Comparison of vestibular and cochlear ototoxicity from transtympanic streptomycin administration

Abstract

Hypothesis: The relative dose-related cochlear and vestibular ototoxicity produced by transtympanically injected streptomycin (SM) compared to that of gentamicin (GM) was assessed.

Background: Although SM, the first aminoglycoside used transtympanically, is thought to be selectively vestibulotoxic, it has been replaced by GM in current clinical use. Little experimental data exist that directly demonstrate the relative cochlear and vestibular ototoxicity resulting from transtympanic administration of SM compared to GM.

Methods: Histologic evaluation was performed on inner ears from Mongolian gerbils to study vestibular and cochlear damage. Comparisons were made between animals receiving single (1 x SM) and five daily (5 x SM) injections of SM/Gelfoam-slurry and similarly injected and noninjected controls. These data were compared to results obtained using GM (1 x GM and 5 x GM) reported previously.

Results: Two weeks after injection, parallel qualitative and quantitative changes were seen in posterior cristae and cochlear sensory epithelia in the 1 x and 5 x SM injected groups, similar to those resulting from GM injections. Statistically significant decreases in number of hair cells were seen when 5 x SM injected ears were compared to 1 x SM injected ears and control ears. Increased damage was seen with increased dosage of each drug. Whenever damage was observed to the posterior crista sensory cells, damage was also seen in cochlear hair cells.

Conclusions: In this model, SM and GM produced significant cochlear damage when vestibular damage occurred. These results suggest that, in the gerbil, SM and GM are ototoxic but not selectively vestibulotoxic. Increasing the number of transtympanic injections generally increases the damage to sensory hair cells in the posterior crista and the cochlea. A variation in interanimal susceptibility to ototoxic effects exists, but the amount of damage is consistent in cochlear and vestibular hair cells from the same animal. No evidence for selective vestibular ototoxicity from transtympanic SM was found.