Many surgical procedures, including otological procedures intended to restore hearing, involve substantial manipulation of components of the middle and inner ear. In particular cochlear implants (CIs), which are increasingly indicated for patients with some residual acoustic hearing, involves direct manipulation of the contents of the inner ear. Unfortunately, up to ~50% of these patients lose their residual acoustic hearing, and a substantial proportion of all CI patients experience balance dysfunction either immediately or some time after surgery. Several studies aimed at determining the cause of the immediate hearing loss have suggested that while the presence of a CI electrode does not substantially alter sound transmission in the cochlea, high-level transients are generated in the cochlea during insertion. These intracochlear pressure (PIC) transients can show peak pressures in excess of 170 dB SPL peak equivalent ear canal SPL (PEAC), thus could cause noise- induced hearing or vestibular loss. These studies were the first to identify the presence of PIC transients, but failed to identify the sources and the risks to hearing from these exposures. The goal of this proposal is to overcome these previous shortcomings using two techniques.
First (aim 1), we will further characterize PIC transients during CI surgery in cadaveric human specimens. PIC measurements will be conducted during insertion, manipulation, and removal of CI electrodes to quantify the number of PIC transients generated, and correlate with features such as CI electrode manufacturer, geometry, and insertion style (i.e. with a sheath or stylet or via forceps). The source of PIC transients will be investigated with live fluoroscopic imaging in a subset of these experiments to correlate PIC events with electrode position in the cochlea.
Second (aim 2), the risk of hearing loss from these PIC transients will be determined in chinchillas. PIC transients recorded in aim 1 will be translating into acoustic stimuli that produce identical intracochlear exposures in chinchilla using the recently quantified relationship between sound transmission into the inner ear in chinchillas and humans. This technique, which will generate acoustic stimuli that produce PIC in chinchillas that are identical to the PIC in human cochlea observed during CI surgery, requires use of two recent innovations:1) characterization of this relationship, and 2) development of a loudspeaker capable of generating the necessary high-level acoustic stimuli. The experiments thereafter follow a standard noise-exposure protocol in which physiological measures of animal hearing are assessed before, and at several time points after noise exposure to assess the resulting permanent hearing loss. These measurements will thus provide a quantitative estimate of the hearing loss expected from exposure to PIC transients generated during CI surgery that may lead to improved electrode design and surgical techniques. This proposal will thereby develop and validate an animal model for assessing human noise induced hearing loss risk. Furthermore, since PIC may be measured for any source that stimulates the inner ear, this proposal develops a technique that may be used to assess noise-induced hearing loss risk for non-traditional (non-air-conducted) noise sources.
Many otology and neurotology procedures, which are commonly performed to remove abnormal growths, drain fluid from the middle ear, and address hearing loss, involve substantial drilling of the mastoid bone, and may involve direct manipulation of the sound conduction pathway to the inner ear. We have recently shown that several such procedures (e.g. incidental drill contact on the ossicular chain and cochlear implant insertion through the round window) produce pressures in the fluid of the inner ear (both the cochlea and the semicircular canals) that are comparable in magnitude to high level sound exposure, suggesting that these procedures may counterproductively risk additional noise-induced hearing loss. Here, I propose using the recently developed relationship between human and chinchilla middle ear sound transmission to generate acoustic stimuli that produce pressures in the chinchilla cochlea that are comparable to those that observed in the human cochleae during such procedures, to investigate the occurrence, sources, and risk of hearing loss for such exposures.
