Hearing loss is either due to insufficient sound conduction from the outer to the inner ear ("conductive hearing loss"), or - much more frequently - to damage to the hair cells and neurons in the cochlea or to the auditory nerve ("sensorineural hearing loss"). The most important causes for sensorineural hearing loss are ageing, acoustic overstimulation by excessive noise or exposure to ototoxic drugs and substances (e.g. certain chemotherapy drugs or antibiotics).

Hearing loss is by far the most prevalent inner ear disorder - according to some estimates, it affects 10% of the population. It is most prevalent among the elderly, with about 30% of people 65 years or older being affected. However, the incidence is rising rapidly among younger people, mostly due to frequent exposure to excessive noise. At the age of 85 years or older, almost everyone is to some extent affected.

In some cases, the sensorineural hearing loss is acute - e.g. after exposure to loud noise - and may disappear again after a while. People affected by it may experience a ringing in the ears, and sounds may become muffled for some time. In most cases however, the hearing loss sets in slowly and progresses insidiously, becoming apparent only after a while. Sounds may become distorted or muffled, and it may be difficult for the affected person to understand speech especially in noisy situations. This is due to the irreversible loss of either hair cells or neurons in the cochlea. At the time of birth, every human being with full auditory capabilities disposes of approximately 3,500 inner hair cells, 13,000 outer hair cells and 30,000 neurons - this number can only decrease in the course of life!