An experimental study on aeration in the epitympanum through the tympanic isthmus was done using human cadaver temporal bone specimens. The tegmen tympani of the temporal bone specimen was opened and covered by wrapping film to observe the attic from above. A thin metal tube was inserted into the Eustachian tube through which smoke was blown. The locus of the smoke flow was observed from above, and recorded by video tape. The epitympanum is divided into the anterior, the medial and the lateral portions by two kinds of vertical type mucosal fold. Normal specimens aeration at the lateral space of the epitympanum was the poorest and delayed. In specimens with a narrowed isthmus, this phenomenon was enhanced. In contrast, in specimens with an enlarged isthmus or aditus, aeration at the lateral space was improved as well as in the entire epitympanum. It is concluded that the lateral portion of the attic is closely related to the delayed aeration, followed by the development of the attic type cholesteatoma.
