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VIII. THE INNER EAR: DISORDERS OF THE INNER EAR

Objectives:

At the end of this section you should be able to:

1. State what is meant by sensorineural hearing loss, and distinguish it from conductive hearing loss.

2. Name the two major categories of inner ear disorders, and the major disorders associated with each of them.

3. Describe the sites of lesions of each of the disorders.

Objective 1: Impairment in the cochlear transduction mechanisms, in auditory nerve transmission, or in both, results in a sensorineural hearing loss

Because both the auditory and vestibular structures of the inner ear are derived similarly embryologically and because they share the same fluid environment, a disorder of one frequently includes a disorder of the other, resulting in a complex of symptoms.  

The inner ear is vulnerable to damage or destruction from a variety of sources. Malformations of the labyrinth may be inherited or acquired. Inflammatory and metabolic processes may disrupt permanently normal vestibular and auditory function at the level of the end organ. Drugs and other substances have teratogenic effects on the inner ears of the fetus and destructive effects on the cochlea and vestibular organs in young and adult individuals. Trauma, either physical or acoustic, can cause hearing loss and vestibular damage. Viral infections may destroy the receptor organs, especially in utero.

Objective 2: Disorders of hearing fall into two broad categories: congenital and acquired

Congenital disorders

Recall that the auditory placodes in humans are formed at three weeks and the membranous labyrinth differentiates between the sixth and seventh weeks. Before and during this period opportunity exists for the maldevelopment of the inner ear.

Dysplasias - there are several morphologic congenital abnormalities of the inner ear. These include maldevelopment of the bony labyrinth, the membranous labyrinth, or both. Within the membranous labyrinth there may be maldevelopment of any or all of the receptor organs and their supporting cells. Severe sensory neural hearing loss results.

Hereditary syndromes - include labyrinthine disorders that are associated with no other abnormalities, and those that are associated with external ear malformations, integument disease, ophthalmic lesions, CNS lesions, skeletal malformations, renal disease, and miscellaneous defects. One example is Usher's Syndrome, which is a diplasia that accounts for about 10% of all hereditary deafness; there is no vestibular involvement and it is associated with retinosis pigmentosa. Another cochlear deformation is associated with Waardenburg's Syndrome. This accounts for 2-3% of all cases of congenital deafness in the U.S. It is associated with a white forelock and widened intercanthal distance.
 

Commonly acquired viremic diseases

Maternal rubella can cause profound congenital sensorineural hearing loss. It is exceptionally virulent to fetal organs, especially the cochlea, when it occurs in the first trimester of pregnancy. The severity of the hearing loss varies considerably from patient to patient and to a lesser degree from one ear to the other in the same individual. The pathogenesis of this disorder is not clear.  

Cytomegalovirus (CMV) - is a common congenital viral infection which is associated with a number of neurological disorders including hearing loss. In temporal bones of infants who died from CMV there are reported changes in the stria vascularis with cochlear duct and saccular hydrops.

Disorders associated with chromosomal abnormalities - Trisomy syndromes

Trisomy refers to the condition in which the nucleus of the cell contains an extra chromosome. Thus, there are under these circumstances 47 chromosomes rather than the normal 22 pairs of autosomes and 2 pair of sex chromosomes.  Depending on the chromosomes, there may be multiple physical anomalies including malformed ossicles and underdeveloped otic capsule and organ of Corti.

Neonatal hyperbilirubinemia - bilirubin encephalopathy

  Bilirubin, a yellow pigment, is the major end product of hemoglobin metabolism. It has long been known that, in human neonates, there is a close association between elevated blood bilirubin levels and disorders of the central nervous system. The most extreme neurological consequence of hyperbilirubinemia is referred to as "kernicterus" - a condition that may include hearing impairment, choreoathetosis, spasticity, oculomotor problems, cognitive dysfunction, and mild forms of mental retardation.  Classical kernicterus in term infants, resulting from Rh incompatibility, has been in many places nearly eliminated by prophylaxis and the use of early exchange transfusion. With the decrease in the incidence of classical kernicterus induced by Rh incompatibility, attention has shifted to the occurrence of this disorder in premature and gravely ill infants. The hearing loss that accompanies hyperbilirubinemia is of the sensorineural type. In studies of temporal bones of humans and animals with this condition there has been no clear-cut evidence of damage to the inner ear structures. Rather, the damage appears to occur in the auditory nuclei of the brainstem; neurons in the cochlear nuclei, in particular are severely damaged or destroyed.  
 

Acquired disorders

Lesions acquired at birth

Trauma - Hearing loss may result from trauma that occurs prenatally or during delivery.

Hypoxia and anoxia - Fetal hypoxia around the time of delivery is believed to play a role in some cases of otherwise unexplained hearing loss. Hypoxia or anoxia has been shown in experimental animals to produce lesions of the auditory pathways of the central nervous system and may also produce cochlear damage. Premature infants are at risk for sensorineural hearing loss. Histological signs associated with neonatal hypoxia or anoxia include atrophy of the organ of Corti, labyrinthine hemorrhage and degeneration of neurons in the central auditory pathways of the brainstem. 

Traumatic Lesions

Noise induced hearing loss - Excessive noise can cause permanent damage to the cochlea. It may occur as the result of a sudden blast (e.g. gun shot) or it may come because of lengthy exposure to high intensity sound (e.g. factory noise). Even relatively brief exposure to a high-noise environment is potentially hazardous to the health of the organ of Corti as evidenced by the studies done at a 4-hour Bruce Springsteen concert in St. Louis and during the 1987 Twins-Cardinals World Series games in domed stadiums (see following journal abstracts).

Temporary threshold shifts from attendance at a rock concert. W. W. Clark and B. A. Bohne, Central Institute for the Deaf and Dept. of Otolaryngology, Washington University School of Medicine, St. Louis, MO 63110). From J. Acoust. Soc. Am., 79:548, 1986.

The relation between exposure level and hearing loss in rock concert attendees was studied. Six volunteer subjects, ages 16-44, participated. All except the 44 year old had normal hearing sensitivity as revealed by audiometric evaluations made immediately before the concert. They attended a Bruce Springsteen concert at the St. Louis Arena and returned to CID for another hearing test within 30 min. following the concert. Noise exposure was assessed by having two subjects seated at different locations in the arena, wear calibrated dosimeters during the event. Sixteen hours after the concert all subjects returned for a final audiometric evaluation. Results indicated the average exposure level was 100-100.6 dBA during the 4 1/2 hr concert. Five of the six attendees had significant threshold shifts (<50 dB) predominately in the 4-Khz region. Measures made 16h after the concert and thereafter indicated that hearing returned to normal in all subjects. Although no PTS was observed, comparison of these data with studies of hearing loss and cochlear damage in animal models suggests that these subjects may have sustained some sensory cell loss from this exposure. (Work supported by NIOSH and NINCDS.)

NOISE EXPOSURE DURING THE 1987 WORLD SERIES: CARDS SOUNDLY BEATEN BY TWINS. W.W. Clark, Central Institute for the Deaf, St. Louis, MO 63110. From paper presented at the 1987 meeting of the Association for Research in Otolaryngology.

The 1987 World Series, matching the St. Louis Cardinals and the Minnesota Twins and played at the Hubert H. Humphrey Metrodome in Minneapolis and Busch Memorial Stadium in St. Louis was the first World Series in history in which the home team won every game. Among the factors most commonly cited by players, fans and the press as contributing to the difference in performance in the two stadiums was the extraordinarily high levels of noise caused by cheering fans inside the enclosed Metrodome. To assess the difference in noise exposure to fans and indirectly to players, measures of noise exposure were made during game 4 of the series held in St. Louis and game 6 of the series, held in the Metrodome. Two subjects wore logging dosimeters (Quest M-27, 90 dR criterion, 80 dB threshold, 5 dB trading ratio, A weighting network) while they attended the game. Seats for both games were in approximately the same location: behind 3rd base in the upper deck.

During the 3 hr 11 min. game in St. Louis, spectators were exposed to an average noise level of 90.6 dBA (49.3% of allowable OSHA dose). The maximum level was 117 dBA, and levels above 95 dBA occurred for 13% of the total time. In contrast, spectators at game 6 in the Metrodome were exposed to an average noise level of 94.4 dBA (90.4% of allowable OSHA dose) during the 3 hr 22 minutes game. The maximum level was 114 dBA and levels above 95 dBA occurred for 28.2% of the time. Inspection of the records for 1 minute averages from each game suggested a different pattern of exposure. In St. Louis, the noise level was fairly constant, peaking only in the middle innings when the Cardinals scored. In game 6 in the Metrodome, however, levels declined to a relatively quiet 77.90 dBA when the Cardinals held the lead, but increased to 95-109 dBA in the later innings when the Twins assumed the lead.

These results show clearly that noise levels recorded in the Metrodome significantly exceeded those measured in St. Louis under similar conditions. They suggest that the excessively high levels of noise in the Metrodome may have differentially affected player communication, concentration, and performance of the St. Louis Cardinals, who were not acclimated to playing in such a noisy environment. Finally, because the levels are sufficient to produce significant temporary threshold shift in most individuals, spectators and players at such events should be strongly advised to wear ear protection.

"Warning: Walkmen May Be Dangerous to Your Hearing Health"
 

There is mounting evidence that miniature Walkman type radios and tape players are a threat to hearing. Powered by as little as a single AA battery, these players can pump out as much as 115 dB at full volume, much of it reaching the ear through foam rubber plugs or earphones that cup the ear. The New York City Health Department took a sound level  meter into Manhattan's streets and subways and found among 35-40 headset wearers' levels ranging from 95 to 112 dB. If you walk past someone  wearing a headset and you can hear any of what they are listening to, that sound could be at a dangerous level. 

The loss is of cochlear origin and is most pronounced in the vicinity of 4 kHz. This frequency corresponds to the frequency region of enhanced sensitivity due to the resonance properties of the external ear.  There is a considerable amount of information now available on the pathophysiology of noise-induced hearing loss. Because of this and because this kind of hearing loss is very widespread in today's noise-filled environment, a separate section on the mechanisms of this disorder is presented.  The consequence of exposure to intense sound is a temporary or a permanent hearing loss. Whether one or the other condition prevails depends on a number of variables including the intensity, frequency, and duration of the sound exposure. It is believed that the structural damage to the inner ear that accompanies a permanent hearing loss arises from the interplay of mechanical and metabolic processes.  

Temporary Threshold Shift

Hearing loss after exposure to long and intense sound often is transient in nature and over time normal hearing returns. This is referred to as a temporary threshold shift (TTS). Many of us have experienced this after, for example, listening to a rock concert. In animals exposed to sounds that create a TTS (as tested in behavioral experiments), there is no evidence for structural changes in the cochlea despite the fact that the hearing loss may be as great as 50 dB. The mechanisms of TTS are, therefore, unclear. It may be that the structural damage is subtle and has so far escaped detection. Perhaps there is no structural alteration at all and it is all in an altered biochemistry which leads to malfunctioning of the transducer mechanisms or synaptic transmission. It is important to know, however, that different individuals exhibit different susceptibilities to intense sounds and that there is probably a fine line between a temporary threshold shift with little residual damage to the cochlea and a permanent hearing loss. 

Permanent Threshold Shift

A permanent threshold shift is accompanied by irreversible cochlear damage. An obvious question to ask is whether there is an orderly relation between exposure conditions, level of permanent hearing loss, and the degree of cochlear injury. While obtaining an answer to this question seems straightforward, it is not, and so far no satisfactory one has been put forward.  

Objective 3: There are several mechanisms that underlie peripheral hearing disorders

Our understanding of the cellular/molecular mechanisms that cause anatomical changes in the organ of Corti after intense sound exposure is incomplete. Two processes, mechanical and metabolic, have been suggested as the principal mechanisms responsible for hair cell damage associated with acoustic overstimulation.  In general, mechanically induced injuries have a rapid onset whereas those produced by "metabolic exhaustion" have a more gradual onset. Mechanical factors related to excessive movements of the cochlear partition would include disruption of the internal structure of the cell. They might result in tears in the basilar membrane or Reissner's membrane, which would result in the cytotoxic mixture of endolymph and perilymph. Excess motion might also separate the organ of Corti from the basilar membrane or from the tectorial membrane. The synaptic junction between the hair cell and eighth nerve fiber could be widened or the OHC stereocilia may lose their connections with the tectorial membrane. 

The long-term changes in hearing consequent to acoustic overstimulation are believed to involve the hair cell metabolic machinery. Structural changes in the endoplasmic reticular system and mitochondria suggest deficits in fuel utilization, protein synthesis, and energy production. There may be disruption in cellular enzyme systems critical to energy metabolism, protein synthesis and ion transport. It is also suggested that acoustic injury may involve regional ischemia, although there is little direct support for this hypothesis. A third possibility has been suggested that relates both to mechanical and metabolic hypotheses, namely that damage results from changes in the cochlear vascular system.

Inner vs. Outer Hair Cells

Outer hair cells (OHCs) are more susceptible than inner hair cells (IHCs) to acoustic over-stimulation. One reason may be that OHCs, because of their greater distance from the fulcrum of the basilar membrane, undergo greater velocity of motion and hence are at greater risk of mechanical damage. Second, the direct mechanical linkage of OHC stereocilia with the tectorial membrane may enhance this cell's susceptibility. Thirdly, the difference may be metabolic, reflecting the differences in internal organelle structure of the IHCs and OHCs. It is noted that OHCs are also more susceptible to ototoxins. Also, the first row of OHCs seems to be at greatest risk. 

Damage to the Stereocilia

Recall that stereocilia act as rigid rods and that this rigidity is imparted on them by compact bundles of microfilaments composed of actin strands cross-bridged with another protein, possibly fibrin. The application of scanning electron microscopy has revealed another dimension to acoustic trauma to the organ of Corti: the disarray, collapse, fusing, elongation, breaking, and/or elimination of sensory hairs. These observations are accompanied by those showing that the crystalline fine-structure of the stereocilia are altered by intense sound, a condition that could reduce the rigidity of these hairs several-thousand-fold. Floppy stereocilia occur when the crystalline lattice along the length of the sensory hair is damaged. The disarray occurs after damage to the base of the hair or its rootlet. The consequences of damage to these cytoskeletal elements would be reduction in efficiency of coupling vibrational energy to the hair and subsequent impairment of hair cell function. It would appear that some forms of stereociliary damage are permanent. Remembering that the tips of the hairs may be location of the ionic current source of the receptor potential, damage to this membrane could have profound effects on the transduction process itself. Fusion and elongation of cilia are not specific to sound exposure and similar observations have been made following ototoxic treatment with antibiotics.

Injury to the Auditory Nerve

Dendritic nerve endings beneath damaged IHCs and OHCs, including their mitochondria, appear swollen after high-intensity noise exposure. Under these sound conditions there may be signs of degeneration of spiral ganglion neurons as well. The question of whether there can be nerve damage or loss without hair cell destruction has not been answered. The mechanisms of nerve damage may include degeneration secondary to hair cell loss, direct mechanical injury, and/or metabolic dysfunction. 

Changes in the central auditory system as a consequence of cochlear damage

Injury to the receptor cells and auditory nerve can result in degenerative changes in the cochlear nuclei of the brainstem and transneuronal atrophy in the superior olivary complex and inferior colliculus. The available evidence also supports the conclusion that the developing ear and brain is more susceptible to acoustic injury than those of the adult. This has been observed in a wide variety of mammalian species and thus there is good reason to believe that it would hold for humans as well. 

Temporal bone fractures

Fractures may occur along the longitudinal axis of the temporal bone or transversely to this long axis. Transverse fractures are usually more serious because they extend into the labyrinth resulting in a total loss of hearing and destruction of the vestibular function. Both kinds of fractures may injure the facial nerve resulting in a facial nerve paralysis.  

Cochlear concussion

A cochlear concussion results from a blow to the head not severe enough to cause a fracture. It may however, produce a mild-to-total hearing loss. This is because of tears of the membranous labyrinth which may involve the vestibular structures as well as the cochlea. In such cases, an individual may also experience vertigo. 

Presbycusis

The term "presbycusis" has been traditionally applied to the hearing loss that normally accompanies aging. Although it  commonly refers to hearing loss resulting from degenerative  changes in the cochlea alone, it is now clear that the aging process affects the whole auditory system and that hearing loss of old age probably involves changes in the middle ear, inner ear and central auditory pathways. Although there is no clear relationship between age-related  changes in the middle ear and the audiographic findings, there are documented cases of ossicular fixation and arthritic changes in ossicular joints with fibrous and calcific changes.  The correlation between the types and patterns of cochlear lesions and the patterns of hearing loss has long been recognized. While there may be wide variations in these patterns, in general, the common finding is a high frequency sensorineural hearing loss associated with degeneration of the organ of Corti in the base of the cochlea. Hearing loss may progress over time to lower frequencies. Figure VIII-1 shows audiograms taken at decade intervals. Note here the gradual and progressive loss of sensitivity at high frequencies.  Presbycustic individuals may also have central nervous system involvement in their hearing disorder. Within the cochlear nuclei, for example, the injury may range from little or no alteration in cellular structure to complete destruction of cells. Whether this occurs independently of a cochlear lesion is currently not known.

Ototoxicity

It has long been known that certain drugs and chemicals can have strong effects on the auditory and vestibular receptors of the inner ear.  The clinical signs of ototoxicity are variable but include  one or more of the following symptoms: sensorineural  hearing loss, tinnitus, "dizziness" of one description or  another, and depressed vestibular function with or without  nystagmus. Over the past 40 years, there has been a steady accumulation of data, from both the clinic and the laboratory, on the mechanisms of action of various ototoxins. With current understanding of the normal cellular-molecular mechanisms of receptor cell action, we are on the threshold of understanding the mechanisms of many of the disorders that  affect hair cells.   A brief description is given a few of some of the more common agents and their actions.  

Aminoglycoside antibiotics

Most of the antibiotics recognized as having ototoxic properties belong to the family of aminoglycosides. The primary ones that require respect are streptomycin, dihydrostreptomycin, neomycin, kanamycin, gentamicin and tobramycin. Figure VIII-2 shows the audiograms from the left and right ears of an individual treated with kanamycin for severe renal failure. Below the audiogram are plots of hair cell and spiral ganglion cell loss in each of the ears taken after postmortem histological preparation of the temporal  bones. Note the correspondence between hair cell loss in the basal half of the cochlea and the high frequency hearing loss which is typical of aminoglycoside ototoxicity. 

Clinical and experimental evidence collected from human and  animal studies over many years has given a picture of the pathophysiological mechanisms which underlie the damage  inflicted by these agents. First, the toxic substances must reach the labyrinthine fluid either via the blood stream or, when applied topically to the middle ear, by direct penetration of the oval and/or round windows. Second, primary damage is to the hair cell; auditory nerve fibers may degenerate secondary to sensory cell degeneration. Both kanamycin and neomycin affect first the outer hair cells of the cochlea base; over time the lesion progresses to the cochlear apex. Inner hair cells seem less vulnerable to these agents. In cases where permanent damage is to outer hair cell regions alone, the physiology of auditory nerve fiber innervating the inner hair cells in the region of the lesion is clearly abnormal. A primary effect is to greatly alter the frequency selectivity of an auditory nerve fiber.  Third, at the cellular-molecular level, the action of aminoglycosides seems to alter plasma membrane permeability for there is microscopic evidence for the swelling of sensory hairs with the deformation of the cell surface.  This may involve several processes upon which cellular integrity depends. Two of them are the cellular metabolic and protein synthesizing machinery, for there is also evidence that mitochondria and ribosomes are damaged.  Another is that the ionic channels which are responsible for mechano-electric transduction to occur may be blocked or otherwise affected.

A number of other antibiotics with differing molecular structure are known to have ototoxic properties.  These include viomycin (polypeptide), polymyxins (polypeptides), vancomycin (contains both sugars and amino acids) and minocycline (a tetracycline). 

Diuretics - Two potent diuretics in general use have well-recognized ototoxic effects. These are ethacrynic acid and furosemide. Rapid intravenous infusion of either of these agents may produce a sensorineural hear loss which is usually immediate in onset and transient, lasting a few hours to several days. There may be vertigo. Severe and permanent hearing loss is reported.  

Animal studies have shown that intravenous injection of ethacrynic acid or furosemide produces within seconds depression of the cochlear microphonic potential (hair cell receptor potential) and auditory nerve action potentials and a decrease in endocochlear potential which is necessary for normal transduction and transmission in the inner ear receptor organs. Anatomical changes include outer hair cell degeneration in basal and middle turns of the cochlea. In those cells that survive there may be distortion of the stereociliary bundle. Moreover, there are marked changes in the stria vascularis, with intra- and extracellular edema and destruction of the intermediate cell layer. Thus, it would appear that diuretic ototoxicity involves changes in the transduction and transmission properties of the hair cells and a breakdown in the intra-labyrinthine secretory mechanisms of the stria vascularis.  

Quinine derivatives - It has long been known that quinine derivatives produce irreversible sensorineural hearing loss with tinnitus as the major symptom. Administration to women in the first trimester of pregnancy has resulted in severe abnormalities of the inner ear of the fetus. There may be a complete absence of hair cells and supporting cells throughout much of the organ of Corti. 

Salicylates - High doses of salicylates predictably produce a bilaterally symmetric, flat hearing loss up to about 40 dB with some reduction in speech discrimination. The magnitude of the hearing loss is directly related to the serum levels of the substance. The hearing loss and accompanying tinnitus are completely reversible within 24-72 hours after the drug is discontinued. There is no consistent morphological change observed in the inner ears of humans or animals subjected to high doses of salicylates. While biochemical changes of the perilymph and endolymph have been noted along with consistently reduced electrical activity of the cochlea and auditory nerve, the precise mechanisms of this form of ototoxicity are not known.  

Meniere's disease (idiopathic endolymphic hydrops)

The prototypic intra-labyrinthine auditory-vestibular disorder is Meniere's disease. The classic description of the symptom complex was given more than a century ago: (1) tinnitus, (2) spontaneous, episodic vertigo and (3) hearing loss. Endolymphic hydrops (i.e. distention of scala media with Reissner's membrane bulging into scala vestibuli) was clearly described as the basic histopathologic lesion associated with the disease (hence the clinical synonym) although a cause-and-effect relationship between the hydrops and the clinical findings remains obscure.  Vertigo of Meniere's disease occurs in sudden attacks and may be accompanied by spontaneous positional nystagmus. Hearing loss is characteristically unilateral, fluctuating, and sensorineural in nature. Bilateral hearing loss occurs in 10-20% of the cases although a figure as high as 40% has been reported. Subjectively, the hearing loss is frequently accompanied by fullness and feelings of pressure in the affected ear. Tinnitus may be roaring, buzzing, whistling or mixed in nature. Histopathologic studies of temporal bones taken postmortem from Meniere's patients usually show hydrops of the scala media and, less frequently, hydrops of the utricle and/or one or more semicircular canal. In severe cases, secondary complications such as rupture of membranes, herniation of the vestibular organs, and degeneration of the organ of Corti may occur. Occasionally, there are noted changes other than hydrops involving the endolymphic sac and its surrounding tissue and the vestibular aqueduct and endolymphic duct. Traditionally, clinical experience indicates that there may be a number of secondary factors that contribute to the syndrome. These include metabolic, infectious, allergic neurogenic and psychosomatic factors.

Labyrinthitis - Invasion of the labyrinth by inflammatory reaction is manifested by vertigo, nystagmus, sensorineural hearing impairment, nausea and vomiting.  Serous (non-suppurative) labyrinthitis represents a serous inflammation of the labyrinth secondary to an acute or chronic infection of the surrounding bony labyrinth. There is not active invasion of the labyrinth by the infecting organism. Normal vestibular and auditory function returns.   Suppurative labyrinthitis occurs secondary to otitis media and mastoiditis and represents a direct extension of the infection into the labyrinth. The symptoms include roaring tinnitus, progressive sensorineural hearing loss to total deafness, and violent vertigo with associated nausea and vomiting. Complications of this disease are meningitis or epidural abscess.  

Tumor of the VIIIth nerve - Lesions of the eight nerve are characterized by tinnitus,  sensorineural hearing loss, mild vertigo, and in some patients, other cranial nerve signs. The classic lesion is the so-called "acoustic neuroma", a benign tumor that is usually not of auditory nerve origin nor is it a neuroma. The tumor is a schwannoma typically arising from the vestibular nerve within the internal auditory meatus. The growth of the tumor in the vestibular nerve does not typically produce vestibular signs and it is not until the tumor compresses the auditory nerve that it is noticed. The most common first symptom is unilateral tinnitus. This may be followed by a high frequency hearing loss. The mechanism for the hearing disorder probably involves the disruption of  normal transmission of action potentials in the fibers of the auditory nerve due to compression by the tumor. Sudden occlusion of the internal auditory artery, which supplies the organ of Corti, may produce a severe or total cochlear hearing loss.  Pressure from the growing tumor may eventually involve cranial nerves VII, VI and V. 

Tinnitus is a major disorder of the peripheral auditory system

What is tinnitus?

No doubt everyone has experienced sounds that do not originate from a source or sources outside of the body. Instead, they originate 'within the head'. These auditory sensations may take many forms, such as roaring noise, tones and clicks, and they may be intermittent or continuous. They all are referred to as tinnitus. Thus, tinnitus can be defined broadly as a conscious experience of sound that originates within the head of its owner.

Tinnitus is not a disease per se, but is a symptom of a wide range of disorders. Severity of tinnitus ranges widely, from being mildly irritating and hardly noticed to being severely debilitating. The incidence of tinnitus is very high. About 32% of all U.S. adults report having tinnitus at one time or another, and about 6.2% of them report it to be severe or debilitating.

What are the causes of tinnitus?

Tinnitus may have a physical basis. That is, sound energy is produced somewhere in the head, which is then heard by a subject. This could include such things as vascular anomalies, muscular contraction, clicking jaws. Also, sounds may be generated within the inner ear that are loud enough to be heard by a subject (or a nearby listener), the so called otoacoustic emissions. In all of these cases, the inner ear and central auditory pathways are normal in their transmission and processing of the unwanted and often annoying sounds.

Tinnitus may have no physical basis. In these cases it is the result of abnormal physiological activity in the inner ear or in the central auditory pathways, which gives rise to the perception of sound even though there is no physical sound present.

Except for those cases for which there is a clear mechanical cause, the physiological mechanisms that underlie tinnitus are unknown.

Because mechanisms are unknown and because tinnitus may arise from any of a number sites in the ear and brain, it is not surprising that there is no single treatment that works in all cases.

Tinnitus has multiple etiologies. It may be associated with:

a. blows to the head. Tinnitus may be transient or long-lasting.

b. drugs and general anesthetics used during surgery may initiate or exacerbate a tinnitus.

c. tumors of the eighth nerve (schwannoma's), and is an early symptom in a high percentage of such cases. Rarely does it go away after surgery.

d. sensorineural hearing loss. In cases of acoustic trauma and presbycusis tinnitus is reported to be high pitched, matching the frequency in the transition zone between regions of greater and lesser hearing loss.

e. otosclerosis, which may be associated with pregnancy.

f. Menier's disease.

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