Otoacoustic Emissions

Description:

Otoacoustic Emissions are acoustic signals that can be recorded in the ear canal.  There are four classes of emissions: Spontaneous, Transient, Stimulus Frequency, and Distortion Product.  Spontaneous emissions are at this point not clinically useful, although a subject of much research regarding cochlear mechanics.   Both Distortion Product (DPOAE) and Transient (TOAE) are used clinically/diagnostically and for hearing testing in infants and special needs populations.

Distortion Product Otoacoustic Emissions (DPOAE) are used at Evergreen Speech & Hearing Clinic, Inc.  The process involves placing a small rubber plug in the patient ear.  This plug contains two speakers for stimulus delivery and a microphone to receive the distortion product, which is generated by the cochlea.  The stimuli consist of two sounds (f2 and f1), which are presented simultaneously to the ear.  The human cochlea may generate many distortion products, but the largest distortion product occurs at a frequency equal to 2f1-f2, which is known at the cubic difference tone.  A response is considered present if the DPOAE is greater than 6dB above the adjacent noise floor.  In humans with normal auditory function, a ratio between f2 and f1 tends to produce the largest distortion products (Gaskill & Brown, 1990).

Responses are present in 100% of normal hearing children, typically reduced in hearing losses of up to 40dBHL, and absent in hearing losses of greater than 40dBHL.

The test procedure takes approximately 30 minutes, and requires only that the child be quiet during testing.

Indications:

Universal Hearing Screening — We support the decision of the 1998 joint taskforce on hearing in children (including representatives from pediatrics, otolaryngology, audiology, education, and speech pathology), which recommended that every child receive a hearing screening at birth.

This test is inexpensive, and non-invasive.  It is an excellent way to quickly rule out hearing loss in your young patients.

Diagnostically, OAEs are helpful in differentiating cochlear from retrocochlear involvement. If a child is suspect of hearing loss secondary to hyperbilirubinemia, the use of Brainstem Auditory Evoked Response (BAER) is recommended for hearing screening.  Middle ear effusion should also be ruled out prior to evaluation.

Significant Findings:

DPOAEs are present in 100% of normal hearing children, typically reduced in hearing losses of up to 40dBHL, and absent in hearing losses of greater than 40dBHL.

Abnormal findings may indicate that the child suffers from significant hearing loss.  Middle ear effusion should also be ruled out using tympanometry in the event of an abnormal OAE.

Although an excellent indicator of cochlear function, OAEs do not completely rule out central auditory dysfunction or retrocochlear involvement. Behavioral testing is required to ensure the integrity of the entire hearing system.

Suggestions:

Due to the need for quiet during testing, this procedure should be done during the child’s natural sleep time.  It is also helpful to schedule the child during the first two months of life.  Waiting until the child is older, increases the potential for resistance or noise contamination during testing.

References:

El-Refaie, A., Parker, D.J.,Barnford, J.M.; “Otoacoustic Emissions versus ABR screening: The effect of external and middle ear abnormalities in a group of SCBU neonates”, British Journal of Audiology, Vol.30.1,page 3, February 1996.

Gaskill, S.A. and Brown, A.M. “The behavior of the acoustic distortion product 2F1-F2, from the human ear and its relation to auditory sensitivity” Journal of the Acoustical Society of America. 88,821-839, 1990.

Gorga, et.al. “From Laboratory to Clinic: A large scale study of DPOAE’s in ears with normal hearing and ears with hearing loss”. Ear and Hearing. Vol. 18, No. 6, 1997.

Kemp, D.T. “Stimulated acoustic emissions from society within the auditory system” Journal of the Acoustical Society of America. 64, 1386-1391, 1978.

Robinette, M.S., and Glattke, T.J.; Otoacoustic Emissions, Clinical Applications, Thieme Publishing, New York, New York, 1997.