Surgery for Vertigo: Chemical Labyrinthectomy
Steven D. Rauch
The vertigo of unilateral Meniere’s disease is controllable by conservative, medical means in 90% to 95% of patients. The remaining 5% to 10% are considered intractable cases, manifesting frequent, severe, or otherwise incapacitating symptoms. These patients are candidates for invasive therapy. Invasive therapies have traditionally been surgical procedures classified as either drainage operations (cochleosacculotomy and endolymphatic sac decompression or shunt) or destructive operations (labyrinthectomy and vestibular neurectomy). Early attempts to capitalize on the ototoxicity of streptomycin to achieve “chemical labyrinthectomy” ablated vestibular function but also caused total hearing loss in the treated ear (1). Newer aminoglycosides, such as gentamicin, offer a fair margin of safety between vestibulotoxicity and cochleotoxicity, and low-dose titration protocols for administration further enhance this margin. Over the last 15 years, a number of investigators have revisited chemical labyrinthectomy using intratympanic gentamicin (2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20). In these reported protocols, administration of gentamicin until onset of labyrinthine symptoms often results in hearing loss. However, gradual administration of very low doses over weeks or months can reliably achieve reduction or elimination of vertigo attacks while preserving hearing. The protocols for treatment vary widely, but there is consensus that the procedure achieves a high degree of successful control of vertigo with acceptably small risk to hearing. Intratympanic gentamicin (ITG) therapy has thus achieved status as a minimally invasive technique in the destructive category. It offers the advantages of high success and low risk plus the added benefit that it is performed in the outpatient setting and has a far more benign course of recovery than that often seen in surgical labyrinthectomy.
TREATMENT RATIONALE
At the outset, intratympanic aminoglycoside therapy was seen as a chemical labyrinthectomy, with every effort made to achieve complete vestibular ablation. When this was performed with streptomycin by Schuknecht, the drug was administered repeatedly until patients developed gait ataxia and oscillopsia (1). Labyrinthine destruction was assured, but it came at the price of total hearing loss in the treated ear; no clear advantage over surgical labyrinthectomy was demonstrated. Treatment with alternative drugs and less aggressive dosing was undertaken in an effort to maintain a high degree of labyrinthine destruction but spare hearing. Gentamicin dosing schemes with two or more injections weekly achieved vertigo control rates of 80% to 97%, but hearing loss rates approached 20% to 40% in some series (3,7, 8, 9,11,18). Some small proportion of aminoglycoside ototoxicity represents an idiosyncratic hypersensitivity to the drug, possibly on a genetic basis. However, the majority of hearing loss cases arise because treating physicians emphasize vestibular ablation as the therapeutic end point, administering gentamicin doses that can fall into a cochleotoxic range. However, it may be feasible to obtain good vertigo control without dosing that achieves a high degree of vestibular ablation (21). This more gentle approach to therapy aims to downregulate vestibular symptoms while preserving hearing. It is unclear whether the efficacy of this very low dose therapy is truly due to ototoxic destruction of hair cells: There are reports that aminoglycosides can have actions as ion channel blockers (22) and can exert an effect on the vestibular dark cells (23) that might alter endolymph homeostasis in a beneficial way. If, as some suggest (11), incomplete vestibular ablation leads to higher relapse rate, it is an easy matter to retreat with more gentamicin when symptoms reactivate to an unacceptable level. Over the long term, giving a single intratympanic injection of gentamicin once or twice yearly appears to give good control of vestibular symptoms with minimal risk to hearing. This philosophy is gaining acceptance because it appears to offer the best balance of symptomatic relief and minimum morbidity.
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