Sensorineural hearing loss

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

Sensorineural hearing loss (SNHL) 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. 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 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. 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.

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 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, being in a loud workplace without hearing protection, or having headphones set to high volumes for a long period.

Differential diagnosis[edit]

The Weber test, in which a tuning fork is touched to the midline of the forehead, 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.

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[edit]

  • Lack of development (aplasia) of the cochlea
  • Chromosomal syndromes (rare)
  • Congenital cholesteatoma - squamous epithelium is normally present on either side of the tympanic membrane. Externally, within the external auditory meatus or ear canal and internally within the middle ear. Within the middle ear the simple epithelium gradually transitions into ciliated pseudostratified epithelium lining the Eustachian tube now known as the pharyngotympanic tube becoming continuous with the respiratory epithelium in the pharynx. The squamous epithelium hyperplasia within the middle ear behaves like an invasive tumour and destroys middle ear structures if not removed.
  • Delayed familial progressive
  • Congenital rubella syndrome, CRS, results from transplacental transmission of rubella (German measles) virus during pregnancy. CRS has been controlled by universal vaccination (measles-mumps-rubella or measles-mumps-rubella-varicella zoster vaccine)
  • Human Cytomegalovirus (HCMV) transmission to a developing fetus during pregnancy (congenital infection) is currently the #1 infectious cause of hearing loss recognized at birth. HCMV congenital infection leads to sensorineural damage that is progressive over the first few years of life. 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 (ref 1). Thus, an estimated 7,000,000 people alive today have suffered hearing loss attributed to HCMV congenital disease, although awareness of this disease is low (re 2). A vaccine to prevent HCMV congenital disease is needed but faces hurdles and has not yet been developed (ref 3).

(ref 1) Mocarski, E.S., T. Shenk, P. Griffiths and R. F. Pass (2013) Cytomegaloviruses. In D. M. Knipe, P. M. Howley, D. E. Griffin, R. A. Lamb, M. A. Martin (Eds.) Fields Virology, 6th Edition. Lippincott Williams & Wilkins, Philadelphia pp 1960-2014 (ref 2) Cannon, M. J., Westbrook, K., Levis, D., Schleiss, M. R., Thackeray, R., Pass, R. F., Awareness of and behaviors related to child-to-mother transmission of cytomegalovirus. Prev Med 54, 351. (ref 3) Krause, P. R., S. R. Bialek, S. B. Boppana, P. D. Griffiths, C. A. Laughlin, P. Ljungman, E. S. Mocarski, R. F. Pass, J. S. Read, M. R. Schleiss, and S. A. Plotkin (2013) Priorities for CMV vaccine development. Vaccine 32:4-10

Acquired[edit]

Long term exposure to environmental noise[edit]

Populations 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 of 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 time factor of 2 per 5 dB is used.
  • Personal audio electronics, such as iPods (iPods often reaching 115 decibels or higher), can produce powerful enough sound to cause significant NIHL, given that lesser intensities of even 70 dB can also cause hearing loss.[1]

Genetic[edit]

Hearing loss can be inherited. More than 40 genes have been identified to cause deafness.[2] There are also 300 syndromes with related hearing loss, and each syndrome may have causative genes.

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[edit]

Medications[edit]

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, sildenafil and NSAIDs, and macrolide antibiotics.

Extremely heavy hydrocodone (Vicodin) abuse is known to cause hearing impairment.

Physical trauma[edit]

  • 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[edit]

Hair cell regeneration using stem cell and gene therapy is years or decades away from being clinically feasible.[4] However, studies are currently underway on the subject, with the first FDA-approved trial beginning in February 2012.[5]

Sensorineural hearing loss can be treated with hearing aids, which amplify sounds at preset frequencies to overcome a sensorineural hearing loss in that range. It can also be treated with cochlear implants, which stimulate cochlear nerves directly, consisting of both internal and external components.[6]

Some research suggests idebenone alone or combined with vitamin E may delay the onset of hearing loss or perhaps reverse it.[7] Use of these agents for this purpose is considered experimental now.

Some audiologists and ENTs have reported if severe noise-induced hearing loss (exposures exceeding 140dB) is treated immediately (within 24 hours) with a course of steroids, it can often be almost completely reversed.[citation needed] This, however, is a new field without proven success.[8]

Researchers at the University of Michigan report that a combination of high doses of vitamins A, C, and E, and magnesium, taken one hour before noise exposure and continued as a once-daily treatment for five days, was very effective at preventing permanent noise-induced hearing loss in animals.[9]

References[edit]

  1. ^ "Sound Output Levels of the iPod and Other MP3 Players: Is There Potential Risk to Hearing?". Archived from the original on October 30, 2007. Retrieved 2007-11-20. 
  2. ^ Matsunaga, T. (2009). "Value of genetic testing in the otological approach for sensorineural hearing loss". The Keio journal of medicine 58 (4): 216–222. PMID 20037285.  edit
  3. ^ Papadakis CE, Hajiioannou JK, Kyrmizakis DE, Bizakis JG (May 2003). "Bilateral sudden sensorineural hearing loss caused by Charcot-Marie-Tooth disease". J Laryngol Otol 117 (5): 399–401. doi:10.1258/002221503321626465. PMID 12803792. 
  4. ^ Parker, M. A. (2011). "Biotechnology in the Treatment of Sensorineural Hearing Loss: Foundations and Future of Hair Cell Regeneration". Journal of Speech, Language, and Hearing Research 54 (6): 1709–1731. doi:10.1044/1092-4388(2011/10-0149). PMC 3163053. PMID 21386039.  edit
  5. ^ "Study Using Stem Cells to Treat Sensorineural Hearing Loss Underway". HealthyHearing. 2 February 2012. Retrieved 8 June 2013. 
  6. ^ "Sensorineural Hearing Loss". HealthCentral. Retrieved 8 June 2013. 
  7. ^ Sergi B, Fetoni AR, Paludetti G, et al. (June 2006). "Protective properties of idebenone in noise-induced hearing loss in the guinea pig". Neuroreport 17 (9): 857–61. doi:10.1097/01.wnr.0000221834.18470.8c. PMID 16738476. 
  8. ^ Haynes, David S.; O??Malley, Matthew; Cohen, Seth; Watford, Kenneth; Labadie, Robert F. (2007). "Intratympanic Dexamethasone for Sudden Sensorineural Hearing Loss After Failure of Systemic Therapy". The Laryngoscope 117 (1): 3–15. doi:10.1097/01.mlg.0000245058.11866.15. PMID 17202923. 
  9. ^ Le Prell, Colleen G.; Hughes, Larry F.; Miller, Josef M. (2007). "Free radical scavengers, vitamins A, C, and E, plus magnesium reduces noise trauma". National Center for Biotechnology Information. 

External links[edit]