My child failed his/her hearing screening (part 2)

Now that we have reviewed how sound makes its way from the outside world into the brain, today we are going to discuss the different types of tests which make up a newborn hearing screen.  The newborn hearing screening program was setup as an early intervention program in an attempt to identify children with early onset hearing loss and facilitate early treatment.  Children with even mild hearing loss can have significant delays in their speech and language development. 

There are 2 main tests which are currently used to perform a newborn hearing screening.  The first test is called otoacoustic emissions, or OAEs.  This test checks the hearing pathway from the outside world to the cochlea.  It does not test cochlear/auditory nerve integrity or any of the pathways from the nerve to the brain.    In order to have a normal OAE test, the hair cells of the cochlea must be present and healthy.  They actually produce a very slight sound, which we cannot routinely hear, in response to a sound administered to the ear through a very small probe.  Therefore the sound administered by the probe must be funneled to the eardrum, vibrate the eardrum, vibrate the hearing bones, and vibrate the membranes of the cochlea to vibrate these hair cells, which, in turn, produce a sound.  The probe that produces the initial stimulus sound is able to detect the sound produced by the hair cells, or the otoacoustic emission. 

OAEs are a good screening test, but have several important limitations.  First, if there is a blockage of the ear canal, whether from ear wax, fluid or vernix, or a developmental malformation of the ear canal, the OAEs will be absent.  Similarly, and abnormality in the ear or eardrum that blocks the conduction of sound will impair OAEs.  That is because the sound cannot be conducted to the inner ear effectively to vibrate the hair cells.  Secondly, one may have a perfectly intact cochlea with normal hair cells and have a profound hearing loss due to a break in the pathway from the cochlea to the brain, an example of which is auditory neuropathy where there is abnormal development of the cochlear/auditory nerve.

The second test is called automated auditory brainstem response testing, or AABR.  This is a newer test which actually checks the integrity of the entire pathway of hearing, including the area beyond the cochlea up to the brain.  During this test, there is a probe which is placed into the ear which administers a broad-band click stimulus at a consistent level.  This device then measures, with probes that are placed on the head, the passing of that sound from the probe, through the ear, into the cochlear/auditory nerve and into the pathway from the brainstem to the brain.  This test is very good at ruling out children who do not have hearing loss and ruling in children who do.  Each ear is measured independently as in the other test.

While this test evaluates the more of the hearing pathway, it, too, has some limitations.  First, this test is a pass/fail test.  There is no interpretation possible.  Therefore a failed test must be followed up with a formal auditory brainstem response (ABR) test.  Second, the click stimulus is not frequency specific.  It generally tests from 1000 to 4000 Hz which are the frequencies thought to be most important for speech and language, but there is much more to our hearing range than is tested with a click stimulus.  Finally, as with the OAE test, the AABR depends on the conduction of the administered sound into the inner ear and hearing pathway.  Therefore any abnormality in the outer or middle ear could compromise the result of this test.

Given the relative strengths and weaknesses of these tests, many institutions are using a combination of tests for newborn hearing screening.  Now that we have discussed these tests and what they measure, in part 3 of our series we will discuss how we deal with a failed newborn hearing screening and what follow-up is involved.