Disorders of hearing

Hearing loss is the commonest sensory disability worldwide, and the World Health Organisation has estimated that 278 million people suffer a moderate to profound hearing loss in both ears, with 80 per cent of deaf and hearing-impaired people living in low- and middle-income countries (WHO 2006). Tinnitus affects approximately 10 per cent of developed populations (Coles 1984) and of these, 5 per cent find the symptom troublesome and seek help (Davis 1995). Tinnitus and hearing loss are primary symptoms of disordered cochlear function, but may also present as a result of central auditory pathology with normal cochlear function. Pathology affecting the central auditory pathways characteristically presents as difficulty hearing in conditions of poor signal-to-noise ratio, for example, in a classroom in the presence of background noise, listening to transmitted sound, for example on the telephone or on a television, and sound localization. As a consequence of multiple relays and bilateral representation above the level of the cochlear nuclei, central auditory dysfunction does not present with hearing loss. Hearing loss and/or tinnitus, with or without associated vestibular abnormalities, will most commonly be the result of otological pathology. However, importantly for the neurologist cochlear, VIII nerve, or central auditory dysfunction may be part of the clinical presentation of a neurological disorder.