Objective Hearing Testing
These methods for detecting hearing loss require no cooperation from the patient. For this reason, objective listening tests are very useful for the study of newborns and infants.
Otoacoustic emission (OAE).
This test reliably determines whether a person’s hearing is healthy or whether the inner ear is damaged.
A probe that emits certain sounds, is inserted into the ear. The outer hair cells in the cochlea (inner ear) move in response to the signals and transmit an acoustic response that can be measured by the probe. This is typically used as a screening tool for hearing impairment most oftenly used with newborns and must be carried out in a very quiet environment. In the event that no otoacoustical emissions (OAE) are discovered, this does not confirm that the baby is hearing impaired. Other possible causes include the following:
- The test environment may not have been quiet enough
- In newborns, especially during the first three days of life, tissue from the mother’s womb or amniotic fluid might block the ear canal, creating a negative OAE result
- A middle ear infection or cold can also cause temporary disturbance of sound condution
Auditory Brainstem Responses or Brainstem Evoked Response Audiometry.
The existence of OAE means that the outer hair cells are working correctly within certain limits. However, the detection of OAE does not allow any conclusion concerning the functional capability of the auditory nerve or related regions of the brain. Auditory evoked potentials (AEP) are used to objectively determine the hearing threshold, either by auditory brainstem responses (ABR) or brainstem evoked response audiometry (BERA). These tests also don’t require a child’s active involvement and are conducted while the child is asleep. It is very similar to an electro-encephalogram (EEG) in that electrodes are attached behind the ear. Headphones are placed over or in the ears and the electrical activity evoked by the test signal is measured and recorded. The test shows whether sound is processed correctly and the hearing threshold for specific frequencies. The duration of the meaurement depends on how many frequencies are measured and the time the child is asleep.
The resulting wave-diagram is analyzed by comparing it to the wave-diagram of subjects with normal hearing. Abnormalities shown give an indirect indication of a possible disease of the auditory pathways in the brain. This test also serves as evidence of other neurological diseases that may result in a loss of hearing in test subjects of any age.
Stapedius reflex test.
In this test the ear is subjected to a loud noise. In healthy ears it results in the contraction of both middle ear muscles that leads to a stiffening of the eardrum. This tension can be measured and an unexpected value can indicate damage to the middle ear.
Tympanometry measures the resistance of the eardrum toward sound. The method used most commonly is the impedence test, during which a probe tube is placed in the ear canal and sealed by a gum plug. During the measurement, an ongoing tone is sent through a loudspeaker in the probe. A tympanometer simultaneously changes and measures the pressure in the middle ear and the resulting compliance curve is used by the physician to assess, among other things, the functioning of the eardrum or if there is fluid present in the middle ear. However, tympanometry is not able to determine the extent of hearing loss.
If a child has middle ear problems, the compliance maximum shifts to negative values (-100 daPA). In this case, the Eustachian tube is not able to provide appropriate ventilation, so bacteria concentrate in the middle ear. This consumes oxygen and causes a retraction of the eardrum toward the middle ear. This process can quickly lead to hearing problems and should be treated urgently, as it could lead to a serious middle ear infection or an effusion. An effusion might cause a conductive hearing loss of up to 40-50dB.