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Hidden Hearing Loss / Cochlear Synaptopathy

To understand Hidden Hearing Loss (HHL) which is the catchy name for cochlear synaptopathy, you need to understand a little bit of anatomy. Stick with me...

hair cells of the inner ear
Copyright: M Charles Liberman, F1000 Research. Diagram of nerve cells of the inner ear.

The inner ear (cochlea) has rows of hair cells, inner hair cells (IHC) and outer hair cells (OHC). When we hear a sound, it quite literally vibrates the parts of our middle and inner ear, setting off a wave in the fluid-filled areas of our cochlea. This wave disrupts a membrane, which is attached to hair-like cells. They aren't hairs, but they look a little like them. When the hair cells move, they turn that mechanical wave into electronic signals - nerve impulses) that the brain can read.

The OHCs are mainly responsible for amplification of incoming sound, and they also work out what pitch the sound is. In age-related hearing loss these are flattened or killed off, much like a field of wheat after a helicopter has landed in it. Because they amplify, the loss of them means that we need sounds to be louder before we can detect them. This is what a standard audiogram measures/shows.

IHCs are mainly responsible for sending the signals to the brain. There are lots of complicated things alongside, such as firing rates and how they respond to loud versus soft sound, but that's basically it. We now know they are damaged by loud noise exposure.

Some time ago, I became interested in why I was seeing patients with mild/moderate hearing loss who really struggled to hear in noise. They struggle beyond what would be expected from their test results. I noticed that they tended to be people with noise-induced hearing loss, and I've been watching the literature ever since.

Since then, I started seeing more patients who are young but have problems hearing in background noise. Often they have tinnitus but no underlying physical damage. Most recently, a patient in Italy has run into problems getting a doctor who understands the complaint.

Time has moved on, and we now know that loud noise does indeed cause damage that doesn't show up on a hearing test, and this is related to the inner hair cells. You can have a combination of OHC and IHC damage, or you can have IHC or OHC damage on it's own.

If your IHCs are damaged but you have normal OHCs, what you get is:

  1. Normal hearing test results

  2. Difficulty hearing in noisy environments (or echo)

  3. Doctors who are stumped because they haven't yet heard about this

  4. Tinnitus, most likely

It's called Hidden Hearing Loss because it doesn't show up on standard tests, however scientists are working on animal models at the moment so we can get human testing.

It can closely resemble other conditions such as Auditory Processing Disorder, which is a different condition related to how well we interpret speech in the brain. However, if you have a history of loud noise exposure, and previous incidents where you:

  • Have been to a concert/used a power tool etc

  • Had a couple of days afterwards where your hearing is dull and you had tinnitus

Then you may be looking at this type of damage.

What is happening is that the synapses of the IHCs (gaps between nerve cells which help them communicate onwards) are damaged and missing. The machinery of your ears is working, but you have problems with transmission. If you imagined your nerves as a bundle of 100 fine copper wires between your ear and your brain, it is as if someone had snipped 50 wires in half.

All this sounds like bad news if you have it, however the hearing impaired population is no stranger to HHL, because a lot of us have it too (along with the other kind). It has been known for years that a section of the elderly patients we see benefit not just from amplifying sound, but also from directional microphones.

With HHL then, you need directional microphones or remote microphones to separate out the speaker from the background noise. And audiologists are there to help you. If you are concerned you may have HHL, book an in-person (preferably) or remote consultation with me and we can investigate how to get you the support you need.

Sally Jackson BSc Hons (Audiology), RHAD, MSHAA

Audiologist/Director at Hearing and Tinnitus Care


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