[Otoacoustic emissions--a step closer to understanding cochlear function]

Otoacoustic emission is a phenomenon that may occur in a large group of normal-hearing subjects. Ear is an organ which collects and treats sounds from the surrounding environment, and nerve fibres transmit the perceived information to the brain in terms of nerve signals. However, ear can also produce a sound (otoacoustic-emission), which can be recorded using very sensitive microphones placed in the ear canal.

Otoacoustic emissions may be spontaneous (SOAE) or may be a response to given auditory stimuli: transiently evoked otoacoustic emissions (TEOAE) and distortion product otoacoustic emissions (DPOAE). Applications of otoacoustic emissions in clinical and experimental medicine are various. Hearing screening program of newborns aims to identify infants with permanent bilateral hearing loss in the first days after birth. Screening programs are based on recordings of transiently evoked otoacoustic emissions (TEOAE) in the first step, and then (if it is necessary) brainstem evoked response audiometry. Transiently evoked otoacoustic emissions may also be used for differential diagnosis of cochlear and retrocochlear disorder.

In cases of substantial retrocochlear pathological findings without cochlear involvement, transiently evoked otoacoustic emissions should be recordable because outer hair cell function is unaffected. It is possible to detect ototoxic effects of drugs before hearing loss can be detected by other methods (pure-tone audiometry). In a similar way, using otoacoustic emissions, it is possible to detect noise induced damages of the outer hair cells. Otoacoustic emissions provide a unique means for assessing the integrity of the medial efferent pathway. Further research into the properties of various types of otoacoustic emissions would contribute to understanding of cochlear function, and in particular, the relative roles of inner and outer hair cells. Evoked otoacoustic emissions are directly related to outer hair cell function and may be somewhat independent of inner hair cells. At present, there is insufficient evidence to indicate whether inner and outer hair cell disorders can exist separately, and if they can, whether they are so strongly correlated that statistically the distinction is meaningless. Applications of otoacoustic emissions in clinical practice is an important route in furthering our understanding.