There are no treatments available, only two main types of prosthesis. First is the hearing aid, which amplifies incoming sounds and can be adjusted to amplify certain frequencies more than others to match the pattern of hearing loss of an individual, and to match the limited dynamic range of the damaged ear to avoid painfully loud sounds being delivered. However, there is much to be learnt about how we use the temporal and frequency cues in speech and other sounds in order to improve programming of hearing aids to facilitate use of these cues. The threshold for provision of a hearing aid is often considered to be around 25 dB hearing level, although many people are not fitted with an aid until their hearing is much worse than this (Davis and Moorjani, 2002). Second is the cochlear implant, which involves surgery to place an extended array of electrodes within the cochlear duct and a subcutaneous receiver for detecting the coded stimulation and transmitting to the electrode array. This is normally considered only for people who have such a profound hearing impairment that they are not helped by hearing aids. For example, the vast majority of children fitted with a cochlear implant have hearing loss of 95 dB or more (Fortnum et al., 2002). Both prosthetic approaches require time to adjust, and in the case of cochlear implants, a considerable period of rehabilitation is needed to maximise benefit.
Neither prosthetic approach is ideal, so there is a need for other approaches to minimise the effects of hearing impairment. Understanding the molecular basis of hearing impairment should allow the development of treatments to stop or slow down progression of hearing loss, if not to restore hearing. Some preliminary work in animal models suggests that biological agents may be useful, at least if applied at around the time of a damaging stimulus (e.g. Wang et al., 2003; Zhai et al., 2004). Treatments might involve some way of triggering regeneration of lost hair cells and their supporting cells, because these cells do not regenerate once they die. Alternatively, a drug-based approach might allow some degree of transcriptional control, such as upregulating alternative genes to replace dysfunctional elements of the hearing process (e.g. Steel, 2000). Slowing down or stopping the advance of a progressive hearing loss seems to be a much more tractable biological problem than developing a treatment to correct an early developmental defect, and most deafness in the human population is progressive, even during early childhood (Fortnum et al., 2001; Johansen et al., 2004). The limited options currently available for hearing-impaired people together with the large numbers of affected individuals argues for further research towards understanding the molecular basis of the disease process.
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