OAE Testing (Otoacoustic Emissions)

Timothy C. Hain, MD

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Introduction

OAE or otoacoustic emission testing is the recording of sounds that the ear produces itself. Otoacoustic emissions were first reported by Kemp in 1978. They appear to be generated by motile elements in the cochlear outer hair cells.

There are 2 types of otoacoustic emissions in clinical use:

• Transient otoacoustic emissions (TOAEs) or transient evoked otoacoustic emissions (TEOAEs) - Sounds emitted in response to an acoustic stimuli of very short duration; usually clicks but can be tone-bursts
• Distortion product otoacoustic emissions (DPOAEs) - Sounds emitted in response to 2 simultaneous tones of different frequencies

Method

OAEs are measured by presenting a series of clicks to the ear through a probe that is inserted in the ear canal. The probe contains a loudspeaker that generates clicks and a microphone that measures the resulting OAEs that are produced in the cochlea and are transmitted through the middle ear into the outer ear canal. The resulting sound that is picked up by the microphone is digitized and processed using signal averaging methodology.

To obtain an OAE one needs an unobstructed outer ear canal, absence of significant middle ear pathology, and functioning cochlear outer hair cells

Utility

The clinical significance of OAE's is that they only occur in a normal cochlea with normal or near normal hearing. If there is damage to the outer hair cells producing mild hearing loss, then OAEs are not evoked. A rule of thumb is that OAEs are present if hearing is 35 dB or better. Because OAEs are evoked by transient signals that have a wide frequency response, a broad region of the cochlea responds, providing information on the frequency range from 1000 Hz to 4000 Hz. OAE's decline with age.(Gates et al. 2002; Cilento et al. 2003)

OAE's are most appropriate for use in difficult-to-test patients: newborn infants, young children, patients who are attempting to feign a hearing loss, and developmentally delayed populations. OAEs only provide information about the activity of the cochlea, and do not assess the status of the rest of the auditory pathway..

TOAE's are commonly used to screen infant hearing, to validate auditory thresholds obtained via other techniques, and to assess the cochlear contribution to hearing.

DPOAE's are also used for infant screening. They can be obtained in persons in whom TOAE's cannot be obtained, and they can be obtained at higher frequencies than TOAE's (i.e. over 1000 hz). DPOAE's may be useful in detecting aminoglycoside ototoxicity, as this condition usually affects high-frequency hair cells first.

OAE's have a special utility in auditory neuropathy. This is a condition in which hearing is impaired but cochlear function is intact.

References:

• Cilento BW, Norton SJ and Gates GA (2003). "The effects of aging and hearing loss on distortion product otoacoustic emissions." Otolaryngol Head Neck Surg 129(4): 382-9.
• Gates GA, Mills D, Nam BH, D'Agostino R and Rubel EW (2002). "Effects of age on the distortion product otoacoustic emission growth functions." Hear Res 163(1-2): 53-60.