Hearing loss, and what we can do about it

0:17Hello, welcome to the Naked Scientist podcast, the programme that brings you the biggest breakthroughs and talks to the major movers and shakers in the worlds of science, technology and medicine. I'm Chris Smith and today we're going to consider why we lose our hearing and what we can do about it.

0:42Hearing loss is one of the most common health problems in the world. It tends to creep up on us as we age, but there are also plenty of younger people who are also affected too. And regardless of how old we are, hearing loss has an undisputed dramatic impact on our quality of life. So today, we're going to examine how we end up losing our hearing and what can be done to help. First, to set the scene for us with a look at the scale of the problem, here's Frankie Oliver, Audiology Manager at National Hearing Loss Charity, the RNID. So we estimate that around 80 million people in the UK have hearing loss, are deaf or have tinnitus.

1:23And that's roughly one in three of us. Hearing loss can happen at any age, but we also know that it is more common in the older population. So when you look at people who are age 55, around half of that population have some level of hearing loss. But when you get to the over 70s, it's around 80%. The World Health Organization estimates around one and a half billion people have hearing loss worldwide. And by 2050, they think this could go up to around two and a half billion. How is the age-related trend changing? Because I keep hearing people saying the headphone generation and the nightclub generation are subject to a different pattern of noise exposure, which is a risk factor for losing your hearing.

2:08and from a younger age therefore we might be brewing up a really big problem here. Yeah that's right and it is something that the World Health Organisation is also quite concerned about as well. So they estimate that around one billion young people are currently at risk of preventable hearing loss through noise exposure. Things like listening to music through headphones or wearing headphones regularly but also going to noisy events kind of recreationally so going to a festival or a gig or a nightclub. When we measure hearing loss, we test over a range of frequencies, and one of these being four kilohertz.

2:43And when you see a little notch, basically, to say, oh, that's probably some noise exposure there. Whether we see that same pattern through different types of noise exposure, I think is yet to be seen. It's something that we'll probably find out in the coming years. How do most people realise or find out that they have a hearing problem? Things like finding that you're turning the television volume up, finding it harder to hear on the phone, maybe having to ask people to repeat when they're talking, especially if you're in a situation where there is a bit of background noise. So say you're in a restaurant or in a kind of a group setting, finding it a bit harder to follow conversations there.

3:22A lot of people find that actually their relatives and loved ones and friends point it out to them and say you might not be hearing so well. These are all signs that you could be experiencing hearing loss. In this country there is a really good provision of health care available so hearing aids and hearing tests and treatment that is all free on the NHS and so if you are starting to experience those signs it's a good sign to go and get your hearing checked. So you've mentioned that you can get treatment what shape does that treatment take? For most people when you go and see an audiologist We'll do something called pure tone audiometry and this is a hearing test and basically what we do is we measure the quietest sound you can hear over a range of different frequencies or pitches and we do this for each ear separately so we can look at the pitches that you might be struggling in and work out exactly what might be the best course of action.

4:22For most people hearing aids is the best thing. They're really the gold standard treatment. And what we can do with hearing aids is we can take the results of that hearing test and turn that into a prescription. So all hearing aids these days are digital, and that means we can fine tune them to your specific hearing loss and your specific needs. They're really amazing bits of kit hearing aids. They've come on a long way from what maybe a lot of people might think of them as, as something that might whistle, something that doesn't work very well. Digital hearing aids these days are really excellent and they can be life-changing for so many people.

4:59And lastly Frankie can we just touch on something that often goes hand in hand with hearing loss and that is hearing things you don't want to hear, tinnitus ringing in your ears. Is that part and parcel of the same problem and how many people does that affect? So it's thought that tinnitus affects around one in seven adults so that's just more than 7 million people in the UK. Tinnitus can often be linked with hearing loss but there's also quite a sizable portion of people that will have tinnitus but no measurable hearing loss as well. So for most people that have tinnitus it's what we call idiopathic which means we can't necessarily identify a cause behind it.

5:42One thing that can be really really helpful especially for people that have hearing loss as well is hearing aids because what we think is happening with a lot of people with tinnitus is that the brain is expecting the sound that it's supposed to hear and if you've got hearing loss that signal does not necessarily get to the brain and then what happens as a result is the brain starts to fill in those gaps and that's where you get this sound some people say it sounds like a ringing some people say it sounds like a buzzing it can be any sound a lot of people find that once they kind of put hearing aids in the tinnitus is kind of drastically reduced in volume or no longer there so they can be really really helpful other things for tinnitus are things like talking therapy so cognitive behavioral therapy that is something that's recommended but a lot of people can also do things that themselves at home so playing pleasant sounds to help distract from the tinnitus try things like relaxation exercises and mindfulness and just reducing stress that can all be extremely helpful some very helpful advice and insights there that was frankie oliver who's at the national hearing loss charity the RNID.

6:45Before we discuss how we lose our hearing and perhaps develop some of the symptoms that Frankie mentioned, we need to consider how we hear in the first place. This all happens in a special structure called the cochlea, which is sensitive to different frequencies of sound vibrations at different points along its length. These vibrations cause populations of inner hair cells to fire off barrages of nerve impulses to the brain whenever they're shaken and that indicates that sound of a certain frequency is present. But even more cunning than that is that sitting alongside these inner hair cells is another population of so-called outer hair cells which can respond to signals coming back from the brain and from the cochlea itself to tune each bit of the cochlea and further sensitize it to certain sounds.

7:36And this is effectively how we're able to focus in on what people are saying to us, even in really noisy places. It's a wonder to behold. And here to talk us through how it works is the University of Manchester's Kevin Munro. So when we talk about hearing, we're talking about being able to detect vibrations. Our ear and our hearing system has to convert these vibrations into electrical signals that go up the hearing nerve and up to the brain. So our hearing system starts by collecting these vibrations, funneling it down our ear canal across a part of the ear called the middle ear and into the inner ear which has a structure called the cochlea and that's where we start to process and analyse the sounds.

8:22So if sounds are arriving as vibrations at the inner ear, the cochlea, there's going to be a whole mass of different mixed up frequencies in there so how does the ear sort that out yes absolutely all the frequencies reach the cochlea it's snail shaped but you could unravel it and it's really a sausage filled with fluid and it has a tuning property it helps to separate out the different frequencies and pitches of sound so actually where the sounds first enter the cochlea. That's where the inner ear of the sausage is tuned to detect high frequency sound. And as the wave moves up the cochlea to the top, that detects low frequency sound.

9:06How does it discriminate along its length in that way? Is that a mechanical thing when you see people playing glasses because they lick their finger and rub a finger around the rim of a glass? If you have big glasses and small glasses, they vibrate best at different notes, as it were. Does the cochlea do it mechanically like that, or is this a clever electrical and neurological processing that's going on? Well, there are probably two important steps. One of them is just as you've described there, Chris, that there's a mechanical tuning. When the sound first enters the cochlea, the membrane is very stiff and very tight, and that means it's very responsive to high-pitched sounds, high-frequency sounds.

9:46But as the sound, the wave travels along the cochlea up towards the top, it becomes less stiff and more receptive to the low frequency sound. But the other thing that's very important and very clever is that part of the cochlea works as an amplifier. It works as a hearing aid. So there are little hair cells that help tune the membrane as the sound travels along. So it helps the membrane become even better than it normally would be. And it's when things go wrong with these hair cells, and we have about 12 ,000 of these, they're called outer hair cells in humans. If they're damaged, then first of all, you might not hear quiet sounds, but you lose that fine tuning.

10:30And that's when things become problematic because you can't separate out two different people speaking, or at least you can't do it very easily. And it requires a lot of effort and some fatigue to be able to do it. So when the sound goes into the cochlear, it's making the tissue vibrate according to frequencies. But how are the vibrations, before we come to sort of tuning in on different sounds, how are the vibrations actually being converted into information the brain can understand? So when the membrane moves up and down, it causes these hair cells to sway around, just like seaweed in the sea.

11:04And these hair cells have little cilia or hairs on their tip. And as they bend one way and the other, they work as a transducer, meaning simply that the vibration gets changed into electrical signal and can then get passed up the hearing nerve. So if that's sending information to the brain, how do those cells also help to do tuning? So are they getting a message back from the brain then, telling them what to tune into? too? Well, they're supplied with energy from the ear and because of their location, when there's vibration, they actually jump up and down a bit more than they normally would do and that gives the extra fine tuning.

11:44And if they're absent, you don't get that fine tuning, so you can't hear quiet sound, but you start to have the blurring of different sounds. And whenever someone has a hearing loss because of damage in the inner ear, in the cochlear, we tend to refer to that as a central neural hearing loss. If there are outer hair cells, does that mean there are also inner hair cells? And if so, what are they doing? Yes, absolutely. There are about 3 ,000 inner hair cells in humans, and the inner hair cell is what carries the signal along the hearing nerve and up to the brain. So they're an important part of the pathway.

12:24And actually, although we're concentrating in the cochlear, we have to remember that the hearing system goes all the way to the brain, And we have two ears, and two ears help us to tell the direction of sound, spatial hearing. That sound came from behind you or from the left or to the right. Before we began this conversation, you told me that you've got a hearing problem yourself. I wouldn't have realised it had you not told me from this conversation and the way you speak. But is that what led you down this path of being interested in this problem? Well, it made me aware that there was a health discipline in hearing science and audiology that probably my friends at school didn't even know existed, Chris.

13:06But I was at a very young age, maybe two or three, diagnosed with absolutely no hearing in one ear. And maybe I was born with it. That sometimes happens. Or maybe it was a virus. Maybe it was mumps when I was one or two years old. So I've gone through life with only having one ear and then more recently, as I got older, started to have a hearing loss in my one remaining ear. If I'm in a quiet environment, like you and me talking together, I tend not to have any problems. But as soon as there's background noise present, I would have considerable difficulty. And also, everything goes into my one ear, so I'm unable to tell where sounds come from.

13:43and that can be disconcerting, especially if it's a warning or an emergency sound and I can't tell where it's coming from. But my main difficulty is in background noise. Unfortunately, today, you and I are in a very quiet environment and that's why I'm not having very much difficulty at all, Chris. It's an amazing system of the year, isn't it? That was Kevin Munro. He's at the University of Manchester. The Naked Scientist podcast is produced in association with Spitfire, Cost-effective voice, internet and IP engineering services for UK businesses. Find out how Spitfire can empower your company at spitfire.co.uk.

14:21Music in the programme is sponsored by Epidemic Sound. Perfect music for audio and video productions. And this is the Naked Scientist podcast with me, Chris Smith. And today we are looking at the subject of hearing loss. and stay tuned because coming up we'll hear why birds can't go deaf and might therefore help us with our own hearing problems. But first we've just heard from Kevin Munro about his personal experience but not all hearing loss is the same. Some people have conductive hearing loss where sound struggles to get into the ear at all while others have so-called sensory neural hearing loss caused by damage to the inner ear.

14:59Some have a mix of both and here's Adam Carlton who's part of the Hearing Research Group at the University of Sheffield to explain. There's three kind of flavours of deafness. Conductive is what usually concerns the outer and the middle ear. So that's the ability of the sound vibrations to actually reach it from the air around us to actually the cells that are going to transform that information into something our brain can understand. A common one would be something like a perforated eardrum, so a hole in the eardrum. You're going to lose a lot of that sound information before it reaches the inside of your skull.

15:33The second form is the ability of these cells to turn the vibrations into something we can understand like an electrical impulse. So this can be the ability of the tiny hairs to actually pick up the sound information. It can be the ability of these cells to move this information to the central nervous system and the brain where we can actually understand it. And the third type would just be a mix of both. So an example would be like an inner ear infection can cause inflammation around the outer ear components and the middle ear components causing a degree of conductive hearing loss, but also can cause inflammation that will damage the cells that detect sound and cause a degree of sensorineural hearing loss.

16:12When a person develops some hearing loss, has it comprehensively affected all of their cochlea so they have a problem with all frequencies? Or do they tend to get problems with just discrete frequencies of sound? Yep, so that entirely depends on how the hearing loss has actually come about. So, for example, things such as ageing and noise-induced hearing loss, those tend to affect the higher frequencies first, so the higher pitches. As you get older, you lose those higher frequencies anyway as just a natural part of aging, which is why those annoying supermarket teenager alarms exist because all the people can't hear them.

16:49When we lose the ability to pick up certain frequencies in that way, if someone like you goes into the cochlea and has a look, what's changed? There's different components that can be damaged, but in general, it's a loss of cells or it's a loss of the wires that carry the information from the cochlea to the actual brain. So a big one is the outer hair cells. To be non-technical, these are basically cells that act like little amplifiers. So as a sound comes in, these cells physically move. They cause a force that makes the sound louder in the cochlea and easier to detect. and these cells are very vulnerable to noise, noise-induced hearing loss.

17:30So if I was to open up the cochlea, I would see that a percentage of those cells have been lost as a result. And the other aspect would be the actual wires that carry sound from the cochlea to the brain. There's a lot of them relative to the number of cells that pick up sound information and they're very vulnerable to sound. So conditions that make you feel like sound is very, very loud when it's not, it's called hyperacusis. And the reason for that is because we've got fewer wires. so that they're very vulnerable again to noise-induced hearing loss. How do you tell, because you've mentioned there are three types of hearing loss, what sorts of experiments can be done to discriminate between them, to work out where a person's problem lies?

18:11But also, how do you then go about fixing the problem in terms of identifying which bit of the cochlea is affected? That's one thing, but trying to overcome some of those difficulties with devices and so on. If you were trying to diagnose that the first tool you'd likely get would be an audiogram, which is you get played frequencies and pitches at certain levels and you press the button when you can hear the frequency, when you can hear the sound. And by building a curve between how loud the sound was when you could hear it and the frequency and the pitch of that sound, you can get an idea as to whether someone has conductive hearing loss or sensorineural hearing loss.

18:51And if you compare that with more thorough examination, so when they look in your ear with that tiny little scope, they can see the kind of inflammation or the kind of things that would cause something like gum here in the middle ear, for example. when it comes to the treating these conditions. So the conductive is actually more straightforward because that's a simple kind of mechanical process of sound moving outside in. You're not having to replace really anything unless you've got to replace the actual middle ear bones, which I'm sure is very complicated, but it's still a, in theory, easier problem.

19:22With gum ear, for example, you drain the fluid. The middle ear bones are now in air as they're supposed to be and your hearing gets better. When it comes to sensorineural hearing loss though, that is more complicated And that's the kind of great goal that we're trying to achieve and that a lot of groups around the world are trying to achieve is to understand how to kind of stop these cells from dying in the first place and possibilities of regenerating what hearing is left after something that causes noise induced or age related hearing loss. and where does tinnitus come into this because many people who have hearing loss will also complain they get unwanted sounds tinnitus which can be worse actually for them psychologically than not being able to hear things why is that happening and what do we think are the parts of the ear involved in making that occur so there's there's two kind of flavors of tinnitus so there's one that's quote-unquote real and there's one that's not the one that's real is an actual vibration in the inner ear that you can detect.

20:21It's occurring spontaneously. It's occurring by itself. But you can actually hear that with equipment. It's there. The other type is occurring in the actual brain itself. So the way in which I always imagine it is if you imagine a volume dial on like a speaker, during normal hearing, you play with that dial to hear things nice and clearly. in quiet environments you turn things up and vice versa and in louder environments you turn them down. If you turn that amplifier all the way up and you hear that hissing tone that's kind of like what tinnitus is. It's the brain turning up the amplifier to try and hear which is why tinnitus is usually associated with hearing loss because the hearing system is damaged and it's not quite as sensitive as it should be.

21:08So your brain is trying to turn up that volume dial and as a side effect you're getting this this kind of hissing sound um it's called central gain but that that one is still up for debate where this arises actually in the brain and exactly how we're perceiving it is still not not really well known and do we know how to turn that volume knob back down so that people get some respite we don't just yet so this this the technology is definitely it's coming across very very quickly now we're getting um the ability to influence how the brain behaves externally without having to put giant metal needles into the skull and things like that.

21:48It's an answer that's going to be coming sooner rather than later, I imagine, because once we know where this arises from, we can start to get an idea of how we can influence it to stop people from hearing these tones. There's quite a few technologies coming out, like even by sound waves, you can use ultrasonic sound waves to influence how the brain behaves. And yeah, once we have an idea of where this is, that would be the way in which we could turn it off. That or by repairing the hearing system itself. So if we can get rid of any sensorineal hearing loss, we can get the brain to stop tinkering with the volume dial and hopefully the tinnitus would go away.

22:25But as we've not cured hearing loss yet, there's no way to know if that would work. Fingers crossed it will. Adam Carlton there from the Hearing Research Group at the University of Sheffield. Finally today, we're looking at some of the emerging treatments for hearing loss. Historically, technologies like hearing aids, which can make the sound frequencies a person struggles to hear selectively louder, and cochlear implants, which can use electrical signals to activate the hearing nerves along the cochlear directly, have been hugely helpful. But now we're entering an era where, as we learn more about the cell biology and the molecular clockwork of the cochlear, we can use that knowledge to engineer the system to solve hearing loss problems.

23:05Scientists can now deliver gene therapies into the inner ear to treat one form of inherited deafness. And approaches are also on the horizon that could enable us to reprogram cells in the cochlea itself to replace the hair cells lost to old age or too many loud music festivals. Konstantina Stankovic is a leading expert in auditory science at Stanford University, where she's pioneering some of these approaches. There are many different biological options and they can involve gene therapy, they can involve stem cell therapies, it can involve drug repositioning, figuring out which of the drugs that are already approved for other indications could be repurposed for hearing restoration and it also entails novel devices.

23:55If I'm reading this correctly, when you say things like gene therapy and stem cell therapy, are you saying we're actually going to put things into the inner ear that would either replace the cells that have been lost so we can get the ability to hear sounds of certain levels and amplitudes and frequencies back again? Or we also use gene therapy to stop cells disappearing in the first place? We are working on both. That's an excellent summary of the state of the art. Indeed, gene therapy works by delivering therapeutic genetic materials directly to the inner ear to restore or modify gene function in cochlear cells.

24:35So you said in one scenario, indeed, the cells have to be there so that the element that's missing and preventing them from working properly is restored. And an example of that is the current gene therapy trials for otopherlin. So the cells have to be there, and autofurline is a protein which is required for neurotransmitter release. Basically, these sensory cells in the inner ear have little vesicles that are filled with neurotransmitter, and these vesicles have to fuse with the cell membrane to allow neurotransmitter release and excitation of the auditory nerve fibers that contact these cells.

25:19And in autofurlin-caused deafness, that process is inhibited because of a genetic mutation. And that can be overcome by directly delivering the gene that works into the inner ear. And early-stage clinical trials have demonstrated sometimes remarkable success and definitely early safety. So that's one approach. And another approach that you alluded to is indeed making new cells, which is relevant for hair cell regeneration. Non-mammalian species like birds and reptiles spontaneously regenerate these cells in their inner ear. And they do it very quickly within days. But mammals do not do that. So by understanding how these lower species do it, we have now identified molecular pathways that guide this regeneration, and we're now tweaking them to promote hair cell regeneration.

26:22That's fascinating. So is it impossible then to make a bird deaf in the long term because it would just regenerate its hearing system? Indeed, yes. It regenerates its hearing system. It's fascinating and remarkable and really inspiring. And basically, the way birds do it, if a hair cell disappears, then a supporting cell around it can do one of two things. One, it can divide into two cells where one cell gives rise to a hair cell and the other is a new supporting cell. and the other way is what's called direct transdifferentiation where a supporting cell directly converts to become a hair cell and that is by reactivating developmental pathways that lead to hair cell birth.

27:13Do we have those supporting cells in our ears or is that an avian specific adaptation? We do. We absolutely have these supporting cells in the mammalian inner ear and studies in animal models have shown that we can now regenerate hair cells in mammals by tweaking these developmental pathways that were initially identified in birds. So what you can turn that pathway on, so you'd put a drug in or inject something that would activate the right pathway so those supporting cells think they're in a bird ear effectively and they start dividing into new hair cells. Exactly. There are several different ways of doing it.

27:59One is indeed to activate these developmental pathways. Another one is what's called transcription factor reprogramming, where we can cause forced expression of hair cell-specific transcription factors in supporting cells. Yet another way is what's called epigenetic modification. And what that means is that there are some age-related epigenetic changes that restrict gene accessibility for hair cell regeneration. So by delivering agents that allow more direct access to these genes would lead to hair cell regeneration. And yet another way is what you alluded to, which is drug-like molecule cocktails to do the same thing.