Principles
- Note that bone has been removed 1 to 2 cm behind the sigmoid sinus which is retracted posteriorly. The amount of cerebellar retraction is considerably less than that required with the retrosigmoid approach. Note that the semicircular canals have been removed en route to exposing the internal auditory canal.
Steps
Theatre preparation
- A, anesthetist
- CUSA, Cavitron Ultrasonic Surgical Aspirator
- N, nurse
- NM, neural monitor
- S, surgeon
- SA, surgical assistant
- TV, television monitor.
Skin incision and muscle dissection
- Skin incision approximately 4 cm behind the postauricular sulcus.
- The incision is carried directly down to bone, thus traversing the posterior portion of the temporalis muscle.
- Leaving the muscle attached to subcutaneous tissue and periosteum affords a more robust closure.
- A wide exposure of the mastoid is required as well as access to the anterior portion of the occipital bone lying behind the sigmoid sinus.
- During flap elevation emissary veins are often divided.
- These can be readily controlled with bone wax.
Cortical Mastoidectomy
- In the initial stages of bone removal the mastoid is decorticated from the posterior aspect of the external auditory canal to a line 1-2 cm behind the sigmoid sinus.
- This is carried out rapidly with a large, high speed, cutting burr.
- The degree of retrosigmoid exposure depends upon both the size of the tumor and the anatomical location of the sigmoid sinus.
- Following skeletonization of the sigmoid sinus with a cutting burr, its bony covering is removed with a large diamond burr. In addition, the retrosigmoid dura is exposed for 1 to 2 cm behind the sinus.
- Larger burrs (preferably 8 to 10 mm in diameter) are favored as they present a broad flat working surface and are less likely to lacerate the dura or vein wall.
- During retrosigmoid exposure it is often necessary to control one or more emissary veins with either bipolar cautery or surgicel packing.
- While the sigmoid sinus and dura usually can be completely freed of their bony covering, it is reasonable to leave a few small flakes of bone attached as long as they do not restrict sigmoid or dural mobility and are not sharp edged.
Labyrinthectomy
- Once the sigmoid has been decompressed, the antrum is opened. Exposure should be carried anteriorly until the entire length of the lateral semicircular canal is visible.
- This reveals the short process of the incus. Excessive opening of the epitympanum should be avoided as this widens the connection between the craniotomy and the middle ear, a circumstance which increases the risk of CSF otorhinorrhea.
- At this stage the posterior fossa dura between the sigmoid sinus and labyrinth is uncovered and tegmen mastoideum and tympani are removed.
- Removing this bone plate from the middle fossa floor is optional in smaller tumors, but is important in larger tumors where maximal cerebellopontine angle exposure is desired.
- Removal of the bony edge along the superior petrosal sinus, where the posterior and middle fossae dura meet, is often more technically challenging than in other areas.
- This elongated bony wedge can usually be dissected free as a piece using dura elevators and sharp dissection as needed.
- Hemorrhage from the inferior petrosal sinus can usually be easily controlled with either bipolar cautery or packing.
- Once the posterior and middle fossa dural faces have been exposed to the level of the labyrinth, elevation of the dura from the surface of the petrous bone is performed.
- Dural elevation permits insertion of retractors which substantially augment the exposure of deeper structures by providing a wider field of action.
- The retractor also serves to protect the sigmoid sinus from inadvertent injury, particularly from the rotating shaft of the drill.
- In this illustration the semicircular canals and facial nerve have been made visible to demonstrate the relationship between these two structures.
- Note that the facial nerve parallels the inferior aspect of the lateral semicircular canal throughout its course and is closely related to only the inferior most portion of the posterior semicircular canal.
- Knowledge of these highly consistent relationships helps the surgeon to perform the labyrinthectomy rapidly and with safety.
- Removal of the semicircular canals is commenced in the sinodural angle.
- As the labyrinth is exceptionally hard bone there is a tendency for the drill to run, particularly when working with the tip of the burr.
- By creating deep troughs parallel to the middle and posterior fossa dural surfaces, it is possible to work with the side of the burr in a controlled manner when approaching the facial nerve.
- The first portions of the labyrinth removed are the upper part of the posterior semicircular canal and the superior aspect of the lateral canal until its full lumen has been uncovered.
- Care must be exercised when working on the anterior edge of the lateral semicircular canal as should the drill run forward in this location injury to the exposed tympanic segment of the facial nerve is possible.
- The labyrinthectomy is then deepened to open and then remove the superior semicircular canal.
- The subarcuate artery is often encountered coursing through the arch of this canal.
- At this stage the endolymphatic sac and duct are exposed. The duct wraps around the common crus (joins the non-ampullated ends of the superior and posterior canals) on its J-shaped route to the vestibule.
- Labyrinthectomy is completed with a diamond burr which is used to identify the horizontal and second genu portions of the facial nerve.
- A thin shell of bone is typically left on the nerve.
- Removal of bone to the level of the facial nerve is important for unhindered exposure of the inferior aspect of the internal auditory canal at a later stage.
- In large tumors the vertical segment of the facial nerve is also skeletonized to facilitate exposure of the anterior cerebellopontine angle.
- Following labyrinthectomy, the vestibule is opened widely.
- This exposes the ampullae of the superior and lateral canals which approximate each other anteriorly and the posterior ampulla which is tucked posteroinferiorly.
- In the floor of the vestibule lies the elliptical recess in which lies the utricle whose glistening white otoconial membrane is often visible.
- Wide opening of the vestibule usually requires drilling slightly under the facial nerve with a diamond burr.
- Once the location of the facial nerve is known, the remaining portion of the sigmoid sinus coursing towards the jugular bulb is uncovered.
- Up until this stage the exposure remains quite restricted.
- Severing the endolymphatic aqueduct at its operculum permits elevation of the dura inferiorly to the level of the jugular bulb, a maneuver which substantially increases exposure.
Opening Internal Auditory Canal
- The internal auditory canal lies in the bone deep to the labyrinth as seen in this translucent illustration. The vestibular nerves originate in the vestibule at the ampullae and otolithic organs. The facial nerve is the only nerve which extends lateral to the vestibule from this surgical perspective. (JB, jugular bulb
- GG, geniculate ganglion
- LFN, labyrinthine segment of the facial nerve
- SVN, superior vestibular nerve
- IVN, inferior vestibular nerve.)
- Identify falciform crest and superior vestibular nerve (common tumor origin).
- Facial nerve lies anterior–inferior to the tumor.
- Exposure of the internal auditory canal commences with a cutting burr. Note that the canal courses posteriorly after taking its origin at the vestibule.
- The initial identification of the canal is performed in its mid-section and towards the porus acusticus.
- To avoid possible injury to the facial nerve in its labyrinthine segment, the lateral extremity of the canal is not dissected until a later stage.
- Once the plane of the internal auditory canal has been identified, troughs are drilled above and below the canal parallel to its long axis. Inferiorly, the deep lateral limit is the cochlear aqueduct.
- Once open, CSF may gush form its lumen, particularly in the tumors smaller than 3cm.
- The cochlear aqueduct serves as a marker for the neural compartment of the jugular foramen which lies medial to the jugular bulb. If additional inferior exposure is desired, it is possible to skeletonize the dural envelope overlying the ninth nerve.
- Superiorly, it is important not to carry the trough too laterally as this jeopardizes premature exposure (and possible injury) to the facial nerve in its labyrinthine segment. (CA, cochlear aqueduct.)
- Bone is removed along the posterior petrous face well deep to the porus acusticus. This maneuver is particularly important for large tumors which require a greater degree of access to the anterior cerebellopontine angle.
- A diamond burr is used to remove the last eggshell thin piece of bone over the canal dura.
- Removal of the bony ring at the porus acusticus is commenced by drilling superiorly and inferiorly to create an osseous semicircle.
- While removing the last ledges of bone, a fenestrated suction many be used to gently retract the internal auditory canal contents to one side and thus reduce the risk of dural disruption while drilling.
- The ring of bone at the porus may then be microdissected off of the dura and removed as a single piece.
- Depending upon the anatomic peculiarities involved, the posterior margin of the porus may be removed in pieces rather than in a single unit as depicted here.
- Prior to removing the remaining bone from the floor and roof of the canal, the dura is elevated from the remaining bony shell.
- The remaining superior and inferior bony plates are removed with a diamond burr while gently displacing the canal contents with a fenestrated suction. As the facial nerve runs in the superior compartment of the canal, it is particularly important to use caution while drilling in this location.
- The final maneuver which complete internal auditory canal exposure is excavation of the fundus.
- The transverse crest is a prominent bony landmark which separates the superior from the inferior vestibular nerves.
- When the distal canal is deeply invaded, exposure may be continued along the superior and inferior vestibular nerves into the vestibule.
- The goal of internal auditory canal exposure is two have exposed approximately two thirds of the circumference of the internal auditory canal.
- This primary reason for this degree of excavation is to permit the unhindered insertion of right angled microinstruments (an arachnoid knife or hook) needed to gently establish the plane between the tumor and facial nerve.
- Once exposure of the internal auditory canal is complete, it is complete accessible to surgeon for microdissection.
- It is particularly important to widely funnel the porus acusticus.
- Ledges left at the porus may obscure the tumor and facial nerve dissection plane as it acutely angulates at the exit of the canal.
- The relationships at the distal end of the internal auditory canal have been made transparent. The superior vestibular nerve terminates at the ampullae of the superior and lateral canals.
- The inferior vestibular nerve terminates at the ampulla of the posterior semicircular canal. Note its branch with passes inferiorly towards the saccule.
- The facial nerve is the only nerve which continues lateral to the vestibule, and it becomes gradually more superficial as it does so.
- A vertical crest of bone, often referred to a Bill’s bar, hangs in the vertical plane between the superior vestibular nerve and the facial nerve.
- As a landmark, it is more easily felt with a hook than seen.
- While many surgeons use it as a landmark for identification of the facial nerve, this author prefers to expose the facial nerve at its entry into fallopian canal rather than relying upon this subtle bony spur.
Dural opening
- For medium and large tumors dural incision is intended to obtain wide exposure of both the internal auditory canal and cerebellopontine angle. The superior incision commences a few millimeters from the anterior surface of the sigmoid and parallels the superior petrosal sinus into the trough above the internal auditory canal. Inferiorly, the incision commences near the sigmoid and courses over the jugular bulb in bony trough inferior to the internal auditory canal.
- The internal auditory canal incision is fashioned like the letter “H” on its side and creates superior and inferior dural flaps.
- Following bipolar cautery of the dura, it is incised with the tip of a No. 11 blade. Once a small opening has been created, a blunt dissection (e.g. Rhoton No. 4) is inserted to gently elevate the dura from the cerebellar surface. Stout scissors are then used to open the remainder of the incision line. Incision is usually much easier for the superior limb of the incision, where the dura is relatively thin, than it is inferiorly where the endolymphatic sac considerably augments dural thickness.
- The internal auditory canal dura is then opened with up-biting and angled scissors. The internal auditory canal and cerebellopontine angle incisions are than connected at the level of the porus acusticus. The cerebellar hemisphere may than be elevated for the tumor surface, thus exposing its cerebellopontine angle component. If CSF has yet to be liberated, the cisterna magna should be entered at this point by dissecting between the lower pole of the tumor and the nerves of the jugular foramen.
- In small tumors, the size of the of the dural opening may be tailored to meet the needs of the tumor at hand. In the case of a small cerebellopontine angle component, a simple Y may be sufficient. Such a limited incision may often be partially or completely suture closed following tumor resection.
- Use of a limited dural incision to expose a small acoustic neuroma.
- Exposure of the internal auditory canal and cerebellopontine angle following completion of a translabyrinthine craniotomy (left side). Posterior fossa exposure spans from the tentorium above to the takeoff of the upper fibers of the cranial nerve 9-10 complex.
- This affords exposure to the lateral aspect of the pons and upper medulla.
- Exposure of a 2-cm acoustic neuroma via a translabyrinthine craniotomy. Sufficient exposure is provided for removal of even the largest acoustic neuromas. Note the deflection of the facial nerve on the anterior surface of the tumor. This is its most frequent course.
Tumor Resection
- Debulk tumor internally, then perform capsular dissection off facial nerve.
- Maintain hemostasis with bipolar coagulation.
- Preserve facial, trigeminal, and lower cranial nerves.
- Use continuous facial nerve monitoring throughout.
Extended translabyrinthine approach
- Involves removal of bone above and below the internal auditory canal deeply into the petrous apex and lateral aspect of the clivus. This permits a somewhat improved view anteriorly into the pre-pontine cistern. Note that the ipsilateral sixth nerve and basilar artery are visible.
Closure
- In closing a translabyrinthine craniotomy, the fossa incudis is occluded with a piece of fascia. Some surgeons remove the incus and insert a piece of muscle into the epitympanum.
- Others open the facial recess and attempt to close the eustachian tube working in the narrow interval between the promontory and the tympanic membrane.
- A strip of fat, previously harvested from the abdomen or hip and placed in bacitracin solution, is inserted into the craniotomy defect.
- In closing a translabyrinthine craniotomy, the fossa incudis is occluded with a piece of fascia.
- Some surgeons remove the incus and insert a piece of muscle into the epitympanum.
- Others open the facial recess and attempt to close the eustachian tube working in the narrow interval between the promontory and the tympanic membrane.
- A strip of fat, previously harvested from the abdomen or hip and placed in bacitracin solution, is inserted into the craniotomy defect.
- Strips fat are then laid, one after the other, into the craniotomy defect both to obliterate the dead space and to discourage CSF leakage.
- A retractor is used to displace the previously placed fat posteriorly in order to open up a passage for subsequent strips.
- The intention of fat packing is to obliterate the craniotomy defect, not to fill the intracranial cavity left behind following tumor resection. (A) axial view.
- Strips fat are then laid, one after the other, into the craniotomy defect both to obliterate the dead space and to discourage CSF leakage. A retractor is used to displace the previously placed fat posteriorly in order to open up a passage for subsequent strips. The intention of fat packing is to obliterate the craniotomy defect, not to fill the intracranial cavity left behind following tumor resection. (B) Surgical view.
- An option while closing a translabyrinthine craniotomy is to cover the dural defect with a fascial graft. This helps to both re-establish the dura and restrain the fat graft from prolapsing into the cerebellopontine angle. In addition, the posterior fossa opening can often be narrowed somewhat, but not completely eliminated, through suture closure of the dura incision.
- Post op MRI with fat graft
Pros
- Suitable for all tumor sizes.
- Lower incidence of postoperative headache.
- Direct approach to cerebellopontine angle → minimal retraction on cerebellum needed
- Removal of large tumour by debunking center of tumour and letting rest of tumour to fall into view
- Early facial nerve identification. Hence can preserve CN7 best
- Patient feel less ill i.e essentially an extra cranial procedure
Cons
- Destroys the labyrinth → complete hearing loss
- Even though the cochlear structure itself is not directly excised—because it destroys the functional integrity of the labyrinth and its hydrodynamic and neural connections that are essential for auditory transduction
- If patient has otitis media in past cant use this technique → infection
- More time consuming
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