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Hearing impairment

The sense of hearing can be impaired in different ways. Damage to hair cells more commonly affects outer hair cells, more so than inner hair cells. Typical damage to the hair cells includes stereocilia fracture, the actin filaments can become depolymerised, the tiplinks can break, the stereocilia may became detached and the cuticular plate may eject from the hair cell. Over the long term, the hair cells can degenerate completely.

Loss of outer hair cells reduces the ability to detect low level sounds, since the active amplification of motion of the tectorial membrane is reduced. The dynamic range of the auditory system will be reduced and the frequency selectivity impaired. The basally located outer hair cells are more sensitive than apically located, resulting in high frequency hearing loss dominating. If the inner hair cells are damaged the sensory function will be reduce.



Normal, uninjured ear.
An electron microscope shows the hair cells with their fine  
stereocilia, which are put into motion by sound waves



Noise damaged ear.
The tiny hairs have been repeatedly exposed to loud noise and eventually broken. Permanent hearing loss has occurred.



Since the ability to hear is crucial for spoken language, a hearing loss might affect speech communication considerably. An extensive loss of outer hair cells will increase the threshold for detection and perception of speech sounds, but will also reduce the audible dynamic range and the frequency resolution, and thereby the speech intelligibility. One major disability associated with outer hair cell loss is reduced ability to understand speech in masking background noise. Hearing impairment may also appear as tinnitus, the perception of a sound (noise, tone) without an acoustic signal causing it. Tinnitus is considered to be caused by different factors. However, tinnitus is often associated with hearing loss.

NOISE-INDUCED HEARING LOSS
Exposure to noise is a major factor causing hearing loss. Long term exposure to extensive noise, damages the hair cells which results in a cochlear hearing loss. Extremely high sound pressure levels could rupture the tympanic membrane and instantly and permanently damage the hair cells in the cochlea. Usually, noise-induced hearing loss initially affects the outer hair cells. The propagating loss of outer hair cells starts in the part of the basilar membrane, corresponding to the frequency region around 4 kHz. An extensive noise-induced hearing loss affects larger areas of the basal outer hair cells and also inner hair cells. Loss of sensory hair cells may also result in secondary neural degeneration.

The disability from noise-induced hearing loss is similar to other sensorineural hearing loss. However, the shift in the hearing threshold in the range 3-6 kHz is characteristic. The degree of susceptibility to noise is individual, as for age-related hearing loss.

PRESBYACUSIS
Presbyacusis is a hearing loss due to a normal age-related degeneration of the auditory system. The major cause of presbyacusis is reduced cochlear function, impairment or loss of hair cells. However, effects on the auditory nerve and the central auditory pathways are also common. Outer hair cells are affected initially, even if pronounced presbyacusis also includes reduced function or loss of inner hair cells.

The initial effect from presbyacusis is high frequency hearing loss, while more pronounced presbyacusis also affects the mid-frequency range. Increased hearing threshold results in reduced dynamic range, but also in reduced frequency selectivity due to outer hair cell loss. The degree of age-related hearing loss is individual.

In a general population, hearing loss due to other factors than age would be expected. Hereditary factors might cause congenital or early progressive hearing loss or interaction with age-related deterioration. Some diseases, such as Menière’s disease, could also affect the auditory system. Other factors inducing hearing loss are exposure to hazardous noise, ototoxic drugs or ototoxic solvents.

© Magnus Johansson, Peltor AB


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 THE AUDITORY SYSTEM

 » How does sound occur
 » The auditory system 
 » The outer ear 
 » The middle ear
 » The inner ear
 » Hearing
 » Exposure to noise 
 » Hearing impairment
 » The concept of equal energy
 » How to measure noise levels
 » Choose right hearing protection
 » Do not over attenuate
 » Noise regulations