Physiological Assessments of the Auditory System

As a follow up to our introduction on creating an assessment battery (see here), this three part series  will take a closer look at the benefits and limitation of the three different assessment options.

Physiological Assessments

Benefits

These tests assess function objectively based on anatomical areas.
This means the person doesn’t have to be actively engaged or necessarily willing, as some of these tests can be done under sedation when necessary.

fMRI

fMRI looks at active blood flow.
fMRI shows us which parts of the brain are being used/activated.  Among other things, fMRI could show us if a person was compensating for an auditory deficit by using their language knowledge and context to improve comprehension.

EEG

EEG looks at the electrical activity of the brain.
EEG simply shows if the area being assessed is functioning normally, similar to the way an EKG of the heart is used.  EEG affords us the ability to look at the 3 major parts of the brain (brainstem, midbrain and central cortex) separately.  This tells us where, anatomically, the auditory dysfunction is. If you have had a baby in the last 20 years it is most likely your baby’s hearing was tested shortly after birth with either EEG looking at the brainstem or OAE’s looking at cochlea function, all while sleeping peacefully.
Using a yet more sophisticated system with a larger number of electrodes one could also measure brain activity in other, non-auditory, areas.  With more advanced functions it is possible to examine where the concentration of electrical activity is.

Sounds easy.
So why don’t we simply use only objective measures?

Limitations

The more sophisticated the equipment the more costly.
Many newborn hearing screening programs are using a screener rather than a more sophisticated diagnostic machine.  Most auditory EEG diagnostic machines look at brain stem only.  The test protocols and the technology to look at midbrain and central cortex is separate.  This adds to the cost significantly.  In a catch 22 the market is smaller, keeping the prices high and resulting in it being offered by fewer companies.
Additionally, these 3 components have to be run as separate tests because of the differences in protocol.  An added consideration is this, the more data you gather the longer it takes to analyze, further adding to the cost.  This time is not a luxury a diagnostician has nor is the cost a luxury the consumer has.

These machines are also highly sensitive and it can be challenging to set up the necessary protocols.
When Hearing Kids obtained our EEG machine with ABR, MLR and LLR capabilities, it took a lot of expert troubleshooting to figure out that the electrical noise interfering with our readings was coming from the sound booth itself.
Adding to the complexity, a person who is too fidgety will create muscle artifact that masks the auditory evoked responses.
Additionally, for brainstem testing we have normative data to compare to, from birth and up.  This allows brainstem testing to be used diagnostically.  However, at the time of this writing, for the midbrain and central cortex testing we do not have enough normative data for all ages.
The results of said tests may tell you there is a dysfunction and where in the brain it is but this information is only useful when the treatment is a medical procedure.

Next time, functional assessments

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