Monday, September 10, 2012

Your Audiology Tutorial: Otoacoustic Emissions

The cochlea, the snail shell shaped organ for hearing sensitivity in the inner ear, produces an echo when stimulated by softer sounds. This organ is lined from base to apex by hair cells (cilia) which are coded for various pitches/frequencies. There are 2 types of hair cells. Outer hair cells amplify incoming stimuli from the eardrum and chain of bones attached to it. OHCs are known as selective amplifiers, allowing fine tuning among multiple frequencies. Inner hair cells take that information and convert it to an electrical impulse which the brain will eventually recognize as sound. For the purposes of not sounding long winded or esoteric, these explanations have been somewhat simplified.

For decades scientists suspected the presence of the aforementioned echo, or an otoacoustic emission (OAE), but it was not until the late 1970s when David Kemp, a Professor in the U.K., experimented with microphones that could measure low intensity signals that the OAE was quantifiable. Since that time, it has become a standard diagnostic tool, often employed by audiologists in their battery for hearing tests (along with tympanometry and behavioral audiometry - sound booth testing). OAE measurement is an effective test to correlate with pure tone test results. Also, it is useful to give clinicians some information about the severity of a hearing loss when the standard "raise your hand when you hear the beep" test is not possible (young children, cognitively impaired). For those individuals who are "malingerers" (faking a hearing loss), the OAE provides objective information the pure tone test does not.

There are 2 main types of OAEs that are used clinically:

DPOAEs (Distortion Product Otoacoustic Emmissions): 2 tones of different intensities and frequencies are presented at the same time, resulting in a distortion product response.

TEOAEs (Transient Evoked Otoacosutic Emissions): Utilizes a "click" (stimulus encompassing all frequencies) or "tone burst" (one particular frequency) to measure response of OHCs.

The results of OAE tests are often predictive of a hearing loss of over 30 or 35 decibels (the patient cannot hear sounds below those levels). Some clinics perform diagnostic TEOAEs and DPOAEs, which measure responses pitch by pitch (usually up to 6000 Hz). With DPOAEs, the distortion product should be at least 6 dB above the noise floor (level of background noise in testing area) This test takes longer than the screening DPOAE, which quickly runs through each frequency and then provides either a "PASS" or "REFER" reading. The DP or TEOAE screener is used extensively in hospitals (often in neonatal intensive care units) for newborn hearing screenings.

There are also what are known as SOAEs (Spontaneous Otoacoustic Emissions), which are recorded without stimuli. Little is known about the physiology behind the mechanism of SOAEs, which often occur in the mid- and high-frequencies. There are theories that there is some positive correlation in patients with measurable spontaneous emissions who report subjective tinnitus ("ringing" or any abnormal sound perceptions not heard externally).

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