Welcome to Module four, disorders of the inner ear. In this module, we're going to review the anatomy of the cochlea. We're going to identify the cochlear cells that are implicated in sensory neural hearing loss. We're going to list causes of sensory neural hearing loss, and understand available treatments, including cochlear implants. Lastly, we're going to discuss tinnitus, including the causes and the available treatments. So we've made it to the inner ear and we get to talk about my favorite organ, the cochlea. This is a fluid-filled structure that's housed deep inside the ear. In fact, to give a sense of scale, the entire cochlea including the balance organs can sit on the surface of the penny. As we transition from the outer and middle to the inner ear, you'll notice a number of differences. The first one is, the cause of hearing loss and the type of hearing loss. In the outer and middle ear, it leads to conductive hearing loss, and in the inner ear, sensory neural hearing loss. In addition, unlike problems with the outer and middle ear, problems with the inner ear cannot be diagnosed by looking inside the ear. This requires a hearing test to determine inner ear function and occasionally other medical and surgical evaluation. Let's talk about the anatomy of the cochlea. The cochlea itself has 2.5 turns. Within these turns, there are three fluid-filled compartments that we call scale-up that differ in their electrolyte composition. These are called the scala vestibuli, the scala media, and the scala tympani. If you remember back to the auditory pathway, the vibration of the ear drum then the vibration of the ossicles, create a fluid wave that occurs with inside the cochlea. Ultimately, the cells of the organ of corti housed in the scala media, are the ones that respond to this vibration and fluid wave. Here's a diagram of the anatomy of the organ of corti. This is where I like to say that the magic of hearing happens. In this slide you can see both the inner and the outer hair cells as well as a number of different anatomic components. I'm going to call your attention to the hair cells first. These are called hair cells because of small fine cilia or hairs that sit on the surface. These are embedded in an overlying membrane called the tectorial membrane. When the basilar membrane on the bottom moves in response to a fluid wave, this ultimately causes those tiny hair cells to move, and this is how mechanotransduction or hearing ultimately occurs. The cochlea is derived from the Latin, meaning snail and it's pretty obvious why you can see it wrapped up there like a snail. The cells of the inner ear can be analogous to that of piano keys with each one corresponding to a different frequency. This very impressive process has something called tonotopic organization. This means that the cells inside the ear correspond to different frequencies and different locations. The physical properties of the basilar membrane allow different frequencies to attain maximal amplitude at different positions. This is important and has important implications for our diagnosis because different causes of hearing loss may preferentially affect different areas of the cochlea and lead to problems with different frequencies inherent. Let's talk about one of those specific causes, noise damage. So, noise damage predominantly affects the high frequency at the location of the cochlea that's closest to the ossicles. Large movements in the tympanic membrane cause big excursions of the ossicles and thus a pretty big wave inside the cochlea. This can lead to shearing of those tiny hairs that are embedded in the tectorial membrane and ultimately lead to loss of those frequencies. Damage from noise induced hearing loss can occur after one very loud noise like an explosion, or it can come from repeated loud noise exposure over time. There are a number of other causes of sensory neural hearing loss, many of which are listed on this slide. There can be a child that's born deaf or born with sensorineural hearing loss and that's called congenital hearing loss. The other aspect of aging such as those in elderly individuals can also experience sensorineural hearing loss. There are a number of other causes including infections, ototoxicity, meaning sensorineural hearing loss that results from medication, as well as different problems with blood supply or vascular or other inner ear or balance problems. Amazingly, in the entire auditory pathway which we see reviewed here on this slide, the problem of sensory neural hearing loss is actually attributed to damage or loss of the inner hair cells more than 95 percent of the time. So, within the entire auditory pathway that we've been through a few times, the problem can be localized to the inner hair cells in the vast majority of individuals. This is important because it has direct implications for the treatment, specifically, cochlear implantation. A cochlear implant bypasses the inner hair cells and transmits electrical impulses directly to the auditory nerve the surgically implanted intra-cochlear electrode. I love to talk about cochlear implants so we're going to go through this slide in detail. A cochlear implant has two parts. One part is an external processor, almost like a hearing aid that you wear behind your ear although it's not a hearing aid. This connects to an internal surgically implanted device which goes under the skin and has a snake-like electrode which goes and is surgically implanted inside the cochlea by a cochlear implant surgeon, such as myself. Let's go through an overview of how sound is heard by an individual with a cochlear implant. Obviously, this slide looks a lot like the auditory pathway we just reviewed. But for cochlear implant individuals, there are a lot of important differences. First, the external sound is captured by the processor or the external portion that's worn behind the ear. This processor converts sound from the environment to digital signals, and then sends these digital signals across the skin or transcutaneously to the internal receiver that's placed under the skin during surgery. These two things are held together by magnets. One magnet that's part of the external processor, and one magnet that's part of the internal part that's placed during surgery. Ultimately, the internal device converts the signal to electrical energy and it sends it to the electrode array which is inside the cochlea. The electrodes stimulate the hearing nerve, they're bypassing the damaged inner hair cells and the brain perceives these signals and that's how a cochlear implant recipient is able to hear sound. Cochlear implant surgery is where there's the placement of the internal device underneath the skin and the electrode inside the cochlea. This is a routine outpatient procedure does last about an hour or an hour and a half. Doesn't require dramatic shaving of the head and in general, the pain is very minimal. It involves a low incision behind the ear and has very few complications. Does it work? Well, that's the most important question and in fact it does. These are not experimental devices. They've been used in more than half a million individuals worldwide. Cochlear implants restore sound perception which enables useful hearing in both children and adults with severe to profound sensorineural hearing loss. In fact, cochlear implants are the most successful neural prosthesis to date. Are there any situations where a cochlear implant may not be indicated? Well, we discussed how the electrode is placed inside the cochlea to bypass the inner hair cells. So, if there is not a cochlea or in the rare case where an individual doesn't have an auditory nerve, a cochlear implant wouldn't be appropriate. However, in the many other causes, such as problems with the outer and middle ear are present but abnormal cochlea, we call that a malformation or a cochlear abnormality and a bunch of other medical conditions that might impact generally anesthesia, none of those are direct contra-indications to cochlear implantation. Those are important for the cochlear implant surgeon and team to consider when they're talking to an individual who might get a cochlear implant. In general, cochlear implants are safe and effective treatment for hearing loss throughout the lifespan for both adults, young children, and even the elderly. In fact, there are many individuals who are candidates for cochlear implantation but may not know about it. It's important if you know someone who wears hearing aids but may not be getting adequate benefit or who has never talked to their audiologist about a cochlear implant, this is something you might want to consider. Let's switch gears a little bit and talk about our final topic and that is of tinnitus. You can say either tinnitus or tinnitus much like potato or potato, both are acceptable. The definition of this is the perception of sound in the ear or the head in the absence of external acoustic stimuli. This comes from the Latin verb 'tinnire' which means to ring and that's why we think of it as ear ringing. But, actually, in many patients describe a multitude of sounds, such as swooshing or shushing, even hussing or fizzing sounds can be described as tinnitus. This can be in one ear or it can be in both or bilateral, and it can occur at any age. It's a pretty widespread problem. The CDC estimates that 50 percent of the population or more than 50 million Americans experienced tinnitus. For most of us, it's just a transient or intermittent phenomenon, but there's a significant portion of the population, maybe up to two million individuals that actually experienced extreme or debilitating symptoms. On this diagram, we see just a few of the causes of tinnitus. As you can see, they occur throughout the ear. Problems of the outer ear and middle ear, including those that we've discussed and some that we haven't, such as a very specialized middle ear tumor called a glomus tumor, problems of the inner ear including problems of the balance organ and also problems that occur behind the ears, such as problems with the auditory nerve and brain. All of these are potential causes of tinnitus. In fact, this slide also includes additional causes of tinnitus. As you can see here, those can include global problems, such as those related to your thyroid or even the vascular or blood system. I want to spend a little time talking about another important cause, and that's medications. On this slide, we see a non-exhaustive list of the medications that have been associated with tinnitus. I don't know if you have ever looked at the back of any medication bottle, but on that long list of things that's printed, you will often see the word tinnitus. For many of these, there are medications that most of us will never come in contact with. But there's a couple I'd like to highlight that are very common. Listed here is aspirin, ibuprofen or motrin and other over-the-counter anti-inflammatory medications. It's important to always review your medications with your doctor if you're experiencing tinnitus because stopping one of these medications may help with your experience of tinnitus. We discussed briefly a bunch of various causes and I'd like to highlight again noise-induced hearing loss. This is something that we discussed before. Ninety percent of people with tinnitus have noise-induced hearing loss. Noise induced hearing loss typically affects the base of the cochlea and that results in high frequency hearing loss. The exact path of physiology of tinnitus is actually unknown at present. We don't know everything about that, but there's a lot of research going on in this area to both figure out the various causes, the pathophysiology, and the treatment. If you or someone you know experiences tinnitus, the most important thing is to get a medical or hearing professional evaluation. Online, you may see a multitude of potential treatments for this problem, but actually, many of those are not effective. Reaching out to a hearing health provider or a physician can help guide you to one of the actually effective remedies for this problem. One of the most effective is hearing aids. As we mentioned before, this can help treat both the underlying hearing loss and also significantly affect that patient's experience of the problem of tinnitus. In addition, masking is a term that's used to describe adding natural or artificial sound to a tinnitus sufferer's environment. This doesn't just mask or cover up the internal noise, but it actually plays a role in helping the brain adjust through distraction or adaptation. Other specific techniques, such as cognitive behavioral therapy or tinnitus retraining therapy, have shown significant promise in the treatment of this difficult problem. They combine counseling and individualized sound therapy to both encourage habituation and treat the individual's response to the tinnitus. As a last thought, I'd like to caution that the most important treatment is actually the one of prevention. We've mentioned a few times about noise-induced hearing loss, and this is a pretty big and important problem. If there's any suggestion that you may be exposed to a loud noise, I would caution you to try to prevent that. In summary, in this final module, we discussed many causes for sensorineural hearing loss and although there are many ideologies, nearly all cause damage to the inner hair cells. Cochlear implants are a safe and effective treatment for adults and children with severe to profound sensorineural hearing loss. Noise-induced hearing loss causes high frequency hearing loss that corresponds to a specific region inside the cochlea. This noise-induced hearing loss is also a common cause of tinnitus. Thank you so much for joining us for this Coursera course, Introduction to Hearing Loss. I hope that you enjoyed it and learned something. Thanks very much.
