March 1, 2013
When an audiologist tests a patient’s hearing, there is an important reality that must be addressed. Conventional audiometry is based on threshold estimation: the tester is seeking the subject’s limits of detection of weak signals, typically tones, in order to determine the intensity at which the subject begins to hear sounds. It is necessarily a behavioral test. Since hearing is a perception and not merely a detection, the tester must necessarily rely on the subject to report the perception. Typically a test subject raises his hand, clicks a hand-held device or signals verbally (“yes”, for example) when a sound is perceived. This requires several trials of manipulation of the intensity of the sound followed by patient responses. The process is necessarily a STATISTICAL ESTIMATE of function.
There are methods of estimating the reliability of these patient responses, such as agreement between different tasks and the reliability of the subject’s responses (was the subject guessing or responding inconsistently?), however, a pure-tone audiogram is still an estimate regardless of the quality of the equipment used, the experience of the tester and the reliability of the responder. It is important to support this data with objective measurements whenever possible. The inner ear in inaccessible to direct observation. We cannot see the damage along the hearing pathways, even by way of imaging studies, other than tumors that can grow along the hearing nerve. The term “sensorineural loss” implies the damage is either sensory (inner ear organ of corti) or neural (along the auditory nerve), but how can we know where the problem really lies?
Otoscopy and ear microscopy can allow us to view the external ear and canal and inspect for signs of malformations that might interfere with hearing. Tympanometry allows us to rule out middle ear fluid, discontinuity of the ossicles (the tiny bones behind the ear drum) or other signs of “conductive loss”. There is one test, however, that allows us to test the integrity of the inner ear “outer hair cells” directly, known as Otoacoustic Emissions (OAE).
This approach was originally demonstrated by a geologist named David Kemp, who had used sonar to determine rock stata. Building upon earlier work suggesting amplification of signals was occurring in the inner ear, the idea emerged from the suggestion that the ear can make its own noise or could reflect external noise which may be measured. Given that sound can be measured in mathematical terms (the frequency and wavelength that determines pitch and the sound pressure that determines intensity are measurable, physical properties that can be described mathematically), specific sounds can be delivered into the ear via a probe, eliciting reflected sound energy that is measurable by the same probe. The resulting sound measured is mathematically related to the stimulus tones delivered to the ear, often the “cubic difference tone”. These emissions can be measured for different frequencies of stimulus tones. No response from the subject is needed.
Is has been found that a healthy set of hair cells will generate much stronger emissions than an impaired inner ear. This makes the test valuable for “site of lesion” determination, when we wish to differentiate between “sensory” and “neural”. This is important in patients who show clinical signs of acoustic neuromas or other “retrocochlear disease” (growths along the hearing and balance nerve. It enables evaluation of patients who are difficult to test, such as infants, young children, patients with physical or mental limitations, etc. OAE is also sensitive to early hair cell damage, often prior to changes in hearing, making it a useful gauge of drug toxicity or early noise damage. The test requires a well-functioning middle ear, therefore, it can gauge middle ear function if tympanometry is not available or cannot not be tested.
Otoacoustic emissions are a quick, non-invasive, objective, and highly useful method of assessing inner ear function. They are another excellent tool for evaluating the health of patients’ ear.