Chapter 52. Sensorineural Hearing Loss

• May affect patients of all ages
• For patients who have unilateral hearing loss
• Weber tuning fork test lateralizes to the unaffected side
• Rinne tuning fork test demonstrates air conduction greater than bone conduction
• Pure-tone thresholds result in equally diminished air and bone conduction
• Speech discrimination testing less than 90% correct.

Hearing loss is extremely common and has a wide spectrum ranging from a nearly undetectable degree of disability to a profound loss of ability to function in society. Nearly 10% of the adult population has some hearing loss. Often, this impairment presents early in life. One to three of every 1000 newborn in the United States is completely deaf, and more than 3 million children have hearing loss. However, hearing loss can present at any age. Between 30% and 35% of individuals over the age of 65 have a hearing loss sufficient to require a hearing aid. Forty percent of people over the age of 75 have hearing loss.

Hearing loss can result from disorders of the auricle, external auditory canal, middle ear, inner ear, or central auditory pathways. In general, lesions in the auricle, external auditory canal, or middle ear cause conductive hearing loss. The focus of this chapter is sensorineural hearing loss that tends to result from lesions in the inner ear or eighth nerve. See Table 52–1 for a list of the common causes of hearing loss.

Sensorineural hearing loss may result from damage to the hair cells caused by intense noise, viral infections, fractures of the temporal bone, meningitis, cochlear otosclerosis, Meniere disease, and aging. The following drugs can also produce sensorineural hearing loss: ototoxic drugs (eg, salicylates, quinine, and the synthetic analogs of quinine), aminoglycoside antibiotics, loop diuretics (eg, furosemide and ethacrynic acid), and cancer chemotherapeutic agents (eg, cisplatin).

Age-Related Hearing Loss (Presbycusis)

Presbycusis, age-associated hearing loss, is the most common cause of hearing loss in adults. Initially, it is characterized by symmetric, high-frequency hearing loss that eventually progresses to involve all frequencies. More important, the hearing loss is associated with a significant loss in clarity.

Congenital Hearing Loss

Congenital malformations of the inner ear cause hearing loss in some adults. Genetic predisposition alone or in concert with environmental influences may also be responsible.

Neural Hearing Loss

Neural hearing loss is due mainly to cerebellopontine angle tumors such as vestibular schwannomas (acoustic neuromas) or meningiomas; it may also result from any neoplastic, vascular, demyelinating (eg, multiple sclerosis), infectious, or degenerative disease, or trauma affecting the central auditory pathways.

HIV-Related Hearing Loss

Human immunodeficiency virus (HIV) infection leads to both peripheral and central auditory system pathology and is associated with sensorineural hearing impairment.

Mixed Hearing Loss

A person can have both conductive and sensory hearing loss, which is termed mixed hearing loss. Mixed hearing losses are due to pathology that can affect the middle and inner ear simultaneously; causes include otosclerosis involving the ossicles and the cochlea, transverse and longitudinal temporal bone fractures, head trauma, chronic otitis media, cholesteatoma, and middle ear tumors. Some inner ear malformations can also be associated with mixed hearing loss. These include large vestibular aqueduct, lateral semicircular canal dysplasia, superior canal dehiscence, and a bulbous lateral end of the internal auditory canal (IAC); the latter is associated with the absence of the bony partition between the basal turn of the cochlea and the IAC (as seen in the stapes gusher syndrome).

Nongenetic Causes

The predominant etiology of hearing impairment in children has evolved with advances in medical knowledge and therapeutics. Historically, infectious disorders such as otitis media, maternal rubella infections, cytomegalovirus (CMV), and bacterial meningitis as well as environmental factors such as intrauterine teratogenic exposure or ototoxic insult were the dominant causes of congenital and acquired hearing losses. The introduction of antibiotics and vaccines, along with improved knowledge and enhanced awareness about teratogens, has led to a decline in hearing loss resulting from infections and environmental agents.

Genetic Causes

Currently, more than half of childhood hearing impairment is thought to be hereditary; hereditary hearing impairment (HHI) can also manifest later in life. HHI may be classified as either nonsyndromic hearing loss, in which hearing loss is the only clinical abnormality, or syndromic hearing loss, in which hearing loss is associated with anomalies in other organ systems.

Hearing occurs by air conduction and bone conduction. In air conduction, sound waves reach the ear by propagating in the air, entering the external auditory canal, and setting the tympanic membrane in motion; the movement of the tympanic membrane, in turn, moves the malleus, incus, and stapes of the middle ear. The structures of the middle ear serve as an impedance-matching mechanism, improving the efficiency of energy transfer from the air to the fluid-filled inner ear. Hearing by bone conduction occurs when the sound source, in contact with the head, vibrates the bones of the skull; this vibration produces a traveling wave in the basilar membrane of the cochlea.

Cochlear neurons send fibers bilaterally to a network of auditory nuclei in the midbrain, and impulses are transmitted through the medial geniculate thalamic nuclei to the auditory cortex in the superior temporal gyri. At low frequencies, individual auditory nerve fibers can respond more or less synchronously with the stimulating tone. At higher frequencies, phase locking occurs so that neurons alternate in response to particular phases of the sound wave cycle. Three things encode the intensity of sound: (1) the amount of neural activity in individual neurons, (2) the number of neurons that are active, and (3) the specific neurons that are activated.

Nearly two thirds of hereditary hearing impairments are nonsyndromic and the remaining one third is syndromic. Between 70% and 80% of nonsyndromic HHI is inherited in an autosomal recessive manner; another 15–20% is autosomal dominant. Less than 5% is X-linked or maternally inherited via the mitochondria.

Extensive progress has been made in the identification of genes responsible for syndromic and nonsyndromic HHI. Over 110 loci harboring genes for nonsyndromic HHI have been mapped with an equal number of dominant and recessive modes of inheritance; of these, 51 different genes have been cloned. In general, the hearing loss associated with dominant genes has its onset in adolescence or adulthood and varies in severity (mimicking presbyacusis), whereas the hearing loss associated with recessive inheritance is congenital and profound. Among the genes associated with human deafness, GJB2 encoding for connexin 26 is significant for being associated with nearly 20% childhood deafness. Furthermore, two frame-shift mutations, 35delG and 167delT, account for more than 50% of the cases making population screening feasible. The 167delT mutation is primarily prevalent in the Ashkenazi Jews where it is predicted that 1:1765 individuals will be homozygous and affected. The hearing loss associated with GJB2 mutations can be variable but is generally severe to profound at birth. In addition, the hearing loss can be variable among the members of the same family, suggesting that other genes likely influence the auditory phenotype.

The contribution of genetics to presbycusis or age-associated hearing loss is also better understood. Several of the nonsyndromic genes are associated with hearing loss that progresses with age. Recently, genome-wide association studies demonstrated that a common allele of GRM7 (gene encoding metabotropic glutamate receptor type 7) contributes to an individual's risk of developing age-related hearing loss. Therefore, it is likely that presbycusis has both environmental and genetic components. Presbycusis is characterized by a loss of discrimination for phonemes, recruitment (abnormal growth of loudness), and particular difficulty in understanding speech in noisy environments. Between 30% and 35% of people over 65 years of age have a hearing loss that is sufficiently great to require a hearing aid.

More than 200 syndromes are associated with hearing loss. Common syndromic forms of hearing loss, among others, include the following: (1) Usher syndrome (retinitis pigmentosa and hearing loss), (2) Waardenburg syndrome (pigmentary abnormality and hearing loss), (3) Pendred syndrome (thyroid organification defect and hearing loss), (4) Alport syndrome (renal disease and hearing loss), and (5) Jervell and Lange–Nielsen syndromes (prolonged QT interval and hearing loss). As a direct result of the rapid advances in the fields of molecular biology and molecular genetics, the responsible genes for the aforementioned syndromes have all been identified. In addition, rapid progress in understanding the basis of these and related disorders has revealed a number of complexities. For example, identification of myosin 7A as the responsible gene for both syndromic and nonsyndromic deafness has led to the abandonment of the “one gene, one disease” dogma. Also, a single gene may cause syndromic or nonsyndromic forms of deafness or may be associated with autosomal dominant or autosomal recessive mode of inheritance.

Vaccination

The vaccination of infants against Haemophilus influenzae type B meningitis prevents a major cause of acquired deafness, as have immunizations for measles, mumps, and rubella. A vaccine against Streptococcus pneumoniae, the most common organism associated with otitis media, is also available and is having a positive impact on the reduction in the incidence of ear infections.

Noise Avoidance

Ten million Americans have noise-induced hearing loss and 20 million are exposed to hazardous noise in their employment. Noise-induced hearing loss can be prevented by avoiding exposure to loud noise or by the regular use of earplugs or fluid-filled muffs to attenuate intense sound. Noise-induced hearing loss results from recreational as well as occupational activities and often begins in adolescence.

High-Risk Activities

High-risk activities for noise-induced hearing loss include wood- and metalworking with electrical equipment as well as target practice and hunting with small firearms. All internal-combustion and electric engines, including snowblowers and leaf blowers, snowmobiles, outboard motors, and chain saws, require that the user wear hearing protectors.

Education

Almost all noise-induced hearing loss is preventable through education, which should begin before adolescence. Industrial programs of hearing conservation are required when the exposure over an 8-hour period averages 85 dB on the A scale. Workers in such noisy environments can be protected with preemployment audiologic assessment, the mandatory use of hearing protectors, and annual audiologic assessments.

Evaluation Goals

In a patient with auditory complaints, the goals in the evaluation are to determine: (1) the nature of the hearing impairment (conductive or sensorineural); (2) the severity of the impairment (mild, moderate, severe, profound); (3) the anatomy of the impairment (external ear, middle ear, inner ear, or central auditory pathway pathology); and (4) the etiology.

Symptoms and Signs

Initially, the history and the physical examination are critical in identifying the underlying pathology leading to the auditory deficit. The history should elicit hearing loss characteristics, including the duration of deafness, the nature of the onset (sudden or insidious), the rate of progression (rapid or slow), and the involvement of the ear (unilateral or bilateral). In addition, the presence or absence of the following conditions should also be ascertained: tinnitus, vertigo, imbalance, aural fullness, hyperacusis, otorrhea, headache, facial nerve dysfunction, and head and neck paresthesia. Information regarding head trauma, ototoxic exposure, occupational or recreational noise exposure, and a family history of hearing impairment may also be critical in the differential diagnosis.

Sudden Onset

A sudden onset of unilateral hearing loss, with or without tinnitus, may represent an inner ear viral infection or a vascular accident. Patients with unilateral hearing loss (sensory or conductive) usually complain of reduced hearing, poor sound localization, and difficulty hearing clearly with background noise.

Gradual Progression

Gradual progression in a hearing deficit is common with otosclerosis, noise-induced hearing loss, vestibular schwannoma, or Meniere disease. People with small vestibular schwannomas typically present with any or all of the following conditions: asymmetric hearing impairment, tinnitus, and imbalance (although rarely vertigo). Cranial neuropathy, especially of the trigeminal or facial nerve, may accompany larger tumors. In addition to hearing loss, Meniere disease or endolymphatic hydrops may be associated with episodic vertigo, tinnitus, and aural fullness. Hearing loss with otorrhea is most likely due to chronic otitis media or cholesteatoma.

Family History

In families with multiple affected members across multiple generations, the family history may be crucial in delineating the genetic basis of hearing impairment. The history may also help identify environmental risk factors that lead to hearing impairment within a family. Sensitivity to aminoglycoside maternally transmitted through a mitochondrial mutation can be discerned through a careful family history. Susceptibility to noise-induced hearing loss or age-related hearing loss (presbycusis) may also be genetically determined.

Physical Examination

Examination of the Ear

The physical examination should evaluate the auricle, external ear canal, and tympanic membrane. In examining the eardrum, the topography of the tympanic membrane is more critical than the presence or absence of the often-cited light reflex. The pars tensa (the lower two thirds of the eardrum) and the pars flaccida (the short process of the malleus) should be examined for retraction pockets that may be evidence of chronic eustachian tube dysfunction or cholesteatomas. Insufflation in the ear canal is necessary to assess tympanic membrane mobility and compliance.

Examination of Other Structures

A careful inspection of the nose, nasopharynx, and upper respiratory tract is indicated. Unilateral serous effusion in the adult should prompt a fiberoptic examination of the nasopharynx to exclude neoplasms. Cranial nerves should be carefully evaluated with special attention to trigeminal and facial nerve function as the dysfunction of these two nerves is most commonly associated with tumors involving a cerebellopontine angle.

Evaluation with a Tuning Fork

Evaluating hearing with a tuning fork can be a useful clinical screening tool to differentiate between conductive and sensorineural hearing loss. By comparing the threshold of hearing by air conduction with that elicited by bone conduction with a 256- or 512-Hz tuning fork, one can infer the site of the lesion responsible for hearing loss. The Rinne and Weber tuning fork tests are used widely both to differentiate conductive from sensorineural hearing losses and to confirm the audiologic evaluation results.

Rinne Tuning Fork Test

The Rinne tuning fork test is sensitive in detecting conductive hearing losses. A Rinne test compares the ability to hear by air conduction with the ability to hear by bone conduction. The tines of a vibrating tuning fork are held near the opening of the external auditory canal, and then the stem is placed on the mastoid process; for direct contact, it may be placed on either teeth or dentures. The patient is asked to indicate whether the tone is louder by air conduction or bone conduction. Normally and in the presence of sensorineural hearing loss, a tone is heard louder by air conduction than by bone conduction. However, with a 30-dB or greater conductive hearing loss, the bone-conduction stimulus is perceived as louder than the air-conduction stimulus.

Weber Tuning Fork Test

The Weber tuning fork test may be performed with a 256- or 512-Hz fork. The stem of a vibrating tuning fork is placed on the head in the midline, and the patient is asked whether the tone is heard in both ears or in one ear better than in the other. With a unilateral conductive hearing loss, the tone is perceived in the affected ear. With a unilateral sensorineural hearing loss, the tone is perceived in the unaffected ear. As a general rule, a 5-dB difference in hearing between the two ears is required for lateralization.

The combined information from the Weber and Rinne tests permits a tentative conclusion as to whether a conductive or sensorineural hearing loss is present. However, these tests are associated with significant false-positive and -negative responses and therefore should be used only as screening tools and not as a definitive evaluation of auditory function.

Audiologic Assessment

The minimum audiologic assessment for hearing loss should include the following measurements: (1) pure-tone air-conduction and bone-conduction thresholds, (2) speech reception threshold, (3) discrimination score, (4) tympanometry, (5) acoustic reflexes, and (6) acoustic-reflex decay. This test battery provides a comprehensive screening evaluation of the whole auditory system. It allows the clinician to determine whether further differentiation of a sensory (cochlear) from a neural (retrocochlear) hearing loss is indicated. Refer to Chapter 45, Audiologic Testing, for additional details on audiologic assessment.

Imaging Studies

Appropriate radiologic studies may be needed to evaluate both the temporal bone and the auditory pathway. The radiologic evaluation of the ear is largely determined by what structures are being evaluated: the bony anatomy of the external, middle, and inner ear; or the auditory nerve and brain. Both computed tomography (CT) and magnetic resonance imaging (MRI) are capable of identifying inner ear malformations; they are equally able to determine the cochlear patency in the preoperative evaluation of patients for cochlear implantation.

CT Scans

Axial and coronal CT of the temporal bone with fine 0.6-mm cuts is ideal for determining the caliber of the external auditory canal, the integrity of the ossicular chain, and the presence or absence of middle ear or mastoid disease, and for detecting inner ear malformations. To reliably identify inner ear malformations, measurement of the cochlear height, lateral semicircular canal bony island width, and the vestibular aqueduct should be routinely performed on all temporal bone studies. CT scanning is also ideal for the detection of bone erosion often seen in the presence of chronic otitis media and cholesteatoma.

MRI

MRI is superior for imaging retrocochlear pathology such as vestibular schwannomas, meningiomas, other lesions of the cerebellopontine angle that may present with hearing loss, demyelinating lesions of the central nervous system, and brain tumors.

Most patients with conductive hearing losses should have axial and direct coronal CT scans of the temporal bones to evaluate the external and middle ear. Patients with unilateral or asymmetric sensorineural hearing losses should have an MRI of the head with gadolinium enhancement to exclude tumors of the cerebellopontine angle. In the presence of vestibular symptoms, patients may require electronystagmography and caloric testing.

Synthesis of the findings on clinical history, otologic and physical examination, and audiologic testing is usually sufficient to establish both the nature and the probable cause of a hearing impairment (Table 52-1).

Creating a Favorable Environment for Hearing

A variety of simple interventions can significantly enhance the ability to understand speech in people who are hard of hearing. A critical first step is to eliminate or reduce unnecessary noise (eg, radio or television) to enhance the signal-to-noise ratio. Speech comprehension is aided by lip reading; therefore, the impaired listener should be seated so that the face of the speaker can be seen at all times. Speaking directly into the ear is occasionally helpful; however, more is usually lost than gained in communication when the speaker's face cannot be seen. In addition, the lighting of the speaker's face should be considered. A person who is hard of hearing should sit with his or her back to the window so that the light is on the speaker's face. Speech should be slow enough to make each word distinct, but overly slow speech is distracting and loses contextual and speech-reading benefits. Although speech should be in a loud, clear voice, in sensorineural hearing losses in general and in elderly hearing-impaired individuals in particular, recruitment (the ability to hear loud sounds normally loud) may be difficult. Above all, optimal communication cannot take place unless both parties give it their full and undivided attention.

Amplification

Patients with mild, moderate, and severe sensorineural hearing losses are rehabilitated regularly with hearing aids that vary in configuration and strength. Hearing aids have been improved to provide greater fidelity and also have been miniaturized. A new hearing aid, the Lyric, has been introduced that is placed deep in the ear canal and is completely hidden.

In general, the more severe the hearing impairment, the larger the hearing aid required for auditory rehabilitation. Digital hearing aids lend themselves to programming for the individual; in addition, multiple and directional microphones at the ear level help some individuals with the difficulty of using a hearing aid in noisy surroundings. Since all hearing aids amplify noise as well as speech, the only absolute solution to the problem is to place the microphone closer to the speaker than to the noise source. This arrangement is not possible with a self-contained, cosmetically acceptable device; it is cumbersome and requires a user-friendly environment. For unilateral deafness, implantation of a bone anchored hearing aid (BAHA) may be considered. Like the contralateral routing of signal hearing aid, the BAHA serves to transmit the sound to the better hearing ear.

In many situations, including at lectures and at the theater, hearing-impaired persons benefit from assistive devices that are based on the principle of having the speaker closer to the microphone than to any source of noise. Assistive devices include infrared and FM transmission; they also include an electromagnetic loop placed around the room for transmission to the individual's hearing aid. Hearing aids with telecoils can also be used with properly equipped telephones in the same way.

Cochlear Implants

In the event that a hearing aid provides inadequate rehabilitation, cochlear implants are appropriate (see Chapter 69). Cochlear implants are neural prostheses that convert sound energy to electrical signals and can be used to stimulate the auditory division of the eighth nerve directly. In most cases of profound hearing impairment, the auditory hair cells are lost, but the spiral ganglion cells of the auditory division of the eighth nerve are preserved. Cochlear implants are a specialized hearing prosthesis for the rehabilitation of profound deafness that convert mechanical sound energy into electrical signals that are delivered to the neurons of the cochlear nerve. The basic operation of the implant is as follows: a microphone is used to pick up acoustic information that is sent to an external speech processor (located on the body or at ear level). This processor converts the mechanical acoustic wave into an electric signal that is transmitted via the surgically implanted electrode array in the cochlea to the auditory nerve. A patient with cochlear implants experiences sound that helps with speech reading, allows open-set word recognition, and helps in modulating the individual's own voice.

The criteria for implantation undergo constant revisions; an adult candidate has severe to profound hearing loss with a score of 60% or less on Hearing-in-Noise Test (HINT) under the best-aided conditions. Children 1 year or older with congenital and acquired profound hearing impairment who have failed a hearing aid trial are also appropriate candidates for cochlear implantation; many are being implanted as early as 6 to 9 months. Usually within 3 months of implantation, adult patients can understand speech without visual cues. With the current generation of multichannel cochlear implants, almost 75% of the patients with these implants are able to converse on the telephone. Bilateral cochlear implants are increasingly common and serve to enhance sound localization and improve understanding of speech in background noise.

Brainstem Auditory Implant

For individuals who have had both eighth nerves destroyed by trauma or bilateral vestibular schwannomas (eg, patients with neurofibromatosis II), severely malformed ears, or patients with congenital absence of the eighth nerve, a brainstem auditory implant placed near the cochlear nucleus may provide auditory rehabilitation. With additional advances in brainstem auditory implant technology, patients may eventually obtain benefits similar to individuals who have cochlear implants.

Therapy for Tinnitus

Tinnitus, a perception of abnormal sounds such as ringing or roaring noises, can often accompany hearing loss. The treatment of tinnitus is particularly problematic; the treatment does not cure it, and therapy is usually directed toward minimizing the appreciation of tinnitus.

The relief of tinnitus may be obtained by masking it with background sound. Hearing aids also are helpful in tinnitus suppression, as are tinnitus maskers, devices that present a sound to the affected ear that is more pleasant to listen to than the tinnitus. The use of a tinnitus masker is often followed by several hours of inhibition of the tinnitus. Antidepressants are also beneficial in helping patients deal with tinnitus although the exact mechanism by which they work is unknown.

Temporary Hearing Loss

Hearing loss due to an incident of noise exposure generally produces a temporary loss that recovers within 24 to 48 hours. However, if the noise is of high enough intensity or is repeated often enough, permanent hearing loss results.

Irreversible Hearing Loss

Sensorineural hearing loss is a condition that is generally irreversible. Presbycusis is a type of sensorineural hearing loss that is both progressive (1–2 dB/year) and irreversible.

Acoustic trauma consists of a single exposure to a hazardous level of noise resulting in a permanent loss without an intervening temporary loss. Given the poor prognosis for most causes of sensorineural hearing loss, the primary goals in management are the prevention of further losses and functional improvement with amplification and auditory rehabilitation. The future holds the promise of molecular therapy to arrest or reverse hearing loss.

We would like to acknowledge Derek D. Mafong, MD, for his contribution to this chapter in the previous editions of CDT.