Histopathology of the tympanic membrane in rabbit experimental otitis media with effusion and cholesteatoma. (Induction of experimental cholesteatoma and developmental mechanism of intramembranous cholesteatoma).

The retraction hypothesis and immigration hypothesis have been proposed as the etiology of acquired cholesteatoma.Our laboratory has supported the retraction hypothesis on the basis of clinical observations, while Sano et al. proved it experimentally.Sano attempted experimental induction of cholesteatoma by obstructing the tympanic orifice of rabbits, followed by raising them for 4-5 weeks, but the rate of induction was low.<BR>We conducted the same experiment employing a longer period of raising, with the objective of the observing the long-term result and increasing induction rate.<BR>The tympanic orifice of rabbits was obstructed with pieces of muscle tissue, and the rabbits were raised for 4-20 weeks.Tympanic perforation occurred in 10 of 28 ears but not in the remaining ears.All perforation cases had suppurative otitis media, but no epithelial invasion from the rim of the perforation was seen.The 18 ears with no perforation were able to be classified into Type I (no change: 2 ears), Type II (retraction of the pars flaccida: 12 ears), Type II a (retraction with effusion: 9 ears), Type IIb (retraction without effusion: 3 ears), and Type III (others; consisting of projection of the pars flaccida in 2 ears and hypertrophy at the normal position in 2 ears).<BR>Type I was considered to be healed otitis media with effusion (OME), Type II a to be OME, Type II b to be a sequela of OME and Type III to be otitis media without negative pressure.The pars flaccida in Type II a showed, regardless of the time after operation, retraction of various degrees, and that of the ear with effusion in the attic had hypertrophy.Intramembranous cholesteatoma of the pars flaccida was seen in one ear 12 weeks after operation.However, no increase in the rate of experimental induction of retraction cholesteatoma could be achieved by extending the observation time after operation.It was surmised that the patency of the eustachian tube is gradually reestablished after the operation, resulting in elimination of the negative pressure, which then permits improvement in the inflammation of the middle ear cavity. Also, because retraction cholesteatoma, even if formed, undergoes retraction in the mesotympanum long after the operation due to the anatomic characteristics of rabbits, the tympanic membrane is perforated, resulting in a pathological change from retraction cholesteatoma to suppurative otitis media.Therefore, it is thought to be necessary to establish a high negative pressure within 4-5 weeks after the operation in order to increase the rate of experimental induction of retraction cholesteatoma.