There are two types of newborn hearing screening tests, which are painless, safe, and quick to perform. They are –
Otoacoustic Emission Test (OAE)
In the OAE test, a minute probe comprising of a speaker and microphone is placed in the ear of the infant. Sounds that are important for comprehending speech are played into the device, which reflects an echo generated by cochlea in the ear canal, which is measured by the microphone.
When the sounds are played, sound travels through ear canal and small bones of the middle ear and reaches to cochlea of the inner ear.
Two things happen at Cochlea , it will process the sound to send signal to brain stem and also generate an echo sent back to the ear.
The sound that echoes back is called otoacoustic emission, which is recorded by the microphone and it is relayed to a computer screen where it is represented pictorially. The screen will show the responses to the sounds played. When the infant has a hearing loss, there is no echo registered by the test.
OAE normally takes 5 min – 10 min.
Auditory Brainstem Response (ABR)
ABR test is conducted for physiologically measuring the response of the brainstem to sound. In this test, sounds are played into the ears and about five electrodes are placed on the infant’s head for detecting response.
Sounds are played through small earphones, and the nerves will carry the sound stimulus to the brainstem. When the nerves respond like this, there is electrical activity taking place in the nerves, which is recorded by the electrodes, and they are represented as waves on a computer screen.
The audiologist will play sounds at different volumes to determine the lowest level that the infant can hear. For testing infants, the sound is only a click played at two levels, one loud, and one soft. The audiologist will determine the response by evaluating the waves on the computer screen. ABR test, normally takes about ten to fifteen minutes.
OAE or ABR
Hospitals may perform one test or both tests for hearing screening.
In High Risk infants ABR testing is to be done.
The main advantage of OAE test is that it is cost effective and easy. On the down side, OAE has a higher false-positive rate, meaning an infant with normal hearing can fail the test. When OAE is performed within three days of the baby being born, the false positive rate is 5 to 21%, whereas it is only 4% in ABR during the same period.
The higher false-positive rate in OAE is mainly due to the high sensitivity of the device to residual vernix and amniotic fluid that is usually present in the ear canal of an infant. Therefore, OAE and ABR testing, best work together, when a thorough hearing screening needs to be done on infants. Since OAE and ABR rely on different mechanisms, they complement each other in providing accurate analysis of infant hearing.
When the infant fails OAE or ABR tests initially, it does not necessarily indicate that the baby has any hearing loss or permanent hearing loss. There are many reasons for failing the tests and therefore retesting and complete ENT & hearing evaluations by a battery of audiology tests is ordered after the failure of the initial test or tests.