Vestibular Neurectomy

Historical Background

The first vestibulocochlear nerve section was performed in 1898 by Krause, although this was an unsuccessful and eventually fatal operation performed for intractable tinnitus. The first posterior fossa vestibulocochlear nerve section for the treatment of vertigo due to Ménière disease was performed by Frazier in 1904. In 1931, McKenzie performed the first selective division of the superior half of the vestibulocochlear nerve with the goal of hearing preservation. The procedure was subsequently popularized by Dandy, who, in 1925, began to section the entire vestibulocochlear nerve but quickly adopted the selective vestibular neurectomy in 1932. Dandy eventually performed a total of 624 neurectomies, with half of the patients undergoing a selective vestibular neurectomy. This series of operations is impressive not only because of the large number of patients but also because the procedures were performed without the assistance of binocular microscopy and modern antibiotics. Following Dandy’s death in 1946, vestibular neurectomy was largely abandoned for decades.

The first microsurgical division of the vestibular nerve through a middle cranial fossa approach was performed in 1961 by House. This approach was later adopted and modified by both Fisch and Glasscock. The middle cranial fossa approach allows the surgeon to identify and selectively section the vestibular nerve within the internal auditory canal (IAC). This technically challenging procedure was not widely adopted because of the higher incidence of postoperative facial nerve weakness and deafness than that in posterior fossa approaches.

The retrolabyrinthine approach for division of the trigeminal nerve was introduced by Hitselberger and Pulec in 1972. In 1978, Brackmann and Hitselberger reported treating vertigo and tic douloureux by selective division of the vestibular and trigeminal nerves respectively, via the retrolabyrinthine approach. Silverstein and Norrell made particular note of a clear cleavage plane between the cochlear and vestibular nerve fibers while resecting a ninth cranial nerve (CN) neurilemmoma via a retrolabyrinthine approach, and suggested that this finding could enhance the success rate of selective vestibular neurectomy. Silverstein’s subsequent reports on the success of retrolabyrinthine vestibular nerve section in eliminating vertigo while avoiding facial nerve damage and hearing loss ushered in a new era for selective vestibular neurectomy. ^,

The success and increasing popularity of the retrolabyrinthine approach to vestibular neurectomy in the 1980s led to efforts to improve the technique. In an effort to prevent the cerebrospinal fluid leak caused by difficulty approximating the dura anterior to the sigmoid sinus, many surgeons adopted a retrosigmoid approach for vestibular neurectomy. This approach also allowed visualization of the nerve more laterally in the cerebellopontine angle cistern, where identification of the vestibulocochlear cleavage plane may be clearer. The retrosigmoid procedure can also be extended to include drilling of the posterior IAC, allowing visualization of the intracanalicular portion of the nerve. This may be helpful in cases where the vestibular and cochlear nerve bundles cannot be distinguished clearly within the posterior fossa. In an effort to avoid postoperative headache syndromes, which more commonly complicate the retrosigmoid approach, Silverstein and colleagues subsequently proposed a combined retrosigmoid-retrolabyrinthine approach.

Preoperative Evaluation

Important preoperative considerations include the patient’s age and general health, the etiology of the vertigo, the degree of disability caused by ongoing imbalance, previous therapy, hearing status, and motivation for recovery. Surgical intervention for vertigo should be considered only in patients with incapacitating vertigo who have not responded to maximal medical therapy and are likely to be able to compensate centrally for the profound postoperative unilateral vestibular deficit. In addition, reliable identification of the offending ear is a crucial component of the preoperative evaluation. This identification is accomplished by audiometry, vestibular testing, and, most importantly, the patient’s history. Lateralizing clues in the history include ipsilateral hearing loss, aural fullness, and tinnitus associated with the vertigo spells. If the patient has auditory symptoms in the contralateral ear, one must suspect that the disease may have become bilateral. Surgery on the poorer ear is relatively contraindicated in this setting.

Patients are often desperate for relief of debilitating symptoms and anxious to try any treatment that offers a chance at cure. Patients should be counseled that a definitive cure cannot be promised with any treatment and that the best course is often a conservative one. This is especially true for patients with vertigo that is not due to Ménière disease because surgery for these patients is considerably less likely to succeed.

If maximal medical therapy has failed, second-echelon treatments such as intratympanic therapy with steroids or gentamicin, intermittent pulsatile pressure device treatment, or endolymphatic sac operations may be considered. These modern treatment options have significantly reduced the need for intracranial procedures to control peripheral vestibular disorders. However, if a patient with intact hearing is eager for relief of episodic vertigo and these other options fail, a selective vestibular neurectomy may be offered. Patients considering vestibular neurectomy should be counseled that this operation does not reverse the underlying disease, but it can eliminate vertigo, which is usually the most disturbing symptom, and it should meaningfully improve the quality and predictability of their daily life. In addition, patients should recognize that they are accepting a profound loss of vestibular function on that side, and that postoperative vestibular rehabilitation is crucial to the central nervous system compensation process.

Because Ménière disease continues to affect the inner ear after surgery, further hearing loss is anticipated unless remission occurs. Preoperative audiometric testing is obtained in all patients, after which the future prognosis and utility of the patient’s hearing must be considered. If the residual hearing is of borderline utility to the patient because additional future hearing loss is anticipated, a less risky treatment, such as labyrinthectomy or intratympanic gentamicin injections, may be a wiser choice.

Radiographic evaluation to exclude retrocochlear pathology is mandatory before considering vestibular surgery. Generally, magnetic resonance imaging (MRI) of CN VIII with the administration of paramagnetic contrast material is preferred. Computed tomography (CT) scanning is often used preoperatively to evaluate the anatomy of the temporal bone.

When a patient has been determined to be a candidate for surgery from an otologic standpoint, the patient must also be medically fit for intracranial surgery. Any procedure performed for vertigo is elective, and every effort should be made to optimize the patient’s medical condition before surgery. A comprehensive discussion of the risks and benefits should be undertaken. In light of the considerations already discussed, providing patients with reasonable expectations regarding their postoperative outcomes is particularly important.

Pertinent Neuroanatomy

The IAC contains the superior and inferior vestibular nerves, the cochlear nerve, the facial nerve, the singular (posterior ampullary) nerve, and the nervus intermedius. The anatomy of these nerves and their relationship to the others are described in this section for the right ear as if the patient were supine with the head turned away from the surgeon ( Fig. 33.1 ).

The transverse (falciform) crest divides the lateral IAC into the superior and inferior halves. The superior half is divided into the anterior and posterior quadrants by the Bill’s bar. The facial nerve, which is joined within the IAC by the nervus intermedius, occupies the anterosuperior quadrant. The superior vestibular nerve, which innervates the superior and horizontal semicircular canal ampullae, utricle, and a portion of the saccule, occupies the posterosuperior quadrant. The inferior half of the IAC is occupied by the cochlear nerve anteriorly and the inferior vestibular nerve posteriorly. The inferior vestibular nerve innervates the saccule and is joined in the IAC by the posterior ampullary (singular) nerve, which carries afferent input from the posterior semicircular canal. This junction occurs near the aperture of the singular canal, where the posterior ampullary nerve emerges into the posteroinferior quadrant of the IAC, approximately 2 mm medial to the transverse crest.

The cochlear nerve, which was initially directly anterior to the inferior vestibular nerve in the fundus of the internal canal, rotates 90 degrees inferiorly to a position directly inferior to the superior and inferior vestibular nerves at the level of the porus acusticus. The superior and inferior vestibular nerve fibers begin to merge medial to the transverse crest. The cochlear nerve fibers enter the brainstem slightly inferior and posterior to the vestibular nerve fibers. A microscopically visible cleft separating the cochlear and vestibular nerve fibers is often identified within the cerebellopontine cistern, usually beginning just medial to the porus acusticus. However, visualization of this cleavage plane cannot be relied upon to guide a selective vestibular neurectomy in all surgical cases, because the separation of the nerve bundles is only evident histologically in 25%. Furthermore, even when a visible cleavage plane is evident, there can be a high degree of nerve fiber overlap ( Fig. 33.2 ). This accounts for some uncertainty about maintaining all cochlear fibers while achieving the complete deafferentation of vestibular inputs when using posterior fossa neurectomy procedures. The nervus intermedius is a landmark that may be useful in identifying the cochleovestibular cleavage plane. This nerve contains the afferent sensory and efferent parasympathetic secretomotor fibers related to the facial nerve and runs between the facial and cochleovestibular nerves along their entire length. It is often located on the anterior portion of the cochleovestibular cleavage plane.

The facial nerve runs proximally from its anterosuperior position in the lateral IAC to its root entry zone, which is actually inferior and slightly anterior to the cochleovestibular nerve at the brainstem. Because of its ventral position, it is hidden by the cochleovestibular nerve along most of its cisternal course. It may be visualized by gentle retraction of the cochleovestibular nerve—retracting inferiorly when working nearer to the porus acusticus, and superiorly when working near the brainstem. The facial nerve is grayer in color than is the cochleovestibular nerve.

The trigeminal nerve is located superior to the facial and cochleovestibular nerves and can be identified by its characteristic striations that are perpendicular to its course. The lower cranial nerves, IX, X, and XI, are located inferior to the facial and cochleovestibular nerves ( Fig. 33.3 ).

Selection of Surgical Approach

The success rates for the control of vertigo by all approaches to the vestibular nerve range from 80% to 95%, which is notably poorer than that expected after a transmastoid labyrinthectomy. In the modern era, most neurotologic surgeons still performing this operation expose the eighth cranial nerve using one of the posterior fossa approaches, usually retrolabyrinthine, retrosigmoid, or a combination of the two. A discussion and illustration of these operations will constitute the emphasis of this chapter, but we will briefly consider two alternative approaches that are used far less frequently. These latter operations are discussed and illustrated in greater detail elsewhere.