Hearing loss affects 1 in 5 people globally, increases the risk of developing dementia, and has no cure. 

An astonishing half a billion people have hearing loss, which impacts their ability to communicate and connect with others. Hearing loss cases are increasing fast, especially among younger people. 

For many, hearing aids and cochlear implants can be life-changing, but they can’t help everyone. But there is hope:

Scientists around the world have been working tirelessly to develop new treatments for hearing loss. 

In this episode, Jonathan speaks to Dr. Marcelo Rivolta, a professor of sensory stem cell biology at Sheffield University. 

He’s on the cusp of a major breakthrough that could mean hearing loss becomes a thing of the past. 

What you’ll learn:

• Why hearing loss is becoming more common

• The shocking link between hearing loss and a 500% increased risk of dementia

• How modern headphones and loud concerts cause permanent, irreversible damage

• Why hearing loss can be more isolating than blindness

• The simple ways you can protect your hearing today

• How current technologies like hearing aids and cochlear implants work

• About the groundbreaking new science that could cure deafness using stem cells

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Transcript

Jonathan: Marcelo, thank you so much for joining me today.

Marcelo: Thank you.

Jonathan: So we have a tradition here at ZOE where we always start with a quick-fire round of questions from our listeners, and we have these rules that are designed to be really hard for professors. The rule is you have to say yes or no, or a one-sentence answer if you have to. Are you willing to give it a go?

Marcelo: Yep, let's do it.

Jonathan: Alright. Does hearing loss only really affect older adults?

Marcelo: No.

Jonathan: Are more of us facing hearing loss than ever before?

Marcelo: Yes.

Jonathan: Do people with hearing loss face an increased risk of dementia?

Marcelo: Yes, that is true.

Jonathan: Can hearing aids help everybody with hearing loss?

Marcelo: No, unfortunately not.

Jonathan: And finally, is a cure for hearing loss possible?

Marcelo: We think it is, and we are developing something which we think could change the landscape.

Jonathan: My podcast producer, Rich, has actually talked to me about how in his teenage years he was running these underground raves, and that means now in his thirties, his hearing is actually damaged.

In my case, I remember the time that my son, aged about 11, joyfully showed me this app on his phone that played these high-pitched noises. I discovered that not only could I not hear lots of noises that he said he could, but really depressingly, there were lots of noises my wife could hear that I couldn't.

Since then, I've really noticed that in noisy restaurants, I can struggle to hear people who are further away from me. I'm aware that my hearing is not quite as good as it was when I was 20.

So when you're telling me that hearing loss is becoming more common, I think that's very worrying. Why are we seeing this rise in cases?

Marcelo: One of the reasons it's becoming more common is because as a population, we are aging; it's more common to have older people that will live longer.

One of the key factors that influences hearing loss is aging. As you said, that loss of the very high-pitch sounds that you tend to have from middle age onwards is just the opening of the door to more substantial hearing loss that will happen later in life.

The other factor is we live in a noisier, louder world. We have a lot of noise exposure. The use of headphones is prevalent, and sometimes we don't respect the volume controls that most devices have.

The exposure to loud noises is really impacting the incidence of hearing loss in general.

Jonathan: I'm interested that you already started talking about headphones. Are headphones safe, or am I inevitably on the path to becoming deaf if I'm using them?

Marcelo: I think the headphones, the way that they are built, have safety limits. There is strong advice built in by the manufacturers of what is safe and what is unsafe.

But we tend to, particularly the younger age groups, use them beyond those safety limits because they get more of a thrill from listening to very loud music.

Also, when we go to gigs and music festivals, we are exposed to very, very loud levels of noise and sound that tend to be unsafe.

Jonathan: So if I listen to this very loud sound, that's not just something temporary that's causing me harm? Can this lead to a permanent impact?

Marcelo: It's a very good point. It's a matter of a spectrum. If you listen to a certain level of sound for a short period of time, it may be a temporary shift.

You may have a temporary loss of perception, what is called your threshold being raised, but it is reversible. But once you get to a certain critical point, then you're producing permanent damage, and that's not reversible.

Jonathan: And how long does it take for this permanent hearing loss to happen?

Marcelo: It depends on the aggression. It can be immediate if you are exposed to a very loud noise, or it can take years or decades of aging for that to develop.

Jonathan: So if I'm listening to really loud music on my headphones all the time for hours a day, over what sort of time period would I expect to see an impact?

Marcelo: That will anticipate the onset of this hearing loss. But if you are exposed, for example, to an explosion, it can be loud enough that it produces permanent deafness there and then. So it all depends on the impact of the aggression.

Jonathan: I want to talk for a minute more about headphones because I don't know at what level my headphones are safe. I've noticed that parents tend to be quite cautious now with little babies, which I never saw when I was young.

Now I've noticed that if you go to a concert or party where there's loud music, you see parents have put these ear protectors over little babies, but I notice nobody else is wearing them.

How do I know at what point my headphones are safe?

Marcelo: It's very difficult to give general advice because it all depends very much on the particular device. Some devices come with very clear guidelines of what is safe and that you should turn your volume down.

As a general rule, I would say you have to keep the level of sound to a comfortable level without getting too loud.

The way that we measure sound intensity is in decibels. The scale is basically the pressure that the sound makes because, at the end of the day, sound is mechanical pressure. I would say probably anything about 60 decibels could potentially be a problem. 70 or 80 will be quite loud.

To give you an idea, about a hundred dB is the sound that a pneumatic drill makes on the street. So we're talking about very, very loud noises. And the scale is logarithmic, so an increment of 10 points is a hundredfold, basically.

Jonathan: So just to play that back, you're saying if I'm listening to music at 60 decibels, that's potentially a problem, and at 70 to 80 that is definitely a problem?

Marcelo: That is definitely a problem. I would not recommend going to those levels.

Jonathan: With headphones attached to phones, I could be in a position where I've got more than 60 decibels in my ear?

Marcelo: It is possible, and in general, the advice would be to just put the volume down.

Jonathan: What's it like to suffer from hearing loss and how does it impact your life?

Marcelo: It can have a very, very substantial impact. One of the things, and I think probably a lot of people in the audience may relate to this, is you start having to put the volume up on certain devices because you cannot get the same kind of information, the same kind of clarity that you had before.

But that, in a way, is more of a symptom. Where it starts impacting your life is when it starts affecting your communication with other people.

There is a very telling sentence from Helen Keller, a teacher who suffered from blindness and hearing loss in the first part of the 20th century.

Because she was blind and deaf, she was asked which one was impacting her life more. She very wisely said, and I'm paraphrasing here, but it was without a doubt hearing loss, because blindness cuts you off from things around you, but hearing cuts you off from people.

It affects communication. You go to a pub to have a conversation with some friends, and all of a sudden you are in that very loud environment and you cannot understand what they're saying. You start asking for them to repeat things.

After a while, you just feel embarrassed. You don't want to ask the same thing over and over again. Then you start feeling more withdrawn or ostracized and then you stop going to the pub because, ‘I can't hear what they're saying. I can't have a conversation.’

That impacts your ability to communicate and interact, and it forces you to almost withdraw from your social life. That has a huge impact on your daily life and on your future welfare.

Jonathan: How does that then affect your health?

Marcelo: In the last few years, an increasing volume of research has identified a link between, for example, hearing loss and dementia.

Roughly, it is said that for those people that develop severe hearing loss in midlife, they have a fivefold enhanced risk of developing dementia later on. If you have moderate hearing loss, it's about a twofold risk later in life.

Why is that? It is still not clear. There are different theories. One is cognitive overload. You need to make more effort to listen and to understand, so your brain uses a lot more brain power, and that produces more intense wear and tear.

There are other theories that perhaps the conditions that lead to developing hearing loss are also underlying the condition that will develop dementia.

But whatever the explanation, it's clear that the link seems to be there, and we can minimize that risk by intervening early.

Jonathan: I find it rather extraordinary that hearing loss should have such a big impact. We've done podcasts talking about dementia and people are often talking about a 30% or 40% increase in risk. Fivefold is enormous.

Marcelo: It is one of the highest links, but also one of the things that can be modified. Other things, like genetics, are more difficult to impact, but hearing loss is one of those things where something can be done to prevent the higher risk later in life.

Jonathan: If I was having hearing loss, but then I had a device that meant I can hear better, does that reduce the risk of dementia?

Marcelo: Yes. Even with the devices which are currently available, for example, the use of hearing aids, they reduce the risk. Because of this theory of cognitive overload, if you need to make a lot more effort to understand what people are saying and use a lot more of your brain space to follow a conversation, you reduce that.

You take it back to more normal levels by using something as available as a hearing aid.

Jonathan: So one theory is my brain is doing so much work to try and understand a conversation that its normal processes are being disrupted.

Marcelo: Yes. It's an overload that then leads to more wear and tear. That seems to be one of the more prevalent theories at the moment.

Jonathan: In simple terms, how do we hear and why does it stop working?

Marcelo: Inside our heads, in what's called the temporal bone, there is an organ called the cochlea. Cochlea comes from a Greek word which basically means snail, and it has the shape of a snail. This cochlea has a lot of important cells, but we can simplify it to two critical cell types. One is called the hair cell.

Remember I said sound is a pressure wave? The hair cell takes the sound that comes through your outer ear and middle ear, and all that pressure gets transmitted to the cochlea. The hair cell is called that because it has something on the top that looks like hair. They move, and that movement converts sound into an electrical signal.

But that electrical signal in the cochlea needs to be transmitted to the brain so the brain can make sense that we are hearing something.

That transmission is done by a second cell, which is called the neuron. So you have the hair cell, which is the mechanotransducer, and then the electrical stimulus is transferred to the brain by the cell called the cochlear neuron. The cochlear neuron is the cable that connects the hair cell with the brain.

Jonathan: So for people who are having hearing loss, what's going wrong?

Marcelo: We'll concentrate only on what is called sensorineural hearing loss, which is the hearing loss that affects the hair cells and the neurons in the cochlea. If you have very loud noises, it could damage and destroy the hair cells.

But in the last 10 to 15 years, it has become more obvious how important the neurons also are. It seems that we tend to lose the neurons, and actually the connection between the neuron and the hair cell, way before we lose the hair cells.

Jonathan: The neurons are like the cables that are plugged into my microphone, which in this case are these hair cells. And you are saying that before you even see damage to your hair cells, it's almost like these cables have been unplugged.

Marcelo: Yes, absolutely. That is a very good analogy. The traditional view was that you lose the hair cells first, and then eventually you start losing the neurons.

But actually, it seems to be the other way around. The neurons become unplugged first, and then you start losing the neurons and the hair cells.

One thing which is very important to understand is that we don't have the capacity to repair or regenerate these cells. We, as a species, have lost the capacity through evolution to regenerate these cells. You are born with the cells which are meant to last your lifetime.

So if you are 85 years old, your hair cells and your cochlear neurons are 85 years old as well. And that's why aging is an important factor in the incidence of hearing loss.

Jonathan: One bit I don't understand yet, Marcelo, why does loud noise cause this damage?

Marcelo: It's a very good point. One impact is that because you are forcing the system to overreact, you are overstimulating it. The connection between the hair cell and the neuron becomes what is called cytotoxic or neurotoxic.

It's stimulated so much that you produce that unplugging between the neuron and the hair cell. If the damage continues, those bundles on the top of the hair cell can also get physically damaged. So the loud noise can act in different ways, but the bottom line is that you will damage and lose your critical sensory cells and neurons.

Jonathan: Can we talk about fixing this? Hearing aids have been around for a long time. Don't they just solve the problem for everybody?

Marcelo: No, I'm afraid not. At the moment, there is no biological treatment for hearing loss. We have two therapies which are basically prosthetic devices. One is the hearing aid.

The hearing aid is pretty much an amplifier. The noise that comes through your ears gets amplified in a very intelligent way. But the bottom line is you need to have quite a bit of residual hearing in order to use them.

Then you have the cochlear implant. The big difference is that the cochlear implant is implanted surgically because it needs to go all the way into your cochlea.

It has a speech processor outside your head that converts sound into an electrical signal. That electrical signal is transported through an electrode, a piece of wire that goes all the way into the cochlea and stimulates the nerve. So it does the job of the hair cell. The cochlear implant replaces the hair cells.

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Jonathan: That's amazing. So if I don't have any hair cells, but I do still have some of these neurons, we can now put an artificial ear on the outside and a wire that goes into the cochlea.

Marcelo: Absolutely. You said something critical there, which is if you have enough neurons. You need to have the cable to be stimulated for the cochlear implant to work. The problem is not all patients that have hearing loss have enough, or have good quality, neurons.

The cochlear implant does an amazing job and has done a lot of good for the deaf population, but if you don't have the neurons, you basically have no alternative. There's nothing that can help us to fix that kind of neural hearing loss.

Jonathan: Could you tell us about what your lab is working on?

Marcelo: Yes, we started this work more than 20 years ago at the University of Sheffield. We are trying to harness the potential of stem cells to develop a treatment for hearing loss.

The problem at the end of the day is that you lose an important cell type, you lose your hair cells or you lose your neurons. Our strategy is to recreate those cells in the lab and then transplant them into a patient.

We do that using stem cells. These are amazing cells, which are basically a complete blank canvas. They are undifferentiated, and the ones which are called pluripotent have the capacity to produce any cell in the human body.

The critical thing is to know which signals you need to give this cell to become the cells that you want. One of our initial developments was to develop methods to drive these stem cells into becoming ear progenitor cells—cells that know they're going to be part of an ear, but they aren't fully mature just yet.

We give them a series of chemical instructions that mimic what happens in embryonic development. We give them the instructions to become what we call otic cells, and those are the cells that we can then make differentiate further into becoming hair cells or neurons.

We tested this in preclinical models, in animals. We decided to focus on producing the cochlear neurons because while we have the cochlear implant for hair cell loss, there is nothing for neuron loss.

In a model that doesn't have these neurons, we transplanted the cells and showed that the cells survived, got to the right place in the cochlea, made connections, and restored auditory function.

Jonathan: Marcello, you talk about a model, but do you actually mean a living animal?

Marcelo: Yes. The model that we use mostly are gerbils.

Jonathan: So you're saying there's a gerbil that is completely deaf. It doesn't have these neurons, these "cables". You've injected these special cells that you've figured out how to train into these gerbils, and they can hear again?

Marcelo: Yes. Absolutely.

Jonathan: You say that very casually. Isn't that rather amazing? How long have you been working on this?

Marcelo: Well, we published those results more than 10 years ago, so it took 10 years to develop it to that point. It's not a full recovery, it's a partial recovery, but it was a substantial achievement.

Since then, we started to develop the whole path of taking this all the way to the clinic to make it into a real clinical solution.

Jonathan: How long do you think it will be until this research actually gets into the clinic for human beings?

Marcelo: We are at a very, very exciting moment. We are preparing the ground to be able to start testing the cells in a human patient for the first time. If the plan goes as we have planned, that will happen next year. From then on, it depends on how things progress through clinical trials.

The first phase is to make sure the cells are safe. The second phase involves more patients in different centers. All that may take perhaps something like five to six years. If it all progresses really well, there will be a solution for that type of patient.

At the moment, we are concentrating on patients that have this neural hearing loss. We are not at this stage trying to replace the hair cells.

The initial phase of this study will be in combination with the cochlear implant, because the implant replaces the hair cell, and we need to make it talk with the new "cable" that we are creating with our neurons.

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Jonathan: So, to conclude, what would your practical advice be to anyone thinking today, how can I reduce my risk of developing hearing loss or having my hearing loss get worse?

Marcelo: Absolutely. You have to protect your hearing. Remember that those cells are your lifetime companions, so you have to look after them. Try not to be exposed to very loud noises. Use headphones in a very sensible way.

Also, and this is very important, when you start having symptoms of losing your hearing, do something about it. Go and see your GP or your audiologist.

They will advise if you need hearing aids. And if you need hearing aids, use them. We talked about how important the use of hearing aids is to reduce the risk of dementia.

And we have to overcome this stigma. We all use glasses at a certain time in our life and nobody will blink an eye. It's completely natural. However, it's not the same with hearing aids.

We need to overcome that presumption that hearing aids make you look different or old. That shouldn't be the case. Hearing aids are there to help, and if you need them, you should wear them.

Jonathan: Marcelo, just before I wrap up, you mentioned that certain antibiotics could hurt your hearing.

Marcelo: Yes, a family of antibiotics called aminoglycosides. They are not commonly used because of this known side effect, but unfortunately for some conditions, they are still an antibiotic of choice. There are genetic predispositions that can make the toxicity more apparent.

But obviously, if you have a very rampant infection and this is the antibiotic that needs to be given to you, you will get it. They are known to be ototoxic, but sometimes the choice is life or death.

There is also a big area of research called otoprotection, which is trying to develop drugs that will prevent the antibiotics or certain cancer treatment drugs from impacting your hearing system.

Jonathan: Marcelo, I'd like to do a little summary.

What I'm most struck by is this extraordinary link between hearing loss and dementia — a fivefold higher risk with severe hearing loss. The leading theory is that the brain is working so hard to understand conversation that this is causing damage. Crucially, wearing a hearing aid if you need one can remove this risk.

This is a powerful reason to protect your hearing. You explained that our hearing cells can't be repaired; what you're born with has to last a lifetime.

And while we often think sight is more important, you pointed out that hearing loss cuts us off from other people, leading to isolation.

The problem is getting worse, partly because we're an older population, but also because we live in a much louder environment with headphones and concerts.

Today, there are two main technologies: hearing aids, which amplify noise but require you to have some hearing left, and cochlear implants, which can work even if your hair cells are gone, but you still need the "cables"—the neurons—connecting to your brain.

Your amazing research is working on solving the problem for people whose neurons are gone. After 20 years, you're going to be testing this on human beings next year, and hopefully, in five or six years, it could be an available treatment.

In the meantime, your key advice is that we should protect our hearing. If you're going to a loud concert, wear ear protection. And every day, think about whether your headphones are too loud. Finally, if you are losing your hearing, see a professional.

If you need a hearing aid, embrace it. It’s a tool to protect your long-term brain health and you should overcome any stigma, just as you would wear glasses if you needed them to see.

Marcelo: I think that's an excellent summary. You've covered all the points that we discussed.