Modelling Hearing Impairment After Ossicular Manipulation and Reducing This Injury

Abstract

When carefully looked for, postoperative sensorineural hearing loss affects one in five middle ear surgery patients and is associated with decreased quality of life. The mechanisms of this loss are poorly understood and animal models are an important way to investigate this injury and reduce iatrogenic harm. However, hearing outcomes are confounded when manipulation of the middle ear limits the reliability of air conducted (AC) evoked auditory potentials, and most currently available bone conduction (BC) transducers have poor output at the frequencies of most interest. Our aims were to develop a high frequency BC animal model of middle ear surgery, then use this model to investigate mechanisms of cochlear injury and whether this injury may be mitigated. We modified a magnetostrictive BC transducer and calibrated the evoked auditory potential output against a known AC stimulus in the guinea pig. We investigated two forms of ossicular manipulation: one with direct coupling to a micromanipulator, the other with burr contact to the incudomalleal complex, and assessed electrocochleography (ECochG) outcomes. We then extended this acute model to a recovery model to investigate whether a single intravenous dose of dexamethasone prior to ossicular manipulation impacted hearing outcomes, as indicated by ECochG. We were able to elicit evoked potentials with BC up to 32 kHz, providing a sensitive indicator of sensorineural hearing. We found these high frequency regions of the cochlea much more susceptible to injury than the lower frequencies traditionally investigated with BC. We identified conductive hearing losses which may otherwise have been assumed sensorineural. Ossicular manipulation with the micromanipulator did not show reliable sensorineural threshold deterioration, but high-speed burr contact did and was accompanied by a reduction in the amplitude of evoked potentials. This threshold shift persisted at follow up and was not altered by dexamethasone. The amplitudes for summating potentials and compound action potentials were, however, significantly better in the dexamethasone group. Surgical manipulation of the middle ear causes high frequency loss of cochlea function which can persist and is mitigated by dexamethasone. This is a useful model to further investigate the influence of middle ear surgery and its treatment.