Sensorineural Hearing Loss in Adults. Etiology of Sensorineural Hearing Loss. I--Developmental and Hereditary Disorders Hereditary Disorders of Adult Onset Nonsyndromic Hereditary Hearing Loss
Sensorineural Hearing Loss in Adults
Etiology of Sensorineural Hearing Loss
dystopia canthorum, the broad nasal root, the confluence of the eyebrows, and the heterochromia iridis
Noise-induced hearing loss (NIHL) is second only to age-related hearing loss as the most prevalent form of hearing loss.
NIHL resulting from relatively brief noise exposures can be reversible, as happens with exposure occurring at an evening spent in a loud entertainment venue.
Permanent NIHL is caused by either an acoustic trauma (i.e., a brief exposure to a very intense (blastlike sound) or a chronic long-term exposure to the loud sounds associated with a noisy occupation.
An accelerating incidence of high-frequency hearing loss in younger individuals points to early, chronic noise exposure, possibly from personal entertainment devices.
NIHL is a complex condition that is influenced by environmental and genetic factors.
Genetic association studies have identified genetic factors primarily related to oxidative stress that influence an individual's susceptibility to NIHL.
Current research on the administration of certain antioxidants before or after noise exposure shows promise for developing a pharmacologic treatment for NIHL in the near future.
NIHL is a preventable condition and the otolaryngologist plays a critical role in educating patients about protecting their ears from the adverse effects of noise overexposure.
NIHL is caused by repeated exposures to sound that is too intense or too long in duration. Each exposure is followed by a TTS, which recovers, but eventually a PTS develops.
Acoustic trauma consists of a single exposure to a hazardous level of noise, resulting in a PTS without an intercurrent TTS.
NIHL almost always results in a symmetric, bilateral hearing loss.
-almost never results in a profound loss.
Early in the course of NIHL, the loss usually is limited to 3 kHz, 4 kHz, and 6 kHz. The greatest loss usually occurs at 4 kHz. As the loss progresses, lower frequencies become involved, but the loss at 3 to 6 kHz is always far worse
The loss progresses most rapidly during the first 10 to 15 years of exposure and thereafter grows at a much-reduced rate
Predicted hearing thresholds (median and extreme values) after 20 years and 40 years of occupational noise exposure at 90 dBA.
Speech frequency average noise-induced permanent threshold shift (NIPTS) as a function of level of exposure (in dBA-TWA) and duration.
One of the most common causes of permanent hearing impairment is exposure to excessive sounds.
Millions of individuals worldwide have noise-induced hearing loss (NIHL), resulting in a reduced quality of life because of social isolation, and possible inexorable tinnitus and impaired communication with family members, coworkers, and friends.
The costs in terms of compensation and early retirement payments for
work-related NIHL are immense. The U.S. Department of Veterans Affairs spends approximately $700 million a year on disability compensation and treatments for NIHL. NIHL is the single largest disability expenditure of the Veterans Benefits Administration.
This steady progression in the knowledge base about NIHL promises to improve significantly the detection and treatment of this disorder over the coming years.
Measurement of Noise
The term noise is commonly used to designate an undesirable sound. In the scientific and clinical fields that deal with hearing, this term has come to mean any excessively loud sound that has the potential to harm hearing. The temporal patterns of environmental noise are typically described as continuous, fluctuating, intermittent, or impulsive.
Continuous or steady-state noise remains relatively constant,
fluctuating noise increases and decreases in level over time,
intermittent sounds are interrupted for varying time periods
Impulsive or impact noises caused by explosive or metal-on-metal mechanical events have rapidly changing pressure characteristics consisting of intense, short-lasting (i.e., milliseconds) wave fronts, followed by much smaller reverberations and echoes that occur over many seconds.
The amount of noise, usually referred to as the sound pressure level (SPL), is conventionally measured by a sound-level meter in decibel (dB) units using a frequency-weighting formula called the A-scale. The dBA-scale metric of sound level essentially mimics the threshold-sensitivity curve for the human ear, so the low-frequency and high-frequency components are given less emphasis as auditory hazards. Standard sound-level meters have
electronic networks designed to measure noise magnitude automatically in dBA, whereas to measure impulse or impact noise, a more intricate peak-reading sound-level meter is needed that is capable of accurately measuring sounds with essentially instantaneous onset times.
The personal noise dosimeter is typically used to measure noise exposure in the workplace. This instrument provides readout of the noise dose or the percent exposure experienced by a single worker, typically over a
specific shift. The logging dosimeter integrates a function of sound pressure over time and calculates the daily (8-hour) dose with respect to the current permissible noise level for a continuous noise of less than or
equal to 85 dBA lasting 8 hours. More recently, personal noise dosimeters have been offered to the consumer as a portable, compact, and affordable device that can be used as hearing protectors. The instrument measures and displays noise dose continuously for 16 hours. The dosimeter provides an early warning that the user is approaching overexposure and should use hearing protection.
A particular noise (e.g., from power tools, music concerts, sporting events) can also be measured for 2 minutes, and then the estimated dose per hour is calculated and displayed to determine if permissible exposure levels would be exceeded. By putting valuable health information into the hands of consumers, such easy-to-use, inexpensive (<$100) dosimeters empower them to take appropriate steps to prevent NIHL.
Nature of the Hearing Loss
Depending on the level of the sound exposure, either reversible or permanent damage can occur to the peripheral auditory end organ.
The reversible loss, typically referred to as a temporary threshold shift (TTS), results from exposures to moderately intense sounds, such as might be encountered at a philharmonic orchestra concert. Hearing problems associated with TTS include elevated thresholds, particularly for the
higher midfrequency region that includes the 3- to 6-kHz frequencies.
The TTS condition is often accompanied by many other common symptoms of hearing impairment, including tinnitus, loudness
recruitment, muffled sounds, and diplacusis.
Depending on the duration of the exposure, recovery from TTS
can occur over periods ranging from minutes to hours and days.
After exposure, if TTS does not recover before the ear is re-exposed to excessive sound, a permanent change in hearing can occur, which is referred
to as a permanent threshold shift (PTS).
In PTS, the elevation in hearing thresholds is irreversible because lasting structural damage occurs to the critical elements of the cochlea. The precise relationship between the TTS and PTS stages of hearing loss
caused by noise exposure is unknown. Although it seems logical to assume that repeated episodes of TTS would eventually lead to PTS, experimental findings imply that the fundamental processes underlying the
development of reversible versus permanent NIHL are unrelated. Nordmann and colleagues using a survival fixation approach showed that the histopathologic manifestations of TTS and PTS noise damage to
the chinchilla cochlea are distinct.
Specifically, TTS was correlated with a buckling of the supporting pillar cell
bodies in the frequency region of the maximal exposure effect.
The morphologic abnormality that was consistently correlated with PTS was a focal loss of hair cells, and a complete degeneration of the
corresponding population of nerve fiber endings.
Because PTS eventually develops from repeated exposures to stimuli that initially produce only TTS, it is likely that the latter condition is also associated with subtle changes to the sensitive outer hair cell (OHC) system that go undetected by conventional light microscopy.
Traditionally, PTS caused by acoustic overstimulation has been separated in two distinct classes.
One type, called acoustic trauma, is caused by a single, short-lasting exposure to a very intense sound (e.g., an explosive blast), and results in a sudden, usually painful, loss of hearing.
The other type of hearing loss is commonly referred to as NIHL, and results from chronic exposure to less intense levels of sound.
A great deal more is known about the anatomic processes underlying the symptoms of and recovery from acoustic trauma than is known about NIHL.
Consequently, it is well established that a single exposure to a severe
sound causing violent changes in air pressure can produce direct mechanical damage to the delicate tissues of the peripheral auditory apparatus, including components of the middle ear (tympanic membrane, ossicles) and inner ear (organ of Corti).
In contrast, regular exposure to less intense but still noisy sounds involves the insidious destruction of cochlear components that eventually and unavoidably leads to an elevation in hearing levels, along with other common symptoms of hearing impairment.
Acoustic trauma was previously a relatively rare event that was typically associated with accidental explosions in industrial settings. Military servicemen and servicewomen caught in roadside bomb explosions in the current armed conflicts in Iraq and Afghanistan are returning home in epidemic numbers, however,
with profound permanent hearing losses and tinnitus.
Consequently, acoustic trauma is a hearing problem that is increasing, at least in combat troops. Because many of these postdeployment cases are being treated
in the private sector, all otolaryngologists may see acoustic trauma in increasing numbers.
Irreversible NIHL is a specific pathologic state exhibiting a recognized set of symptoms and objective findings.
NIHL includes (1) a permanent sensorineural hearing loss with damage principally to cochlear hair cells, and primarily to OHCs;
(2) a history of a long-term exposure to dangerous noise levels (i.e., >90 dBA for 8 hours/day) sufficient to cause the degree and pattern of hearing loss described by audiologic findings;
(3) a gradual loss of hearing over the first 5 to 10 years of exposure;
(4) a hearing loss involving initially the higher frequencies from 3 to 8 kHz before including frequencies less than or equal to 2 kHz;
(5) speech-recognition scores that are consistent with the audiometric loss; and
(6) a hearing loss that stabilizes after the noise exposure is terminated.
A patient with NIHL commonly consults a physician because of difficulties in hearing and understanding ordinary speech, especially in the presence of background noise.
The beginning region of impairment involves the sensitive midfrequency range, primarily 3 to 6 kHz, and the corresponding hearing loss is classically described as the “4-kHz notch.” This pattern of maximal hearing loss, with little or no loss at less than 2 kHz, typically occurs regardless of the noise-exposure environment. -thresholds for bone-conducted stimuli are essentially identical to the thresholds for air conduction. The profile of noise-induced threshold hearing is
usually symmetric for both ears, particularly for individuals who have been working in noisy industrial settings in which there are “surround” sounds.
this left-handed patient, note greater impairment in the right (yellow circles) rather than the left (red circles) ear because of protective head-shadow effect.
The development of a hearing loss caused by habitual exposure to moderately intense levels of noise typically consists of two stages.
Initially, the middle to high frequencies exhibit the resulting hearing loss.
As the length of time of exposure to loud noise increases, hearing loss becomes greater and begins to affect adjacent higher and lower frequencies
A, Spectra of noise produced by hammer (red circles) and press (yellow circles) equipment, with maximal energy centered in 0.2- to 1-kHz and 0.125- to 0.5-kHz regions. B and C, Resulting hearing losses for press (B) and hammer (C) operators. Noise-induced hearing losses occurred at frequencies above peak energy in the exposure. Geometric symbols represent experimental subjects according to years of noise exposure. Shaded areas indicate effects of aging on hearing levels in control subjects of similar age (i.e., 23 to 54 years old), who worked in non-noisy parts of the same drop-forging plants.
The primary site of anatomic damage is at the level of the mechanosensory receptors of the auditory system's end organ. Loud sound damages the inner hair cells and OHCs of the organ of Corti, with the OHCs in particular being most affected in the initial stages. In instances involving very intense acoustic stimulation, supporting-cell elements also can be directly affected. Depending on the physical attributes of the exposure stimulus (e.g., time-varying characteristics or the intensity, frequency, or spectral content, duration, or schedule), noise can cause damage to hair cells ranging from total destruction to effects evident only in the ultrastructure of specialized subcellular regions (e.g., the fusing or bending of the individual cilia that make up the stereociliary bundle). Whenever degenerative processes or structural modifications to the cochlea reach a significant level, an associated reduction in hearing capability can be detected.
The sharp transition in the basal end (following the uncoiling to the right) from
the normal-looking organ of Corti (a darkish stripe corresponding to the region of inner hair cells and OHCs), with its dense network of nerve fibers, to the complete absence of hair cells and their corresponding nerve fibers (the much lighter adjacent area) can be noted. Figure 151-3B graphically reconstructs the
histopathologic features of this cochlea as a cytocochleogram by depicting the number of remaining hair cells, in the form of percentages, averaged over 1-mm sections. A typical finding in individuals exposed to the occupational noise exemplified in this case is the almost symmetric pattern of degeneration observed for the two ears. The inset at the top right of the cytocochleogram shows the patient's audiogram obtained about 1 year before his death, which shows the severity of the anatomic damage in functional terms by revealing
an abrupt hearing loss for test frequencies greater than 2 kHz.
organ of Corti from the left cochlea of a 50-year-old man exposed extensively to occupational noise, showing a pattern of abrupt degeneration of basal region. Arrow indicates a small patch of remaining organ of Corti near basal end.
, Modified cytocochleograms for two ears along with an audiogram that was measured 1 year earlier, showing sharp pattern of hair cell degeneration (expressed as percentage remaining per millimeter of length of basilar membrane measured from basal end) and nerve fiber egeneration. Note relative symmetry of corresponding abrupt high-frequency loss of cochlear elements. Separate curves represent inner (solid lines) and outer (dashed lines) hair cells (averaged over three rows of outer hair cells) for left (X) and right (O) ears. Yellow horizontal line along abscissa indicates presence of nerve fibers in osseous spiral lamina.
Some authors believe that perilymphatic fistulas develop spontaneously.
Diagnosis is made by middle ear exploration. Visualization of fluid in the region of the oval or round windows is not definitive evidence of a fistula because serous fluid can ooze from the middle ear mucosa, or lidocaine from the local anesthetic can collect in the vicinity
Treatment consists of packing the area in question with tissue.
Because of the lack of a definitive diagnostic test for the presence of a fistula, and because even surgical exploration does not reliably diagnose or exclude the possibility of a fistula, there is considerable controversy regarding management
In the 1980s and early 1990s, it was commonly believed that spontaneous perilymphatic fistulas were a common cause of otherwise unexplained
hearing loss and vertigo. Many surgical fistula repairs were performed as a result of this belief.
It has since become clear that spontaneous perilymphatic fistula is rare
No clear consensus exists regarding diagnosis or management.
Finally, labyrinthine fistula can result from erosion by cholesteatoma, or may develop spontaneously as in the superior semicircular canal dehiscence syndrome.
Median audiograms for patients with little to no noise exposure as a function of gender, frequency, and age.
has defined four separate types of presbycusis on the basis of pathologic findings in human temporal bones.
1--sensory presbycusis, hair cells are progressively lost beginning at the base of the
cochlea. Patients with this abnormal pattern tend to have steeply sloping high-frequency hearing losses.
2--Neural presbycusis implies a loss of auditory nerve fibers. These patients tend to have reduced speech discrimination out of proportion to their pure-tone thresholds.
3--strial presbycusis- Atrophy of the stria vascularis was seen in these patients-
relatively flat audiograms.
4--cochlear conductive or mechanical presbycusis. No light microscopic
abnormalities are seen in these specimens, and Schuknecht theorized that an age-related change in the stiffness of the basilar membrane resulted in the hearing loss. These patients have gradually descending (approximately 25 dB/octave) pure-tone thresholds.
These patterns are not useful clinically because there is variability of audiometric shape and severity in individuals with age-related hearing loss, and clinically the
losses do not fall naturally into these patterns.
Circulatory disorders have long been proposed as the cause of hearing loss in aging persons. In the Framingham cohort, coronary artery disease, stroke, intermittent claudication, and hypertension were linked to hearing loss.
However, there is insufficient histopathologic evidence of this etiology for confirmation.
The relationship between high-frequency sensorineural hearing loss and the degree of cerebral atherosclerosis has been used to support this theory; unfortunately, both may be independent but age-related. Atherosclerotic disease of renal vessels and inner-ear vessels has also been related to age.
They found a similarity between the degeneration of inner ear vessels with analogous changes in the retina due to microangiopathy, and they demonstrated that the plugging of vascular canals by bony tissue is a generalized phenomenon that is related to aging. They believed that the plugging of vascular canals was one of the major causes of presbycusis.
Much like arteriosclerosis, diabetic angiopathy may contribute to presbycusis. In this disorder, disseminated proliferation and hypertrophy of the intimal endothelium of arterials, capillaries, and venules causes significant narrowing of the lumen; there is also the precipitation of lipids and other substances in the vascular wall. In addition, arteriolosclerosis is more common and more extensive in patients with diabetes.
Even though recent epidemiologic studies demonstrate a higher incidence of sensorineural hearing loss among diabetic patients than age-matched controls, this effect is mitigated in populations older than 60 years. Additionally, hemoglobin A1C levels did not correlate with hearing loss.
Serum cholesterol may also play a role in presbycusis. In Rosen's studies of Finnish patients on long-term controlled diets, the reduction of saturated fat resulted in a significant lowering of serum cholesterol and an improvement in auditory threshold testing. However, the link between serum lipids and hearing loss is not definitive
Presbycusis has been found to cluster in families, and in fact approximately half of the variability in presbycusis may be attributed to genes.
The effect of genes is more pronounced for the strial atrophy pattern of hearing loss (flat audiogram) than the sensory phenotype (high-frequency loss).
Genes that may play a role include those that protect against oxidative stress, in that this stress plays a significant role in presbycusis. Proposed genes in recent studies include those that code for glutathione peroxidaseand superoxide dismutase, two antioxidant enzymes that are active in the cochlea.
Genes responsible for monogenic deafness may also play a role.
Hearing and Dementias
Recent studies of the cochlea in temporal bones from patients with confirmed Alzheimer's disease showed a lack of degeneration in the cochlea, which is typical of Alzheimer's patients. This finding is distinguished from findings in the peripheral olfactory and visual systems, which show the typical neurofibrillary tangles and
Conversely, a possible relationship between central auditory dysfunction was found in the Framingham follow-up study of 1662 subjects. However, that study is weakened by the absence of objective testing in competing message tests.
Amplification remains the mainstay of treatment for presbycusis.
The correction of other health factors that may impact age-related hearing loss, such as smoking, hypertension, and cholesterol levels, may play a role.
Although dietary measures over the long term may be effective in reducing the progression of hearing loss in certain aging patients, further data are necessary before this treatment modality is clinically accepted.
Cochlear implantation may play a role in treating older adults with severe to profound sensorineural deafness. Such a degree of hearing loss is most often due to an underlying pathologic process such as Meniere's disease or otosclerosis in combination with presbycusis; the latter does not produce this degree of
hearing impairment on its own.
A recent study of 749 adolescent and adult cochlear implant recipients found
that age was a clinically insignificant predictor of audiologic outcome from cochlear implantation, compared with duration of profound deafness and residual speech recognition.
Quality-of-life scores between elderly and nonelderly cochlear implant recipients are also similar.
As discussed previously, comorbid chronic health conditions seen more commonly in older populations will play a role in surgical planning and perioperative management.
Presbystasis, which is the dysequilibrium of aging, is a group of disorders that affect the mobility of a large number of older persons. Due to the degeneration of the vestibular, proprioceptive, and visual senses, the ability to walk and drive can be reduced to the point of incapacitation; lessening spatial-orientation abilities
contribute to this as well. Loss of balance is the most common manifestation of vestibular dysfunction in older adults.
Although attempts have been made to categorize the dysequilibrium of aging as a single specific entity, a large number of vestibular disorders are seen in older patients. These include vascular disease, Meniere's disease, benign positional vertigo, and adaptation deficits. Input from the vestibular, visual, proprioceptive,
and other systems can be thought of as providing input into a common central processor that, in turn, controls posture and eye movement. This adaptive control system alters afferent signals from the various receptors at both visual-vestibular interfaces as well as proprioceptive-vestibular interfaces. Control circuits
can be affected by disturbances in the general condition of the patient, the availability of the neurotransmitter, and in pathologic disorders.
Other feedback loops help control visual tracking and postural adjustment in response to motion. Cognitive controls also exist and contribute primarily in the areas of spatial orientation, the hallucination of motion, and the development of athletic skills.
Disorders of these sensory organ systems have traditionally been treated by otolaryngologists, neurologists, and ophthalmologists, depending on the organ system causing the most obvious dysfunction. However, development of the unifying discipline of neuro-otology has led to an integrated approach to, evaluation of,
and care for older persons with dysequilibrium
. Otolaryngologists must be aware of other causes of dysequilibrium or dizziness, because a variety of organ systems may contribute to these difficulties, including
vestibular, ocular, proprioceptive, musculoskeletal, central processing, cardiovascular, and neuromotor. For example, side effects of psychotropic medications, abnormalities in blood pressure, leg muscle weakness, neuromotor disorders such as Parkinson's disease, and generalized loss of coordination can contribute to
feelings of dysequilibrium and dizziness. The failure of one organ system can be overcome with compensation, but with multisystem failure, increasingly severe deficits occur.
Falls are one of the most common concerns relating to imbalance in older adults. Approximately one-third of adults older than age 65 in the community and one-half of adults older than age 80 in institutionalized settings fall each year. One-third of these falls result in injuries that require medical attention or the restriction
of activities for at least 1 day, and 10% to 15% of these falls result in fracture. As a result, the medical costs of fall-related injuries totaled $19 billion in 2000, with nearly $9 billion for hip fractures alone.
Moreover, falls lead to functional decline, anxiety, depression, and social withdrawal. In particular, one-half of older adults hospitalized for a hip fracture do not return to prior levels of function.
Age-related degeneration has been noted in hair cells, neurons, and supporting structures of the peripheral vestibular system, as well as more centrally in the vestibular nuclei and cerebellum. Hair cell loss has been found in the semicircular canals, the utricle, and the saccule. This degeneration is most noted in the central
area of the cristae, whereas macular degeneration is more diffuse. Degeneration of the saccule may be greater than that of the macula. A decrease in the total number of peripheral vestibular neurons, as well as a decrease in the size of myelinated nerve fibers, has been described in patients who are older than 65 years
old. Degenerative changes also occur in otoconia of the human maculae, deformities of the vestibular end organs, and degeneration of the synaptic structures of afferent dendrites.
These degenerative changes are considered to be the vestibular equivalent of presbycusis. Unlike presbycusis, however, asymmetric loss of vestibular function can result in incapacitation.
The use of objective tests to identify the etiologic basis of presbystasis is essential. Vestibular function studies described elsewhere are applicable in older adults as well. Avoidance of a “trash basket” diagnosis of presbystasis and continuing clinical research into etiologic diagnosis are essential. In particular, studies of
electronystagmography, platform posturography, and sinusoidal harmonic acceleration in older adults are ongoing.
In cases of presbystasis arising in the peripheral labyrinth, generalized hypofunction is often found.
Symmetric maximum slow-phase velocity responses to warm and cool caloric stimulation of less than 10ýC per second per irrigation can be empirically used to identify this condition. In cases of peripheral hypofunction, the use of vestibular nerve suppressants may be contraindicated. Such treatment further reduces the already reduced vestibular input, thereby resulting in further incapacitation.
Nonvestibular causes of presbystasis need to be identified and treated specifically. Examples include postural hypotension associated with antihypertensive medications, endocrine imbalances, malnutrition, and cardiovascular insufficiency.
Because of the adaptive control feedback mechanism in the complex vestibular system, treatment modalities have been developed to allow for compensation. Vestibular habituation training involves “exercises” based on feedback control initiated by the habituation effect. Mechanisms of adaptation and compensation are
stimulated through repeated elicitation of minor degrees of vertigo.
Other goals of vestibular exercise programs include the improvement of visual tracking when the head is stationary, gaze stability during head movement, and visual-vestibular interactions during head movement and general balance. These exercises are designed to incorporate visual and proprioceptive experiences
with vestibular cues.
The twin goals of these exercises are the reestablishment of balance and the reduction of the symptoms of dizziness and disorientation. In many cases, consultation and therapy with a physical or occupational therapist trained in vestibular compensation exercises can be extremely helpful.
Another important consideration that must be stressed to the patient is the prevention of falls. Precautions include the use of night lights (especially en route to the bathroom), the removal of throw rugs, the avoidance of stairs, and the use of ambulatory assistance devices when necessary.
Patients who have bilateral disease usually develop bilaterality early in the course of the disease. Patients who maintain hearing loss in only one ear for the first few years rarely develop it in the contralateral ear. This finding supports the idea that
the syndrome we have labeled as Meniere's disease is actually the manifestation of several different pathologies.
Although vestibular destructive therapy (chemical or surgical labyrinthectomy, vestibular nerve section) is effective in controlling the episodic vertigo associated with Meniere's disease, no therapy to date has been proven to be effective in the treatment of the hearing loss. The most widely accepted medical therapy for
Meniere's disease, a sodium-restricted diet and diuretic administration, is based on the hypothesis that the hydropic distention of the membranous labyrinth can be reduced by altering body water distribution, as in the management of hypertension. This regimen of sodium restriction and diuretics was first proposed by
Furstenberg and has gained wide acceptance.
Despite this wide acceptance, few studies have been able to show definitively that there is a beneficial therapeutic effect with regard to improvement or preservation of hearing.
The creation of an intralabyrinthine shunt (cochleosacculotomy) has been proposed as a mechanism to control the hydrops and prevent the recurrent membrane ruptures thought to traumatize the organ of Corti.
This procedure has been shown to be beneficial in controlling vertigo, but has been associated with an unacceptably high incidence of severe hearing loss.
Surgical decompression of the endolymphatic sac, with or without shunting of the sac into the mastoid or the subarachnoid space, has been proposed as a
way to correct the presumed defective sac physiology
The unpredictable, fluctuating nature of Meniere's disease, the lack of an objective diagnostic test, and the high incidence of spontaneous resolution of symptoms make it extremely difficult to come to valid statistical conclusions with regard to therapeutic efficacy in Meniere's disease.
Meniere's disease may be considered to be idiopathic endolymphatic hydrops. Although it is the most common cause of endolymphatic hydrops, many other entities result in similar clinical presentations and pathologic findings.
The syndrome of delayed endolymphatic hydrops consists of the initial development of a profound SNHL in one ear, followed by development of symptoms of endolymphatic hydrops years later in either the ipsilateral ear (ipsilateral delayed endolymphatic hydrops) or the contralateral ear (contralateral delayed endolymphatic hydrops).
Other pathologic processes have been associated with development
of endolymphatic hydrops, including
temporal bone trauma,