Experimental induction of middle ear cholesteatoma in rats.

We induced cholesteatoma in two groups of rats by instilling different concentrations of propylene glycol into the middle ear cavity. Fifteen rats were exposed to 50% propylene glycol (group I), while pure propylene glycol was applied to six others (group II). The group I rats were killed 1 month after instillation. Seven of the 15 showed cholesteatoma in the middle ear with accumulation of keratin debris. The group II rats were killed 3 months after instillation. All six animals showed inflammation in the experimental ears, and five of the six experimental ears showed cholesteatoma in the middle ear cavity. Six experimental ears in group I and five in group II revealed retraction of the tympanic membrane, possibly due to eustachian tube obstruction. Bone resorption was seen along with cholesteatoma and inflammatory cells and osteoclasts in the middle ear of all 11 of these rats. The seventh cholesteatoma of group I can be classified as a microcholesteatoma, a pearl-like cyst within the tympanic membrane. The microcholesteatoma was formed by an invasion of basal cells from the tympanic epidermis and the proliferation of these cells in the fibrous layer of the tympanic membrane. Our findings suggest that cholesteatoma in the middle ear cavity is a response to the inflammation produced by high concentrations of propyleme glycol.