Sensorineural hearing loss
|Sensorineural hearing loss|
Cross section of the cochlea.
|Classification and external resources|
Sensorineural hearing loss (SNHL) is a type of hearing loss, or deafness, in which the root cause lies in the inner ear (cochlear),vestibulocochlear nerve (cranial nerve VIII), or central processing centers of the brain. Sensorineural hearing loss can be mild, moderate, severe, profound or total.
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. 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.
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.
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 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.
Sensory 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. Exposure to a very loud noise such as a bomb blast can cause noise-induced hearing loss.
The 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. The Rinne test, which tests air conduction vs. bone conduction is positive, because 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 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 central hearing impairments.
Sensorineural hearing loss may be congenital or acquired.
- Genetic causes. 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 ie m.1555A>G which makes the individual sensitive to the ototoxic effects of aminoglycoside antibiotics.
- Congenital infections:
- 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 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 (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). The majority of cases do not have recognisable hearing loss at birth but develop it in the first decade of life.
- Congenital toxoplasmosis
- Congenital herpes infection
- Unknown - cases of hypoplastic auditory nerves or abnormalities of the cochlea are often of unknown cause.
(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
- Difficulties around the time of birth can be associated with factors which can give rise to sensorineural hearing loss. These are much more common in premature babies, particularly those under 1500 gms at birth.
- Anoxia or hypoxia (poor oxygen levels) - if prolonged or severe
- Bacterial meningitis - rarely
- Ototoxic medication - rare with monitoring of levels unless the child carries the m.1555A>G mutation
- Intracranial haemorrhages (intraventricular haemorrhages)
- Hyperbilirubinaemia (jaundice) - if severe. This can also be a cause of hearing loss in term babies with ABO or Rhesus incompatibility or G6PD deficiency.
- Autoinflammatory disease, such as Muckle-Wells syndrome, can lead to hearing loss. This is rare.
- Suppurative labyrinthitis
- Bacterial Meningitis may damage the cochlea
- 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 loss
- Syphilis is commonly transmitted from pregnant women to their fetuses, and about a third of the infected children will eventually become deaf.
- Ototoxic drugs
- Physical trauma - either due to a fracture of the temporal bone affecting the cochlea, or a shearing injury affecting cranial nerve VIII.
- Noise-induced - prolonged exposure to loud noises (>90 dB) causes hearing loss which begins at 4000 Hz (high frequency). The normal hearing range is from 20 Hz to 20,000 Hz.
- Presbycusis - is deafness due to loss of perception to high tones, in the elderly. Presbycusis is hearing loss that occurs in the high frequency range (4000 Hz to 8000 Hz).(See impact of environmental noise exposure below.)
- Sudden sensorineural hearing loss (SSNHL or SSHL)
- Idiopathic: sudden deafness, also called sudden idiopathic hearing loss (SIHL) and idiopathic sudden sensorineural hearing loss (ISSHL or ISSNHL)
- Vascular ischemia of the inner ear or CN 8
- 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
- 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)
- 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.
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.
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.
A 2015 review recommends comprehensive genetic testing should be part of a tiered approach to clinical evaluation.
Both dominant and recessive genes exist which can cause mild to profound impairment. If a family has a dominant gene for deafness, the individual only needs one copy of the gene for deafness to be affected and so it is passed from generation to generation and manifests itself in the offspring of each generation. An affected parent will pass the gene for deafness onto one in two of the children. If a family has genetic hearing impairment caused by a recessive gene, it is only apparent when a child inherits two copies of the gene, one from each parent. If both parents carry a copy of the gene the offspring have a one in four chance of having the condition. Subsequent generations are not affected unless both partners carry the same mutated gene. 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 dominant syndromic forms of hearing impairment include Stickler syndrome and Waardenburg syndrome.
- The most common recessive syndromic forms of hearing impairment are Pendred syndrome and Usher syndrome.
- MT-TL1 mutations cause hearing loss, along with diabetes and other symptoms.
Disease or illness
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- 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.
- AIDS and ARC patients frequently experience auditory system anomalies.
- HIV (and subsequent opportunistic infections) may directly affect the cochlea and central auditory system.
- 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 can be associated with problems that result in sensorineural hearing loss such as anoxia or hypoxia(poor oxygen levels), jaundice, intracranial haemorrhages, meningitis
- Otosclerosis is an inherited condition usually characterised by fixation of the stapes (or stirrup) in the middle ear causing a conductive hearing loss. However, sensorineural hearing loss can also be seen in this condition.
- Posterior circulation infarct
- Charcot–Marie–Tooth disease
- 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). A rare mitochondrial mutation, m.1555A>G, can increase an individual's susceptibility to the ototoxic effect of aminoglycosides.
Extremely heavy hydrocodone (Vicodin) abuse is known to cause hearing impairment.
- 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.
- Repeated 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.
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.
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.
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 set to the individual hearing loss to give maximum benefit. More severe hearing losses may be amenable to management by cochlear implants, which stimulate cochlear nerve endings directly. These consist of both internal implanted electrodes and magnets and external components.
- "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.
- Matsunaga, T. (2009). "Value of genetic testing in the otological approach for sensorineural hearing loss". The Keio journal of medicine 58 (4): 216–222. doi:10.2302/kjm.58.216. PMID 20037285.
- 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.
- 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.
- "Study Using Stem Cells to Treat Sensorineural Hearing Loss Underway". HealthyHearing. 2 February 2012. Retrieved 8 June 2013.
- "Sensorineural Hearing Loss". HealthCentral. Retrieved 8 June 2013.
- Sensorineural Hearing Loss, Dr Peter Grant