If you’re like me, you probably have no clue how you could possibly test hearing on an infant who can’t even speak. What’s even more fascinating is that they can test infants a DAY AFTER they are born! Incredible.
How does it work?
Well actually there are 2 different kinds of tests. The first test, conducted 1 day after Fiona’s birth and again a week later as a retest was called the Otoacoustic Emission (OAE) Test. In my layman’s non-medical understanding, what they basically did was insert a small probe into her ear that generates sounds and captures the “echo” back from that sound. They can then measure if that echo looks normal or not. Its pretty quick and non-invasive, provided the baby can stay quiet for 15 minutes.
All new infants receive this test today, but it is not uncommon to fail due to ear blockage from the birth. This is why we were not too concerned when Fiona failed her first (and even second) tests.
So after failing the first 2 tests, they pulled out the big guns 1 month after her birth. This was a test called the Auditory Brainstem Response (ABR) test. Basically it measures how sound introduced into the baby’s ear translates into electrical responses received by the brain. Here’s a great explanation from MedicineNet.com.
The test is performed by placing four to five electrodes on the infant’s head, after which a variety of sounds is presented to the infant through small earphones. As the hearing nerve fires, the sound stimulus travels up to the brain. This electrical activity generated by the nerve can be recorded by the electrodes and is represented as waveforms on a computer screen. The audiologist can then present different loudness levels of each sound and determine the softest levels at which the infant can hear. For infant-screening purposes, only one sound is used to test the hearing, commonly referred to as a “click.” The click is a grouping of several sounds to test a wider area of the hearing organ at one time. The click is typically presented at a loud level and a soft one. If a healthy response is recorded, then the infant has “passed” the hearing screen.
Here’s what the equipment looks like (stock picture, not Fiona). As you can guess, “Fiona was not impressed” with this setup:
ABR Response Test Equipment
Sitting through this test was not pleasant – it took about 3 hours to run, while you’re sitting there in darkness and silence watching the audiologist fidgeting with various graphs on a screen, all the while having little clue if the graphs are “good” or “bad”. In addition, the baby needs to remain quiet and still, which is an immensely taxing chore, especially for mom.
The screen the audiologist is looking at looks something like this:
I don’t pretend to understand 1/100th of how this works, but I’ll tell you what I learned: they test each ear individually, measuring responses to different frequencies at different volumes (dB levels). Low frequencies (like the booming of a subwoofer) are easier to hear while higher frequencies (breaking of glass) are harder. This is why you can hear the ground booming when walking up to a concert well before you hear the singer, especially annoying teeny-boppers like Bieber…
During this test they are basically measuring 3 different variables:
- The frequency: can the infant hear some frequencies (base drums) better then others (glass breaking)?
- The volume (dBs): are some frequencies just “quieter” then others and can be amplified?
- The ear: is the left different then the right?
Each different graph captures a different combination of the above 3. I don’t know how to read those graphs beyond saying “the bigger the peaks and drops the better”. If the line is essentially flat, that means no response at all.
Most infants have various responses at various frequencies and volume levels. Some may look just fine, while others can be corrected by “turning up the volume”, which is basically what hearing aids do.
Fiona was flat at all frequencies and all volumes, in both ears. This means “profound” hearing loss, and something that is likely not correctable by hearing aids. (to be fair there was one low frequency in one ear that got a small result at very high volume level, but that could be an artifact of the testing precision. We’ll know more in a later retest).
The good news is there are now options (beyond sign language, which is also awesome) for profound hearing loss in the form of cochlear implants. I’ll talk more about that in a future post, but we’re very excited there.
Hearing loss is the most common birth defect today. I had no clue. Apparently 3 out of 100 babies have some level of hearing deficiency. This is why they test so early now – and it is true wisdom. Babies begin learning language within their first 6 months. As you can imagine, hearing is a huge component of this, and the earlier you are aware of the problem, the sooner you can act on it to help speech develop normally.
In this regard we feel truly fortunate. Knowing for sure within 1 month (1 month!) after birth has set us on a course NOW that we’d likely not have otherwise started until 1-2 years old, well past the start of language learning. This makes me much more optimistic in our ability to educate her well in these early stages.