Sarah Frisina, B.S., 4th Year Audiology Extern
Doctoral Candidate, Long Island Consortium of Audiology
“I can’t hear my friends and family when there’s background noise! ”
This is a common complaint in our offices. Many people come to get their hearing tested expecting the audiologist to diagnose them with hearing loss. Sometimes we do. Other times, hearing impairment does not appear on a traditional pure-tone audiogram. Why is that?
Hearing loss can be a result of many different types of pathologies and/or damage to the anatomy in any part of the auditory pathway of the ear – from the ear canal up to the primary auditory cortex of the brain. The most common type of hearing loss is a result of damage to the hair cells in the inner ear. These hair cells are a primary force that send an auditory signal to neurons which eventually reaches the brain for processing. Hearing loss on an audiogram is largely attributed to damage to these hair cells . When audiologists perform a hearing test, they typically test the most important frequencies for understanding speech – 250 to 8,000 Hz. However, the human ear can hear from as low as 20 Hz to as high as 20,000 Hz. Due to the anatomy of the ear, hearing loss from noise exposure and aging typically affect the higher frequencies first. This means that sometimes hearing loss goes undetected because it is only present in the ultra-high frequencies like 10,000, 12,000 or 16,000 Hz. This type of hearing loss is deemed Hidden Hearing Loss or Cochlear Synaptopathy. Although the information we hear at 10,000 Hz and above is not too critical for communication, it does provide important speech cues – especially in the presence of background noise (Liberman et al., 2016).
Hidden Hearing Loss is also characterized by damage to the synapses between the hair cells and the auditory nerve fibers. Cochlear synaptopathy does not cause a patient’s responses to “beeps” during a hearing test to be elevated above the normal intensity levels (0-25 dB HL) until it becomes extreme. What this means is that a patient can have this type of impairment in their auditory system and it will not be identified until years later. Why is this important? The synapses between the inner ear and the auditory nerve help us immensely in difficult listening situations – like trying to converse with your loved ones during a large family gathering (Liberman et al., 2016). They contribute to how we hear others in alternating levels of ambient noise.
Noise exposure is partially to blame for Hidden Hearing loss. A research study in which mice were exposed to a loud level sound (100 dB SPL – about as loud as a snowmobile) for two hours showed significant loss in their hair cell/auditory nerve synapses in the regions dedicated to high frequency stimulation. Further testing (Auditory Brainstem Response) indicated that auditory nerve function in the mice was abnormal for medium to high level sounds; however, testingelicited normal results for low intensity sounds. The functioning for medium to high intensity sounds is required for hearing conversations in background noise. The neuropathy seen in the mice subjects can be assumed in humans with a normal audiogram and a history of noise exposure; thus, the cause for issues hearing in difficulty listening environments (Plack, Barker & Prendergast, 2014).
Reduced output from the cochlea to the vestibulocochlear nerve may also be a reason for tinnitus in the absence of concomitant hearing loss. When patients report a constant ringing in the ears, we typically expect to find some degree of hearing loss. A theory behind this being, in simple terms, that if the brain is receiving less auditory input, it tends to produce a phantom sound to fill in the gaps of absent sound stimulation (Schaette & McAlpine, 2011). Therefore, less input to the brain as a result of hidden hearing loss can be the explanation for that annoying ringing in your ears!
If you are experiencing any ear-related issues, it is advised to make an appointment at ENT and Allergy Associates. Our ENTs and audiologists work closely together to ensure the best hearing healthcare for you and your family!
References
Liberman, M. C., Epstein, M. J., Cleveland, S. S., Wang, H., & Maison, S. F. (2016). Toward a
Differential Diagnosis of Hidden Hearing Loss in Humans. Plos One, 11(9). doi:
10.1371/journal.pone.0162726
Plack, C. J., Barker, D., & Prendergast, G. (2014). Perceptual Consequences of “Hidden” Hearing
Loss. Trends in Hearing. https://doi.org/10.1177/2331216514550621
Schaette, R., & Mcalpine, D. (2011). Tinnitus with a Normal Audiogram: Physiological Evidence
for Hidden Hearing Loss and Computational Model. Journal of Neuroscience, 31(38),
13452–13457. doi: 10.1523/jneurosci.2156-11.2011
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