Integration of acoustic and electrical hearing.

INTRODUCTION Over the past quarter of a century, cochlear implants have become recognized as a highly successful treatment for deafness. The accompanying articles in this issue of JRRD provide ample evidence of the past and current success of electrical stimulation of the inner ear as a treatment for profound hearing loss. However, the majority of persons with hearing loss, including most veterans with hearing loss, are not profoundly deaf; that is, they have some remaining usable hearing. In the present article, we will update our recent experiences at the University of Iowa in using electrical stimulation of the inner ear via a cochlear implant as a supplement to a patient's existing hearing. In this approach, the goal is to preserve the patient's existing "acoustic" hearing and the "electrical hearing" of the cochlear implant merely adds perception of some of the speech sounds that are missed because of the partial hearing loss. HIGH-FREQUENCY HEARING LOSS The traditional method of treating high-frequency sensorineural hearing loss has been to fit a hearing aid that amplifies the high-frequency regions of speech. With the advent of better hearing aid components in the 1980s, as well as digital technology in the 1990s, it became possible to provide relatively large amounts of gain to these higher frequency regions. However, clinical experience, and some reports in the literature, evidenced that providing audible speech to high-frequency regions of hearing loss did not always restore speech understanding. Pavlovic [1] and Kamm et al. [2] were some early examples, demonstrating that for some individuals with more severe high-frequency hearing loss, the increase in speech recognition with added high-frequency audibility did not correspond with what would be expected based upon the increase in audible speech information. Hogan and Turner [3] and Ching et al. [4] expanded this idea with more systematic studies, showing that when the hearing loss in the high frequencies (approximately 2,500 Hz and above) exceeded 60 to 80 dB, some patients exhibited no improvement in speech recognition from high-frequency amplified speech. These results suggest that high-frequency speech cues, which usually involve recognizing consonants, are affected by severe hearing loss to the point where even when amplification is provided, the information transmitted to the brain is insufficient or incorrect because of the loss of sensory cells. The most common type of sensorineural hearing loss is a loss of hearing sensitivity that increases as frequency increases; this is also known as a "sloping" or "high-frequency" hearing loss. This hearing loss configuration can be linked to many different causes, including aging, noise exposure, and treatment with ototoxic drugs, all of which often occur in the veteran population. In typical high-frequency sensorineural hearing loss, the damage occurs primarily in the basal end of the cochlea, either to the hair cells themselves, or in the case of aging, apparently also to the endocochlear potential that serves to depolarize the hair cells when they are stimulated by sound [5]. When the hearing loss exceeds approximately 60 to 80 dB, this damage to the hair cells begins to affect not only the more susceptible outer hair cells but also results in missing inner hair cells [6]. These inner hair cells are the sensory receptor cells that are responsible for sending signals to the central auditory system; thus, in the case of severe-to-profound hearing loss, information is no longer being transmitted to the brain for the cochlear regions involved. Therefore, electrical stimulation of the auditory nerve, via a cochlear implant inserted only in the base of the cochlea, can provide a means of restoring hearing sensitivity for these high-frequency sounds. This is the basis for the Iowa/Nucleus Hybrid or "short-electrode" cochlear implant developed at the University of Iowa in cooperation with Cochlear Limited (Centennial, Colorado). …