Objective: To ascertain whether the severity of toxin-related hearing loss and the interval between instillation of toxin and surgical trauma affect hearing recovery capacity following semicircular canal (SCC) surgery in the rat.
Material and methods: Twelve rats were injected with Pseudomonas aeruginosa exotoxin A (PaExoA). Auditory brainstem responses (ABRs) were measured 72 h and 3 weeks later. Depending on the severity of hearing loss, the rats were divided into two groups: those with moderate (Group A; n = 6) and severe (Group B; n = 6) hearing loss. Three rats from Group A were then operated on 3 weeks after toxin exposure and the other three 3 months after instillation of toxin. All Group B rats were operated on after 3 months.
Results: In Group A, post-surgical hearing loss recovered to a varying degree but rats in Group B showed little or no hearing recovery capacity. This difference was statistically significant. When the six rats with moderately toxin-affected ears were compared, statistical differences in recovery capacity between those operated on at 3 weeks and at 3 months were also detected. The group with a shorter interval showed significantly less hearing recovery of inner ear function following surgical trauma.
Conclusion: When the toxin causes severe hearing damage there is no capacity for cochlear recovery following additional surgical trauma. When the rat inner ear is moderately affected by PaExoA, the interval between toxin exposure and SCC destruction plays a significant role in the ultimate hearing outcome. Cochlear recovery potential seems to be weakened in close temporal proximity to toxin exposure, but recovers with the passage of time.