7 Cranial Nerve VIII: Hearing Disorders
Abstract
Cost-effective evaluation of audiovestibular disorders relies on a detailed and directed history and physical examination. Audiologic evaluation is the primary ancillary examination for auditory and vestibular disorders. Laboratory testing and imaging of the inner ear and brain should be guided by specific findings on history, physical examination, and audiological evaluation. Hearing loss is common, especially in patients older than 60 years. Patients with a complaint of hearing loss should undergo audiological evaluation. Patients with asymmetric sensorineural hearing loss (SNHL) noted on audiogram, including sudden SNHL (SSNHL), should undergo MRI to evaluate for retrocochlear pathology (most commonly vestibular schwannoma). Patients with conductive hearing loss (other than cerumen impaction that can be resolved by the primary care physician) should be referred to an otolaryngologist for further evaluation. Treatment options for conductive hearing loss include observation, hearing aid, and surgery. SNHL is common in older adults but can occur at any age. Treatment options for SNHL include observation, hearing aids, and cochlear implants. All newborns should be screened for hearing loss. Newborns with abnormalities on hearing screening should be further evaluated until hearing loss is definitively ruled in or out. Loss of follow-up is a major concern in this population. Stepwise, algorithmic evaluation of congenital hearing loss is critical to perform a thorough evaluation in a cost-effective manner.
7.1 Introduction
Hearing and vestibular sensation are functions of the ears. While there are disorders that affect both hearing and vestibular function (Meniere’s disease, vestibular schwannoma [VS] tumors, labyrinthitis), most disorders affect only one system. Therefore, hearing and balance disorders will be addressed separately. Most of the content applies to patients of all ages. There are, however, special sections dedicated to newborn hearing evaluation and cochlear implantation. These are divided into separate sections in this chapter.
7.2 Hearing Loss
7.2.1 Auditory Anatomy and Physiology
The ear can be thought of comprising three components: the outer ear (pinna and external auditory canal), the middle ear (from the tympanic membrane [TM] to the stapes footplate), and the inner ear (the cochlea and the semicircular canals). In a healthy ear, sound first arrives at the outer ear, transmitted as air vibrations. Sound then travels past the pinna through the external auditory canal (ear canal) to the TM. Air vibrations are transformed into mechanical vibrations, first of the TM and then of the ossicular chain (malleus, incus, stapes bones) in the middle ear. The energy of the oscillating stapes is transferred through the oval window of the cochlea. Within the cochlea, the sound energy is transmitted as inner ear fluid waves. These waves cause the movement of hair cells within the cochlea. Hair cell movement leads to excitation of the cochlear nerve. The signal from the cochlear nerve travels through the internal auditory canal (IAC) to the cochlear nucleus in the brainstem (retrocochlear pathway). The central auditory pathway then carries the neural input onto appropriate regions of the brain.
7.2.2 Epidemiology and Economic Impact of Hearing Loss
Hearing loss is one of the most common and debilitating cranial neuropathies worldwide. In the United States alone, it is estimated that 68% of adults older than 70 years experience hearing loss but that only approximately 20% seek treatment. 1 , 2 The economic burden related to hearing loss is enormous in the billions of dollars in the United States. 3 Medical expenses alone related hearing loss are estimated to be $12.8 billion. 4 This figure includes indirectly related medical expenses as individuals with hearing loss are at increased risk for falls, hospitalizations, and cognitive decline. 5 , 6 , 7
In addition to simply not being able to hear the surrounding environment, hearing loss can lead to a reduction in overall quality of life (QOL) and reduced ability to engage with others and can predispose to development of depression. 8 A study of individuals with severe hearing loss prior to cochlear implantation showed an average health utility index (HUI; a measure of disability with 1 being perfect health and 0 being dead) score of 0.49 to 0.54, similar to stroke survivors or those with end-stage renal disease. 9
Beyond the direct and indirect medical costs are estimated losses in productivity. Kochkin estimated that the amount of lost income attributable to hearing loss in the United States may be as high as $176 billion with an additional $26 billion of unrealized federal taxes. 10 Treatment of hearing loss is of paramount importance and potentially will prevent billions of dollars’ worth of disability and, more importantly, restore a fundamental ability to interact with one’s environment.
7.2.3 Clinical Approach of Hearing Loss
Hearing loss can be conductive, sensorineural, or mixed. Conductive hearing loss (CHL) is caused by a problem with transference of sound energy from the air to the cochlea. CHL generally relates to a problem of the external or middle ear. Sensorineural hearing loss (SNHL) is caused by a problem within the cochlea (inner ear), the cochlear nerve, or the auditory pathway (the latter two of which comprise the retrocochlear pathway). There can be simultaneous conductive and SNHL, which is considered a mixed hearing loss.
Hearing loss can be caused by a myriad of conditions. Some causes are easily diagnosed and treated such as cerumen impaction, a mechanical obstruction of the external auditory canal by wax build-up. Other pathologies, such as VS, are rare causes of hearing loss that present diagnostic challenges to the physician. In order to choose appropriate ancillary tests and to arrive at an accurate diagnosis of the underlying etiology of hearing loss, a pertinent history and physical examination must be performed. In addition, an audiological evaluation is an essential tool for the evaluation of hearing loss.
7.2.4 History
Essential elements of a history for a patient with hearing loss include description of hearing loss onset (sudden vs. gradual), duration, severity, unilateral versus bilateral, progressive versus stable, and stable versus fluctuating. Additionally, the patient should be asked about otalgia, tinnitus, drainage from the ear canals (otorrhea), aural fullness, and dizziness. History related to head trauma, barotrauma, noise exposure, exposure to ototoxic medications, previous ear infections, previous otologic surgery, and family history of otologic problems should also be ascertained. A full history including current medications, past medical and surgical history, social background, and family history should be performed.
7.2.5 Physical Examination
A complete head and neck examination should be performed on patients complaining of hearing loss, including an examination of the cranial nerves, face, scalp, eyes, nose, oral cavity, neck, pinna, external auditory canals, and TMs. Examination of the external auditory canals and TMs should be performed with a microscope as opposed to a handheld otoscope whenever possible. Pneumatic otoscopy should be performed to test for TM mobility. A tuning fork examination should be performed, typically with a 512-Hz tuning fork. In the Weber test, the tuning fork is placed on the center of the forehead. The sound of the tuning fork lateralizes away from the side with a SNHL and toward the side with a CHL. In the Rinne test, the sound intensity is compared between air conduction and bone conduction by placing the tuning fork in front of the ear and directly onto the mastoid bone behind the ear to see which is subjectively louder to the patient. If the air conduction condition is considered louder, the result is positive. If the bone conduction condition is considered louder, the result is negative and typically reflects at least a 25-dB CHL on that side.
7.2.6 Audiological Evaluation
An audiological evaluation should routinely be performed for those complaining of hearing loss, except in the case where the hearing loss can be resolved by removing cerumen from the external auditory canals. 11 Audiometric evaluation includes pure-tone audiometry, tympanometry, and speech audiometry.
7.2.7 Pure-Tone Audiometry
Pure-tone audiometry is a behavioral test in which the patient responds to pure-tone stimuli. Pure-tone audiometry allows for determination of the type of hearing loss (SNHL, CHL, or mixed), the pattern, and the severity of the hearing loss. In pure-tone audiometry, thresholds are obtained from the patient at a sequence of pure-tone frequencies. Thresholds are typically assessed at 250, 500, 1,000, 2,000, 4,000, and 8,000 Hz. A threshold is defined as the lowest intensity at which the patient is able to respond to the pure-tone stimulus at least 50% of the time. Pure-tone thresholds are obtained for both air conduction (through earphones or ear inserts) and bone conduction (through vibration of the mastoid bone). Pure-tone average (PTA) is a measure of hearing loss and is defined as the average of the 500-, 1,000-, and 2,000-Hz thresholds.
7.2.8 Speech Audiometry
Speech reception threshold (SRT) is the quietest level at which the patient can correctly recognize two-syllable words at least 50% of the time. Word recognition score (WRS) is the percentage of words correctly identified by the patient from a standard phonetically balanced word list presented about 40 dB above the PTA. WRS is routinely tested on the Central Institute for the Deaf (CID) Auditory Test W-22. WRSs are considered excellent (90–100%), good (80–90%), fair (70–80%), or poor (<70%). WRSs are expected to increase to a maximum accuracy as the intensity level that they are presented at increase from the PTA with the exception for patients with eighth cranial nerve pathology (such as a VS), where increasing the sound intensity above a certain point can lead to a decrease in WRS (also known as rollover).
7.2.9 Acoustic Immittance Testing
Acoustic immittance testing is generally composed of tympanometry and acoustic reflex testing.
Tympanometry
Tympanometry is a test by which energy is transmitted through the ear canal and the reflection of the energy back to the probe is measured. Tympanometry provides information about middle ear function by giving an estimate of admittance, which can be thought of as the compliance of the TM as well as an estimate of the ear canal volume. Low compliance (e.g., flat tympanogram) with a large estimated ear canal volume can be seen in a TM perforation. Low compliance with a normal ear canal volume can be seen in patients with middle ear effusion.
Acoustic Reflex Testing
The acoustic reflex, also known as the stapedial reflex, is the contraction of the stapedius muscle in response to high-intensity sound. The acoustic reflex is triggered bilaterally when high-intensity sound is presented to either ear. The reflex can be detected by a change in compliance of the TM. The acoustic reflex threshold is the lowest intensity level at which a change in TM compliance can be measured. In a patient with bilateral CHL, acoustic reflexes are generally absent bilaterally. In a patient with unilateral CHL, stimulation of the unaffected ear will result in an ipsilateral acoustic reflex, while stimulation of the affected ear may or may not lead to an acoustic reflex depending on the severity of the CHL. In a patient with sensory (cochlear) hearing loss greater than 80 dB, acoustic reflexes are generally absent. In a patient with sensory hearing loss less than 80 dB, acoustic reflexes are generally present at higher intensity thresholds. Acoustic reflexes will be diminished or absent in patients with facial nerve disorders proximal to the branch of the nerve to the stapedius muscle.
7.2.10 Otoacoustic Emissions
Cochlear outer hair cell movements generate sounds called otoacoustic emissions (OAEs). The presence of OAEs therefore gives objective evidence to the presence of outer hair cells in the cochlea. OAEs can be measured by placing a microphone in the ear canal. Transient-evoked otoacoustic emissions (TEOAEs) are evoked by use of a broadband click. TEOAEs are generally used for newborn hearing screening (▶Fig. 7.1). TEOAEs are present in most normal hearing ears. The presence of TEOAEs suggests hearing thresholds of at least 20-dB hearing level. Distortion product otoacoustic emissions (DPOAEs) are evoked by simultaneous application of two pure-tone frequencies. DPOAEs are able to assess higher frequencies than TEOAEs. DPOAEs are used for hearing screening for patients with concern for ototoxicity and noise-induced hearing loss.
7.2.11 Auditory Evoked Potentials
Auditory evoked potentials are neuroelectrical events that can be measured in response to auditory stimulus that reflects transmission of signal in the auditory system. Both electrocochleography (ECochG) and auditory brainstem response (ABR) are tests that rely on auditory evoked potentials.
Electrocochleography
During the first 2 to 3 milliseconds after an auditory stimulus is delivered to the ear canal, it will result in the ECOG response that can be measured with electrodes placed onto or near the TM. The summating potential (SP) and the compound action potential (AP) are measured. An elevated SP/AP ratio greater than 0.45 is generally considered abnormal and suggests increased labyrinthine pressure, although institutions may have a higher or lower limit of normal. This can be used to aid in the diagnosis and monitoring of Meniere’s disease and superior semicircular canal dehiscence syndrome.
Auditory Brainstem Response
Auditory stimulus delivered to the ear canal will result in an ABR during the first 10 milliseconds after delivery of the stimulus. The response is measured by multiple electrodes placed on the scalp. The response is characterized by five waves approximating different locations along the auditory pathway: (1) wave I corresponds to the distal portion of CN VIII; (2) wave II to the proximal portion of CN VIII; (3) wave to III the cochlear nucleus, (4) wave IV to the superior olivary complex, and (5) wave V the inferior colliculus. ABR can be used for hearing screening and pure-tone threshold estimation in all ages (including newborns) by determining the lowest threshold at which wave V of the ABR is present. ABR can also be used to monitor hearing during neurotologic procedures (e.g., middle cranial fossa VS resection).
7.2.12 Imaging
Computed Tomography Scan of the Temporal Bone
High-resolution computed tomography (HRCT) of the temporal bones without intravenous contrast provides excellent structural detail of the bony anatomy of the auditory pathway.
Magnetic Resonance Imaging
Magnetic resonance imaging (MRI) with attention to the IACs with and without intravenous contrast such as gadolinium provides detailed soft-tissue evaluation of the auditory pathway.
7.3 Cost-Effective Clinical Approach to Specific Hearing Loss Disorders
7.3.1 Presbycusis
Progressive hearing loss with age is common, affecting greater than 50% of individuals older than 60 years. 3 , 12 The majority of cases are thought to be caused by degeneration of hair cells within the cochlea due to age and cumulative noise exposure. While there is limited evidence to support the cost-effectiveness of routine audiometric hearing screen in adults older than 50 years, any older adult who complains of hearing loss should be referred for audiometric evaluation. 4 , 13 Patients who are found to have symmetric down-sloping SNHL should be referred to an otolaryngologist for evaluation (▶Fig. 7.2). Further workup including imaging is not needed in cases of gradual, symmetric down-sloping hearing loss in older adults as this is the pattern associated with age-related hearing loss. However, a sudden decline in hearing bilaterally should be investigated as this is not typical of aging. Treatment options include observation, hearing aid amplification, and cochlear implants in bilateral severe hearing loss.
Hearing Aids
For those who are suffering from mild to severe forms of hearing loss, hearing aids are often the best and most cost-effective option. A hearing aid is defined by the U.S. Food and Drug Administration (FDA) as “any wearable instrument or device designed for, offered for the purpose of, or represented as aiding persons with or compensating for, impaired hearing.” 11 , 14 Essentially, hearing aids consist of a directional microphone, amplifier, microprocessor, and speaker. Over the last approximately four decades, the efficacy of hearing aids has increased, while their size has decreased. Starting with the digitization of signal in the 1980s and 1990s, hearing aid technology has advanced greatly with recent advances in battery capability, noise reduction, signal compression, and device compatibility via Bluetooth. 12 , 15 The FDA has established requirements regulating hearing aid including technical standards, device capability, as well as the requirement of preintervention medical clearance in certain circumstances.
Unfortunately, hearing aid use in the United States can be expensive, with estimates of average cost for a pair of hearing aids in 2013 being approximately $4,700 (range: $3,300–6,000). 3 , 16 This estimate includes a package of professional consultation with a licensed audiologist, device selection, fitting, and ongoing adjustments. Approximately 3 million hearing aids are dispensed every year in the United States with the largest single provider being the Department of Veteran’s Affairs (VA) Medical System, responsible for about 20% of the total volume of hearing aids dispensed. 3 , 16 The VA’s bulk purchasing power allows it to negotiate much lower prices for each aid compared to what is sold privately. In 2014, for example, the VA reported it paid an average of $369 per device, up to $1,900 cheaper than comparable pricing for aids sold in the surrounding area. 4 , 17 This pricing is separate from the services bundle provided by VA audiologist and otolaryngologists, however, and thus gives the best “real-world” example of at-cost device pricing. Indeed, much of the criticism leveled at the cost of hearing aids involves the lack of clarity in pricing structure. An inability to separate the actual technology from the service bundle, vertical integration of hearing aid companies with vendors, and changing insurance coverage structures with insurance plans all contribute.
Hearing aids are specifically not covered by Medicare Part A or B in the original language of the statute: “Notwithstanding any other provision of this title, no payment may be made under Part A or B for any expenses incurred for items or services … where such expenses are for … hearing aids or examinations therefore.” 13 , 18 Starting at age 65, U.S. citizens covered by Medicare are eligible for an Annual Wellness Check performed by a qualified medical provider which can include screening for hearing or vestibular disorders and testing thereof. 14 , 19 Audiologists can be reimbursed for performance of testing (audiogram) as directed by a physician or other medical provider. Hearing aids must then be paid for out-of-pocket or with private insurance. An increasingly popular option is Medicare Part C (Medicare Advantage) programs, which allow for recipients to opt out of traditional Medicare coverage and instead have those funds directed toward a private insurer.
Private insurances, however, have only marginally better rates of coverage for hearing health and vary widely in total dollar amount covered often with omission of key services for fitting, support, and continued service. Employer-sponsored health plans, covered by the Employee Retirement Income Security Act of 1974, are similarly variable in their coverage of hearing services and often vary widely between states. Currently, only three states, Arkansas, New Hampshire, and Rhode Island, mandate coverage of hearing aids and hearing services for adults. 4 , 17 Some federal employees, such as members of Congress, covered by Federal Employee Health Benefits Program have coverage for hearing aids. Members of the military and their families who are covered by TRICARE have access to hearing aid coverage for qualifying hearing loss. These benefits are often accessed through the VA system.
The VA continues to be one of the largest and most important disbursers of hearing aids in the United States, and audiological services are one of the highest-demand services within the VA itself. 15 , 20 The VA covers audiological services and hearing aids if hearing loss is determined to be related to military service and is the single largest reason for service-connected coverage within the VA system. 4 , 17
A cost-effective alternative for those with mild, minimally bothersome hearing loss are personal sound amplification products (PSAPs). The FDA differentiates a PSAP from a hearing aid as a “wearable electronic product that is not intended to compensate for impaired hearing, but rather is intended for non-hearing impaired consumers to amplify sounds in certain environments.” 16 Classically, these have included devices used for bird-watching, hunting, sports watching, and others. They often consist of a simple directional microphone and an amplifier. PSAPs are not subject to FDA regulation as medical devices and thus are available essentially as over-the-counter (OTC) options for amplification. Costs of these devices generally range from approximately $25 to $300. For those with minimally bothersome hearing loss, mild hearing loss, or inability to afford more expensive options for treatment of SNHL, PSAPs may represent a viable alternative for therapy in certain situations. In 2017, the Over the Counter Hearing Aid Act was signed into law, designed to provide greater public accessibility and affordability with OTC hearing aids. The impact of this on access to amplification will surely be substantial, but difficult to quantify this early on.
Despite their high initial cost and continuing cost of maintenance, hearing aids have been shown to mitigate the economic effects of hearing loss, when controlled for demographic data. A 2010 study of the income from 48,000 households showed clear gap in income between normal hearing individuals and those with hearing loss. This gap widened with increasing severity of hearing loss. The authors also questioned whether using hearing aids would help to close the income gap. While the income differential for hearing loss households starts at $2,000 for individuals with mild loss, it increases to $31,000 for profound loss. When treated with a hearing aid, there is no income differential for people with mild to moderate hearing loss, meaning complete salary equity. After that, it increases to a loss of $1,000 a year to only $11,000 for those with profound loss, a $20,000 salary gain per year from use of hearing aids. 10 This indicates that, while hearing aids may be expensive, they may be cost-effective on both microeconomic and macroeconomic levels compared to the cost of nonintervention.
7.3.2 Asymmetric Sensorineural Hearing Loss
Individuals complaining of long-standing or gradually occurring unilateral hearing loss should undergo audiological evaluation. Asymmetrical sensorineural hearing loss (ASNHL) is often defined as (1) a 10-dB difference at three continuous frequencies, (2) 15-dB difference at two continuous frequencies, (3) 15-dB difference a 3,000 Hz, (4) 15% difference between ears in WRSs, or (5) greater than 10-dB difference between ears across the average of multiple frequencies. 21 Patients found to have ASNHL should be referred to an otolaryngologist for further evaluation (▶Fig. 7.3).
While in most cases of ASNHL an underlying etiology is never found, about 2% of patients will have an IAC and/or cerebellopontine angle (CPA) tumor and thus require thorough evaluation. The most common IAC/CPA tumor (92%) is VS (see the following text), which is benign but could cause tinnitus, imbalance, or even facial nerve weakness, hydrocephalus, or even life-threatening brainstem compression.
MRI imaging is considered standard of care to evaluate for IAC/CPA pathology in patients with ASNHL and has been shown to be cost-effective in some studies (▶Fig. 7.4). 21 , 22 The standard protocol for MRI includes pre- and postcontrast T1-weighted images with submillimeter slices through the IAC and CPA.
However, although most patients with VS will have asymmetric hearing, most patients who present with asymmetric hearing will not have VS. According to estimates, from less than 0.1 to 3% of patients with asymmetric hearing loss will have evidence of VS by MRI. 23 , 24 , 25 Therefore, the clinician must have a reasonable index of suspicion to warrant further workup of potential retrocochlear pathology.
Throughout the 1980s and 1990s, the ABR was the standard screening modality for detecting retrocochlear pathology. However, the sensitivity was relatively poor for tumors less than 1 cm. With the growing use of gadolinium-enhanced MRI in the 1990s, detection of even small tumors improved to nearly 100% and remains the gold standard method for the diagnosis of VS. Given improvements in resolution and decreased cost of MR, most neurotologists do not consider ABR a cost-effective screening tool for CPA pathology. 26
Because of the relative frequency of ASNHL, the potential morbidity of VS tumors, and relatively low yield of costly MRI, investigators have sought a most cost-effective way to radiographically evaluate these patients.
Gadolinium-enhanced MRI has been shown to be cost-effective, though high-resolution T2-weighted imaging may also be a reasonable and cost-effective alternative. 21 , 26 While many otolaryngologists order gadolinium-enhanced MRI scans for initial testing, there is evidence to support T2-weighted MR as a more cost-effective initial screen for CPA pathology. Not only does this decrease cost, but also associated risks of gadolinium (including allergy, nephrogenic systemic fibrosis, and central nervous system deposition) can be avoided.
Many authors have also suggested strategies to more effectively screen patients. 27 , 28 Over 20 years ago, Fisher and colleagues, citing cost concerns of increasing usage of MRI screening for asymmetric hearing loss, suggested stratifying patients into categories based on risk, particularly in those with unexplained ASNHL. 29 A similar study was published only a few years later by Astor et al adding unilateral low-frequency tinnitus and imbalance as potential risk factors for the presence of a tumor. 27 , 30 For the general population, Saliba et al described the “Rule of 3,000” where interaural threshold asymmetries on audiogram were found to be most sensitive at 3,000 Hz. 28 , 31 These findings were corroborated by Ahsan and colleagues. 29 , 32 In an effort to cut costs, or in situations where wait time for MRI is unacceptably high, ABR has re-emerged as a reasonable and cost-effective modality for patients with low to moderate suspicion of VS. 30 , 33
In our practice (DHC and ENA), we stratify patients into “lower risk” and “higher risk” patients. This is largely based on history, age, and audiogram results. Patients with lower clinical suspicion of retrocochlear pathology are offered a follow-up audiogram in 1 year to rule out atypical progression or an ABR. MRI is discussed, but with proper education most patients understand the likely low yield of imaging. Patients at higher risk are sent for MRI.
Ultimately, the burden of proof lies on the physician, and in a litigious society such as that found in United States, gadolinium-enhanced T1-weighted MRI remains the standard of care. However, with proper counseling as to the reasoning behind a rationale diagnostic algorithm, personal and anecdotal experience shows that the vast majority of patients will agree to alternative methods of investigation, including observation. This is especially true as patient out-of-pocket costs rise.
Treatment
Depending on the hearing status of both ears, treatment for ASNHL can include traditional hearing aid amplification, contralateral routing of signal (CROS) hearing aids, bone-anchored hearing device (BAHD), or, in some rare cases, a cochlear implant.
In situations of unilateral hearing loss, CROS aids are a popular option for treatment. CROS aids route signal from the hearing-impaired side to the hearing side to rehabilitate the users’ sound field. These systems are similar to slightly more expensive in price compared to regular hearing aids. Most CROS aids that previously relied on a hardware connection to route signal to the hearing side now use Bluetooth connection.
BAHD rehabilitation relies on bone conduction and direct stimulation of the cochlea fluid. When compared to CROS users, BAHD users showed increased ease of conversation, listening in reverberant conditions and background noise, and less aversion to noise. 20 , 34 While there are no studies directly comparing the cost efficacy of CROS devices to BAHD, their profile should be similar to that of conventional hearing aids as they are similar in price and are governed by the same regulations and patterns of disbursement.
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