Methods of screening infant hearing

An infant’s hearing is usually ‘screened’ by one or both of the following methods:

• Auditory Brainstem Response (ABR) test, or
• Oto Acoustic Emission (OAE) test.

Both tests are effective and accurate. Importantly, neither test requires an invasive procedure or even an observable response from the infant. The two tests differ in the way they measure hearing.

The OtoAcoustic Emission (OAE) Test

In this test, a small microphone and speaker device is fitted into the infant’s ear. A sound is played into the speaker and the test measures the response that is produced by the cochlea (in the inner ear: see our page on Hearing Loss: The Facts ).

Throughout, the infant is resting quietly undisturbed. As we know, in the human body, sound is passed from the cochlea by an electrical stimulus into the brain. But there is also a second sound that does not travel to the brain at all but bounces back out into the ear canal. This is called an otoacoustic emission. The microphone in the infant’s ear records this secondary sound that has been bounced back into the ear canal and this recording is then analyzed by the audiologist on a computer. Essentially, the audiologist sees which sound has generated an emission and how strong that emission is. Some sounds are essential to understanding speech. If those sounds yield an emission, the infant has passed the test.

The Auditory Brainstem Response (ABR) Test

This test measures the brain’s response to a sound stimulus. In this test, the infant is fitted with a pair of small earphones and several electrodes on its head. A spectrum of sound is fed through the earphones. These sounds are stimuli which are received by the infant’s ear and travel to the infant’s brain. As all of this nerve-activity is essentially electrical, the audiologist can measure it through the electrodes. A computer representation of this electrical activity is analysed by the audiologist. The audiologist determines the softest sound audible to the infant — ie, the least sound that generates an acceptable level of electrical activity. In actuality, while screening an infant, only one type of sound is used (known as a “click”) but it is applied at a loud level and at a low level, split into groups. Depending on the results, the audiologist determines the quality of the infant’s hearing.

Which test is right for my baby?

The OAE is easy and inexpensive but the ABR is frequently more accurate. For a truly proper scrutiny and maximum accuracy, both tests should be used together.

My infant failed the initial screening test. Is my infant deaf?

Not necessarily. This may be the result of a number of factors in newborn infants—for example, if there is still amniotic fluid in the ear canal, blocking the sound path; a middle-ear infection; etc. Often, a re-screen is ordered after a few days—which the baby passes.

Remember, too, that both tests record very minute sounds. If the baby makes too much noise or movement during the test, it’s possible that the test results will be negative. That’s why these tests are done when the baby is quiet or sleeping (for example, just after a feed).

My infant does have a hearing loss. What do I do next?

A full diagnostic exam must be done to determine the type and amount of hearing loss.

What is a diagnostic test? How is it different from a hearing screen? Does it involve surgery?

The diagnostic test does not require surgery. It is a longer session (sometimes over an hour) and there is more participation with the baby. In diagnostics, both the above tests (the ABR and the OAE) are re-used. The ABR is usually the principal test and a variety of testing sounds are administered. The OAE is used to cross-verify the results. A diagnostic test harvests a huge amount of information and can report if there is really a hearing loss and, if so, if it is permanent or reversible/correctable.

What is the sequence of testing I should follow?

It is important to follow this sequence:

1. initial screen (ABR or OAE or both);
2. re-screen if initial screen failed (after a few days);
3. full diagnostic if re-screen fails.

How can IHF help me?

IHF has a close association with major hospitals in Bombay which provide facilities for both these tests. We routinely refer infants to these hospitals (Nair Hospital and Hinduja Hospital) for screening and diagnostics. IHF plans to offer both screen tests in-house in the near future.

However, IHF also has a high-quality audiological facility soundproofed to well below 18db. This is used for in-depth testing of hearing in slightly older babies (not infants), preparatory to fitting them with appropriate hearing aids.

See also