34 Surgical Refinements to the Translabyrinthine Approach to the Cerebellopontine Angle

10.1055/b-0039-169188

34 Surgical Refinements to the Translabyrinthine Approach to the Cerebellopontine Angle

Sampath Chandra Prasad, Alessandra Russo, Abdelkader Taibah, Enrico Pasanisi, Francesco Galletti, and Mario Sanna

34.1 Introduction

The translabyrinthine approach (TLA) is a lateral approach used for accessing various tumors of the cerebellopontine angle (CPA), most commonly vestibular schwannoma (VS). This approach was first described by Rudolf Panse in 1904 and was first performed for VS in 1911 by Franciscus Hubertus Quix from Utrecht University, in the Netherlands. However, due to unsatisfactory results, this approach was criticized by Harvey Cushing and Walter Dandy, two of the pioneers of neurosurgery of that time, and thereafter this approach fell into disrepute.s. Literatur After the advent of microscopes in otology, William House at the House Ear Institute in Los Angeles rediscovered the TLA, marking the beginning of the era of modern skull base microsurgery.s. Literatur ^, s. Literatur ^, s. Literatur ^, s. Literatur Subsequently, there was a developing opinion that while small tumors could be removed through the TLA, larger lesions were best approached through a suboccipital approach as the TLA failed to provide a large surgical field.s. Literatur To overcome this limitation of the TLA, the proponents of this approach enlarged the surgical corridor by additional bone removal over the middle and posterior fossa dura and by the addition of the transapical extensions (where bone is drilled out to various degrees around the internal auditory canal [IAC]), thereby obtaining a wider surgical view and better control over the tumor and surrounding structures. This facilitated removal of even very large tumors with anterior and medial extensions.s. Literatur ^, s. Literatur ^, s. Literatur ^, s. Literatur ^, s. Literatur Furthermore, this approach could be combined with other skull base approaches such as the transcochlear approaches described by William Houses. Literatur or transotic approaches described by Ugo Fischs. Literatur ^, s. Literatur for additional exposure (Chapter 38).

Over the years, we have introduced some technical modifications to the TLA for very large tumors that have allowed us to reduce the mortality rate to <1% with minimal morbidity.s. Literatur Identifying the facial nerve by using the ampullary nerve as a landmark rather than the vertical crest (Bill’s bar), lowering the high jugular bulb, and extending bone removal around and anterior to the IAC (transapical extension) were the major refinements added to the original technique.

The standard TLA approach is reviewed in detail in Chapter 33. This chapter will discuss the two significant modifications that have been developed at the Gruppo Otologico to the conventional TLA described by House. We refer to these modifications as the enlarged translabyrinthine approach (ETLA) and the ETLA with transapical extension (ETLA + TAE). Both of these approaches will be described and discussed separately. This chapter is based on the experience accumulated in the management of over 3,000 VSs using the TLA performed between 1986 and 2016 at the Gruppo Otologico.

34.2 The Enlarged Translabyrinthine Approach

34.2.1 Rationale

The limited view provided by the classic TLA prevented considering this valuable approach for the removal of large VS, more so in the presence of limiting anatomical obstacles such as a high jugular bulb, an anteriorly placed sigmoid sinus, low middle fossa dura, and a small mastoid cavity.s. Literatur

The purpose of the ETLA approach is the same as with the conventional TLA—i.e., to obtain lateral access to the IAC and the CPA, thereby allowing removal of CPA lesions without cerebellar retraction. In the conventional TLA, a thin shelf of bone is sometimes left over the middle cranial fossa dura, the sigmoid sinus, and the jugular bulb. In the ETLA, bone is removed over these structures to completely expose the middle cranial fossa dura, the posterior fossa dura, sigmoid sinus, retrosigmoid dura, and the jugular bulb along with the contents of the IAC. This allows retraction of structures during surgery, thereby enabling exposure and hence removal of even very large VSs. The advantage in exposure obtained by the enlarged approach when compared to the conventional TLA is shown in Fig. 34‑1 . Another difference between the two approaches is that in the enlarged approach, the bone around the IAC is drilled by 270 degrees or more compared to 180 degrees in the conventional approach, providing the surgeon with additional anteromedial exposure (Fig. 34‑2 ).

**Fig. 34.1** Exposure of the CPA and the surrounding areas obtained by the ETLA is much larger than that obtained by the classic TLA.

**Fig. 34.2** Illustrations showing the difference between the conventional 180-degree exposure **(a)** and the 270-degree exposure **(b)** of the internal auditory canal.

34.2.2 Indications

Tumors of any size with poor preoperative hearing: This approach is indicated for tumors wherein hearing preservation is not essential due to nonserviceable preoperative hearing. With the wide exposure that this approach provides, the surgeon can focus on disease removal.
Large tumors irrespective of preoperative hearing: While opinions regarding this vary, we feel that the ETLA is indicated in VSs greater than 1.5 cm in diameter in the extracanalicular portion, regardless of the preoperative hearing, since the probability of hearing preservation with gross total resection is low. Very large tumors, even those larger than 4 cm in diameter, can be safely removed by the ETLA.
Neurofibromatosis type 2 (NF2) with large tumors: NF2 commonly presents with bilateral VSs, making hearing preservation or rehabilitation very important. While hearing can be preserved in small tumors by opting for hearing preservation surgeries such as the retrosigmoid or the middle cranial fossa approaches, it is difficult to ensure this in larger tumors.s. Literatur In such a scenario, the ETLA may be used, wherein large tumors can be removed and preservation of the cochlear nerve can be attempted for simultaneous cochlear implantation (CI). Likewise, when it is not possible to preserve the cochlear nerve, an auditory brainstem implant (ABI) can be placed in the same sitting, by this approach.s. Literatur

34.2.3 Contraindications:

Only hearing ear: This procedure is contraindicated in the only hearing ear. However, this has become a relative contraindication currently due to the option of simultaneous ipsilateral CI or ABI, as mentioned above, or a contralateral CI.s. Literatur
Ipsilateral chronic otitis media: This procedure is contraindicated in the presence of active infection. However, the approach can be used in cases of simple tympanic membrane perforation with no active infection, while the external auditory canal is closed as a cul-de-sac. If there is an active infection, a subtotal petrosectomy is performed, with eradication of the infection, obliteration of the cavity with abdominal fat, and closure of the external auditory canal as a cul-de-sac. The ETLA is then performed as a second-stage surgery.

34.2.4 Surgical Technique

Mastoidectomy (Left Ear)

A C-shaped postauricular skin incision is placed starting from 2 to 3 cm superior to the attachment of auricle, at the level of the dome of the helix, continuing 4 to 5 cm posterior to the retroauricular sulcus, and ending just below the mastoid tip (Fig. 34‑3 ). The skin flap is reflected anteriorly and maintained in position with fish hooks. The musculoperiosteal layer is incised in a T-shaped fashion using cautery (Fig. 34‑4 ) and is elevated. Monopolar cautery is applied to transect the muscles attached to the mastoid tip that helps to raise the flap. Bleeding from the mastoid emissary vein usually occurs at this step and it can be controlled by the use of bone wax. The posterior margins of the musculoperiosteal flaps are sutured to the skin, which helps in achieving hemostasis from the skin incision and at the same time keeps the flaps retracted (Fig. 34‑5 ). We do not recommend the use of the conventional retractors, which limit access and exposure.

**Fig. 34.3** Right postauricular skin incision for the ETLA.

**Fig. 34.4** The musculoperiosteal flap is incised using a monopolar diathermy in a “T” fashion. The vertical incision extends down to the mastoid tip.

**Fig. 34.5** The musculoperiosteal flaps are held by sutures.

An extended mastoidectomy is then performed. The middle cranial fossa dura and the sigmoid sinus are identified, leaving a thin shell of bone overlying them. Bone 2 to 3 cm posterior to the sigmoid sinus is also drilled using a large cutting burr (Fig. 34‑6 ). The mastoid air cells are exenterated, and the antrum is widely opened. The digastric ridge is identified, the facial nerve is skeletonized, and the retrofacial air cells are drilled out. Using a large diamond burr, the remaining bone covering the middle cranial fossa and the sigmoid sinus as well as the posterior fossa dura posterior to the sinus is further drilled and removed. The sinodural angle must be clearly exposed to know the exact location of the superior petrosal sinus exiting the sigmoid sinus. The sigmoid sinus is followed down to the jugular bulb, and the three semicircular canals are delineated.

**Fig. 34.6** **(a)** Extensive removal of bone over the middle cranial fossa as well as posterior to the sigmoid sinus (SS) is essential in the ETLA. **(b)** Mastoid air cells are well exenterated to demonstrate the labyrinth (L) and the mastoid segment of the facial nerve (FN). An adequate bone exposure will extend to about 3 cm posterior to the sigmoid sinus. DR, digastric ridge; MFD, middle fossa dura; PFD, posterior fossa dura.

Labyrinthectomy, access to the IAC and the CPA. The labyrinthectomy starts by opening the lateral semicircular canal using a medium-sized cutting burr. The posterior semicircular canal is next opened, followed by opening of the superior semicircular canal (Fig. 34‑7 ). The anterior end of the lateral semicircular canal is left in situ to protect the facial nerve lying anteriorly. The last part of bone over the middle cranial fossa dura adjacent to the labyrinth is finally removed using a rongeur after separating the bone from the dura by the use of a freer dissector. The ampullae of the lateral and superior semicircular canals are drilled. However, the anterior part of the ampullae of these two canals should be left to protect the labyrinthine segment of the facial nerve and to serve as a landmark for the superior vestibular nerve.

**Fig. 34.7** **(a)** Note the suction-irrigator is pushing the sigmoid sinus away from the working burr. **(b)** The three semicircular canals have been opened. Note the endolymphatic duct (EL) extending from the medial surface of the posterior canal (PSC) to the posterior fossa dura (PFD). FN, facial nerve; LSC, lateral semicircular canal; MFD, middle fossa dura; SS, sigmoid sinus; SSC, superior semicircular canal.

The vestibule is then opened widely. It is important to avoid drilling the floor of the vestibule in order to avoid inadvertent entry into the IAC. Similarly, drilling the roof of the vestibule may result in injury to the facial nerve, as the nerve runs immediately lateral to the vestibule. The endolymphatic duct is transacted by the use of Beaver knife (Fig. 34‑8 ). This step facilitates the complete drilling of bone overlying the posterior fossa dura and the subsequent dural retraction. The bone left over the posterior and middle fossa is successively drilled out. During this step, the cochlear aqueduct can be identified. This is an important landmark for the glossopharyngeal nerve, which lies immediately inferior to it. In small or medium-sized VSs, opening the cochlear aqueduct allows the cerebrospinal fluid (CSF) to drain. This does not occur in large tumors, because the duct is occluded by the tumor. After a wide exposure of the dura is accomplished, following the posterior fossa dura allows the porus of the IAC to be identified. The ampulla of the superior semicircular canal serves as a landmark for the superior border of the IAC at the fundus. The inferior border of the canal is identified by drilling the retrofacial air cells up to the cochlear aqueduct. Further cautious drilling at the level of the fundus leads to the identification of the horizontal crest as well as the superior ampullary canal. Bone between the superior border of the IAC and the middle fossa dura and between the inferior border of the IAC and the jugular bulb can be further drilled, depending on the size of the tumor. We routinely drill the bone 270 degrees around the IAC (Fig. 34‑2 ).

**Fig. 34.8** A Beaver knife is used to cut sharply the endolymphatic duct (EL) to allow dural retraction. The semicircular canals have been drilled out and the vestibule (Ve) can be seen. FN, facial nerve; JB, jugular bulb; MFD, middle fossa dura; SS, sigmoid sinus.

Identification of the Facial Nerve at the Fundus

The original technique described by House for identifying the facial nerve at the fundus depends on the identification of the vertical crest (Bill’s bar). However, we have modified this technique. By drilling inferiorly at the fundus, the inferior vestibular nerve is exposed, while in a more superior plane the horizontal crest is identified, which separates the inferior vestibular nerve from the superior vestibular nerve. The superior vestibular nerve is followed laterally where it leaves the fundus of the IAC. The nerve then lies in a tiny canal that enters the ampulla of the superior semicircular canal as the superior ampullary nerve (Fig. 34‑9 ). The superior ampullary nerve is dissected from its canal using a 90 degrees hook with the tip facing inferiorly (Fig. 34‑10 a). The superior ampullary nerve is separated from the facial nerve by a vertical crest of bone, called Bill’s bar, which protects the facial nerve while this step is being carried out. The hook is moved medially toward the fundus, with subsequent separation and establishment of a good plane of cleavage between the superior vestibular nerve and the anteriorly lying facial nerve. Once the superior ampullary nerve is detached and reflected medially and posteriorly, the facial nerve can be clearly observed (Fig. 34‑10 b). The relationship between the facial and cochlear nerves and the horizontal and vertical crests is well appreciated. Careful dissection of the superior vestibular nerve continues medially. Often, here, one may encounter adhesive bands between the superior vestibular nerve and the facial nerve (Fig. 34‑11 ), which may bleed on manipulation, thus obscuring the plane of dissection. To control this, a piece of dry Gelfoam is placed on the facial nerve, which also helps to protect the nerve during further dissection. Our technique of identification of the facial nerve by identification of the superior ampullary nerve and the horizontal crest rather than the vertical crest ensures proper identification and hence greater protection to the facial nerve.