Sustained stimulation of the higher neural pathways linking the inner ear and the auditory cortex is necessary for development of normal sound and speech perception. Most causes of deafness involve the hair cells within the cochlea, with the higher pathways remaining intact. However, hearing losses that occur very early in life are associated with poor maturation of these pathways, particularly at a cortical level. Hearing loss acquired before the acquisition of cortical speech perception (prelingual) is usually associated with disordered higher pathways and characterised by abnormal speech quality, which in extreme cases can be completely unintelligible. Hearing loss acquired later in life, after the acquisition of speech (postlingual), is usually associated with well-formed higher pathways and characterised clinically by normal speech quality.
The separation of these two broad categories of hearing loss (prelingual versus postlingual) is an important factor in the selection and counselling of prospective cochlear implantation candidates. In general, candidates with postlingual deafness have predictable and satisfactory outcomes. With prelingual hearing losses, outcomes are more difficult to predict and can vary since they are dependent on many factors, including the age of the patient at implantation. Although the outcomes with regard to speech perception may be limited because cortical perception of speech sounds is poorly developed, the individual’s appreciation of environmental sounds allows many adults with prelingual deafness to derive considerable auditory benefit from cochlear implantation. However, the prospective candidate should receive careful counselling to ensure their expectations are realistic.
How common is hearing loss?
Hearing disorders are common in Australia, with over 3 million people (14%) reporting at least one long-term hearing disorder.3 The proportion of people with long-term hearing disorders increases with age from 3% of children aged up to 14 years to 49% of adults aged 75 and over, and is higher in men (18%) than women (11%).3 The prevalence of hearing loss in the adult community increases from 17% in 50-year-olds to 48% in 60-year-olds and 64% in 70-year-olds.4 The most common hearing disorder is complete or partial deafness, which affects one in 10 people in Australia.3
Most of the affected population has a mild to moderate degree of hearing loss that is highly suitable for assistance with a hearing aid. Severe hearing losses (greater than 70 dB) affect a smaller proportion of the community but carry a greater disability burden. In the elderly, hearing loss is often combined with visual failure and cognitive decline. The impact of this combination of sensory losses on quality of life and independent living can be profound.