Delayed Ototoxicity

Delayed ototoxicity from topical gentamicin has been documented. It may persist within the inner ear of animals for more than 6 months. So gentamicin ototoxicity may continue for several months after the gentamicin therapy has stopped.

The duration of middle ear disease can affect the absorption of materials into the inner ear. The permeability of the round and oval windows increases early in the course of otitis media, allowing increased amounts of ototoxins to pass easily into the inner ear. With chronicity, granulation tissue and mucus accumulation within the middle ear may prevent the diffusion of the same medication into the inner ear. In a chinchilla study, when experimentally induced otitis media resolved there was less susceptibility to ototicity by polymyxin.¹⁹ Some ototoxic medications and chemicals are also pro-inflammatory, causing thickening of the mucosa and the membranes of the round and oval windows, which limits absorption of these compounds into the inner ear.

With so many otic products available to veterinarians, it is prudent to look at the ingredients of an ear cleaner to see whether it may have ototoxicity potential. Many manufacturers are aware of the potential ototoxicity of the ingredients in their prod­ucts and so they have been placing warnings on their labels stating “Caution: Do not use if the eardrum is ruptured.”