Epidemiology and Classification

The World Health Organization estimates that nearly 300 million people – 5% of the world’s population – have a disabling hearing impairment. When classifying the severity of deafness, the hearing level in the better hearing ear is most relevant, as this is the ear the patient relies on.

In developed western countries about 1 in 1000 children is born deaf (congenital deafness). This is much more common in the developing world. The majority of these children have permanent sensorineural loss. This can be part of a syndrome – syndromic deafness (e.g. Usher’s syndrome) or it can be an isolated problem that is not part of any definite pattern of anomalies – non-syndromic deafness. More and more cases of non-syndromic deafness are now known to result from a genetic cause (e.g. connexin 22 defects). Early diagnosis of congenital deafness is essential for the best outcome, hence the importance of detecting hearing loss in the newborn infant.

Many people become deaf later in childhood or as they progress through adult life (acquired deafness). Some deterioration in hearing is a part of ageing – presbyacusis (Figure 4.2).

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