Anteriorly it is carried to a point in line with the tragus, as this provides access to the posterior root of the zygoma, deep to which lies the anterior part of the epitympanum. Below the temporal line the incision continues straight down to the periosteum; above it the plane of dissection is immediately superficial to the temporalis fascia. This is facilitated by an incision down to bone along the temporal line. A large piece of temporalis fascia, for dural closure, is harvested and a 3-cm-long vertical incision is made through the posteroinferior part of the temporalis muscle, continuous with the previous incision along the temporal line. The muscle is then mobilized in an anterosuperior direction so that the posterior root of the zygoma and the temporal squama are exposed (Fig. 15.4). It is important to minimize mobilization of the membranous ear canal to reduce the chance of subsequent stenosis and recurrent otitis externa. The soft tissues may be held with either self-retaining retractors or hooks connected via elastic bands to towel clips attached to the mattress of the operating table.

Fig. 15.2 Operating room setup for left translabyrinthine approach. 1, patient table; 2, surgeon; 3, microscope; 4, instrument table; 5, surgeon; 6, OR nurse; 7, facial nerve monitor; 8, surgical technician; 9, anesthesia nurse; 10, anesthesia cart; 11, anesthesiologist; 12, infusion stand; 13, high-speed drill; 14, bipolar cautery device. (Adapted from Dart CM. Major Ear Surgery Resource Manual. Self-published. Reprinted by permission.)

Fig. 15.3 The skin incision is marked.

♦ Mastoidectomy

A very wide intact canal wall mastoidectomy is performed. Cortical bone removal is continued both 2 cm (and occasionally 4 cm) behind the sigmoid sinus (the occipital squama) and on to the squamous temporal bone to expose a crescentic area of vertical middle fossa dura approximately 3 to 4 cm in maximum height (Fig. 15.5). This is best achieved with a large (8 or 9 mm) bur, which will reduce both the time taken and the chance of injury to the underlying dura. Safety is further enhanced by changing from a cutting to a diamond bur as the dura is approached. It is particularly important to saucerize the edges of the mastoidectomy and not to leave vertical edges or overhangs that will hamper access later in the procedure.

Fig. 15.4 The temporal squama and the posterior root of the zygoma (RZ) are exposed after soft tissue mobilization.

Bone is then removed from over the sigmoid sinus, the posterior fossa plate, and the middle fossa plate. These latter areas join over the superior petrosal sinus (the sinodural angle). Bone is removed from the angle, and from above and behind the point at which the sigmoid, transverse, and superior petrosal sinuses meet. This maneuver produces a “sac of dura,” which facilitates extradural retraction of the temporal lobe and cerebellum later in the procedure.

Any bleeding at this stage comes either from mastoid emissary veins or, rarely, a torn venous sinus. The former is readily controlled with bone wax. The latter can be stopped by gentle pressure over a large piece of a collagen-based absorbable hemostat supported by a neurosurgical cottonoid.

The mastoidectomy is continued under the operating microscope. The mastoid antrum is entered and the horizontal semicircular canal identified. The superior aspect of the body of the incus is exposed and the epitympanum opened anteriorly so that the head of the malleus is clearly visible. It is important at this stage to remember that the middle fossa dura curves sharply downward over the anterior part of the epitympanum, exposing it to a drill injury in this area.

An important step is positive identification of the facial nerve. It is found in its descending portion, well inferior to the external genu. In the region of the horizontal semicircular canal the nerve is changing direction and therefore at risk of injury. Inferiorly its straighter course means that the nerve can be identified here quite safely. A cutting bur is used in the manner of a paintbrush, carefully brushing off layers of bone along the expected course of the nerve until the fine blood vessels of the epineurium or a more diffuse “blush” can be seen through the bone. In this way a layer of bone can be left over the nerve. If the position of the sigmoid sinus allows, the nerve should be approached from posteriorly or posterolaterally rather than laterally, as this enables the surgeon to use the equator rather than the pole of the bur, improving visualization. Copious irrigation is essential to prevent thermal damage to the nerve, especially if a diamond bur is used at this stage. Once the nerve has been found, it is followed superiorly in the same fashion until the entire descending portion is clearly on view.

Fig. 15.5 The sigmoid sinus (SS) and vertical middle fossa dura (MFD) are exposed. PCW, posterior canal wall.

A facial recess opening (posterior tympanotomy) is then created (Fig. 15.6). This is achieved by drilling between the facial nerve and chorda tympani to fashion an oval-shaped opening into the middle ear. The incudostapedial joint is disarticulated and the incus removed. These maneuvers allow the eustachian tube to be packed with muscle under direct vision to eliminate one source of postoperative cerebrospinal fluid (CSF) leakage. The bridge of bone over the fossa incudis is removed and the horizontal segment of the facial nerve can clearly be seen.

Fig. 15.6 The facial nerve (FN) is on view, and the facial recess opening is created. HC, horizontal semicircular canal; IN, incus; M, malleus; SS, sigmoid sinus.

♦ Perilabyrinthectomy

The approach to the internal auditory canal, of which labyrinthectomy is a vital step, is most safely done in a wide field. The authors have coined the term perilabyrinthectomy to describe the removal of bone above and below the labyrinth prior to labyrinthectomy.

Inferiorly this involves creating a space bounded by the jugular bulb below, the posterior semicircular canal above, the posterior fossa dura behind and in front, and laterally the facial nerve and more medially the solid bone in which runs the cochlear aqueduct. The bone is removed from over the jugular bulb and from the angle between it and the sigmoid sinus. If the bulb is particularly high and limiting access, this maneuver allows it to be depressed inferiorly and held in position with bone wax.4

Above the labyrinth, the superior semicircular canal is skeletonized by removing bone from the middle fossa plate and from the angle between the superior and lateral semicircular canals, the so-called solid angle of Citelli.

These essential steps result in the labyrinth projecting into space (Fig. 15.7), as it were, allowing for meticulous removal of bone in the exposure of the internal auditory canal.

♦ Labyrinthectomy

The labyrinthectomy is commenced by opening the horizontal semicircular canal. Because the external genu and horizontal segments of the facial nerve are clearly on view, they should be safe from injury. The dissection continues anteriorly until the ampulla is reached. Directly superior to this is the ampulla of the superior semicircular canal, which is now opened. This is an extremely important landmark and delineates the anterior limit of the superior part of the dissection, as anterior to the ampulla lies the labyrinthine segment of the facial nerve.

Fig. 15.7 The perilabyrinthectomy is completed. The superior semicircular canals (SC) are on view. FN, facial nerve; HC, horizontal semicircular canal; JB, jugular bulb; M, malleus; PC, posterior semicircular canal; SS, sigmoid sinus.

The superior semicircular canal is then opened along its entire length until the crus commune is reached. This leads the surgeon naturally into opening the posterior canal. The upper part of the vertical segment of the facial nerve can be seen lateral to the ampulla of the posterior canal. All three semicircular canals should now be clearly on view and open (Fig. 15.8), as is the vestibule, in which the membranous saccule and utricle can often be seen. During the labyrinthectomy it is usually possible to identify the endolymphatic sac on the posterior fossa dura. This can be a useful teaching exercise, and it facilitates identification of the intraosseous portion of the endolymphatic duct. The rugose portion of the sac is divided as it enters the bone, enabling the posterior fossa dura to fall back, away from any residual bone, which needs to be drilled out.

♦ Approach to the Internal Auditory Canal

When the labyrinthectomy has been completed, only two landmarks remain in a mass of bone: the stump of the ampulla of the superior semicircular canal, and the open vestibule. The former is superior to the lateral end of the internal auditory canal (IAC), and the latter is directly lateral to the lateral end of the canal.

The IAC is approached by drilling through this mass of bone on a broad front, aiming initially to identify it posteriorly and inferiorly, furthest away from the facial nerve.

Fig. 15.8 The semicircular canals (SC) are opened. HC, horizontal semicircular canal; PC, posterior semicircular canal.

The anterior limit of the inferior dissection is the aforementioned cochlear aqueduct. Beyond it lie the glossopharyngeal, vagus, and accessory nerves in the neural compartment of the jugular foramen. The cochlear aqueduct is not always readily seen, but when encountered usually rewards the surgeon with a flow of CSF. This helps to decompress the posterior fossa, facilitating both drilling and extradural retraction.

As the IAC, full of tumor, is approached, it is seen as a color change in the bone. Once it has been clearly identified, posterior and superior bone removal can proceed with a degree of confidence. The aim is to expose the canal around 270 degrees of its circumference, leaving a thin eggshell of bone overlying it for protection (Fig. 15.9). The bone in the angle between the IAC and the posterior fossa dura, overlying the porus, forms something of a collar that eventually can be lifted free without tearing the underlying dura. It is also important at this stage to ensure that the medial bone removal is continued far enough anteriorly round both the superior and inferior aspects of the IAC.

Fig. 15.9 The internal auditory canal (IAC) is skeletonized. FN, facial nerve; M, malleus.

Once the IAC has been fully skeletonized, the remaining bone is removed using a diamond bur and picks or elevators. It is safer to start medially, at the porus end of the IAC, concentrating again initially on the inferior aspect of the canal, which is furthest away from the facial nerve. Medially and superiorly it is essential to ensure that bone removal is complete, as later dissection of tumor from the facial nerve in this area will be compromised by any residual bone that interferes with visualization of the nerve. Exposure of the IAC continues until the crista falciformis (horizontal crest) is exposed. This is an important landmark for the facial nerve at the lateral end of the IAC, the nerve lying anterosuperior to the crest.

The other landmark at the lateral end of the IAC is Bill’s bar, which is a spur of bone lying in the coronal plane separating the facial nerve in front from the superior vestibular nerve behind. Bill’s bar, however, is much more difficult to identify with certainty than is the horizontal crest

All bone removal must be completed before the dura is opened to prevent contaminating the CSF with bone dust or chips. The suction should also be changed to a Brackmann-type side fenestrated sucker-irrigator.⁵ The posterior fossa dura is initially incised using a scalpel, in line with the IAC, taking care not to injure the underlying cerebellum. The incision is continued anteriorly and laterally above and below the porus acusticus using dural scissors. The tumor should now be on view. Depending on the size of the tumor, the dural opening may need to be enlarged by making cuts at right angles to the original incision.

These authors believe that the dura of the IAC should not be opened until the end of the tumor dissection, nor should positive identification of the facial nerve at the lateral end of the IAC take place until that time. This prevents undue mobilization of (and traction on) the facial nerve while tumor debulking is taking place medially.

♦ Tumor Removal

Tumor removal is a gradual, stepwise process. Unlike surgery of malignant neoplasms, the tumor is removed piecemeal, and extensive intracapsular debulking is the key to the removal. The posterior surface of the tumor is first tested with the facial nerve stimulator to ensure that there are no facial nerve fibers stretched across this surface, and that the rare diagnosis of a facial nerve neuroma has not been missed. An opening is then made in the tumor after cautery with the bipolar diathermy. The debulking is then started though this opening, using a combination of cupped tumor forceps and bipolar diathermy.

Fig. 15.10 Insulated and uninsulated Fisch-type dissectors.

As the debulking proceeds it becomes possible to mobilize the capsule of the tumor, separating it from the underlying cerebellum and brainstem before removal. It is important that intracapsular debulking proceeds as far as possible before a search is made for the facial nerve at the brainstem. This minimizes the need for later retraction on the nerve.

The facial nerve is likely to lie anteriorly and inferiorly to the bulk of the tumor. The tumor will almost certainly distort the course of the nerve, which may be attenuated significantly near the porus. The nerve is nearly always found on the brainstem anterior and slightly inferior to the vestibulocochlear nerve, which is a regularly identified landmark as it runs into and blends with the tumor. The nerve ascends along the brainstem, deep to the tumor until it takes a near right-angled bend to run across the cerebellopontine angle to the porus. It is at the porus itself that the facial nerve is most at risk, which emphasizes the need for maximal bone removal at this point. The nerve monitor and stimulator are invaluable in the presence of such altered anatomy, and the authors find the use of an insulated Fisch-type middle ear dissector 6 particularly helpful, as it reduces the need to change between instruments and provides early warning should the nerve be encountered unexpectedly during dissection (Fig. 15.10).

Debulking and mobilization continue from medial to lateral. The anterior inferior cerebellar artery runs in or outside of the arachnoid mesh that surrounds the tumor. Generally a ready plane of cleavage is found deep to the arachnoid, allowing the artery to drop away out of harm’s way. The vestibulocochlear nerve is best left intact at the brainstem until such time as this compromises access, as it will protect the facial nerve from some of the traction caused by tumor mobilization. Dissection of the tumor from the lateral end of the IAC is performed last. As the facial nerve is firmly fixed in its labyrinthine segment, even mild traction here may result in some loss of function; if there is still a significant bulk of tumor present at this stage, the pull of its weight alone may be enough to damage the nerve. This again emphasizes the need for extensive debulking before formal tumor removal. Once the facial nerve has been identified here, the final remaining fragments or tumor are removed, along with the vestibular nerves. When tumor removal is complete, the integrity of the facial nerve is once again tested with the stimulator, as this has been shown to be of valuable prognostic significance; evidence of conduction at a stimulator current of 0.05 mA is a favorable sign.⁷

♦ Closure and Postoperative Care

After meticulous hemostasis the cerebellopontine angle is filled with Hartmann’s solution to prevent pneumocephalus,⁸ and the dural defect is covered with a temporalis fascia graft. If not already done, the eustachian tube and middle ear are packed firmly with muscle. Subcutaneous fat is harvested from the left iliac fossa (to prevent confusion with an appendicectomy scar) and used to fill the mastoid cavity. The wound is closed in layers, aiming for a watertight closure. The combination of interrupted muscle and subcutaneous layers of an absorbable braided suture material (2–0 Vicryl) and a continuous subcuticular layer of absorbable monofilament (4–0 Monocryl) generally produces highly satisfactory results. A firm head bandage is applied and left in situ for 3 days.

If the total duration of surgery has been less than 5 hours, the patient does not usually go to the intensive care unit unless dictated by comorbidity. The urinary catheter is removed and the patient encouraged to mobilize the second day after surgery. Prophylactic antibiotics are given for 24 hours, but steroids or mannitol are not used. An aperient is routinely prescribed to avoid straining and the risk of precipitating a cerebrospinal fluid leak.

Acknowledgments

The senior author (P.A.F.) wishes to pay tribute to three surgeons whose experience and generosity encouraged and guided him to an interest in translabyrinthine surgery and one hopes a certain degree of skill. They are Bill House, who began it all; Derald Brackmann; and the late and much missed Antonio de la Cruz.