Permeability and ototoxicity across the round window membrane was studied in guinea pigs by applying neomycin directly to the round window membrane, and the following results were obtained.
1) Neomycin passed the round window membrane freely, and caused damage to the cochlea.
2) Damage of the cochlea was confined to the organ of Corti. In general, the damage was severest in the basal turn, and was observed more frequently in the outer hair cells.
3) In some cases, the severest damage was observed in the second turn.
4) When neomycin was applied to the round window membrane on both sides, the damage was often quite severe.
5) Damage was observed as early as 4 hours after application of neomycin, and it increased in frequency as the time of application became longer.
6)Damage was compared in normal ears and in ears with experimental otitis media with effusion, and no significant difference was observed. The result appeared to indicate that neomycin passed the round window membrane quite freely regardless of condition of the middle ear mucosa.
7) Analysis of the concentration of neomycin in the perilymph disclosed that neomycin on the round window membrane penetrated into the perilymph quickly, and that its concentration in the perilymph became extremely high, reaching a peak after 2 hours, and then decreased gradually.
8) No consistent relationship was observed between the concentration of neomycin in the perilymph and the presence or absence of the damage at each time interval of neomycin application.
9) Results of this study suggested probable existence of individual variations among guinea pigs.
