Combined vestibular neurectomy and endolymphatic sac shunt via the retrosigmoid approach in the treatment of Meniere's disease.

Abstract

The nature of surgical treatment for Miniere's disease has evolved from destructive to conservative in the interest of preserving hearing. We have performed a combined procedure that involves a vestibular neurectomy to control vertigo, which is followed by an endolymphatic sac shunt to control hydrops in 26 patients. Both procedures are performed via the retrosigmoid approach during the same surgical step. We believe this combination procedure is a worthwhile option to consider in order to achieve good control of vertigo and endolymphatic hydrops and to preserve hearing in patients with Meniere's disease.

Introduction

In its guidelines published in 1985, the American Academy of Otolaryngology-Head and Neck Surgery defined the diagnostic criteria for Meniere's disease as two or more episodes of spontaneous vertigo lasting 20 minutes or longer, a documented hearing loss on at least one occasion, and the presence of tinnitus or aural fullness. [1] Although the etiology of Meniere's disease is unknown, its pathology is seen in the inner ear as endolymphatic hydrops. There is a distention and distortion of the endolymph-containing structures of the labyrinth. In diagnosing Meniere's disease, it is necessary to exclude other conditions, such as recurrent vestibulopathology, vestibular neuritis, and sensorineural hearing loss. [24]

The possible etiologies of Meniere's disease symptoms have been studied. Vertigo can be caused by a reduction in endolymph absorption by the endolymphatic sac and by an abnormal secretion of osmotically active proteins by the sac. [5] The immune reactivity of the inner ear is also centered in the endolymphatic sac. Repeated inflammatory reactions, such as those seen in patients with allergies, can cause sac dysfunction and lead to endolymphatic hydrops. [6] This multifactorial nature of Meniere's disease requires more investigation to define its etiology. [7]

Methods of evaluation

There are two somewhat specific tests for Meniere's disease or endolymphatic hydrops: the glycerol dehydration test and electrocochleography (ECoG). In the glycerol dehydration test, the patient ingests a dehydrating agent such as glycerol or mannitol, and the physician looks for subsequent changes in symptoms, particularly a measurable improvement in hearing.

ECoG exposes the ear to a train of click or tonal stimuli recorded by an electrode situated close to the cochlea. The electrode records action potentials (APs) from the firing of auditory neurons. The normal ECoG response is very complex, consisting of an AP, a summating potential (SP), and cochlear microphonic (CM) responses. The AP represents the simultaneous firing of cochlear neurons, while the SP and CM response correspond to end-organ responses by the organ of Corti. Although there is some overlap between normal and pathologic values, SP values typically rise ([greater than]10 mV) in patients with inner ear dysfunction. The increase in SP leads to a corresponding increase in the SP/AP ratio. Hydrops is suggested when the ratio is greater than 35%. Patients with Meniere's disease usually have an SP greater than 5 mV, and they often have an SP/AP ratio that is higher than normal. The SP is a strong indicator of endolymphatic hydrops and an occasional indicator of its secondary form, which features perily mphatic depletion. Endolymphatic hydrops causes the waveform to widen and show multiple peaks, which is quite different from a normal result.

Transtympanic ECoG can show evidence of cochlear involvement. ECoG measures the ratio of SP (probably arising from the movement of the basilar membrane) to AP on the nerve in response to auditory stimuli. Thus, ECoG is a useful tool for determining postoperative changes in cochlear pathology. [8]

Treatment options

The mainstays of treatment are sodium restriction, diuretics, vestibular suppressants, vasodilators, and physical rehabilitation. [9] When patients remain refractory to medical management, surgery becomes the best option to relieve the disabling symptoms of vertigo and unsteadiness. [10] There are two general types of surgical procedures: preservative and ablative. Preservation procedures include endolymphatic sac surgery and cochleosacculotomy. Ablative procedures include labyrinthectomy, vestibular nerve section, streptomycin perfusion of the labyrinth, and gentamicin or streptomycin instillation into the middle ear.

In all patients who have unilateral persistent otologic symptoms, magnetic resonance imaging is required to exclude acoustic neuroma, which can mimic Meniere's s disease. When Meniere's disease is identified, it is sensible to confirm the diagnosis with an ECoG.

Ablative procedures can be further classified into two categories: preservative and destructive. [11-44] Procedures that preserve hearing include vestibular nerve section, streptomycin perfusion of the labyrinth, and gentamicin or streptomycin instillation into the middle ear. Those that destroy hearing are labyrinthectomy and translabyrinthine nerve section.

The main consideration in choosing a neurotologic procedure is the patient's preoperative hearing level. In our practice, we perform a transmastoid endolymphatic sac shunt to the subarachnoid space or to the mastoid cavity for patients whose auditory threshold is approximately 30 dB or better. When the auditory threshold is between 40 and 80 dB, we perform a vestibular neurectomy via a retrosigmoid approach. This technique has become our routine procedure to relieve the disabling vertigo of Meniere's disease. When the patient has no useful hearing, we perform a labyrinthectomy. [15]

Even though vestibular neurectomy is considered a "conservative" procedure with regard to auditory function, we never can be sure how the endolymphatic hydrops underlying Meniere's disease will affect the remaining auditory function in the postoperative period. Therefore, we use an additional and complementary procedure--the endolymphatic sac shunt--to control this pathophysiology. During this operation, the increased fluid pressure in the endolymphatic spaces of the inner ear is released to the subarachnoid space or mastoid cavity. This allows the organ of Corti to function without the stress of that pressure.

The standard procedure for an endolymphatic sac shunt to either the subarachnoid space or the mastoid requires opening the mastoid cavity. [16,17] In order to avoid the risks associated with surgical trauma, in addition to seeking better results in vertigo control and hearing preservation, we have been performing the endolymphatic sac shunt after the vestibular neurectomy. Both procedures are performed via the retrosigmoid approach and at the same surgery.

Surgical technique

Vestibular neurectomy. We obtain access to the vestibular branch of the VIIIth cranial nerve in the usual way for a retrosigmoid-suboccipital approach. We make a 5-to 7cm linear incision 7 cm behind the postauricular crease. After we obtain a proper surgical field, we perform a suboccipital craniectomy, exposing the posterior fossa dura, keeping a bone table, and recovering the bone dust. Then we retract the cerebellum and expose the cerebellopontine angle. We identify the Vth, VIIth, VIIIth, IXth, Xth, and XIth cranial nerves. Then we use a dissector to separate the vestibular branch of the VIIIth nerve from the cochlear nerve, and we transect it with fine-tipped neurosurgical scissors (figure 1).

Endolymphatic sac shunt. After the vestibular neurectomy, we proceed to find the endolymphatic sac via the same approach. Dissections performed in our temporal bone laboratory have shown that we can find the endolymphatic sac by following anatomic landmarks on the posterosuperior wall of the petrous pyramid. We find the angle formed by the posterior edge of the sigmoid sinus and tentorium cerebelli and move 1 cm toward the IXth, Xth, and XIth cranial nerves from the apex of this angle. There are wide variations in the size, shape, and location of the endolymphatic sac.

Once we find this structure, we open the medial wall of the sac until we see the intraluminal space (figure 2). Then we place a Silastic sheet into the sac so that it protrudes through the incision to produce a permanent fistula between the lumen of the sac and the cerebellopontine angle. The Silastic can be fixed to the medial wall of the sac with Nurolon (figure 3).

The final steps are standard. We re-expand the cerebellum, close the dura, replace the bone plate, and seal the defect. We suture the soft tissue and skin and cover the wound with a compression dressing.

Discussion

Neurotologists have tried many surgical techniques to better control vertigo, which is probably the most disturbing symptom of Meniere's s disease. Our own experience has convinced us that vestibular neurectomy is the best surgical option to control the symptoms of meniere's s disease. However, we feel that this procedure does not address the issue of controlling endolymphatic hydrops, which has adverse effects on hearing and ear pressure. Furthermore, there is always the possibility that not all fibers of the vestibular nerve have been sectioned during this procedure.

In an attempt to achieve better control of vertigo and to preserve residual hearing, we have performed an endolymphatic sac shunt after vestibular neurectomy, via the retrosigmoid-suboccipital approach, as a complementary procedure during the same surgical step in 26 patients. This combination procedure has lowered postoperative SP/AP ratios on ECoG in nine patients for whom we have complete followup (table). [18,19] All patients have had stable audiograms, and none has complained about the procedure.

This combination procedure can also be performed via the retrolabyrinthine approach, but with this approach there is much less room in the cerebellopontine angle in which to work. The two main advantages of the retrosigmoid approach are the avoidance of the risks of drilling near the inner ear structures and facial nerve and the avoidance of acoustic trauma that can be associated with mastoidectomy. In addition, by shunting the sac via this approach, we can avoid a cerebrospinal fluid (CSF) leak into the middle ear and eustachian tube. CSF is less likely to leak because the soft tissues on the bone defect are thicker than they would be after a postauricular incision. By adding a compression dressing, we believe we have virtually eliminated the possibility of this complication.

Acknowledgment

The authors thank Mr. Cesar Tamayo for preparing the manuscript, table, and figures and Mrs. Karen Berliner for her assistance and patience in reviewing the English version of this paper.

From Centro Medico Otologico, Santafe de Bogota, Colombia.

References

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