Histopathology of chloroform-induced inner ear damage.

Inner ear function loss was caused in guinea pigs and rats by injecting chloroform into the middle ear. After symptoms for cochlear and vestibular deficit had been registered, the animals were permitted to survive for one day to five months. Ear histopathology was then studied in celloidin sections. In both species, hair cells and afferent nerve fibers were intact at all survival times. The acute stage of functional loss in guinea pigs was associated with inner ears of normal histological appearance. Within days after chloroform injection a severe otitis media developed which led to fibrous occlusion of the round window and eventually to new bone growth in the middle ear space around the otic capsule. A secondary labyrinthitis was also observed, resulting in endolymphatic hydrops at longer survival times. Different histopathological changes were seen in rats. The tectorial membrane appeared swollen in all cases, the swelling being more severe in more apical turns at longer survival times. It is concluded that only secondary sequela of the initial functional insult can be detected by standard light microscopic histopathology. Chloroform does not cause a chemical labyrinthectomy as previously assumed, although it is severely ototoxic.