Diagnostic and Rehabilitative Audiology

The long-term research objectives for the Audiology Section at Mayo Clinic are centered in the general topic areas of assessment and treatment of Auditory Sensory Function. Currently, approximately 28 million persons in the United States have measurable hearing loss, and this number is expected to grow over the next thirty years, as the growth of senior citizens (65+ years) is projected to double. Although hearing loss is viewed primarily as an older person’s handicap, an increasing number of younger persons suffer from hearing loss. Perhaps more alarmingly, younger persons are less likely to wear amplification devices than their older counterparts (Kochkin, 1999), perhaps because of the stigma associated with use of hearing aids. Figure 1. Age, in years, as a function of hearing aid ownership (millions).

Research in the audiology section at Mayo has focused on current and future hearing aid technology, including digital amplification, directional/beamforming microphone arrays for noise reduction, and development of fitting and assessment techniques for hearing aid verification. This research has led to a considerable number of Medical/Industry extramurally funded protocols and subsequent publication. Mayo is regarded as a national leader in the area of applied research with hearing aid strategies. Further work is being conducted on new techniques to increase hearing aid use by younger persons, including remote (Internet) programming of digital hearing aids, disposable or partially disposable hearing aids, and self-adjustment of hearing aid parameters. All of these methods improve access and efficiency to patient care, and are especially suited to the Mayo patient, who often travels some distance to receive hearing health care.

Another direction for future research is in the diagnostic tests available for assessment of hearing loss to assist with determination of inner hair cell damage. Currently, the most sensitive and specific measures for hearing assessment are unchanged from the techniques used for the past fifty years. Pure-tone signals are varied in intensity to determine behavioral thresholds for frequencies between 250 and 8000 Hz. Although functionally accurate, these measures provide no information regarding inner- and outer- hair cell survival in the Organ of Corti. Figure 2: The Organ of Corti.

Inner hair cells are the afferent fibers primarily responsible for sensory perception. Outer hair cell function, which is primarily efferent in nature, is primarily to modulate inner hair cell function and provide greater dynamic range of hearing for low-input stimuli. Figure 3: Effect of hair cell damage on auditory neural tuning.

Clinically, there is a well-established method for assessment of outer hair cell function, and this measure is used by audiologists at Mayo for the newborn hearing screening program. Otoacoustic emissions (OAE) data are collected on every baby born on the Mayo Clinic campus, and these results provide accurate and efficient information regarding the presence of normal hearing at birth. At issue, however, is that this result is a "pass/fail" test that does not provide information regarding the degree of hearing loss, when present. Future research will be directed towards establishment of electrophysiological measures that may be used to determine objectively the degree of hearing loss, when present. In addition to being useful for pediatric and difficult-to-test populations, it would provide an excellent clinical tool for all patients with measurable hearing loss. Work underway will investigate the use of specialized auditory brainstem response (ABR) testing to assess partial damage to inner hair cell function. If successful, this test will be useful for determination of hearing aid candidacy, and also aid in the development of hybrid hearing aid/cochlear implant strategies.

If both inner and outer hair cell function is obliterated by extensive cochlear damage, cochlear implants provide an interface to surviving spiral ganglion cells in the auditory nerve for persons with severe-to-profound hearing loss. If, however, noise trauma, presbycusis, and/or ototoxicity causes damage only in certain (usually high) frequency regions. Figure 4: Precipitous high-frequency sensorineural hearing loss.

Persons with this type of hearing loss currently do not qualify for cochlear implants, and yet they do not function well with traditional or digital hearing aids. Research is planned to develop a hybrid, shallow-insertion depth cochlear implant that would stimulate basal regions of the cochlea electrically, while providing traditional acoustic amplification for more basal regions. Discussions have begun to develop extramural funding sources for this work, which could potentially benefit many hard-of-hearing persons.

In summary, Audiology research is directed towards a combination of applied and basic topics, and future progress is dependent on time away from clinical responsibilities and funding sources. Currently, the four Ph.D. audiologists are all on 100% clinical appointments, and this is the greatest challenge to future developments.