Investigations into the Effects of Middle Ear Surgery on Inner Ear Function
Degree GrantorUniversity of Canterbury
Degree NameDoctor of Philosophy
Middle ear surgical procedures are typically associated with a high rate of improvement in air-conduction thresholds and a low rate of sensorineural hearing loss in the conventionally assessed frequency range (0.25 – 8 kHz). Hearing loss in the extended high-frequency (EHF) range (8 – 16 kHz), however, may be common, although its characteristics are not well understood. To elucidate the effects of middle ear surgery on auditory function, prospective investigations were performed to provide data that allowed transient and permanent changes in EHF hearing to be distinguished, and to establish the nature of EHF hearing loss. Changes in hearing at 0.25 to 16 kHz were documented in 88 patients following stapedectomy, ossiculoplasty, and tympanoplasty. Hearing was measured preoperatively, and 1 week, 1, 3, 6, and 12 months postoperatively. Results showed that elevation of EHF air-conduction thresholds occurred frequently following all three surgeries and was most severe one week postoperatively. Although significant recovery of hearing was recorded by three months, 12 months after surgery, 50% of patients who underwent stapedectomy, 42% who had a tympanoplasty and 20% who underwent ossiculoplasty retained a reduction in their highest audible frequency. A TEAC HP-F100 bone-conduction transducer was modified for use in EHF audiometry and used in a small pilot study to demonstrate that EHF hearing loss following stapedectomy may be composed of both conductive and sensorineural elements. It was hypothesised that changes to utricular responses reflective of trauma to the vestibular portion of the inner ear may also occur following middle ear surgery. Measurements of tap-evoked ocular vestibular evoked myogenic potentials (oVEMPs) were performed in the same group of patients in which audiometric data was collected. Overall, the oVEMP data provided no evidence of a postoperative change in utricular responses. To assess clinical importance of EHF hearing loss, its role in one aspect of auditory function, localisation ability, was investigated in 46 participants; 23 with EHF hearing loss and 23 with normal EHF hearing. Overall, the results agreed with previous studies that localisation accuracy decreased when was EHF spectral content was removed by filtering, however the difference between hearing groups was significant only when speakers were positioned in the lateral vertical orientation. Regardless of the clinical consequences, the increased vulnerability of EHF hearing acuity to the effects of middle ear surgery provides a useful model which could be used to efficiently assess the effect of technical factors of surgery or the efficacy of ototherapeutic treatments on hearing outcomes.