Hearing HealthRobert Thayer Sataloff, MD, DMA
The human ear is amazing. Although it is one of the smallest and most complex organs in the body, it is capable of using the tiniest disturbances in air molecules, inducing them into a form the brain can understand, and doing so instantaneously over an enormous range of pitch and loudness. Considering the ear's delicacy, it is remarkably resilient. Nevertheless, problems can occur which impair our ability to hear properly. In recent years, substantial advances have made it possible to determine the cause of hearing impairment in nearly all cases, and to treat the hearing loss in many ears. It is helpful for patients to understand a bit about how the ear works, and to be familiar with many of the reasons why it may cease to work properly. Such knowledge allows us to recognize and understand ear problems when they occur, and to judge whether we are really receiving the latest and best in medical care.
The ear is divided into three major anatomical divisions: (a) the outer ear, (b) the middle ear, and © the inner ear (Figure 1). The outer ear has two parts: (a) the “trumpet-shaped” apparatus on the side of the head called the auricle or pinna, and (b) the tube leading from the auricle into the temporal bone called the external auditory canal. This opening is called the meatus. The tympanic membrane, or “eardrum,” stretches across the inner end of the external ear to the inner ear. A bony bridge is held in place by muscles and ligaments. The middle-ear chamber is filled with air and opens into the throat through the eustachian tube. The eustachian tube helps to equalize pressure on both sides of the eardrum. The inner ear is a fluid-filled chamber divided into two parts: (a) the vestibular labyrinth, which functions as part of the body's balance mechanism, and (b) the cochlea, which contains the hearing-sensing nerve. Within the cochlea is the organ of Corti, which contains thousands of minute, sensory, hair-like cells (Figure 2). The organ of Corti functions as the switchboard of the auditory system. The eighth cranial or acoustic nerve leads from the inner ear to the brain, serving as the pathway for the impulses the brain will interpret as sound. Sound creates vibrations in the air somewhat similar to the waves created when a stone is thrown into a pond. The outer-ear “trumpet” collects these sound waves, and they are funneled down the external ear canal to the eardrum. As the sound waves strike the eardrum, they cause it to vibrate. The vibrations are transmitted by mechanical action through the middle ear over the bony bridge formed by the malleus, incus, and stapes. These vibrations, in turn, cause the membranes over the openings to the inner ear to vibrate, causing the fluid in the inner ear to be set in motion. The motion of the fluid in the inner ear excites the nerve cells in the organ of Corti, producing electrochemical impulses that are gathered together and transmitted to the brain along the acoustic nerve. As the impulses reach the brain, we experience the sensation of hearing. The sensitivity of the hearing mechanism is most extraordinary. Near threshold (the softest detectable sound), the eardrum only moves approximately 1/1,000,000th of an inch. Our intensity range spans extremes from the softest sounds, to sounds of jet engine intensity, covering an intensity range of approximately 100,000,000 to1. Over this range we are able to detect tiny changes in intensity, and in frequency. Many young, healthy humans (through teens and early twenties) can hear frequencies from about 20 Hz to 20,000 Hz, and can detect frequency differences as small as 0.2%. That is, we can tell the difference between a sound of 1000 Hz, and one of 1002 Hz. Consequently, it is no surprise that such a remarkably complex system can be damaged by various illnesses and injuries.
Patients diagnosed as having conductive hearing loss receive a much better prognosis than those with sensorineural loss because modern techniques make it possible to cure or at least improve the vast majority of cases in which the damage occurs with the outer or middle ear. Even if they are not improved medically or surgically, these patients stand to benefit greatly from a hearing aid, because what they need most is amplification. They are not bothered by distortion and other hearing abnormalities that may occur in sensorineural loss.
The word “sensorineural” was introduced to replace the ambiguous terms “perceptive deafness” and “nerve deafness.” It is a more descriptive and more accurate anatomical term. Its dual character suggest that two separate areas may be affected, and , actually, this is the case. The term “sensory” hearing loss is applied when the damage is localized in the inner ear. Common synonyms are “cochlear” or “inner-ear” hearing loss. “Neural” hearing loss is the correct term to use when the damage is in the auditory nerve proper, anywhere between its fibers at the base of the hair cells and the auditory nuclei (relay stations in the brain). This range includes the bipolar ganglion of the eighth cranial nerve. Other common names for this type of loss are “nerve deafness” and “retrocochlear hearing loss.” These names are useful if applied appropriately and meaningfully, but too often they are used improperly. Although at present, it is common practice to group together both sensory and neural components, it has become possible in many cases to attribute a predominant part of the damage, if not all of it, to either the inner ear or the nerve. Because of some success in this area and the likelihood that ongoing research will allow us to differentiate between even more cases of sensory and neural hearing loss, we shall divide the terms and describe the distinctive features of each type. This separation is advisable because the prognosis and the treatment of the two kinds of hearing impairment differ. For example, in all cases of unilateral sensorineural hearing loss, it is important to distinguish between a sensory and neural hearing impairment, because the neural type may be due to an acoustic neuroma (benign tumor in the ear-brain interface) which could become serious. Those cases which we cannot identify as either sensory or neural and those cases in which there is damage in both regions we shall classify as sensorineural. There are various and complex causes of sensorineural hearing loss, but certain features are characteristic and basic to all of them. Because the histories obtained from patients are so diverse, they contribute more insight into the etiology (cause) than into the classification of a hearing loss. Sensorineural hearing loss is one of the most challenging problems in medicine. A large variety of hearing impairments fall under this category. The prognosis for restoring a sensorineural hearing loss with presently available therapy is poor. However, in a minority of cases specific treatable causes are found, and dramatic improvements are occasionally achieved. Although some spontaneous remissions and hearing improvements with therapy have occurred, particularly in cases involving sensory loss, a great need for further research still exists.
Although information about central hearing loss is accumulating, it remains somewhat a mystery in otology (the medical specialty of ear medicine and surgery; a sub-specialty of otolaryngology [ear, nose and throat]). Physicians know that some patients cannot interpret or understand what is being said and that the cause of the difficulty is not in the peripheral mechanism but somewhere in the central nervous system. In central hearing loss the problem is not a lowered pure-tone threshold but in the patient's ability to interpret what he or she hears. Obviously, it is a more complex task to interpret speech than to respond to a pure-tone threshold; consequently, the test necessary to diagnose central hearing impairment must be designed to assess a patient's ability to handle complex information. Most of the tests now available were not created specifically for this purpose, and, so, it still requires a very experienced and almost intuitive judgment on the physician's part to make an accurate diagnosis. One common central condition frequently leads people to think they have hearing loss when their hearing is actually normal. It is called a central auditory processing disorder. Despite the fact that this problem is extremely common and present in many highly successful people (including this author), it is actually classified as a learning disability. Basically, the problem involves a person's inability to filter out competing auditory signals. People with central auditory processing disorders have difficulty “hearing” when there are several conversations going on, can't study with the radio or television on, have difficulty reading if someone turns on a vacuum cleaner or air conditioner near them, and generally miss the first sentence from people talking to them if they are involved in an auditory attention task (such as watching television). Although such people (and their families and friends) frequently suspect that they have a hearing loss, the function of the ears is usually normal, and routine hearing tests are normal. Naturally, people with this condition may also develop hearing loss from other causes which sometimes makes it even more difficult for them to function under everyday circumstances. There is no good treatment for central auditory processing disorders other than educating the patient, family and friends, and trying to control the environment. This is especially important for children whose grades may go from F to A if they are provided with a silent place in which to do their homework!
The hearing difficulty may have an entirely emotional or psychological etiology, or it may be superimposed on some mild organic hearing loss, in which case it is called a functional or a psychogenic overlay. Often, the patient really has normal hearing underlying the functional hearing loss. A carefully recorded history usually will reveal some hearing impairment in the patient's family or some reference to deafness which served as the nucleus for the patient's functional hearing loss. The most important challenge in such a case is to classify the condition properly. It may be quite difficult to determine the specific emotional cause, but if the classification is made accurately, the proper therapy can be instituted. Too often, the emotional origin of a functional hearing loss is not recognized, and patients receive useless otologic treatments for prolonged periods. In turn, this process may aggravate the emotional element and cause the condition to become more resistant to treatment. Therefore, early and accurate classification is imperative.
Each type of hearing loss has specific distinctive characteristics which make it possible to classify in the vast majority of cases. These features are identified through a comprehensive history, physical examination, hearing tests, and sometimes other tests.
There are many other symptoms that may be related to hearing loss. Hearing loss is not always slowly progressive and stable from day to day. It may be sudden, rapidly progressive, or even fluctuating (good times and bad times). Sometimes difficulty understanding speech is more prominent than inability to hear speech. Other related symptoms commonly accompany hearing problems. These may include ear fullness or pressure, ear noises (ringing, buzzing, crickets, seashells, steam, and others) and dizziness.
The ear doctor will also perform a physical examination. Ordinarily this includes a complete ear, nose and throat examination with special attention to the ears. The otologist usually includes an informal hearing assessment with a tuning fork, and sometimes other assessments of balance, sensation, and other functions. An audiogram (hearing test) will virtually always be obtained. Depending upon the results of the history, physical examination and hearing test, the doctor may order numerous other tests. Sometimes these seem overwhelming at first; but there is ordinarily a good reason for each of them.
Site of lesion testing involves the regular audiometer, using a variety of other test protocols to help determine where a problem lies. This kind of testing may involve comparison of hearing in one ear with the other, detection of small changes in signal intensity, testing ability to hear in the presence of noise, testing the ability to hear sentences placed in both ears at the same time, and many other test scenarios. Brainstem evoked response audiometry (BERA or ABR) involves sophisticated, computerized equipment. Sounds are placed in the ear, and the brainstem's response is recorded from electrodes pasted to the patient (like electrocardiogram electrodes). This testing is extremely helpful in distinguishing sensory (inner ear) from neural (nerve) causes of hearing loss, and for helping to localize problems in the brainstem auditory pathway. In selected cases, it is also helpful in determining hearing threshold (ability to hear soft sounds). Tinnitus (ear noise) can be tested in several ways. Sometimes it is possible to measure the frequency and intensity of the tinnitus. There are also tests that help determine whether it can be suppressed or masked. Since the inner ear is divided into hearing and balance sections which are related, balance system testing is often appropriate when sensorineural problems are present. Such testing may be useful even in patients who do not have obvious balance problems. The most common balance tests are electronystagmography (ENG) and computerized dynamic posturography (CDP). ENG involves eye movements that can be measured, and stimulating the vestibular pathways through visual and caloric stimuli. The caloric stimuli are usually cool and warm water placed in the ears. Additional related information can also be obtained from rotational testing, using a special rotating chair. This kind of testing is not widely available , but may be valuable for some patients. CDP tests overall balance function using a computerized testing platform. It provides invaluable information that is especially useful in combination with an ENG.
From my perspective as a tertiary care medical school professor, I think the personal cost of allowing one individual to go undiagnosed from a serious treatable condition is greater than the money saved by not ordering tests that might prove normal. In general, the larger the tumor at the time of diagnosis, the less the chance of saving hearing, and the greater the chance of the patient suffering facial paralysis, stroke, or worse at the time of surgery. Personally, I think the most cost effective way to practice medicine is to be certain that no one leaves my office without a diagnosis, regardless of what tests may be medically necessary to establish that diagnosis. This approach used to be fairly universal, until the current emphasis on controlling medical costs became such a dominant force. Nevertheless, it is important for consumers of health care to understand the price they pay for cost effectiveness, and legitimate differences in philosophy among health care providers. Such understanding allows patients to position themselves so that they can get the kind of care they want and access to physicians whose approach is consistent with their needs.
Many other problems can cause ear canal occlusion and hearing loss. They include infections with swelling that shuts the ear canal, foreign bodies in the ear, trauma, birth defects, tumors (including cancer) and other causes. Resulting hearing loss is conductive. That is, it interferes with sound conduction and is generally correctable. It causes decrease in volume but does not generally produce distortion.
Ear fluid not caused by infection accumulates commonly in children, and sometimes in adults. The condition is known as serous otitis media. This is the condition that leads to myringotomy and tube placement, one of the most common operations performed in the United States. It is useful to understand the mechanism behind serous otitis media. Usually it is due to malfunction of the eustachian tube which connects the ear with the back of the throat. The eustachian tubes' job is to keep the pressure in the middle ear approximately the same as that in the ear canal and outside world. The eustachian tube does so by opening momentarily when we swallow or yawn. If it fails to open because of swelling, allergy, structural developmental reasons, the air already in the middle ear becomes trapped. Gradually, this air is resorbed into the blood stream. The middle ear space is bony except for the eardrum. As resorption occurs and less and less air is present, its disappearance creates a vacuum (negative pressure) which sucks the ear drum into the middle ear (retracted ear drum). When the negative pressure is great enough, it causes fluid to seep out of the blood vessels in the mucosa lining the middle ear. This is the serous fluid that constitutes serous otitis media. The condition causes hearing loss, and sometimes frequently recurring ear infections. In some cases, the eustachian tube starts to work again spontaneously and the condition resolves. In others, decongestants may be helpful. When allergy is the cause, allergy treatments may improve the problem. When serous otitis media causes frequent infections (recurrent acute otitis media) some physicians recommend the long-term use of antibiotics although this treatment is currently controversial. The most widely accepted treatment for persistent middle ear fluid especially in children is myringotomy and tube placement. This ordinarily eliminates the hearing loss almost instantly, decreases the incidence of recurring infections and prevents the reformation of fluid. The tiny tubes placed in the eardrum to keep the ear drainage hole open are not drainage tubes. They are ventilating tubes. They act in lieu of a functioning eustachian tube to maintain normal pressures within the middle ear. Hence, no vacuum forms, and the fluid is not created in the first place. This is why children with tubes don't have constant problems with fluid draining out of their ears. Treatment of this condition is important for many reasons. In addition to eliminating disturbing hearing loss and frequent infections, there is evidence to suggest that such hearing loss in children interferes with learning, perhaps more than we might expect it to from the mild to moderate severity of the hearing loss in most cases. Middle ear effusion in adults deserves special mention, too. It is not an uncommon consequence following barotraumas, such as a poorly pressurized airplane descent. However, occasionally we find unexplained middle ear effusion in one ear of an adult. It is essential to determine the cause of this problem, and especially to rule out the presence of a mass (such as a cancer) in the nasopharynx occluding the opening of the eustachian tube. Otosclerosis is a common hereditary disease that deposits bone around the footplate of the stapes. This prevents normal bone transmission of sound from the eardrum to the inner ear, and consequently conductive hearing loss. This hereditary condition is present in about 10 percent of white Americans, and it causes significant hearing loss in about 1 percent. It occurs less commonly in black and Asian people. Otosclerosis generally becomes apparent during early adult life, and the severity of hearing loss is accelerated by pregnancy. The hearing loss can be cured by an operation called stapedectomy, in which the stapes bone is replaced by an artificial bone. In good hands of the surgeon, this procedure usually only takes fifteen or twenty minutes, and it is performed under local anesthesia. When all goes well, hearing may return to normal. Various birth defects may also involve the middle ear and cause hearing loss. Some are major and cause obvious external defects. Many involve only the inner ear structures and CT scan or even surgery may be necessary to establish the diagnosis. In any case, most conductive forms of hearing loss are surgically treatable. It is also important to recognize that the tumors of the middle ear may be responsible for hearing loss. They may be malignant, such as rhabdomyosarcoma in children or squamous cell carcinoma in adults. It may also be benign, such as glomus tumors (which also cause pulsating ear noises) or neuromas of the facial or acoustic (hearing and balance) nerves. Cholesteatoma is a common benign skin cyst that grows slowly and dissolves adjacent bone as it grows. It is a fairly common problem, especially in people who have had repeated ear infections; but it may also occur from birth in the absence of infections. It generally requires mastoid surgery, as discussed below.
The natural aging process also causes sensorineural hearing loss. Beginning shortly after birth, we begin to lose hair cells and nerve endings in the basal turn of the cochlea (the region that hears very high frequencies). As this loss pattern progresses over a lifetime, sensorineural hearing loss develops. There are also other age-related causes of hearing loss including stiffening of the cochlear partition in the inner ear and loss of nerve endings in the acoustic nerve. A fistula (opening) is an abnormal connection between the inner ear and middle ear. The inner ear is filled with fluid, and the middle ear is filled with air. If a fluid leak occurs allowing the inner ear fluids out, hearing loss and dizziness commonly result. This kind of sensorineural hearing loss can often be cured by surgically repairing the fistula. Such leaks are usually caused by trauma. The trauma may be direct, such as a blow to the ear or a head injury in a car accident. However, it may also be the result of barotrauma caused by an airplane trip, a forceful sneeze, or lifting a heavy object. Direct head trauma, particularly trauma severe enough to cause unconsciousness, can cause inner ear concussions and sensorineural hearing loss. Ménière's syndrome is a condition characterized by fluctuating sensorineural hearing loss (usually more prominent in the lower frequencies where we hear speech), episodic vertigo, fluctuating ear pressure, and tinnitus (usually of a seashell variety). It is due to endolymphatic hydrops, which is a swelling and fluid overload of the middle compartment of the inner ear. There are many treatable causes of Ménière's syndrome. So, exhaustive evaluation in search of them is recommended. When all tests have revealed none of the known causes, the condition is classified as Ménière's Disease. Noise is an important cause of hearing loss. It is estimated that there are 7 to 10 million people in American industry with noise-induced hearing loss, virtually all of which was preventable. In addition to industrial noise, recreational noise can damage hearing. Such noise is encountered commonly from gunfire, power tools, snow blowers, motorcycles, loud music (especially with insert earphones) and other causes. In some cases, the playing of musical instruments can damage hearing. This has been reported not only with loud, electrical rock and roll instruments, but also with classical music performance such as violin playing and flute playing. One can minimize such problems by using ear protection whenever practical, such as during selected practice sessions. Infections involving the inner ear and hearing nerve can also produce deafness. Middle ear infections can spread to the inner ear causing loss of hearing, and usually dizziness. Viral infections may also involve the hair cells or auditory nerve causing hearing loss and even sudden total deafness. Sudden deafness is a special problem. It may be caused by a variety of problems and demands immediate, aggressive and comprehensive evaluation. Treatment is controversial, but there is at least some evidence that suggests that aggressive treatment may improve the chances for hearing recovery even after a sudden profound loss. In any case, even those of us who believe in treating sudden deafness aggressively agree that once the condition has been present for more than 2 to 3 weeks, even the most aggressive treatments do not work. So, prompt attention is essential. Neural problems may also produce hearing loss. Among the more common are acoustic neuroma, multiple sclerosis, autoimmune sensorineural hearing loss (in which the body attacks its own ear) and ototoxicity. Ototoxicity is hearing loss caused by a substance that gets into the body. Most often the substance is a medication, particularly certain antibiotics and diuretics. However, other toxins (such as lead) may also cause hearing loss.
Lyme disease can cause similar problems. This increasingly common infection is caused by a spirochete (as is syphilis) carried most commonly by ticks. Lyme disease often causes a rash and joint pain; but these may be minor enough to escape notice. Appropriate blood tests should be obtained for people with sensorineural hearing loss of unknown origin, and for patients with other unexplained nerve problems, such as facial paralysis. Numerous other systemic infections including herpes, cytomegalo virus (CMV), measles, mononucleosis, varicella, mycoplasma, influenza, and fungal diseases may cause hearing problems as well.
When hearing loss occurs early in childhood, its devastating consequences are more obvious than when it occurs insidiously late in life. A substantial hearing deficit in infancy interferes with the normal process of psychological and emotional development. Severe hearing loss makes learning a mammoth task for the child, and frustration or isolation frequently results. The personality distortion that results from this sequence of events affects the person and his family throughout their lives. Even more mild forms of hearing loss early in life can cause great difficulties, including poor attention and bad grades in school. Frequently, such children are considered “not too bright,” before anyone realizes that a hearing loss is present. When it is corrected, the changes in the child's performance, attitude and interactions are often remarkable.
Myringotomy and tube placement involves making a small incision in the eardrum, suctioning fluid out of the middle ear, and placing a flanged ventilating tube within the eardrum. In children, the procedure requires general anesthesia. In adults, it can be done as an office procedure and ordinarily takes only a few seconds. After tubes have been placed, they generally fall out by themselves in 3 to 12 months. Most physicians agree that is important to keep the ears dry (avoid water) while the tubes are in. Stapes surgery is performed under local anesthesia. Like all ear surgery, it requires the use of an operating microscope. The eardrum is moved aside, and the immobile stapes is either mobilized or replaced with an artificial bone (usually made out of teflon and stainless steel). As with all ear surgery, there are potential risks including worse hearing loss, tinnitus, dizziness, facial paralysis (extremely rare), bleeding, infection, and others. However, the overall success rate of stapes surgery is in the range of 97 or 98 percent; and many people prefer surgical restoration of hearing to wearing hearing aids. Mastoid surgery usually involves general anesthesia (although it can be performed under local anesthesia when necessary). An incision is usually made behind the ear. The mastoid bone is located immediately behind the external ear. It is removed using high speed drills, and any disease is eliminated from the mastoid. Mastoidectomy also allows access to the middle ear. In old fashioned mastoid surgery, the ear canal is removed and a cavity created. This is still occasionally necessary, but not often. Cavities often necessitate cleaning (1 to 4 times per year by a physician), and many people with mastoid cavities cannot get water in them without causing infection. It is possible to do mastoid surgery leaving the ear canal intact. Following such procedures, hearing can often be restored to normal; and people can shower, swim, and perform other activities without restriction. Translabyrinthine surgery (through the inner ear) for removal of acoustic neuromas provides excellent access to the tumor with the best chance of preserving facial nerve function and totally removing the tumor in many cases. This approach involves a mastoidectomy extended through the inner ear labyrinth to enter the brain cavity. However, it nearly always results in total loss of hearing. In most cases, the tumor has caused a significant hearing loss; and total tumor removal is not possible without removing the roots of the tumor imbedded in the hearing nerve thereby necessitating loss of hearing in order to cure the tumor. This approach has many advantages, including minimizing trauma to the brain. However, any acoustic neuroma surgical team utilizes translabyrinthine surgery in combination with other approaches, depending upon the anatomy of the tumor and the needs of any individual patient.
Having reviewed many (but certainly not all) of the health problems that may adversely affect the ear, it is clear that maintenance of good general health is important to the ear as well as to the rest of the body. Maintaining appropriate blood pressure, healthy eating and exercise habits, and close surveillance on bodily health (such as an annual physical examination) are extremely helpful. If there is reason to suspect hearing loss such as with a strong family history, hearing tests should be included in the physical examination so that hearing problems can be recognized and addressed early. Like so many other things, we never appreciate the value of hearing until it is lost. Through sensible preventive measures, many potential causes of hearing loss can be eliminated. When hearing loss occurs, its progression can sometimes be prevented or slowed. In every case, early diagnosis and optimal management minimize the psychological and social trauma so common in people with hearing impairment. Maintaining contact with a physician specializing in hearing is wise for any patient with hearing impairments. A part from all the things we have learned in the last decade or two, hearing experts are constantly working to learn more about the conditions we don't understand, yet. Even for patients we can't cure today, there is always hope for tomorrow.
|