Inflammatory effects of topical antibiotic suspensions containing propylene glycol in chinchilla middle ears

This study examines the occurrence of inflammatory changes and cholesteatoma in the middle ears of seven chinchillas after the application of topical antibiotic suspensions containing two different concentrations of propylene glycol. The preparations used were Cortisporin otic suspension, which contains neomycin, polymyxin B, hydrocortisone, and 10.5% propylene glycol, and Cortisporin ophthalmic suspension, containing the same ingredients, but only a 2% concentration of propylene glycol. Six weeks after the administration of the Cortisporin preparations, applied to the middle ear through a transbulla approach, no cholesteatomas were found in the seven ears treated with the ophthalmic suspension. Evidence of mild inflammation was present in only two of these ears. In the seven contralateral ears treated with the otic suspension, middle ear adhesions were found in six, cholesteatoma was present in four, serous effusions were found in three, and one had a large tympanic membrane perforation. The ears that showed cholesteatomas also had histologic evidence of squamous metaplasia, granulation tissue, and erosion of the underlying bone. We submit that the pathologic responses of the middle ear mucosa treated with the otic suspension, were due to an inflammatory response to the higher concentration of propylene glycol compared to that of the ophthalmic suspension.