HISTORICAL BACKGROUND

The first vestibulocochlear nerve section was performed in 1898 by Krause,² and the first posterior fossa vestibulocochlear nerve section for the treatment of vertigo in a patient with Meniere’s disease was performed by Frazier in 1904.³ McKenzie⁴ was the first to perform selective division of the superior half of the vestibulocochlear nerve in 1931. This procedure was popularized by Dandy, who first sectioned the vestibulocochlear nerve in 1925, and later began performing selective vestibular neurectomies in 1932. Dandy eventually performed 624 vestibular neurectomies; half of these patients underwent selective vestibular neurectomy.⁵ This series of patients is impressive not only because of the large number of patients, but also because the procedures were performed without microscopic assistance and modern antibiotics. With Dandy’s death in 1946, vestibular neurectomy was largely replaced by peripherally destructive procedures. Although highly successful in treating vertigo, residual hearing was compromised with these procedures.

The first microsurgical division of the vestibular nerve was performed in 1961 by House⁶ through a middle cranial fossa approach; this approach was modified further by Fisch⁷ and Glasscock.⁸ The middle cranial fossa approach allows the surgeon to identify and section selectively the vestibular nerve within the internal auditory canal (IAC). The procedure is technically challenging, however, and has a high incidence of postoperative facial nerve weakness and deafness.

The retrolabyrinthine approach for division of the trigeminal nerve was described by Hitselberger and Pulec in 1972,⁹ and in 1978, Brackmann and Hitselberger reported treatment of vertigo and tic douloureux by selective division of the vestibular and trigeminal nerves via a retrolabyrinthine approach.¹⁰ Silverstein and Norrell¹¹ noted a clear cleavage plane between the cochlear and vestibular nerve fibers while resecting a ninth cranial nerve neurilemmoma via a retrolabyrinthine approach, and suggested that selective vestibular neurectomy was feasible in this fashion. Silverstein’s subsequent reports^12,13 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 further. In an effort to prevent cerebrospinal fluid leak, which was caused by difficulty reapproximating the dura anterior to the sigmoid sinus, many surgeons adopted a retrosigmoid approach for vestibular neurectomy. This approach also allows division of the nerve more laterally in the cerebellopontine angle cistern, where identification of the vestibulocochlear cleavage plane may be easier. The retrosigmoid procedure can be extended in some cases to include drilling of the posterior IAC, allowing visualization of the intracanalicular portion of the nerve; this may be helpful in cases where the nerve bundles cannot be distinguished within the posterior fossa. In an effort to avoid problems with postoperative headache, which was a common complication after retrosigmoid vestibular neurectomy, Silverstein and colleagues^14–16 subsequently proposed a combined retrosigmoid-retrolabyrinthine approach.

PREOPERATIVE EVALUATION

Important preoperative considerations include the etiology of vertigo; the degree of disability; previous therapies; and the patient’s age, overall health, hearing status, and motivation. Surgical intervention for vertigo should be considered only in patients with incapacitating vertigo who have failed maximal medical therapy and are believed to be able to compensate centrally for the expected 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. Historical clues to diagnosis of laterality in vestibular disease 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, and surgery on the poorer ear is relatively contraindicated in this setting.

Patients are often desperate for relief of debilitating symptoms, and are anxious to try any treatment that offers a chance at cure. Patients should be counseled that there is no promise of a cure with any treatment, and that often the best course is a conservative one. This is especially true for patients with vertigo that is not due to Meniere’s disease because surgery for these patients is much less likely to succeed.¹

If maximal medical therapy has failed, especially in classic unilateral Meniere’s disease, the patient may be a suitable candidate for surgery. Patients considering vestibular neurectomy should be counseled that this form of treatment does not address the underlying disease, but simply endeavors to eliminate one major symptom of the disease. The Meniere’s disease process continues to affect the inner ear even after surgery, and further hearing loss is anticipated, unless remission occurs. In addition, patients should know that they are trading a partial unilateral vestibular lesion for a total loss of vestibular function on that side, and that postoperative vestibular rehabilitation is crucial to the central nervous system compensation process.

Preoperative documentation of hearing with audiometric testing is obtained in all patients. Because the audiogram measures only the patient’s current hearing ability, however, the utility of the patient’s future hearing should also be considered. Because the disease process may cause a continued deterioration in hearing postoperatively, a practical approach to determining surgical candidacy takes this future hearing loss into consideration. If residual hearing is of borderline usefulness to the patient, and future hearing loss is anticipated, a procedure that is less risky than vestibular neurectomy, such as labyrinthectomy or intratympanic gentamicin treatments, 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 administration of paramagnetic contrast material is preferred. Computed tomography (CT) scanning may also be useful to evaluate the bony anatomy of the temporal bone, but is not routinely required preoperatively.

When a patient has been determined to be a candidate for surgery from an otologic standpoint, the patient must be determined to 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. Before proceeding with surgical intervention, the patient should understand the risks and benefits of the procedure. Providing patients with reasonable expectations regarding their postoperative outcome is also important.

PERTINENT NEUROANATOMY

Within the IAC lie 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 relationships with each other are described in this section for the right ear as if the patient is supine with the head turned away from the surgeon (Fig. 36-1).

FIGURE 36-1 Anatomy of CN VII and VIII from the brainstem to the internal auditory canal as viewed from a position posterior to the nerves. The facial nerve begins posteroinferior to the cochleovestibular nerve and rotates around the cochleovestibular nerve until it reaches the anterosuperior quadrant of the internal auditory canal. The cochlear nerve fibers begin to separate from the vestibular nerve fibers in the cerebellar cistern. The cross sections show the rotation of the nerves. The superior vestibular nerve is divided to show the facial nerve anteriorly. Co, cochlear nerve; F, facial nerve; IV, inferior vestibular nerve; PAN, posterior ampullary nerve; PSCC, posterior semicircular canal; SV, superior vestibular nerve; VN, vestibular nerve.

The transverse (falciform) crest divides the lateral IAC into superior and inferior halves. The superior half is divided into anterior and posterior quadrants by Bill’s bar. The facial nerve and nervus intermedius occupy the anterosuperior quadrant, and the superior vestibular nerve, which innervates the superior semicircular canal, the horizontal semicircular canal, the 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 innervates the saccule, and is joined by the singular nerve, which innervates the posterior semicircular canal. This junction occurs where the singular canal emerges from its canal 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 so that it is 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 just medial to the transverse crest. A gross separation of the cochlear and vestibular nerve fibers usually exists medial to the porus acusticus¹⁷; however, in 25% of cases, the separation is evident only histologically. Even in cases of a visible cleavage plane, there can be a high degree of nerve fiber overlap (Fig. 36-2).¹⁸ The cochlear nerve fibers enter the brainstem slightly inferior and posterior to the vestibular nerve fibers.

FIGURE 36-2 A-F, Variability in the separation of the cochlear and vestibular fibers. The presence of a cleavage plane does not correspond to a complete segregation of cochlear and vestibular fibers.

Rights were not granted to include this figure in electronic media. Please refer to the printed book.

(Modified from Rasmussen A: Studies of the VIIIth cranial nerve of man. Laryngoscope 50:667, 1940. Used by permission from Wolters-Kluwer.)

A landmark that may be useful in identification of the cochleovestibular cleavage plane is the nervus intermedius. This nerve contains the afferent and parasympathetic fibers of 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. It enters the brainstem near the cochleovestibular nerve, and its fibers terminate in the salivatory and glossopharyngeal nuclei.

The facial nerve runs proximally from its anterosuperior position in the lateral IAC to a position that is 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 intracisternal course; however, it may be visualized by gentle retraction of the cochleovestibular nerve or with the use of an endoscope. It is slightly grayer in color than the cochleovestibular nerve.

CN V is located superior to the facial and cochleovestibular nerves and can often be identified by its characteristic striations that are perpendicular to its course. CN IX, X, and XI are located inferior to the facial and cochleovestibular nerves (Fig. 36-3).

FIGURE 36-3 Anatomy of the cerebellopontine angle as viewed through a large retrosigmoid craniectomy showing CN V, VII, VIII, IX, and X. SS, sigmoid sinus; ce, cerebellum; 5, cranial nerve V; 6, cranial nerve VI; 7, cranial nerve VII; 8, cranial nerve VIII; 9, cranial nerve IX; 10, cranial nerve X; 11, cranial nerve XI.

RETROLABYRINTHINE APPROACH

The patient is positioned in the standard fashion for mastoidectomy after removing approximately 3 to 5 cm of hair. The planned surgical site is marked and injected with a mixture of local anesthetic and epinephrine, and the scalp is prepared and draped in the standard fashion (Fig. 36-4A). The facial nerve is monitored electromyographically throughout the procedure. Auditory function is generally monitored by recording intraoperative auditory brainstem responses, although near-field CN VIII electrodes are preferred by some surgeons after the nerve is exposed. A wide postauricular scalp flap is elevated and turned forward, exposing the temporal bone. The authors prefer distinct scalp and musculoperiosteal incisions, each of which can be individually closed at the end of the case.