Sensorineural hearing loss

Noise

Most people living in modern society have 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 70 dB (about twice as loud as normal conversation) if there is prolonged (24-hour) or continuous exposure. 125 dB (a loud rock concert is ~120 dB) is the pain level; sounds above this level cause instant and permanent ear damage.

Noise and ageing are the primary causes of presbycusis, or age-related hearing loss, the most common kind of hearing loss in industrial society.^[10] 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.^{[Note 1]} Sound intensity or sound pressure level (SPL) is measured in decibels (dB). For reference:

An increase of 6 dB represents a doubling of the SPL, or energy of the sound wave, and therefore its propensity to cause ear damage. Because human ears hear logarithmically, not linearly, it takes an increase of 10 dB to produce a sound that is perceived to be twice as loud. Ear damage due to noise is proportional to sound intensity, not perceived loudness, so it's misleading to rely on 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.^{[Note 2]}

The safe amount of exposure is reduced by a factor of 2 for every exchange rate (3 dB for NIOSH standard or 5 dB for OSHA standard) increase in SPL. For example, the safe daily exposure amount at 85 dB (90 dB for OSHA) is 8 hours, while the safe exposure at 94 dB(A) (nightclub level) is only 1 hour. 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 65 dB defines a noise-impacted area.
• Personal audio electronics: Personal audio equipment such as iPods (iPods often reach 115 decibels or higher), can produce powerful enough sound to cause significant NIHL.^[11]
• 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: The 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 how diabetes is related to hearing loss. It is 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 affecting the vestibulocochlear nerve
    • Meningioma – benign tumour of the pia and arachnoid mater
• 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. It is relatively rare and commonly over diagnosed.
• Bacterial meningitis e.g. pneumococcal, meningococcal, haemophilus influenzae may damage the cochlea – Hearing loss is one of the most common after-effects of bacterial meningitis. It has been estimated that 30% of bacterial meningitis cases result in mild to profound 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 mixed (sensorineural plus conductive) hearing loss, and can be bilaterally.
  • Ramsay Hunt syndrome type II (herpes zoster oticus)
• Bacterial
  • Syphilis is commonly transmitted from pregnant women to their fetuses, and about a third of the infected children will eventually become deaf.

Ototoxic and neurotoxic drugs and chemicals

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 does not recover when the drug is stopped. 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.). Cytotoxic agents such as carboplatinum, used to treat malignancies can give rise to a dose dependent SNHL, as can drugs such as desferrioxamine, used for haematological disorders such as thalassaemia; patients prescribed these drugs need to have hearing monitored.

Prolonged 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 gasoline; 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 head 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 tinnitus.

Perinatal conditions

These are much more common in premature babies, particularly those under 1500 g 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 endemic hypothyroidism are associated with hearing loss.^[12] If a pregnant mother has insufficient iodine intake during pregnancy it affects the development of the inner ear in the foetus leading to sensorineural deafness. This occurs in certain areas of the world, such as the Himalayas, where iodine is deficient in the soil and thus the diet. In these areas there is a high incidence of endemic goitre. This cause of deafness is prevented by adding iodine to salt.

Brain stroke

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

Cochlear hair cells

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', or cilia, 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.^[13]

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.^[14]

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.^[13][14] It is therefore clear that the OHCs play a major role in the active processes of the cochlea.^[13] 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.^[14]

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.^[13] 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).^[14]

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.^[14]

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).^[13]

IHC vs OHC hearing loss

Figure 5: Neural tuning curve for OHC loss. Adapted from.^[14]

Figure 6: Neural tuning curve for OHC front row loss and IHC loss. Adapted from.^[14]

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).^[14]

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.^[14]

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.^[15]

Pure tone audiometry (PTA)

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.^[15]

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).^[15]

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.^[15]

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.^[16][17] 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.^[15]

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.^[15] Although it is clear that PTA is not the best test to identify a dead region.^[18]

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

Although some debate continues regarding the reliability of such tests,^[19] it has been suggested that 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).^[13] 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).^[18]

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.^[18] However, the traditional method of obtaining PTCs is not practical for clinical use, and it has been argued that TENs are not accurate enough.^[18][19] 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.^[14] 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.^[20]

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.^[21]