Audition: Disorders

The World Health Organisation (WHO), in its factsheet on deafness and hearing impairment, refers to hearing impairment as “the complete or partial loss of the ability to hear from one or both ears. The level of impairment can be mild, moderate, severe or profound.”

The classifications are based on hearing thresholds (see Table 1, next page), most commonly measured using a standard procedure known as pure tone audiometry. In this approach, a measure of the quietest sound an individual can perceive is recorded at a number of different frequencies of sound. Plotting these values creates the audiogram.

WHO estimates from 2001 suggest that if an audiogram were to be recorded for every individual on the planet, approximately 600 million people worldwide would have a hearing loss affecting both ears, and 250 million of these would be of moderate-to-profound severity.

The actual level of disability resulting from a hearing loss is influenced by many factors beyond hearing thresholds. These factors include the amount of distortion in audible sounds, difficulties in processing and filtering auditory information, and the complexity of listening environment exposed to in a hearing impaired person's daily life. In general, a hearing loss of moderate or worse severity in both ears is deemed of particular clinical significance because, to be audible, a significant proportion of speech sounds must be raised beyond normal conversational levels. This has important consequences for the development of speech and language in children, which may be limited unless measures are taken to overcome the loss.

Additional to the level of hearing loss, the affected site or sites along the auditory pathway are used when labeling hearing impairments. Divisions are usually made into conductive (affecting the outer or middle ear), cochlear, retrocochlear (affecting the nerve of hearing), and central (affecting the cognitive processing of sound information).

This entry describes different types of hearing loss as well as some information on the relevant management options available.