My child failed his/her hearing screening! (Part 1)

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Amid all of the excitement of a newborn child, there are several tests that are run shortly after birth while your baby is in the newborn nursery that are mandated by the state.  One of those tests is a hearing screen which checks to see if the most basic parts of the hearing mechanism are intact.  We are going to discuss the newborn hearing screening, how it works, what the results mean, and what should be done in follow-up of an abnormal test in a 3-part posting.  Please be sure to check back for the second and third parts to follow shortly.

To understand the hearing screening test, we should first talk about how sound gets from the outside world into the brain.  There are many important pieces which are needed in order to hear sound.  Sound is actually a pressure wave.  That wave gets funneled into the ear canal by the ear which you see on the side of the head, or pinna/auricle.  Once in the ear canal that wave is transmitted down to the ear drum, which it vibrates.  The eardrum is connected to 3 of the tiniest bones in the body: the malleus, incus and stapes (or hammer, anvil and stirrup bones).  When the sound wave hits the eardrum, the eardrum vibrates, and, as a result, the 3 hearing bones vibrate as well.  The 3^rd hearing bone (stapes) is connected to the inner ear and transmits that wave into the inner ear. 

Here is where all of the magic happens.  The sound wave that is transmitted into the inner ear, or cochlea, vibrates delicate membranes that are within the body of the cochlea.  There are cells along those membranes which respond to different frequencies of vibration.  When that frequency of sound is present, those cells, called hair cells, activate and change that mechanical wave into an electrical signal.  This electrical signal is then collected by the cochlear, or auditory, nerve which then takes that information into the brainstem and relays it all the way to the temporal lobe of the brain where that signal is perceived as sound.

For a basic animation to understand this process, please visit: http://www.youtube.com/watch?v=tkPj4IGbmQQ

In the next part of this series, we will discuss the types of newborn hearing screening tests and what type of information these tests gives us as ear, nose, and throat doctors.

Thank you very much for reading our blog!  Please check back again for parts 2 and 3 of this series.