When first developed, the focus of cochlear implantation was on paediatric age groups. However, the past 20 years has seen an increasing number of adult candidates identified and successfully implanted, with the largest number being aged 60 years or older (Figure 1). With an ageing population in Australia, this trend is likely to continue (Figure 1 and Figure 2).
How do we hear?
Hearing requires sound to be transmitted mechanically through the external and middle ear components (the conductive components) to the inner ear (Figure 3). Within the inner ear, the mechanical sound energy is converted to electrical signals by the hair cells (the sensory components) in the organ of Corti (the end organ of hearing within the cochlea), which in turn leads to stimulation of the auditory nerve and higher neural pathways, ultimately reaching the auditory cortex in the temporal lobe.2
Disruption to the conductive hearing mechanisms (e.g. caused by otitis media, perforations or otosclerosis) produces hearing loss that is mild-to-moderate in degree and responds well to treatment with surgery or hearing aids. In contrast, problems affecting the sensory mechanisms in the inner ear (e.g. caused by ageing, genetics, noise exposure, Ménière’s disease, trauma, meningitis or ototoxicity) may produce severe-to-profound hearing loss, which responds poorly to hearing aids and is associated with a significant reduction in quality of life. It is for patients with this latter type of hearing loss that the cochlear implant has been designed and developed.
How do we go deaf?
Most causes of acquired hearing loss involving the sensorineural parts of the hearing pathway are due to hair cell lesions of the inner ear (Figure 4a and b). The hair cells within the organ of Corti are vulnerable to a variety of toxicities. In some cases of profound hearing loss there may be a single identifiable causative agent that leads to hair cell loss and subsequent deafness, such as exposure to ototoxic agents (e.g. gentamicin, cisplatin) or following inflammation caused by meningitis. More often there are a variety of factors that together produce hair cell loss and profound hearing loss or, alternatively, the deafening aetiology is unknown. One common pattern of hearing loss is the effect of ageing (presbycusis) added to a pre-existing, nonaging factor for hearing loss, such as noise exposure with a background genetic susceptibility leading to early hair cell dysfunction.