Facts about Hearing Tests
Dr. M. Vijayalakshmi, M.D (Peds), M.D (USA), F.A.A.P
Facts about Hearing Tests
- What is Hearing Testing or Hearing Screen?
- What is Audiometry
- What is Brainstem Auditory Evoked Responses (BAER)
- What is Electrocochleography (ECOG)
- What is Otoacoustic Emissions
Hearing can be impaired due to lesions in the external ear canal, the middle ear, or the inner ear. The purpose of hearing testing is to evaluate hearing function and, if it is impaired, to attempt to localize the site of lesion.
What is Hearing Testing?
Hearing testing is a means of evaluating an individual's overall hearing function. The tests are used to determine if there is something wrong with the hearing portion of the inner ear. They are often used as an initial screening to decide if more expensive tests like magnetic resonance imaging are needed.
The following hearing tests are briefly described below:
- Brainstem auditory evoked response (BAER)
- Otoacoustic emissions
Audiometry is the term used to describe formal measurement of hearing. The measurement is usually performed using an "audiometer" . In audiometry, hearing is measured at frequencies varying from low pitches (250 Hz) to high pitches (8000 Hz). An example of an audiogram in a person with normal hearing is shown in figure 3. The hearing level is quantified relative to "normal" hearing in decibels (DB), with higher numbers of DB indicating worse hearing. The DB score is not really percent loss, but neverthless 100 dB hearing loss is nearly equivalent to complete deafness for that particular frequency. A score of 0 is normal. It is possible to have scores less than 0, which indicate better than average hearing.
When there is a hearing loss, the next step is to try and determine whether the loss is caused by a sensory problem (sensorineural hearing loss) or a mechanical problem (conductive hearing loss). This distinction is made by using a bone vibrator, which bypasses the mechanical parts of the middle ear. If hearing is better using bone than air, this suggests a conductive hearing loss.
There are a number of special subtests that are optionally included in the audiometry procedure. Speech audiometry involves reading a list of words to see if patients can discriminate words. By comparing speech comprehension with anticipated speech comprehension, inferences can be made about central processing and central hearing deficits.
Acoustic reflex testing consists of subjecting the ear to a loud sound and determining if it causes the stapedius muscle to tighten the stapes. Acoustic reflexes are mainly useful as a crude but non-subjective method of evaluating hearing, as the stapes should tighten for a given level of perceived loudness. Acoustic reflexes can also be a sign of brainstem dysfunction. Click here for more detail.
Brainstem Auditory Evoked Responses (BAER or ABR)
Brainstem auditory evoked responses (BAER),also known as auditory brainstem evoked response (ABR), measures the timing of electrical waves from the brainstem in response to clicks in the ear. Three waves are plotted for each ear. Delays of one side relative to the other suggests a lesion in the 8th cranial nerve between the ear and brainstem or the brainstem itself. The main indication for BAER is when an acoustic neuroma is suspected. However, MRI is a superior test for this purpose. BAER testing may also be useful in situations where an auditory neuropathy is suspected. In this case, it may be combined with otoacoustic emission testing. BAER testing requires reasonable high-frequency hearing.
ECOG is a variant of BAER where an electrode is placed on or in the ear drum to increase the size of wave 1 of the BAER. ECOG is a difficult technical procedure and for this reason is not always helpful. ECOG is also only useful diagnostically when positive.
This is a relatively new test used to assess hearing in newborns as well as being a method of determining whether the cochlea is functioning. With this test, a probe that contains both a tiny speaker and a tiny microphone is inserted into the ear canal. Quiet tones are sent from the speaker, which travel through the middle ear and stimulate the hairs in the cochlea The hairs respond by generating their own minute sounds, which are detected by the microphone. If there is a hearing loss, the hairs in the cochlea do not generate these minute sounds.
- Northwestern University
- ENT Journal