Middle Cranial Fossa-Vestibular Neurectomy

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Chapter 35 Middle Cranial Fossa—Vestibular Neurectomy

The treatment of Meniere’s disease continues to evoke controversy, as evidenced by a multitude of treatment modalities and their claims of efficacy. Medical management in an attempt to alter or stall the course of this condition has proved ineffective, with the exception of symptomatic relief of vertiginous attacks with the use of pharmacologic agents. In recent years, in light of the questionable efficacy of endolymphatic sac operation and the undesirable sequelae of myriad ablative procedures, vestibular neurectomy has been accepted as the most effective means to manage recalcitrant and disabling Meniere’s disease.

With either the middle fossa or the posterior fossa approach, ablation of vestibular functions and vertigo is reported to be 85% to 99%,¹^–⁴ greatly exceeding the results of other treatment protocols. Technical difficulties have been cited by many authors as the major reason for abandoning the middle cranial fossa approach in favor of a predominantly neurosurgical approach from the posterior, especially as collaborative efforts between otoneurology and neurosurgery increase. Many centers in Europe and South America continue to use the middle fossa approach, however, with great success.³^–⁶

The middle cranial fossa vestibular neurectomy performed at the University of Zurich is also known as the transtemporal-supralabyrinthine approach. In contrast to the middle fossa approach of House,⁷ which involves significant elevation of the middle fossa dura and retraction of the temporal lobe, the transtemporal-supralabyrinthine access to the internal auditory canal (IAC) is gained through bony reduction, with only minimal retraction of the dura.

PATIENT SELECTION

Patients with unilateral Meniere’s disease who have incapacitating attacks of vertigo of at least 6 months’ duration despite maximal medical therapy are candidates for vestibular neurectomy. These patients usually have residual and fluctuant hearing. Patients with severe to profound hearing loss and extremely poor speech discrimination are better managed by translabyrinthine cochleovestibular neurectomy. The severity of the vertiginous attacks indicative of surgical management is subjective, and depends more on the patient’s functional capacity than on the frequency of attacks.

In bilateral Meniere’s disease, surgery could still be contemplated if a dominant side can be identified. Four patients at the University of Zurich underwent bilateral vestibular neurectomies in stages (separated by at least 1 year after good vestibular compensation); postoperative vestibular compensation was shorter and easier after the second operation. These patients have remained symptom-free for 16 to 20 years since surgery. Patients with Meniere’s disease suitable for a vestibular neurectomy account for approximately 10% of the whole group.

Unilateral peripheral vertigo without the full spectrum of Meniere’s disease may also benefit from vestibular neurectomy.⁸ In these patients, it is important to confirm the side of pathology with vestibular function tests if there is no hearing loss to provide a lateralizing sign. Other indications for vestibular neurectomy are rare. Labyrinthine trauma with residual hearing and disabling vertigo after successful stapedectomy with good hearing could be considered. Contraindications of surgery include an only hearing ear, signs of central vestibular dysfunction, and poor medical condition. Age older than 70 years is a relative contraindication subject to individual assessment.

PREOPERATIVE EVALUATION AND COUNSELING

Before surgery, the patient generally undergoes a full auditory-vestibular evaluation that includes full audiometry, electronystagmography, and auditory brainstem response. Electrocochleography and dehydration tests are not part of the test battery performed at the University of Zurich. A high-resolution computed tomography (CT) scan and magnetic resonance imaging (MRI) are obtained to rule out a cerebellopontine angle lesion, and to determine perilabyrinthine pneumatization. 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. Immunologic evaluation is obtained if autoimmune disorders or significant allergies are suspected. Immunologic evaluation is not a standard part of the test battery.

Patients are usually referred from other otologists and have been managed conservatively for a prolonged period without relief. They are made aware of the efficacy of vestibular neurectomy and its related complications, particularly complications pertaining to hearing and facial nerve.

PREOPERATIVE MEDICATION

The patient is premedicated with clonidine, metoclopramide, and midazolam before surgery. A perioperative antibiotic, ceftriaxone (Rocephin), 2 g intravenously for 24 hours, is started at the time of surgery and continued for the duration of the intravenous infusion, usually for 5 days.

SURGICAL SITE PREPARATION, POSITIONING, AND DRAPING

The surgical site is prepared in the operating room after the induction of anesthesia. Hair over the temporal region is shaved 9 cm above and 5 cm behind the pinna. The skin is washed with povidone-iodine (Betadine). The patient is secured on the Fisch operating table (Fig. 35-1) in supine position with the head turned to the side. Draping is standard except for a large-reservoir plastic bag, which is at the head of the table to catch excess irrigation fluid and blood.

FIGURE 35-1 Fisch table.

(From Fisch U, Mattox DE: Microsurgery of the Skull Base. New York, Thieme, 1988, p 17.)

INTRAOPERATIVE MONITORING AND ANESTHETIC CONCERNS

Intraoperative facial nerve monitoring using the Xomed nerve integrity monitor with percutaneous electromyography needles is standard. Unipolar and bipolar stimulating forceps are available. Intracranial pressure is controlled by deep anesthesia induced intravenously before introducing inhalation anesthetic. The partial pressure of carbon dioxide is maintained between 30 mm Hg and 40 mm Hg. Pharmacologic manipulation with dexamethasone (Decadron), 4 mg every 8 hours perioperatively for 4 days, and mannitol, 0.5 mg/kg intravenously intraoperatively, are also standard. Furosemide is added when necessary. Lumbar cerebrospinal fluid drainage is not routinely performed. Hypotensive anesthesia with nitroglycerin or clonidine (Catapres) or both is used in most cases to maintain a systolic blood pressure between 80 mm Hg and 90 mm Hg.

SPECIAL INSTRUMENTS

1. Fisch operating table (Contraves/Zeiss): Motorized table with the center of rotation at the temporal bone, allowing easy remote control for optimal positional changes that can be operated by the scrub nurse or the anesthesiologist (see Fig. 35-1)

2. Articulated middle fossa retractor (Fischer-13-18-200): Self-retaining retractor that allows adjustments in the angle of retraction, tilting, and sideways shifting of the retractor blade (Fig. 35-2)

3. Temporalis muscle retractor (Fischer-13-18-200): Strong, self-retaining retractor with a wide opening span, essential for the exposure of the zygomatic root (Fig. 35-3)

4. Angled microraspatory (Fischer-13-18-308 [left], 13-18-306 [right]): Specially designed to facilitate dural elevation; its shoulder retracts the dura away, and the tip is used to separate dural attachments to bone (Fig. 35-4)

FIGURE 35-2 Middle fossa retractor. (From Fisch U, Mattox DE: Microsurgery of the Skull Base. New York, Thieme, 1988, p 426.)

FIGURE 35-3. Temporal muscle retractor. (From Fisch U, Mattox DE: Microsurgery of the Skull Base. New York, Thieme, 1988, p 426.)

FIGURE 35-4. Angled raspatory. (From Fisch U, Mattox DE: Microsurgery of the Skull Base. New York, Thieme, 1988, p 427.)

FIGURE 35-5. A, Middle cranial fossa (MCF) topography. B and C, Correct (B) and incorrect (C) approaches.

(Redrawn from Fisch U, Mattox DE: Microsurgery of the Skull Base. New York, Thieme, 1988, p 430.)

SURGICAL TECHNIQUE

The objective of the transtemporal-supralabyrinthine approach for vestibular neurectomy is to gain access to the IAC through the exenteration of the supralabyrinthine bone, while dural elevation and retraction are limited to no more than 1.5 cm (Fig. 35-5).

Skin Incision

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 (Fig. 35-6). The depth of the incision is made to the temporalis fascia, and branches of the superficial temporal artery are divided and clamped. Retraction of the skin edges is provided by securing arterial clamps to the drapes.

FIGURE 35-6 Skin incision.

FIGURE 35-7. A and B, Muscle flaps.

FIGURE 35-8. Craniotomy.

Temporal Muscle Flap

After some undermining, the temporalis muscle is well exposed with a self-retaining retractor. Five muscle flaps (Fig. 35-7) are developed, elevated from the temporal squama, and retracted away with stay sutures. The temporal squama should be exposed from the root of the zygoma to the parietosquamous suture line. A temporalis muscle retractor should be used for the inferior exposure, where the identification of the zygomatic root is essential in the accurate positioning of the craniotomy.