CDIS 4027 Disorders of the Inner Ear


So we are continuing with our lecture on the disorders of the ear. So in the previous lecture, we talked about the conditions that might affect the external ear and the middle ear, and that most of these conditions result in a conductive hearing loss. So in the next few slides, we will talk about some of the conditions that may result in a sensorineural hearing loss because of damage in the inner ear. The majority of the hearing loss that an audiologist might encounter is in the sensorineural region. Here, pathology is primarily in those inner and outer hair cells. Unlike a conductive hearing loss, where there is a sensitivity loss, in other words, the sounds are softer because of energy being lost in the external or middle ear, with a sensorineural hearing loss, not only do we see a loss in the sensitivity, but beyond that, there is also a distortion of the speech or sound. So in that way, just making the sounds louder does not completely rectify the situation. So let’s begin by talking about some of the causes of inner ear conditions. Broadly, the causes for sensorineural hearing loss can be endogenous, meaning that it could be because of reasons within the organism, or, in this case, the human auditory system, or the reason can be exogenous, reasons from outside. Examples of endogenous hearing loss is like the hereditary hearing loss, the hearing loss that we see with some syndromes, or hearing loss that many individuals have seen from birth. Examples of exogenous or external hearing loss could be because of trauma, noise exposure, ototoxicity (excessive drugs, especially some antibiotics). The causes for hearing loss can divided in time stages. So it could be because of some prenatal causes and that’s where the hereditary type of hearing loss is found, but then hearing loss can occur independently or at birth or it could be because it’s a part of a syndrome. Some of the environmental causes, even during prenatal stages, in other words, when the fetus is still in the womb, could be trauma to the mother, could be some viral infections such as rubella or autoimmune deficiency syndrome (AIDS) or could be a cytomegalic virus, which is notorious for creating hearing loss, especially if the mother has been suffering with this condition through the first trimester. Perinatal causes refers to the causes that happen during the birth process. It could be because of anoxia, lack of oxygen to the newborn/infant, another cause could be premature birth, so whatever reason results in premature birth could also be associated with a hearing loss that’s one of the reason why premature birth and low birth weight are considered high risk factors and these are usually children who we continuously monitor for a few months, even if they pass a hearing screening at birth. Because there are certain classes of hearing losses that are acquired or later developmentally, especially if those children have these high risk factors. Another cause for hearing loss during birth could be because of trauma such as a breech delivery or because of a forceps delivery where they have to use a prong to breakout the baby. Postnatal causes for hearing loss in early years could be because of a longstanding or untreated otitis media, could be meningitis, could be because of viral infections like measles or mumps, or chicken pox, could be because of syphilis, which is a bacterial infection, or it could be because of an infection of the labyrinth, and an infection of the labyrinth is known as labyrinthitis, which could be restricted to the cochlea or involving the vestibular apparatus also. Another reason for postnatal hearing loss could be because of a tumor growing in the vicinity of the inner ear. Ototoxicity, or the hearing loss because of prolonged use of some classes of antibiotics, is also a common cause for hearing loss. Some antibiotics are known to specifically target the cochlea, while others can specific target the vestibular apparatus. Examples of cochlear toxic antibiotics include kanamycin, neomycin, viomycin, while streptomycin and gentamycin are antibiotics that specifically affect the vestibular apparatus. These drugs should be avoided; however, in some cases, they might be the only drug that helps curtail the infection in the body somewhere else, but if they have to be prescribed, then the dosage needs to be monitored and the hearing system/status needs to be monitored periodically with follow-up audiograms. Another cause for ototoxicity could be chemotherapy; the drugs that they use for chemotherapy are also known to cause hearing loss affecting the inner ear. In ototoxicity, the hearing loss typically starts with the higher frequencies, and that is usually the case with many types of hearing loss that affects the inner ear. The higher frequencies are the ones that are very susceptible to damage first. The American Speech and Language Hearing Association has some guidelines for what needs to be done to monitor the hearing status of ototoxic drugs. That includes timely identification of these patients that are susceptible to this ototoxicity, pretreatment counseling, working with the physician and counseling the patient that these drugs are known to cause hearing loss, so it’s important that we periodically monitor your hearing status, and then of course checking them for any emerging hearing loss, and, if so, then making adjustments in the dosage or substituting the drug. Another reason for inner ear damage could be the progression of otosclerosis, which we talked about in the previous lecture. Here, if it was undiagnosed and if the spongy bone growth progresses into the inner ear, it’s going to result in a mixed hearing loss to begin with and eventually a sensorineural hearing loss, completely. Some cases of sensorineural hearing loss could be because of factors related to the surgery that was preformed; it could be because of an accident during the surgery, or it could be some consequence that happened following the surgery. One of the reason for inner ear damage could be a sudden shift, a sudden change, in the pressure , not only affecting the tympanic membrane, in this condition known as barotrauma, it actually can result in a rupture of the inner ear, and thereby all the fluid in the inner ear can be mixed or leaked, resulting in this significant hearing loss. One of the most common reason for sensorineural hearing loss is, of course, noise induced hearing loss. The hearing loss could be temporary if it was just one or a few instances of noise exposure, but if there’s a prolonged exposure, it’s going to result in a permanent threshold shift. There are some factor that are known to aggravate the degree of loss, for example, aspirin that is commonly given to patients with cardiac conditions is known to aggravate or increase the degree of loss. And historically, men have been known to have more noise induced hearing loss than women, and it could be because the traditional roles that the men take in the previous era, where they were primarily factor workers and more exposed to noise, and also the men because they like to go shooting and practice hunting. The specific case of noise induced hearing loss could be acoustical trauma, where the exposure is from one or a few episodes, but to impulsive sounds, like being in close proximity to an explosion; in those cases you would see a bilateral hearing loss. Typically the frequencies that are very prone to damage to being with is between 3000 and 6000 Hz, and often, you’ll see a notch is the hearing, particularly at 4000 Hz. Such pattern is commonly seen in noise induced hearing loss. In some cases, you might see that the damage is more in one ear than the other, and that is commonly seen in patiens or individuals involved in practice shooting with a rifle, so, for instance, if they are right handed, their left ear is more closer to the end of the barrel, so you’re looking to see more of a hearing loss in the left ear than the right ear, with a notch around 4 kHz. So the damage with noise induced hearing loss begins at the level of the stereocilia, so here are some illustrations of how the stereocilia are actually disarranged or bent because of noise exposure. Typically, this is what would happen, like in this figure over here, it would be been and that would result in this temporary hearing loss. But prolonged exposure and continuous exposure and unprotected exposure to noise might eventually result in the loss of stereocilia and the hair cells in the Organ of Corti, like this for example, where this section of the hair cells are completely removed or lost. Here is another illustration, where what you expect to see in a normal cochlea, the outer hair cells over here, the inner hair cells; again, the outer hair cells are the ones that are more susceptible to damage to begin with, so if there was acoustical trauma or prolonged exposure to noise, the hair cells would be completely lost over time, resulting in that sensorineural hearing loss. So here’s a typically audiogram that you see with early noise induced hearing loss. As I said, in most cases, you will see that notch around4 kHz; and you can see for both the air conduction thresholds and the bone conduction thresholds, hence it’s a sensorineural hearing loss. Known as a common complaint along with the hearing loss would be that high pitched tinnitus. Often that’s the early warning sign of the emergence of a sensorineural hearing loss. Occupational noise exposure is a major cause of noise induced hearing loss. Although now with personal stereo systems, it could be the case that those have an even higher risk for noise induced hearing loss. In the United States, the Occupational Safety and Health (OSHA) sets prescriptions of how much hours a worker can work in different noise levels. That’s what we call as a damage risk criteria. So if the noise level in the machine is about 85 dB A, the worker can work as long as 8 hours, but for every 5 dB increase, the time that they can be exposed to is half of that, so for 90 dB A it’s only 4 hours, so for every 5 dB increase, half the permitted time, but for 105 dB A, exposure should be limited to 30 minutes. So if you were an audiologist finding yourself in an occupational setting, then your role would be to implement this damage risk criteria, and monitor those individuals who work in these noisy environments with levels above 85 (dB A). Again, there is a number of variables that influence how much hearing loss a person will have, it could be related to genetics, some individuals are more susceptible to noise exposure and hearing loss, age, advanced age also makes you more prone to damage, and lifestyle after work, for example, you could be up all night performing as a rockstar, so these individuals, of course, are more prone to have noise induced hearing loss. One of the initial symptoms for any cochlear pathology is this tinnitus, typically reported as a high frequency squeaky ringing sound. Noise induced hearing loss typically has consequences in communication, but there’s a lot of studies that show it’s also related to increased anxiety levels and problems in concentrating. Some might even have disturbed sleep, if they were exposed to prolonged periods of excessive noise. A less common, but an interesting profile of sensorineural hearing loss, is those that have a sudden hearing loss. So there are some individuals who wake up in the morning and suddenly they feel that their ear is stuffed in one side. If they were the astute ones, they’d get it tested as early as they could, while they might be surprised that they have a severe or profound hearing loss in one ear. Often the reason for such hearing losses is not known; some suspect it could be a transient viral infection, and in some cases it could be kind of a miniature spasm kind of stroke in those blood arteries that supply the inner ear. About 50% of these patients, if they were immediately care for, immediately meaning within 48 to 72 hours, if they get some kind of medication that improves their blood supply along with antibiotics to deal with the infection, they might see a completely or partial reversal of the hearing loss. But often the longer the period of time that it’s identified and identify medical intervention, the less chance of the hearing loss reversing. Ménière’s disease is also a prevalent type of inner ear related hearing loss. Typically, Ménière’s disease, you would see a unilateral hearing loss and what is more striking than the hearing loss is these sudden attacks of vertigo. So these patients have paroxysmal, meaning that they’re unpredicted and sudden, attacks of dizziness and vertigo that might be total incapacitating for them and quite traumatized. As far as a hearing loss, they would report a fullness in the ear, and often it’s associated with a low frequency, kind of roaring tinnitus. Because of the nature of the hearing loss, they would have poor speech recognition skills, and, again, what’s more disturbing for these patients is their violent stints of vertigo that might be associated with vomiting too. Unfortunately, this type of hearing loss is progressive and is known to run through families. The main pathology with Ménière’s disease is an over-secretion or poor absorption of endolymph. Ménière’s disease is also known as endolymphatic hydrops. So the endolymph that circulates in the scala media, either because it’s not absorbed correctly or because it’s an over-secretion, makes this scala media distended, in other words, filled. And that’s what results in this symptoms of vertigo and hearing loss. It’s more often seen in males, and is very unusual to be seen in children. It’s also been known to be related to hormonal changes, especially in women. It’s a paroxysmal, sudden attacks of vertigo, that makes it a very handicapping condition. So here’s a schematic illustration of how the endolymph is within the vestibular organs and within the cochlea of the inner ear. What’s presumed to be happening in the Ménière’s disease is the dilation of this scala media and the lymphatic sacs over here. This results in making them hypersensitive to movement and triggering this violate attacks of vertigo. There’s no none treatment that completely eradicates Ménière’s disease. Some kind of diet control, limiting fluid retention and salt intake, is known to decrease the severity of the attacks. In some extreme cases, you might have to do some surgical procedures that would include decompressing or removing the excessive endolymph from the inner ear. In some cases, you might have to put a shunt to periodically alleviate some pressure in those endolymph failures. In extreme cases where there’s sudden attacks of vertigo often, especially in cases where the cause of these vertigo attacks are leading to suicidal tendencies, the surgeon might resort to completely removing the labyrinth or sectioning the auditory-vestibular nerve. Of course, that’s going to be related to a sudden, complete hearing loss in that ear. But that might be an option to alleviate the symptoms in these extreme cases. So in Ménière’s diseases, you expect to see a sensorineural hearing loss, and often, you might see this pattern where there’s some better hearing around 2 kHz. That’s an interesting finding that you’ll see in many of those cases with Ménière’s disease. As I said, there’s no known treatment, mostly diet control can help a few of those individuals and in extreme cases you might have to result to surgical intervention. Head trauma can also result in sensorineural hearing loss. The configuration of this hearing loss is typically to what you would see with noise induced hearing loss. But again, depending on the damage, if the damage has affected the middle ear structures or resulting in a tympanic membrane perforation or disarticulation of the ossicles, you might see a mixed hearing loss because of the conductive component. Of course, the most prevalent type of hearing loss is the hearing loss we receive with advanced aging. Especially with now, with the life expectation that being increased in the past few decades. We are seeing a larger proportion of individuals with this presbycusis. The hearing loss usually beings in the early 60s for males and a little later for females on average. About 25% of the age range of 45-64 years, the middle years, are expected to have some degree of hearing loss, and, of course, as age progresses, the percentage that have a hearing loss also increases. Again, the higher frequencies are the ones prone to have a hearing loss, especially in those earlier stages. But, progressively, the lower and middle frequencies can become affected. So here’s an average hearing loss that you would expect to see with age, at 65 years and 90 years, of course, you would expect to see a larger degree of hearing loss. As with the case with many sensorineural hearing losses, a common complaint with presbycusis is a speech sounding distorted, so just making it loud and increasing the volume on the TV does not help them, and that’s because of the distortion that is happening at the level of the inner ear and the auditory nerve. Often, hearing aids would be the first step to take if presbycusis is diagnosed. In cases where there has been progressive hearing loss and if someone has reached the point that they’re not getting much benefits from hearing aids, then a cohclear implant might be an option. In the next lecture when we talk about hearing aids and cochlear implants, I’ll kind of explain what the cutoff is. And when do we determine that hearing aids is not enough and start to use cochlear implants. Here’s just an audiogram for a patient with ototoxicity, which affects the higher frequencies to begin with. Some believe that testing at the extended higher frequencies, in other words, frequencies above 8 kHz, and some audiometers you can do that, you can test all the way up to 20 kHz, so if you do extended high frequency audiometry, then they believe that you can detect or remediate ototoxicity at earlier then just using traditional audiometry.

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