Science Friday - Why Is Tinnitus So Hard To Understand And Treat?

Episode Date: May 15, 2024

Tinnitus, a condition commonly described as a persistent ringing in the ears, affects millions of people around the world. In the US, the prevalence of tinnitus is estimated at around 11% of the population, with 2% affected by a severe form of the condition that can be debilitating. But despite it being so common, the exact causes of some tinnitus, and how best to think about treating the condition, are still unclear. In some cases, it’s brought on by exposure to loud noise, while in others, an ear infection or even earwax can be to blame.Dr. Gabriel Corfas, director of the Kresge Hearing Research Institute at the University of Michigan, joins guest host Sophie Bushwick to talk about current research into the condition and possible treatments, from regrowing nerve cells, to devices that provide electrical stimulation.Transcripts for each segment will be available the week after the show airs on sciencefriday.com. Subscribe to this podcast. Plus, to stay updated on all things science, sign up for Science Friday's newsletters.

Tinnitus, no matter how you pronounce it, the persistent ringing in the ears can be annoying or even debilitating. A simple way to think about it is a phantom sensation of sound in the absence of an external sound. I'm SciFri producer Charles Berkwist. It's Wednesday, May 15th, and you're listening to Science Friday. Tinnitus affects millions of people around the world, but despite it being so common, the executive causes of some tenetis and how best to think about treating the condition are still unclear. Dr. Gabriel Corfus, director of the Creske Hearing Research Institute at the University of Michigan, joined guest host Sophie Bushwick to talk about tinnitus and some research into possible ways to mute the buzz. Welcome to Science Friday.

Thank you very much for having me. Let's start with some definitions. What is tinnitus? A simple way to think about it is a phantom sensitive. of sound in the absence of an external sound. And I've said ringing in the ears, but it can have other sounds as well, right? Yes. Some people feel it like constant buzzing.

Some people feel like chirping. It can be of different frequencies as well as different intensities. And sometimes they are constant. And for some people, they come up and go away in Jamaica. And these phantom sounds, do we know where in the auditory system they're being produced? Like, is it something in the ear or in the brain or somewhere in the connections between them? All the evidence points out that tinnitus or tinnitus is created in the brain. most probably caused in response to changes in the periphery in the inner ear.

But the sound itself is generated by activity most likely in the brainstem. If we were talking about a mechanical system, you know, something like a microphone, as you turn up the gain to try to get a stronger signal, you also get more noise. Could it be that tinnitus is the result of the brain kind of acting like? like a mic, it's trying to turn up a quiet signal, but turning it up too much? That is pretty accurate in some way. The way most researchers are thinking about is that when there is a significant shearing loss and there is less input from the ear to the brain, some circuits in the auditory pathway

respond to that by increasing the game. And that can generate this phantom sound. So it sounds like hearing loss could be one of these causes of tinnitus, but apparently there are a lot of other ones. Can you tell me about some of them? Today we consider hearing loss as the key cause, which could be working together with others.

For example, some people may have the onset of their tinnitus after a stressful situation. The idea in that case would be that there was already a defect in hearing that was contributing to changes in the brain, but then the stressor could have been a second hit that enhanced either. the perception or the strength of the genitis. So what are some causes of hearing loss that you know can end up resulting in tinnitus? There are many reasons why people lose hair. Ageing is one common, and the cause of age rate of hearing loss is not yet clear, but there are several potential contributors, of course, normal aging, during normal aging,

our tissues and organs start getting significant damage through just regular life or insults like noise, antibiotics, other drugs that could affect the inner ear. And some of your work involves studying how nerves can be regrown. How does that connect to tinnitus? So in the inner year, we have very specialized cells that are core cells. These are the cells that respond to the movement of waves within the ear, and they are activated through these mechanical processes, and they release neurotransmitters and activate the auditory neurons in the year, that then they send the signals to the brain.

The connection between the inner hair cells and this very specialized auditory neuronal, called spiroagangular neuron, the synapses that connect them, which are critical for the information to move from the airspace to the neuron, are very sensitive to noise, and they can be destroyed by noise exposure. For example, in animal models, we know that putting a mouse in conditions that would be similar to a rock hunter, let's say two hours of sounds of about 100 decibels, is sufficient to cause the loss of these synapses, and these loss, unfortunately, in large part, cannot be repaired by itself. By studying the mechanisms that regulate the formation of these synapses,

we discover certain molecules, in particular at neurotrophic parts, factor called nt3 of neurotrophin 3, that is very critical in regulating the formation of this synapses. And then we were able to demonstrate that by increasing the amount of neurotrophine 3 in the ear after noise exposure, a very significant portion of the lost synapses can be regenerated. So normally these cells wouldn't regrow after damage, but you have a lot of the loss of damage, but you add this compound and then you're sort of like coaxing them to regrow? To reconnect. To reconnect.

Exactly. Yes. To reform the synapses and therefore the signals can move from the herstles to the growths. And once you've regrown it, do you see changes in how the mice, for example, respond to sound? Yes. So we can test mice for their ability to hear. by measuring the physiological responses to noise, and also by using behavioral tests that tells us

if the animal is processing the auditory signals correctly. What we have found is that in mice that have been exposed to noise, and if we increase the amount of NT3, the electrophysiological responses of the mouse to sounds are increased and go back to what it was normal before the noise exposure. When we take animals that are at middle age for a mouse that would be one year old, if we increase the amount of 23 in the year, the following week their hearing is improved, and by the end of their lives, their ears look much younger functionally and structurally that they're no one counterpards.

So we believe that the evidence indicates that increasing NTP3 or other molecules could help us not only restore hearing after noise exposure in some cases, but as well as improved hearing in middle age and preserve hearing as age. I think it's fascinating that you're able to reverse hearing loss in this way, but I'm also curious about how this impacts tinnitus. And are there even animal models of tinnitus? You know, how can we tell if a mouse is experiencing ringing in its ears? That is a great question. And it's challenging. But people have done many studies to do this. And primarily there are two approaches that most laborers use.

In one case, you train animals to be able to recognize a short gap. of silence when there is a background noise. So the minds have to either respond to the gap in the background noise by performing a specific behavior. And the idea is that if the animals have the tinnitus, that basically feels the gap because there is a sound in their brains, then they fail in the behavior. that's primarily the way we can interrogate this. We cannot ask the animal, are you hearing the sound?

Right. But we are using the lack of ability to report behaviorally that there was a gap in the background noise as a surrogate for measuring the pinatus. And when it comes to measuring tinnitus in people, are there ways to tap into the brain signals, you know, using an MRI or other imaging methods? Like, can you see this constant buzz signal in their auditory systems? Today, the most common way to test it is to put the patient in a sound chamber and let the patient move dials of sound in order to, create the kind of a sound that they are sharing in that the tinnitus has. So I have to say I have tinnitus and I have done that myself. And I've been able to identify the frequency and the intensity of my tinnitus by using that approach. Tinnitus seems like a really tough

condition to deal with. And it's understandable that researchers are looking around for a lot of different kinds of treatment. So there's also researchers who are working on using different types of electrical stimulation to try to treat tinnitus. Can you tell us about that technique? Yes, absolutely. Part of that work was the pioneering work of Dr. Susan Shore, who until recently was a professor in our institute. And her work identified some of the key mechanisms in the brainstem that subserved the tinnitus. What she discovered is that in animals that were exposed to noise that produced this mild hearing loss, they develop an increased excitability in specific nuclei in the brainstem. And she discovered this particular mechanism in which this over-excitation could be

quiet down by the specific presentation of sound, as well as stimulation to the skin in the face of the neck. And these protocols were very effective to quiet down the behavior that she used to measure tinnitus in animals. And then she developed a specific device and already run to clinical trials in Jouvas that show remarkable effects. Similar approach was developed by a company in Ireland. And they, the only difference is that instead of doing this very subtle stimulation to the skin of the neck or face, they use a stimulation of the tongue.

Wait, how does that work? So the idea is that the specific presentation of the sound and the electrical stimulation produces what is called a sensory adaptation or quieting of specific connections between somatosensory stimuli and the auditory system. circuit, which are the ones that have been increased in response to the hearing laws and are

believed to be the cause of the diners. I mean, is it sort of like how one sense can damp down another? Like feeling heat or cold might affect how someone feels pain. And in this case, they're doing something with stimulating the tongue to affect how the ears are processing information? Not exactly. The way we think about this is that because of the human race, these somatosensory sequence have kind of invaded the auditory circuits.

And by stimulating them in a very particular timing, these misconnections become quiet down by a process that is called synaptic plasticity. So this is not something that occurs instantaneously, but is through repetition that the synapses in the brainstem are changing. And these changes are not going to be permanent, but they are long-lasting. So when patients use these devices chronically and keep using them,

these effects can be very significant and can be persist even after you stop using the machine for a while. So you're essentially retraining that part of the brain? Absolutely. That would be a very good way to put it. This technique is a lot less scientific, but I've seen some posts on social media that talk about pressing on your head a certain way as a technique to temporarily, relieved tinnitus symptoms. Is there anything to that? And if so, why would something like that work? Well, this is kind of the basis for the concept of this somatic dimples, which according to the literature, maybe 70% of the patients, which is where, and this is my case, when I vote my

jobs, the intensity of mighty nitous changes. And this is because this somatosensory system, you know, the nerves that take care of the sensation by face are misconnected now to the auditory system. And by stimulating both the somatosensory and the auditory systems at the same time with this particular time, what we are doing is inducing a deep, increase in the effectiveness of that connection and that's reducing the thing. What's the cutting edge in this field? What do you really want to know?

Well, we need to understand exactly what about the hearing loss is treating this plasticity because they may be in ways eventually in which drugs could be used as well to treat denig One of the really interesting questions remaining is why some people develop in items, while others with similar hearing notes do not. Are there genetic predisposition that we could identify and try to help people with that knowledge? Dr. Gabriel Corfus is Director of the Kresge Hearing Research Institute and the Lynn and Ruth Townsend Professor of Communication Disorders at the University of Michigan in Ann Arbor. Thank you so much for taking time to talk with me today. Thank you very much.

That's it for today's episode. Lots of folks help make this show happen, including Dee Petersman, Sandy Roberts, Beth Ramney, John Dancosky. On tomorrow's episode, how changing climates around the world could affect your favorite sports. I'm SciFry producer Charles Burkrist. Thanks for listening. We'll see you soon.