Sensorineural hearing loss

Sensorineural hearing loss
File:Cochlea-crosssection.png Cross section of the cochlea.
Classification and external resources
Specialty	Otorhinolaryngology
ICD-10	H90.3-H90.5
ICD-9-CM	389.1
DiseasesDB	2874
MedlinePlus	003291
Patient UK	Sensorineural hearing loss
MeSH	D006319

Sensorineural hearing loss (SNHL) is a type of hearing loss, or deafness, in which the root cause lies in the inner ear (cochlea and associated structures), vestibulocochlear nerve (cranial nerve VIII), or central auditory processing centers of the brain. SNHL accounts for about 90% of hearing loss reported. A hallmark of such hearing loss is that it is asymmetrically distributed usually toward the high frequency region, or may have a notch at some frequency. SNHL is generally permanent and can be mild, moderate, severe, profound, or total.

Sensory hearing loss is in the great majority of cases due to poor cochlear hair cell function. The hair cells may be abnormal at birth, or damaged during the lifetime of an individual. There are both external causes of damage, like noise trauma and infection, and intrinsic abnormalities, like deafness genes. A common cause or exacerbating factor in sensory hearing loss is prolonged exposure to environmental noise, for example, being in a loud workplace without wearing protection, or having headphones set to high volumes for a long period. Exposure to a very loud noise such as a bomb blast can cause noise-induced hearing loss.

Neural hearing loss occurs because of damage to the cochlear nerve (CVIII). This damage may affect the initiation of the nerve impulse in the cochlear nerve or the transmission of the nerve impulse along the nerve. Hearing loss that results from abnormalities of the central auditory processing system in the brain is called auditory processing (or central auditory processing) hearing impairment. Since the auditory pathways cross back and forth on both sides of the brain, deafness from an auditory processing cause is unusual.

SNHL is characterized by gradual rolloff of frequency response thresholds occurring over years to decades, eventually affecting large portions of the frequency range. It may be accompanied by other conditions such as ringing in the ears(tinnitis), dizziness or lightheadedness(vertigo), or flat hearing loss(conductive hearing loss). SNHL can be inherited or congenital, age-related, or acquired (result from external causes like noise or disease). The most common kind of sensorineural hearing loss is age-related (presbycusis), followed by noise-induced hearing loss(NIHL).

Frequent symptoms of SNHL are loss of acuity in distinguishing foreground voices against noisy backgrounds, difficulty understanding on the telephone, some kinds of sounds seeming excessively loud or shrill (hyperacusis), difficulty understanding some parts of speech (fricatives and sibilants), loss of directionality of sound, esp. high frequency sounds, perception that people mumble when speaking, and difficulty understanding speech of women and children. Similar symptoms are also associated with other kinds of hearing loss; audiometry or other diagnostic tests are necessary to distinguish sensorineural hearing loss.

A diagnosis of sensorineural hearing loss is usually made via pure tone audiometry (an audiogram) in conjunction with speech audiometry administered by a medical doctor or audiologist.

There is no proven or recommended treatment or cure for SNHL; management of hearing loss is usually by hearing aid. In cases of profound or total deafness, a cochlear surgical implant may restore a functional level of hearing. SNHL is at least partially preventable by avoiding environmental noise, ototoxic chemicals and drugs, and head trauma, and treating or inoculating against certain triggering diseases and conditions like rubella and diabetes.

Signs and symptoms

Since the inner ear is not directly accessible to instruments, symptomatology is by patient report and audiometric testing. Within all the people who go to their doctor with sensorineural hearing loss, 90% report having diminished hearing, 57% report having plugged feeling in ear, and 49% report having ringing in ear (tinnitis). Itching ears, while reported in only a small percentage of cases, is a hallmark symptom of SNHL not present in other types of hearing loss. About half report vestibular (vertigo) conditions.

For a detailed exposition of symptoms useful for screening, a self-assessment questionnaire was developed by the American Academy of Otolaryngology, called the Hearing Handicap Inventory for Adults (HHIA). It is a 25-question survey of subjective symptoms.

Causes

Sensorineural hearing loss may be genetic, congenital, age-related, or acquired (i.e. disease, noise-induced, trauma, etc.).

Genetic

Hearing loss can be inherited. More than 40 genes have been identified to cause deafness. There are also 300 syndromes with related hearing loss, and each syndrome may have causative genes.

Recessive, dominant or X-linked genetic mutations can affect the structure or metabolism of the inner ear. Some may be single point mutations whereas others are due to chromosomal abnormalities. Some genetic causes give rise to a late onset hearing loss. Mitochondrial mutations can cause SNHL i.e. m.1555A>G which makes the individual sensitive to the ototoxic effects of aminoglycoside antibiotics.

Both dominant and recessive genes exist which can cause mild to profound impairment. Rarely X-linked recessive genes for hearing loss occur and these are passed from unaffected mothers onto sons who then have hearing loss. Daughters are unaffected carriers because the second X chromosome will provide the second normal copy of the gene, whereas the shorter Y chromosome does not. Dominant and recessive hearing impairment can be syndromic or nonsyndromic. Recent gene mapping has identified dozens of nonsyndromic dominant (DFNA#) and recessive (DFNB#) forms of deafness.

• The most common type of congenital hearing impairment in developed countries is DFNB1, also known as Connexin 26 deafness or GJB2-related deafness.
• The most common syndromic forms of hearing impairment include (dominant)Stickler syndrome and Waardenburg syndrome, and (recessive) Pendred syndrome and Usher syndrome.
• MT-TL1 mutations cause hearing loss, along with diabetes and other symptoms.
• Charcot–Marie–Tooth disease an inherited sensorineural disorder with delayed onset that can affect the ears as well as other organs
• Muckle-Wells syndrome, a rare inherited autoinflammatory disorder, can lead to hearing loss.
• Autoimmune disease: although probably rare, it is possible for autoimmune processes to target the cochlea specifically, without symptoms affecting other organs. Granulomatosis with polyangiitis, an autoimmune condition, may precipitate hearing loss.

Congenital

• Infections:
  • Congenital rubella syndrome, CRS, results from transplacental transmission of rubella (German measles) virus during pregnancy. CRS has been controlled by universal vaccination (MMR or MMRV vaccine).
  • Human Cytomegalovirus (HCMV) transmission to a developing fetus during pregnancy (congenital infection) is currently the most common infectious cause of congenital hearing loss. HCMV congenital infection can lead to sensorineural hearing loss that may be identified shortly after birth although many affected children have no hearing loss until later. Classically the hearing loss is progressive over the first decade and possibly later. Worldwide, HCMV congenital infection impacts between 0.5 and 2% of all live births, with sensorineural hearing loss estimated to occur in 10 to 20% of infected newborns. Thus, an estimated 7,000,000 people alive today have suffered hearing loss attributed to HCMV congenital disease. The majority of cases do not have recognisable hearing loss at birth but develop it in the first decade of life.
  • toxoplasmosis, a parasitic disease affecting 23% of the population in the U.S. that can cause sensorineural deafness to the fetus in utero.
• hypoplastic auditory nerves or abnormalities of the cochlea

Presbycusis

Main article: Presbycusis

Normal progressive age-related loss of hearing acuity or sensitivity starting as early as age 18, primarily affecting the high frequencies, and men more than women. Such losses may not become apparent until much later in life. Presbycusis is by far the dominant cause of sensorineural hearing loss. Hearing loss that accumulates with age but is caused by factors other than normal aging, such as noise-induced hearing loss, is not presbycusis, although differentiating the individual effects of multiple causes of hearing loss can be difficult. One in three persons have significant hearing loss by age 65; by age 75, one in two. Age-related hearing loss is not preventable or reversible.

Acquired

Noise

Main article: Noise-induced hearing loss

Most people living in modern society suffer from some degree of progressive sensorineural (i.e. permanent) noise-induced hearing loss (NIHL) resulting from overloading and damaging the sensory or neural apparatus of hearing in the inner ear. NIHL is typically a drop-out or notch centered at 4000 Hz. Both intensity (SPL) and duration of exposure, and repetitive exposure to unsafe levels of noise contribute to cochlear damage that results in hearing loss. The louder the noise is, the shorter the safe amount of exposure is. NIHL can be either permanent or temporary, called a threshold shift. Unsafe levels of noise can be as little as 70db (about twice as loud as normal conversation) if there is prolonged (24 hour) or continuous exposure. 125db (loud rock concert ~120db) is the pain level; sounds above this level cause instant and permanent ear damage.

Noise, along with heredity, may be a primary factor in presbycusis, or age-related hearing loss, the most common kind of hearing loss in industrial society.

Noise-related causes of hearing loss are divided into extrinsic (nosocusis) and intrinsic (sociocusis) causes: In the auditory system, the extrinsic component (nosocusis) includes hearing loss due to otologic disease, hazardous noise exposure, acoustic trauma, and ototoxic agents. The intrinsic component (sociocusis) indicates the wear-and-tear effects of exposure to the everyday sounds of normal living. People who live in nonindustrialized regions avoid both nosocusis and sociocusis and demonstrate excellent hearing into old age.

The dangers of environmental and occupational noise exposure are widely recognized. Numerous national and international organizations have established standards for safe levels of exposure to noise in industry, the environment, military, transportation, agriculture, mining and other areas. Sound intensity or sound pressure level (SPL) is measured in decibels (db). For reference,

db levels

• 45db ambient noise level around the home
• 60db quiet office,
• 60-65db normal conversation
• 70db city street noise at 25' or average TV audio,
• 80db a noisy office
• 95-104db nightclub dance floor,
• 120db close in thunder, loud rock concert
• 150-160db gunshot (handheld sidearm)

An increase of 6db represents a doubling of the SPL, or energy of the sound wave, and therefore its propensity to cause ear damage. Because our ears hear logarithmically, not linearly, it takes an increase of 10db to produced a sound that is perceived to be twice as loud. Ear damage due to noise is proportional to sound intensity (SPL), not perceived loudness, so it's misleading to rely on our subjective perception of loudness as an indication of the risk to hearing, i.e., it can significantly underestimate the danger.

While the standards differ moderately in levels of intensity and duration of exposure considered safe, some guidelines can be derived.

The safe amount of exposure is reduced by a factor of 2 for every exchange rate (3 dB for NIOSH standard or 5db for OSHA standard) increase in SPL. For example, the safe daily exposure amount at 85 dB (90db for OSHA) is 8 hours, while the safe exposure at 94 dB(A) (nightclub level) is only 1 hours. Noise trauma can also cause a reversible hearing loss, called a temporary threshold shift. This typically occurs in individuals who are exposed to gunfire or firecrackers, and hear ringing in their ears after the event (tinnitus).

• ambient environmental noise - Populations living near airports, railyards and train stations, freeways and industrial areas are exposed to levels of noise typically in the 65 to 75 dBA range. If lifestyles include significant outdoor or open window conditions, these exposures over time can degrade hearing. U.S. Dept. of Housing and Urban Development sets standards for noise impact in residential and commercial construction zones. HUD’s noise standards may be found in 24 CFR Part 51, Subpart B. Environmental noise above 65db defines a noise-impacted area.
• Personal audio electronics, such as iPods (iPods often reaching 115 decibels or higher), can produce powerful enough sound to cause significant NIHL.
• Repeated exposure to loud noise (90-95 dB or more) can cause progressive hearing loss. Sound levels at places where live or dance music is performed typically range from 95db to 104db.
• acoustic trauma - Exposure to a single event of extremely loud noise (such as explosions) can also cause temporary or permanent hearing loss. A typical source of acoustic trauma is a too-loud music concert.
• workplace noise - OSHA standards 1910.95 General Industry Occupational Noise Exposure, and 1926.52 Construction Industry Occupational Noise Exposure identify the level of 90 dB(A) for 8 hour exposure as the level necessary to protect workers from hearing loss.

Disease or disorder

• Inflammatory
  • Suppurative labyrinthitis or otitis interna (inflammation of the inner ear)
• diabetes mellitus
  Main article: Diabetes mellitus and deafness
  A recent study found that hearing loss is twice as common in people with diabetes as it is in those who don't have the disease. Also, of the 86 million adults in the U.S. who have prediabetes, the rate of hearing loss is 30 percent higher than in those with normal blood glucose. It has not been established know how diabetes is related to hearing loss. It's possible that the high blood glucose levels associated with diabetes cause damage to the small blood vessels in the inner ear, similar to the way in which diabetes can damage the eyes and the kidneys. Similar studies have shown a possible link between that hearing loss and neuropathy (nerve damage).
• tumor
  • Cerebellopontine angle tumour (junction of the pons and cerebellum) - The cerebellopontine angle is the exit site of both the facial nerve(CN7) and the vestibulocochlear nerve(CN8). Patients with these tumors often have signs and symptoms corresponding to compression of both nerves.
  • Acoustic neuroma (vestibular schwannoma) - benign neoplasm of Schwann cells afflicting the vestibulocochlear nerve
  • Meningioma - benign tumour of the pia and arachnoid maters
• Ménière's disease - causes sensorineural hearing loss in the low frequency range (125 Hz to 1000 Hz). Ménière's disease is characterized by sudden attacks of vertigo, lasting minutes to hours preceded by tinnitus, aural fullness, and fluctuating hearing loss.
• pneumococcal Meningitis may damage the cochlea - Hearing loss is one of the most common after-effects of meningitis. It has been estimated that 30% of meningitis cases result in mild to severe hearing loss. Children are most at risk: seventy percent of all bacterial meningitis occurs in young children under the age of five.
• viral
  • AIDS and ARC patients frequently experience auditory system anomalies.
  • Mumps(epidemic parotitis) may result in profound sensorineural hearing loss (90 dB or more), unilaterally (one ear) or bilaterally (both ears).
  • Measles may result in auditory nerve damage but more commonly gives a conductive hearing loss or mixed hearing loss.
  • Syphilis is commonly transmitted from pregnant women to their fetuses, and about a third of the infected children will eventually become deaf.
  • Ramsay Hunt syndrome type II (herpes zoster oticus)

Ototoxic and neurotoxic drugs and chemicals

Main article: ototoxicity

Some over-the-counter as well as prescription drugs and certain industrial chemicals are ototoxic. Exposure to these can result in temporary or permanent hearing loss.

Some medications cause irreversible damage to the ear, and are limited in their use for this reason. The most important group is the aminoglycosides (main member gentamicin). A rare mitochondrial mutation, m.1555A>G, can increase an individual's susceptibility to the ototoxic effect of aminoglycosides. Long term hydrocodone (Vicodin) abuse is known to cause rapidly progressing sensorineural hearing loss, usually without vestibular symptoms. Methotrexate, a chemotherapy agent, is also known to cause hearing loss. In most cases hearing loss is not recovered when off the drug. Paradoxically, methotrexate is also used in the treatment of autoimmune-induced inflammatory hearing loss.

Various other medications may reversibly degrade hearing. This includes loop diuretics, sildenafil (Viagra), high or sustained dosing of NSAIDs (aspirin, ibuprofen, naproxen, and various prescription drugs: celecoxib, etc.), quinine, and macrolide antibiotics (erythromycin, etc.).

Prolongued or repeated environmental or work-related exposure to ototoxic chemicals can also result in sensorineural hearing loss. Some of these chemicals are:

• butyl nitrite - chemical used recreationally known as 'poppers'
• carbon disulfide - a solvent used as a building block in many organic reactions
• styrene, an industrial chemical precursor of polystyrene, a plastic
• carbon monoxide, a poisonous gas resulting from incomplete combustion
• heavy metals: tin, lead, manganese, mercury
• hexane, an industrial solvent and one of the significant constituents of gasoline
• ethylbenzene, an industrial solvent used in the production of styrene
• toluene and xylene, highly poisonous petrochemical solvents. Toluene is a component of high-octane gasolne; xylene is used in the production of polyester fibers and resins.
• trichloroethylene, an industrial degreasing solvent
• Organophosphate pesticides

Head trauma

There can be damage either to the ear itself or to the central auditory pathways that process the information conveyed by the ears. People who sustain head injury are susceptible to hearing loss or tinnitus, either temporary or permanent. Contact sports like football (U.S. NFL), hockey and cricket have a notable incidence of hear injuries (concussions). In one survey of retired NFL players, all of whom reported one or more concussions during their playing careers, 25% had hearing loss and 50% had tinnitis.

Perinatal conditions

These are much more common in premature babies, particularly those under 1500 gms at birth. Premature birth can be associated with problems that result in sensorineural hearing loss such as anoxia or hypoxia(poor oxygen levels), jaundice, intracranial haemorrhages, meningitis. Fetal alcohol syndrome is reported to cause hearing loss in up to 64% of infants born to alcoholic mothers, from the ototoxic effect on the developing fetus, plus malnutrition during pregnancy from the excess alcohol intake.

Iodine deficiency / Hypothyroidism

Iodine deficiency and hypothyroidism are associated with hearing loss. Depending on age of onset, may be correctable by iodine or thyroid medication.

Brain stroke

Brain stroke in a region affecting auditory function such as a posterior circulation infarct has been associated with deafness.

Diagnosis

Case history

Before examination, a case history provides guidance about the context of the hearing loss.

• major concern
• pregnancy and childbirth information
• medical history
• development history
• family history

Otoscopy

Direct examination of the external canal and tympanic membrane (ear drum) with an otoscope, a medical device inserted into the ear canal that uses light to examine the condition of the external ear and tympanic membrane, and middle ear through the semi-translucent membrane.

Differential testing

Differential testing is most useful when there is unilateral hearing loss, and distinguishes conductive from sensorineural loss. These are conducted with a low frequency tuning fork, usually 512Hz, and contrast measures of air and bone conducted sound transmission.

• Weber test, in which a tuning fork is touched to the midline of the forehead, localizes to the normal ear in people with unilateral sensorineural hearing loss.
• Rinne test, which tests air conduction vs. bone conduction is positive, because both bone and air conduction are reduced equally.
• less common Bing and Schwabach variants of the Rinne test.
• absolute bone conduction (ABC) test.

Table 1. A table comparing sensorineural to conductive hearing loss

Criteria	Sensorineural hearing loss	Conductive hearing loss
Anatomical site	Inner ear, cranial nerve VIII, or central processing centers	Middle ear (ossicular chain), tympanic membrane, or external ear
Weber test	Sound localizes to normal ear in unilateral SNHL	Sound localizes to affected ear (ear with conductive loss) in unilateral cases
Rinne test	Positive Rinne; air conduction > bone conduction (both air and bone conduction are decreased equally, but the difference between them is unchanged).	Negative Rinne; bone conduction > air conduction (bone/air gap)

Other, more complex, tests of auditory function are required to distinguish the different types of hearing loss. Bone conduction thresholds can differentiate sensorineural hearing loss from conductive hearing loss. Other tests, such as oto-acoustic emissions, acoustic stapedial reflexes, speech audiometry and evoked response audiometry are needed to distinguish sensory, neural and auditory processing hearing impairments.

Tympanometry

A tympanogram is the result of a hearing test with a tympanometer. It tests the function of the middle ear and mobility of the eardrum. It can distinguish conductive hearing loss from other kinds of hearing loss including SNHL.

Audiometry

An audiogram is the result of a hearing test called pure tone audiometry, the most common type of hearing test. It charts the thresholds of hearing sensitivity at a selection of standard frequencies between 250 and 8000Hz. There is also high frequency pure tone audiometry which tests frequencies from 8000-20,000Hz. It can be used to differentiate between conductive hearing loss, sensorineural hearing loss, auditory processing hearing loss, and mixed hearing loss. Hearing loss which shows up as a relatively flat (but lowered) line on the audiogram is called a "flat loss". It means that thresholds are higher (sounds are harder to hear) at all frequencies. A flat loss is relatively uncommon and is often caused by a conductive problem rather than a sensorineural problem. Sensorineural hearing loss is characterized by a notch or notches in the audiogram, or asymmetrical audiogram with progressively greater loss with rising frequency. A relatively new refinement of pure tone audiometry is Bekesy audiometry.

There are also other kinds of audiometry designed to test hearing acuity rather that sensitivity (speech audiometry), audio processing and nerve transmission (evoked response audiometry).

Magnetic resonance imaging

To refine clinical findings and after audiometry, a contrast-enhanced MRI may reveal clinically unsuspected inflammatory, auto-immune or tumoural disease.

Pathophysiology

The great majority of human sensorineural hearing loss is caused by abnormal structure or function of the hair cells of the organ of Corti in the cochlea. There are also very unusual sensorineural hearing impairments that involve the eighth cranial nerve (the vestibulocochlear nerve) or the auditory portions of the brain. In the rarest of these sorts of hearing loss, only the auditory centers of the brain are affected (auditory processing disorder). In this situation, cortical deafness, sounds may be heard at normal thresholds, but the quality of the sound perceived is so poor that speech cannot be understood.

Cochlear dead regions in sensory hearing loss

Hearing impairment may be associated with damage to the hair cells in the cochlea. Sometimes there may be complete loss of function of inner hair cells (IHCs) over a certain region of the cochlea; this is called a "dead region". The region can be defined in terms of the range of characteristic frequencies (CFs) of the IHCs and/or neurons immediately adjacent to the dead region.

Cochlear hair cells

File:Cochlea-crosssection.png

Figure 3: Cross-section of the cochlea.

Outer hair cells (OHCs) contribute to the structure of the Organ of Corti, which is situated between the basilar membrane and the tectorial membrane within the cochlea (See Figure 3). The tunnel of corti, which runs through the Organ of Corti, divides the OHCs and the inner hair cells (IHCs). OHCs are connected to the reticular laminar and the Deiters’ cells. There are roughly twelve thousand OHCs in each human ear, and these are arranged in up to five rows. Each OHC has small tufts of hairs on their upper surface known as stereocilia, and these are also arranged into rows which are graded in height. There are approximately 140 stereocilia on each OHC.

The fundamental role of the OHCs and the IHCs is to function as sensory receptors. The main function of the IHCs is to transmit sound information via afferent neurons. They do this by transducing mechanical movements or signals into neural activity. When stimulated, the stereocilia on the IHCs move, causing a flow of electric current to pass through the hair cells. This electric current creates action potentials within the connected afferent neurons.

OHCs are different in that they actually contribute to the active mechanism of the cochlea. They do this by receiving mechanical signals or vibrations along the basilar membrane, and transducing them into electrochemical signals. The stereocilia found on OHCs are in contact with the tectorial membrane. Therefore, when the basilar membrane moves due to vibrations, the stereocilia bend. The direction in which they bend, dictates the firing rate of the auditory neurons connected to the OHCs.

The bending of the stereocilia towards the basal body of the OHC causes excitation of the hair cell. Thus, an increase in firing rate of the auditory neurons connected to the hair cell occurs. On the other hand, the bending of the stereocilia away from the basal body of the OHC causes inhibition of the hair cell. Thus, a decrease in firing rate of the auditory neurons connected to the hair cell occurs. OHCs are unique in that they are able to contract and expand (electromotility). Therefore, in response to the electrical stimulations provided by the efferent nerve supply, they can alter in length, shape and stiffness. These changes influence the response of the basilar membrane to sound. It is therefore clear that the OHCs play a major role in the active processes of the cochlea. The main function of the active mechanism is to finely tune the basilar membrane, and provide it with a high sensitivity to quiet sounds. The active mechanism is dependent on the cochlea being in good physiological condition. However, the cochlea is very susceptible to damage.

Hair cell damage

SNHL is most commonly caused by damage to the OHCs and the IHCs. There are two methods by which they might become damaged. Firstly, the entire hair cell might die. Secondly, the stereocilia might become distorted or destroyed. Damage to the cochlea can occur in several ways, for example by viral infection, exposure to ototoxic chemicals, and intense noise exposure. Damage to the OHCs results in either a less effective active mechanism, or it may not function at all. OHCs contribute to providing a high sensitivity to quiet sounds at a specific range of frequencies (approximately 2–4 kHz). Thus, damage to the OHCs results in the reduction of sensitivity of the basilar membrane to weak sounds. Amplification to these sounds is therefore required, in order for the basilar membrane to respond efficiently. IHCs are less susceptible to damage in comparison to the OHCs. However, if they become damaged, this will result in an overall loss of sensitivity.

Neural tuning curves

Frequency selectivity

Figure 4: Neural tuning curve for normal hearing.

The traveling wave along the basilar membrane peaks at different places along it, depending on whether the sound is low or high frequency. Due to the mass and stiffness of the basilar membrane, low frequency waves peak in the apex, while high frequency sounds peak in the basal end of the cochlea. Therefore, each position along the basilar membrane is finely tuned to a particular frequency. These specifically tuned frequencies are referred to as characteristic frequencies (CF).

If a sound entering the ear is displaced from the characteristic frequency, then the strength of response from the basilar membrane will progressively lessen. The fine tuning of the basilar membrane is created by the input of two separate mechanisms. The first mechanism being a linear passive mechanism, which is dependent on the mechanical structure of the basilar membrane and its surrounding structures. The second mechanism is a non-linear active mechanism, which is primarily dependent on the functioning of the OHCs, and also the general physiological condition of the cochlea itself. The base and apex of the basilar membrane differ in stiffness and width, which cause the basilar membrane to respond to varying frequencies differently along its length. The base of the basilar membrane is narrow and stiff, resulting in it responding best to high frequency sounds. The apex of the basilar membrane is wider and much less stiff in comparison to the base, causing it to respond best to low frequencies.

This selectivity to certain frequencies can be illustrated by neural tuning curves. These demonstrate the frequencies a fiber responds to, by showing threshold levels (dB SPL) of auditory nerve fibers as a function of different frequencies. This demonstrates that auditory nerve fibers respond best, and hence have better thresholds at the fiber's characteristic frequency and frequencies immediately surrounding it. The basilar membrane is said to be ‘sharply tuned’ due to the sharp ‘V’ shaped curve, with its ‘tip’ centered at the auditory fibers characteristic frequency. This shape shows how few frequencies a fiber responds to. If it were a broader ‘V’ shape, it would be responding to more frequencies (See Figure 4).

IHC vs OHC hearing loss

Figure 5: Neural tuning curve for OHC loss. Adapted from.

Figure 6: Neural tuning curve for OHC front row loss and IHC loss. Adapted from.

A normal neural tuning curve is characterised by a broadly tuned low frequency ‘tail’, with a finely tuned middle frequency ‘tip’. However, where there is partial or complete damage to the OHCs, but with unharmed IHCs, the resulting tuning curve would show the elimination of sensitivity at the quiet sounds. I.e. where the neural tuning curve would normally be most sensitive (at the ‘tip’) (See Figure 5).

Where both the OHCs and the IHCs are damaged, the resulting neural tuning curve would show the elimination of sensitivity at the ‘tip'. However, due to IHC damage, the whole tuning curve becomes raised, giving a loss of sensitivity across all frequencies (See Figure 6). It is only necessary for the first row of OHCs to be damaged for the elimination of the finely tuned ‘tip’ to occur. This supports the idea that the incidence of OHC damage and thus a loss of sensitivity to quiet sounds, occurs more than IHC loss.

Dead region audiometry

Pure tone audiometry (PTA)

When the IHCs or part of the basilar membrane are damaged or destroyed, so that they no longer function as transducers, the result is a ‘dead region’. Dead regions can be defined in terms of the characteristic frequencies of the IHC, related to the specific place along the basilar membrane where the dead region occurs. Assuming that there has been no shift in the characteristic frequencies relating to certain regions of the basilar membrane, due to the damage of OHCs. This often occurs with IHC damage. Dead regions can also be defined by the anatomical place of the non-functioning IHC (such as an “apical dead region”), or by the characteristic frequencies of the IHC adjacent to the dead region.

Dead regions affect audiometric results, but perhaps not in the way expected. For example, it may be expected that thresholds would not be obtained at the frequencies within the dead region, but would be obtained at frequencies adjacent to the dead region. Therefore, assuming normal hearing exists around the dead region, it would produce an audiogram that has a dramatically steep slope between the frequency where a threshold is obtained, and the frequency where a threshold cannot be obtained due to the dead region.

Figure 7: Response of the basilar membrane to a pure tone.

Figure 8: Response of the basilar membrane to a pure tone, when there is a dead region.

However, it appears that this is not the case. Dead regions cannot be clearly found via PTA audiograms. This may be because although the neurons innervating the dead region, cannot react to vibration at their characteristic frequency. If the basilar membrane vibration is large enough, neurons tuned to different characteristic frequencies such as those adjacent to the dead region, will be stimulated due to the spread of excitation. Therefore, a response from the patient at the test frequency will be obtained. This is referred to as “off-place listening”, and is also known as ‘off-frequency listening’. This will lead to a false threshold being found. Thus, it appears a person has better hearing than they actually do, resulting in a dead region being missed. Therefore, using PTA alone, it is impossible to identify the extent of a dead region (See Figure 7 and 8).

Consequently, how much is an audiometric threshold affected by a tone with its frequency within a dead region? This depends on the location of the dead region. Thresholds at low frequency dead regions, are more inaccurate than those at higher frequency dead regions. This has been attributed to the fact that excitation due to vibration of the basilar membrane spreads upwards from the apical regions of the basilar membrane, more than excitation spreads downwards from higher frequency basal regions of the cochlea. This pattern of the spread of excitation is similar to the ‘upward spread of masking’ phenomenon. If the tone is sufficiently loud to produce enough excitation at the normally functioning area of the cochlea, so that it is above that areas threshold. The tone will be detected, due to off-frequency listening which results in a misleading threshold.

To help to overcome the issue of PTA producing inaccurate thresholds within dead regions, masking of the area beyond the dead region that is being stimulated can be used. This means that the threshold of the responding area is sufficiently raised, so that it cannot detect the spread of excitation from the tone. This technique has led to the suggestion that a low frequency dead region may be related to a loss of 40-50 dB. However, as one of the aims of PTA is to determine whether or not there is a dead region, it may be difficult to assess which frequencies to mask without the use of other tests.

Based on research it has been suggested that a low frequency dead region may produce a relatively flat loss, or a very gradually sloping loss towards the higher frequencies. As the dead region will be less detectable due to the upward spread of excitation. Whereas, there may be a more obvious steeply sloping loss at high frequencies for a high frequency dead region. Although it is likely that the slope represents the less pronounced downward spread of excitation, rather than accurate thresholds for those frequencies with non-functioning hair cells. Mid-frequency dead regions, with a small range, appear to have less effect on the patient’s ability to hear in everyday life, and may produce a notch in the PTA thresholds. Although it is clear that PTA is not the best test to identify a dead region.

Psychoacoustic tuning curves (PTC) and threshold equalizing noise (TEN) tests

File:Psychoacoustical tuning curvesA.jpg

Figure 9: Psychoacoustical tuning curve.

Although some debate continues regarding the reliability of such tests, it has been suggestedthat psychoacoustic tuning curves (PTCs) and threshold-equalising noise (TEN) results may be useful in detecting dead regions, rather than PTA. PTCs are similar to neural tuning curves. They illustrate the level of a masker (dB SPL) tone at threshold, as a function of deviation from center frequency (Hz). They are measured by presenting a fixed low intensity pure tone while also presenting a narrow-band masker, with a varying center frequency. The masker level is varied, so that the level of masker needed to just mask the test signal is found for the masker at each center frequency. The tip of the PTC is where the masker level needed to just mask the test signal is the lowest. For normal hearing people this is when the masker center frequency is closest to the frequency of the test signal (See Figure 9).

In the case of dead regions, when the test signal lies within the boundaries of a dead region, the tip of the PTC will be shifted to the edge of the dead region, to the area that is still functioning and detecting the spread of excitation from the signal. In the case of a low frequency dead region, the tip is shifted upwards indicating a low frequency dead region starting at the tip of the curve. For a high frequency dead region, the tip is shifted downwards from the signal frequency to the functioning area below the dead region. However, the traditional method of obtaining PTCs is not practical for clinical use, and it has been argued that TENs are not accurate enough. A fast method for finding PTCs has been developed and it may provide the solution. However, more research to validate this method is required, before it can be accepted clinically.

Perceptual consequences of a dead region

Audiogram configurations are not good indicators of how a dead region will affect a person functionally, mainly due to individual differences. For example, a sloping audiogram is often present with a dead region, due to the spread of excitation. However, the individual may well be affected differently from someone with a corresponding sloped audiogram caused by partial damage to hair cells rather than a dead region. They will perceive sounds differently, yet the audiogram suggests that they have the same degree of loss. Huss and Moore investigated how hearing impaired patients perceive pure tones, and found that they perceive tones as noisy and distorted, more (on average) than a person without a hearing impairment. However, they also found that the perception of tones as being like noise, was not directly related to frequencies within the dead regions, and was therefore not an indicator of a dead region. This therefore suggests that audiograms, and their poor representation of dead regions, are inaccurate predictors of a patient’s perception of pure tone quality.

Research by Kluk and Moore has shown that dead regions may also affect the patient’s perception of frequencies beyond the dead regions. There is an enhancement in the ability to distinguish between tones that differ very slightly in frequency, in regions just beyond the dead regions compared to tones further away. An explanation for this may be that cortical re-mapping has occurred. Whereby, neurons which would normally be stimulated by the dead region, have been reassigned to respond to functioning areas near it. This leads to an over-representation of these areas, resulting in an increased perceptual sensitivity to small frequency differences in tones.

Vestibulocochlear nerve pathology

• congenital deformity of the internal auditory canal,
• neoplastic and pseudo-neoplastic lesions, with special detailed emphasis on schwannoma of the eighth cranial nerve (acoustic neuroma),
• non-neoplastic Internal Auditory Canal/CerebelloPontine Angle pathology, including vascular loops,

Prevention

Presbycucis is the leading cause of SNHL and is progressive and nonpreventable, and at this time, we do not have either somatic or gene therapy to counter heredity-related SNHL. But other causes of acquired SNHL are largely preventable, especially nosocusis type causes. This would involve avoiding environmental noise, and traumatic noise such as rock concerts and nightclubs with loud music. Use of noise attenuation measures like acoustic ear plugs is an alternative.

Treatment

Treatment modalities fall into three categories: pharmacological, surgical, and management. As SNHL is a physiologic degradation and considered permanent, there are as of this time, no approved or recommended treatments.

There have been significant advances in identification of human deafness genes and elucidation of their cellular mechanisms as well as their physiological function in mice. Nevertheless, pharmacological treatment options are very limited and clinically unproven. Such pharmaceutical treatments as are employed are palliative rather than curative, and addressed to the underlying cause if one can be identified, in order to avert progressive damage.

Profound or total hearing loss may be amenable to management by cochlear implants, which stimulate cochlear nerve endings directly. A cochlear implant is surgical implantation of a battery powered electronic medical device in the inner ear. Unlike hearing aids, which make sounds louder, cochlear implants do the work of damaged parts of the inner ear (cochlea) to provide sound signals to the brain. These consist of both internal implanted electrodes and magnets and external components. The quality of sound is different than natural hearing but may enable the recipient to better recognize speech and environmental sounds. Because of risk and expense, such surgery is reserved for cases of severe and disabling hearing impairment

Management of sensorineural hearing loss involves employing strategies to support existing hearing such as lip-reading, enhanced communication etc. and amplification using hearing aids. Hearing aids are specifically tuned to the individual hearing loss to give maximum benefit.

Research

Pharmaceuticals

• Tanakan - a brand name for an international prescription drug extract of Ginko biloba. It is classified as a vasodilator. Among its research uses is treatment of sensorineural deafness and tinnitis presumed to be of vascular origin.
• Coenzyme Q10 - a substance similar to a vitamin, with antioxidant properties. It is made in the body, but levels fall with age.
• ebselen, a synthetic drug molecule that mimics glutathione peroxidase (GPx), a critical enzyme in the inner ear that protects it from damage caused by loud sounds or noise

Stem cell and gene therapy

Hair cell regeneration using stem cell and gene therapy is years or decades away from being clinically feasible. However, studies are currently underway on the subject, with the first FDA-approved trial beginning in February 2012.

Sudden sensorineural hearing loss

Sudden sensorineural hearing loss (SSHL), commonly known as sudden deafness, occurs as an unexplained, rapid loss of hearing—usually in one ear—either at once or over several days. Nine out of ten people with SSHL lose hearing in only one ear. It should be considered a medical emergency. Delaying diagnosis and treatment may render treatment less effective or ineffective.

Experts estimate that SSHL strikes one person per 5,000 every year, typically adults in their 40s and 50s. The actual number of new cases of SSHL each year could be much higher because the condition often goes undiagnosed.

Presentation

Many people notice that they have SSHL when they wake up in the morning. Others first notice it when they try to use the deafened ear, such as when they use a phone. Still others notice a loud, alarming "pop" just before their hearing disappears. People with sudden deafness often become dizzy, have ringing in their ears (tinnitus), or both.

Diagnosis

SSHL is diagnosed via pure tone audiometry. If the test shows a loss of at least 30db in three adjacent frequencies, the hearing loss is diagnosed as SSHL. For example, a hearing loss of 30db would make conversational speech sound more like a whisper.

Causes

Only 10 to 15 percent of the cases diagnosed as SSHL have an identifiable cause. Most cases are classified as idiopathic, also called sudden idiopathic hearing loss (SIHL) and idiopathic sudden sensorineural hearing loss (ISSHL or ISSNHL) The majority of evidence points to some type of inflammation in the inner ear as the most common cause of SSNHL.

• Viral - The swelling may be due to a virus. A herpes type virus is believed to be the most common cause of sudden sensorineural hearing loss. The herpes virus lays dormant in our bodies and reactivates for an unknown reason.
• Vascular ischemia of the inner ear or Cognitive Normal VIII (CN8)
• Perilymph fistula, usually due to a rupture of the round or oval windows and the leakage of perilymph. The patient will usually also experience vertigo or imbalance. A history of trauma is usually present and changes to hearing or vertigo occur with alteration in intracranial pressure such as with straining; lifting, blowing etc.
• Autoimmune - can be due to an autoimmune illness such as systemic lupus erythematosus, granulomatosis with polyangiitis

Treatment

About half of people with SSNHL will recover some or all of their hearing spontaneously, usually within one to two weeks from onset. Eighty-five percent of those who receive treatment from an otolaryngologist (sometimes called an ENT) will recover some of their hearing.

• vitamins and antioxidants
• vasodilators
• betahistine (Betaserc), an anti-vertigo drug
• hyperbaric oxygen (HBO)
• anti-inflammatory agents, primarily oral corticosteroids such as prednisone, methylprednisone

See also

• Otosclerosis, a sometimes associated or predecessor conductive hearing loss condition of the middle ear
• Conductive hearing loss, hearing loss caused primarily by conditions in the middle ear
• Hearing loss
• Cortical deafness, another kind of nerve deafness
• Tinnitis, ringing in the ears, a common accompaniment of SNHL
• Inner ear, the innermost portion of the ear containing the sensorineural apparatus of hearing