Vestibular Neurectomy Technique
- Author: Jayita Poduval, MS, MBBS, DNB(ENT), DORL; Chief Editor: Arlen D Meyers, MD, MBA more...
A number of approaches can be used for vestibular neurectomy depending on specific advantages and disadvantages. The different approaches are as follows:
Middle cranial fossa
Combined retrolabyrinthine and retrosigmoid
Only one approach, the translabyrinthine, may be considered as one that might not require the collaboration of a neurosurgeon, but it can only be performed in cases in which the hearing loss is profound or total and the ear is essentially nonfunctional, as it involves complete destruction of the labyrinth and vestibular apparatus.
The middle cranial fossa approach is a neurosurgical approach. The main advantage of this procedure is that the vestibular nerve can be definitively identified and selectively sectioned in the internal acoustic meatus, as it is housed in the posterior compartment and separated by bony partitions from the cochlear and facial nerves. The vestibular nerve fibers in the distal IAC are distinct and a complete sectioning can be performed while preserving the cochlear nerve.
Resection of the entire vestibular nerve, including Scarpa’s ganglion, makes regeneration less likely. Bleeding is also less in this approach because large vessels are fewer in the distal portion of the IAC. The disadvantages are temporal lobe retraction, with its attendant neurological sequelae, and the technical complexity of the surgery, requiring a steep learning curve to achieve expertise. The problems associated with temporal lobe retraction have been somewhat circumvented with the transtemporal supralabyrinthine variation of the middle cranial fossa approach.
The internal acoustic meatus (IAM) is a short bony canal lined with dura and measuring about 1 cm in length. It extends laterally from the cerebello-pontine angle into the petrous temporal bone and is closed laterally at the fundus by a bony plate that has several perforations for the passage of nerves and vessels, ie, the facial, auditory, and vestibular nerve and the internal auditory artery and vein.
At the fundus, the IAM on its inner medial aspect has a bony crest called the transverse crest or crista falciformis, which divides the IAM into a smaller upper and a larger lower compartment. The upper compartment is further divided by a vertical partition called Bill’s bar to contain the facial nerve anteriorly and the superior vestibular nerve posteriorly. The lower compartment contains the cochlear nerve anteriorly and the inferior vestibular nerve posteriorly; the singular nerve passes below and behind the inferior vestibular nerve through a separate foramen.
The superior vestibular nerve supplies the superior and lateral semicircular canals, the utricle and part of the saccule, while the inferior vestibular nerve supplies the saccule, and the singular nerve supplies the posterior semicircular canal; all these branches make up the vestibular part of the auditory-vestibular or vestibulocochlear or eighth cranial nerve, supplying the vestibular apparatus housed in the bony labyrinth.
The central portion of the bony labyrinth is the vestibule, which is about 5 x 5 x 3 mm. It has several openings, ie, the fenestra vestibuli laterally, 5 openings posteriorly that lead into the semicircular canals, and the opening of the vestibular aqueduct posteroinferiorly. A ridge of bone called the vestibular crest separates the saccule from the utricle and is overlaid by the vestibular (Scarpa’s) ganglion. Proximal to the vestibular ganglion, the vestibular nerve is a single bundle that runs from the porus or opening of the IAM through the cerebello-pontine angle up to the brain stem. At the porus of the IAM, the vestibular nerve is medial to the cochlear bundle, and together they form the vestibulocochlear nerve, separated only by a shallow groove running longitudinally along the bundle.
To experienced eyes, the cochlear nerve appears whiter than the vestibular nerve, which is slightly grayish. This distinction is a useful guide during vestibular neurectomy, though the outermost high-frequency fibers of the cochlear nerve may be sacrificed without any significant damage to the hearing.
The facial and vestibulocochlear nerves rotate through 90° by the time they reach the brain stem, where the facial nerve is rostral to the vestibulocochlear nerve, but almost always separated by the anterior inferior cerebellar artery (AICA) so that they are more easily identified in the posterior approaches, and due care can be taken to protect the facial nerve, and the AICA, which gives off the labyrinthine artery.
Middle Cranial Fossa Approach
In the middle cranial fossa approach, after shaving and draping, a preauricular incision is made from approximately the lower edge of the zygomatic root and extended to the temporal area at an angle for about 7 cm to the depth of the temporalis fascia, and branches of the superficial temporal artery are divided and clamped. The temporalis muscle is well exposed with a self-retaining retractor, and after elevation of muscle flaps, the temporal squama is exposed from the root of the zygoma to the parietosquamous suture line.
The craniotomy is placed anteriorly for better exposure, and a bone flap based on the zygomatic root is created while avoiding any laceration of the dura. Dural elevation is performed with the microscope, taking care to avoid the area of the middle meningeal artery, which is the anterior extent of dissection. Dural elevation is continued up to the meatal plane, which is bounded by the arcuate eminence, superior petrosal sulcus, and facial hiatus; the operating table is placed in the Trendelenburg position for better visualization. The superior SCC is blue-lined, and the area of the meatal plane overlying the IAC is defined. The superior lip of the porus and the medial roof of the IAC are progressively thinned out, until they are blue-lined.
The lateral exposure of the IAC involves the removal of bone over the meatal fundus and tegmen tympani. The tegmen may be opened to expose the malleus head and incus for better orientation. Up to one third of the bone of the upper circumference of the IAC over the meatal foramen and the distal superior vestibular nerve (SVN) is removed to identify the vertical crest, called Bill’s bar, and to optimally perform the vestibular neurectomy. When the IAC is opened from the fundus to the porus, the SVN is identified and cut sharply with a neurectomy knife distal to the vestibular crest. The cut end of the SVN is then retracted to expose the saccular and singular branches of the inferior vestibular nerve, the vestibulofacial anastomoses are cut, and the vestibular nerve is cut along with Scarpa’s ganglion sharply with neurectomy scissors.
Damage to the facial nerve is minimized by delineating the meatal plane meticulously and avoiding bone removal over the facial nerve. In spite of necessary precautions, facial nerve injury is higher in this approach compared to the others. Sharp sectioning of the nerve, and not its avulsion, after identification helps to avoid collateral injury to the cochlear nerve. Postoperative pain and trismus are also more common in this approach due to the division of the temporalis muscle.
Hemostasis is ensured, the bone defect plugged, the dura and craniotomy flap replaced, and the wound is closed is layers with suction drain. Lumbar cerebrospinal fluid drainage is usually not required.
Retrolabyrinthine, Retrosigmoid, and Combined Retrolabyrinthine-Retrosigmoid Approaches
The retrolabyrinthine, retrosigmoid, and combined retrolabyrinthine-retrosigmoid approaches are posterior approaches performed either by the neurosurgeon alone or a team of neurosurgeon and otologist. In these approaches, the dura of the posterior fossa is incised after a posterior craniotomy, and the area of the cerebello-pontine angle is approached.
The facial nerve is demarcated from the vestibulocochlear nerve by the nervus intermedius and is somewhat grayer in color. Superiorly, the trigeminal nerve is identified by characteristic striations across its length, while the last 4 cranial nerves lie inferior to the vestibulocochlear bundle. In the vestibulocochlear nerve bundle, the vestibular nerve is grayer compared to the cochlear nerve due to the more dense arrangement of the nerve fibres.
The retrolabyrinthine approach is technically easier, and the risks of facial and cochlear nerve injury is significantly lesser. A significant advantage of this approach is that the endolymphatic sac is in the same region, and a sac decompression can be done in the same sitting. The disadvantages are that cerebrospinal fluid (CSF) leak is more common, as is postoperative headache; both of which may be avoided by the retrosigmoid variation of the retrolabyrinthine approach. It is less time-consuming and also easier to identify the nerve root in the retrosigmoid approach, as it is found more laterally in the cerebello-pontine angle cistern in this method.
After exposure, the posterior wall of the IAM is removed up to the singular canal, and the superior vestibular and singular branches of the vestibular nerve are sectioned. The inferior vestibular branch is not divided, as it is closely related to the cochlear nerve and supplies the saccule, which is not known to have any vestibular function in human beings and would not cause postoperative vertigo if not sectioned.
Combined Retrolabyrinthine-Retrosigmoid Approach
The combined retrolabyrinthine-retrosigmoid approach combines the best features of the retrolabyrinthine and retrosigmoid approaches with less bone removal and cerebellar retraction.
The retrosigmoid approach has been found to be simpler and as effective for selective vestibular neurectomy in patients with refractory vertigo but having good hearing.
Endoscopic-Assisted Vestibular Neurectomy
Endoscopic-assisted vestibular neurectomy offers a magnified view of relevant structures and the ability to use angled or flexible scopes to allow visualization of structures not seen well with the microscope. It also helps to identify and obliterate any mastoid air cells entered while drilling the IAC, and thus minimize CSF leak. The disadvantages of using the endoscope include keeping it free of blood, fluid, and secretions; the limited choice of instruments for manipulation; and a relatively 2-dimensional field compared to using the microscope.
Translabyrinthine Vestibular Nerve Section (TLVNS)
Translabyrinthine vestibular nerve section (TLVNS) results in a complete labyrinthectomy by dual denervation (ie, preganglionic and postganglionic) of the vestibular system. It can be undertaken when no preoperative residual hearing exists and also in cases of previously failed labyrinthectomy or vestibular nerve section in which partial removal of the neuroepithelium or partial sectioning of the vestibular nerves was accomplished.
Thus, TLVNS normally follows a labyrinthectomy in which the semicircular canals and vestibule are opened and all the soft-tissue elements of the membranous labyrinth are removed. The IAM is approached by blue-lining first the vestibule and then the IAM so that the latter can be skeletonized and its contents clearly identified; only then are all the branches of the vestibular nerve dissected away from the facial nerve and sectioned proximal to Scarpa’s ganglion.
Since hearing is not a consideration in this method, the cochlear nerve may be sacrificed in cases in which disabling tinnitus also exists as a feature of advanced Meniere disease. This precludes the possibility of cochlear implantation in the future and also does not guarantee relief of tinnitus; hence the cochlear nerve is preserved.
TLVNS is considered the criterion standard for disabling spontaneous vertigo in end-stage Meniere disease, and patients must be counseled that this complete de-afferentation will lead to some loss of balance permanently that may not be fully compensated by central mechanisms, resulting in chronic unsteadiness.
A full audio-vestibular evaluation is used, including pure tone and speech audiometry, electronystagmography (ENG) and brainstem evoked-response audiometry (BERA), vestibular-evoked myogenic potential (VEMP) and platform posturography. Electrocochleography and dehydration tests are not routinely performed now because they do not impact the decision to perform vestibular neurectomy.
HRCT and MRI
A HRCT and contrast MRI Temporal bone is taken to rule out any intracranial pathology like a cerebellopontine lesion and to determine the degree of perilabyrinthine pneumatization. If a middle cranial fossa approach is planned, then a Stenvers view is routinely obtained to show the contours of the floor of the middle cranial fossa and the relationship of the meatal plane and superior semicircular canal (SCC) with respect to the arcuate eminence.
Serological tests may be performed in specific cases in which autoimmune disorders are suspected, although it does not impact the decision to operate unless there is any significant co-morbidity.
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