Sensorineural_hearing_loss

Sensorineural hearing loss

Sensorineural hearing loss is a type of hearing loss in which the root cause lies in the vestibulocochlear nerve (Cranial nerve VIII), the inner ear, or central processing centers of the brain.

The Weber test, in which a tuning fork is touched to the head, localizes to the normal ear in people with this condition. The Rinne test, which tests air conduction vs. bone conduction is positive (normal), though both bone and air conduction are reduced equally.

Sensorineural hearing loss can be mild, moderate, or severe, including total deafness.

The great majority of human sensorineural hearing loss is caused by abnormalities in the hair cells of the organ of Corti in the cochlea. There are also very unusual sensorineural hearing impairments that involve the VIIIth 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. In this situation, central hearing loss, sounds may be heard at normal thresholds, but the quality of the sound perceived is so poor that speech can not be understood.

Most sensory hearing loss is due to poor 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.

Sensory hearing loss (also called sensorineural hearing loss) may also result from abnormalities of the VIII cranial nerve.

Sensory hearing loss that results from abnormalities of the central auditory system in the brain is called Central Hearing Impairment. Since the auditory pathways cross back and forth on both sides of the brain, deafness from a central cause is unusual.

This type of hearing loss can also be caused by prolonged exposure to very loud noise. For example, wearing headphones on full blast for a few hours.

Differential diagnosis

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 Sound localizes to affected ear (ear with conductive loss)
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)

Sensorineural hearing loss may be congenital or acquired.

Congenital

Acquired

Long term exposure to environmental noise

Populations of people living near airports or freeways 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. The U.S. EPA and various states have set noise standards to protect people from these adverse health risks. The EPA has identified the level of 70 db(A) for 24 hour exposure as the level necessary to protect the public from hearing loss (EPA, 1974).

  • Noise-Induced Hearing Loss (NIHL) typically is centered at 4000 Hz.
  • The louder the noise is, the shorter the safe amount of exposure is. Normally, the safe amount of exposure is reduced by a factor 2 for every additional 3 dB. For example, the safe daily exposure amount at 85 dB is 8 hours, while the safe exposure at 91 dB(A) is only 2 hours (National Institute for Occupational Safety and Health, 1998). Sometimes, a factor 2 per 5 dB is used.
  • Personal electronic audio devices, such as iPods (iPods often reaching 115 decibels or higher), can produce powerful enough sound to cause significant Noise-Induced Hearing Loss, given that lesser intensities of even 70 dB can also cause hearing loss.

Genetic

Hearing loss can be inherited. Both dominant and recessive genes exist which can cause mild to profound impairment. If a family has a dominant gene for deafness it will persist across generations because it will manifest itself in the offspring even if it is inherited from only one parent. If a family had genetic hearing impairment caused by a recessive gene it will not always be apparent as it will have to be passed onto offspring from both parents 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.

Disease or illness

  • Measles may result in auditory nerve damage
  • Meningitis may damage the auditory nerve or the cochlea
  • Autoimmune disease has only recently been recognized as a potential cause for cochlear damage. Although probably rare, it is possible for autoimmune processes to target the cochlea specifically, without symptoms affecting other organs. Wegener's granulomatosis is one of the autoimmune conditions that may precipitate hearing loss.
  • Mumps (Epidemic parotitis) may result in profound sensorineural hearing loss (90 dB or more), unilateral (one ear) or bilateral (both ears).
  • Presbyacusis is deafness due to loss of perception to high tones, mainly in the elderly. It is considered by some to be a degenerative process, although there has never been a proven link to aging. (See impact of environmental noise exposure above.)
  • Adenoids that do not disappear by adolescence may continue to grow and may obstruct the Eustachian tube, causing conductive hearing impairment and nasal infections that can spread to the middle ear.
  • AIDS and ARC patients frequently experience auditory system anomalies.
  • HIV (and subsequent opportunistic infections) may directly affect the cochlea and central auditory system.
  • Chlamydia may cause hearing loss in newborns to whom the disease has been passed at birth.
  • 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.
  • Premature birth results in sensorineural hearing loss approximately 5% of the time.
  • Syphilis is commonly transmitted from pregnant women to their fetuses, and about a third of the infected children will eventually become deaf.
  • Otosclerosis is a hardening of the stapes (or stirrup) in the middle ear and causes conductive hearing loss.

Medications

See also Ototoxicity
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).

Various other medications may reversibly affect hearing. This includes some diuretics, aspirin and NSAIDs, and macrolide antibiotics.

Extremely heavy Vicodin abuse is known to cause hearing impairment. There has been speculation that radio talk show host Rush Limbaugh's hearing loss was at least in part caused by his admitted addiction to narcotic pain killers, in particular Vicodin and OxyContin.

Physical trauma

  • There can be damage either to the ear itself or to the brain centers that process the aural information conveyed by the ears.
  • People who sustain head injury are especially vulnerable to hearing loss or tinnitus, either temporary or permanent.
  • Exposure to very loud noise (90 dB or more, such as jet engines at close range) can cause progressive hearing loss. 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.

Treatment

At present, sensorineural hearing loss is treated with hearing aids, which amplify sounds at pre-set frequencies to overcome a sensorineural hearing loss in that range; or cochlear implants, which stimulate the cochlear nerve directly.

Some research suggests that idebenone alone or combined with Vitamin E may delay the onset of hearing loss or perhaps reverse it. Use of these agents for this purpose is considered experimental at this time.

References

External links

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