Transcochlear approach

For the purposes of this book the transcochlear approach will be illustrated as an extension of the translabyrinthine approach

Although in actual practice labyrinthectomy and exposure of the IAC may take place after removal of the ear canal.

In this initial view, a translabyrinthine craniotomy has been completed and the ear canal and middle ear are in view.

In the transcochlear approach it ear canal is ligated closed (see chapter on ear canal closure) and the ear canal, tympanic membrane, and ossicles are removed.

In this transparent view through the medial wall of the middle ear the relationships of the cochlear to the intrapetrous carotid artery and eustachian tube are revealed. Note that the cochlear nerve originates from the modiolus of the cochlea and then passes deep to the inferior vestibular nerve in the internal auditory canal.

The sequence of events which lead up to posterior re-routing of the FN are commenced with the identification and decompression of the descending portion of the nerve. The nerve is first rapidly identified with a cutting burr. The epineurium is exposed with a diamond burr. The least traumatic approach is frequently to leave small islands of bone on the nerve which are microdissected from the epineurium. A disposable myringotomy knife is an excellent instrument for this maneuver as is sharp on one side to lyse stubborn adhesion, while the opposite curvature serves well as a blunt dissector.

Opening of the fallopian canal without injuring the facial nerve is an essential element of the transcochlear approach.

A cutting burr is used to rapidly locate the nerve, leaving a thin bony covering in place. A diamond burr is then used to remove the last eggshell of bone from the epineurium.

A diamond burr can also be used to define small islands of bone which can then be microdissected from the epineurium.

The cochlea is then excavated to the mid-modiolar plane. The anterior limit of exposure is the genu of the internal carotid artery which is located anterior to the cochlea immediately inferior to the eustachian tube. The horizontal segment of the FN is also skeletonized during this stage. Two guides to the location of the FN when approaching from below are the oval window and cochleariform process, both of which lie just inferior to the nerve.

A diamond burr is them used to skeletonize the FN from the stylomastoid foramen to the geniculate ganglion. Elevation of the FN from its bony channel proceeds from inferior to superior. Bleeding of the sytlomastoid artery is common. This is usually best managed by gentle pressure rather than cautery.

Elevation of the tympanic segment requires transection of the nerve to the stapedius muscle.

Anterior transposition of the facial nerve is then performed as a prelude to more proximal decompression of the nerve.

Opening the acute angle of the peri-geniculate segment greatly simplifies exposure of the labyrinthine segment.

Bone overlying the labyrinthine segment is removed until it joins the dura of the internal auditory canal.

The greater superficial petrosal nerve is then transected, permitted posterior displacement of the nerve.

It is usually at this point that the nerve becomes unstimulatable electrically, presumably as a consequence of vascular compromise.

The dura of the internal auditory canal is then opened and the eighth nerve (cochlear and both vestibular branches) are then transected.

The facial nerve is laid on the posterior fossa dural surface. After posterior translocation, the nerve typically drapes over the terminal section of the sigmoid sinus and jugular bulb. The remaining cochlear remnant is then removed. Aggressive osseous exenteration is then carried out through the apical petrous bone into the lateral aspect of the clivus. Typically diffuse venous oozing is encountered from the clival bone marrow. This can be controlled with bone wax and Surgicel packing.

The anterior limits of bone removal superficially are the genu of the carotid artery and the orifice of the eustachian tube.

The deep limit is the point at which the dura-bone interface “straightens” indicating that the posterior face of the clivus in the midline has been reached.

The marrow space of the apical petrous bone and clivus often bleeds prodigiously during these later stages of the bone removal.

Dural incision superficially is like that of the translabyrinthine approach: paralleling the superior petrosal sinus superiorly, following the sigmoid sweep over the jugular bulb inferiorly, and connecting across the porus acusticus.

Medial to the porus, a Y-shaped incision opens the dura overly the apical petrous bone.

Intracranial exposure with the transcochlear approach is illustrated with a bulky prepontine tumor in place. Exposure includes (in addition to the contents of the cerebellopontine angle) the lateral and anterior faces of the pons, the basilar artery, and both sixth nerves. Care must be exercised to protect the rerouted facial nerve from drying or inadvertent injury. To accomplish this it can be covered by a moistened Telfa strip. Closure is similar to that with the translabyrinthine approach, except that the external auditory canal must be sutured closed and the eustachian tube obliterated.

Closure of the eustachian tube in a manner which is watertight to withstand cerebrospinal fluid pressure is necessary with the transcochlear approach, as it is with many cranial base procedures.

In preparation of eustachian tube occlusion, the orifice of the tube is widely drilled opened and the mucosa is stripped out from the infundibulum.

A series of small spheres of bone wax are then deeply impacted into the canal using a cottonoid and a blunt dissector. A piece of fascia is then laid over the orifice of the tube.