Intratympanic application of botulinum toxin: experiments in guinea pigs for excluding ototoxic effects

The aim of the study was to exclude ototoxic side effects of intratympanally applied botulinum toxin. The background is that a transection of the tensor tympani tendon (tenotomy) may relieve symptoms of tinnitus due to myoclonic tensor contractions. Moreover, there are certain indications that in some cases tenotomy may influence the course of Menière's disease positively. In such cases, a temporary (probatory) inactivation of the muscle with botulinum toxin might be better than a definitive surgical solution. Although in theory botulinum toxin should not have ototoxic side effects, a study on animals (guinea pigs) should prove this assumption. On eight guinea pigs (16 ears), the middle ear spaces (bullae) were opened and botulinum toxin was introduced. Hearing thresholds were measured via ABR recordings, prior to 1 and 3 weeks, respectively, after botulinum toxin application. Histological examinations of the middle ear mucosa were performed on each animal. In our series, the hearing thresholds remained essentially unchanged. Furthermore, no middle ear pathologies could be found in histology. No negative effects of botulinum toxin on hearing could be observed in our series. This is a precondition for the further development of the concept of intratympanical applications of the drug, to inactivate the tensor tympani muscle or for other options.