Sensorineural hearing loss may be genetic or acquired (i.e., as a consequence of disease, noise, trauma, etc.). People may have a hearing loss from birth (
congenital) or the hearing loss may come on later. Many cases are related to old age (age-related).
Genetic Hearing loss can be inherited. More than 40 genes have been implicated in the cause of deafness. There are 300 syndromes with related hearing loss, and each syndrome may have causative genes.
Recessive,
dominant,
X-linked, or
mitochondrial 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, i.e., m.1555A>G, which makes the individual sensitive to the ototoxic effects of
aminoglycoside antibiotics. • The most common cause of recessive genetic congenital hearing impairment in developed countries is
DFNB1, also known as Connexin 26 deafness or
GJB2-related deafness. • The most common syndromic forms of hearing impairment include (dominant)
Stickler syndrome and
Waardenburg syndrome, and (recessive)
Pendred syndrome and
Usher syndrome. • Mitochondrial mutations causing deafness are rare:
MT-TL1 mutations cause
MIDD (Maternally inherited deafness and diabetes) and other conditions, which may include deafness as part of the picture. •
TMPRSS3 gene was identified by its association with both congenital and childhood-onset autosomal recessive deafness. This gene is expressed in fetal cochleae and many other tissues, and is thought to be involved in the development and maintenance of the
inner ear or the contents of the
perilymph and
endolymph. It was also identified as a tumor-associated gene that is overexpressed in
ovarian tumors. •
Charcot–Marie–Tooth disease an inherited neurological disorder with delayed onset that can affect the ears as well as other organs. The hearing loss in this condition is often ANSD (auditory neuropathy spectrum disorder), a neural cause of hearing loss. •
Muckle–Wells syndrome, a rare inherited
autoinflammatory disorder, can lead to hearing loss. •
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.
Congenital • Infections: •
Congenital rubella syndrome, CRS, results from transplacental transmission of the
rubella virus during pregnancy. CRS has been controlled by universal vaccination (
MMR or
MMRV vaccine). •
Cytomegalovirus (CMV) infection is the most common cause of progressive sensorineural hearing loss in children. It is a common viral infection contracted by contact with infected bodily fluids such as saliva or urine and easily transmitted in nurseries and thus from toddlers to expectant mothers. CMV infection during pregnancy can affect the developing foetus and lead to learning difficulties as well as hearing loss. •
Toxoplasmosis, a parasitic disease affecting 23% of the population in the U.S., can cause sensorineural deafness to the fetus in utero. •
Hypoplastic auditory nerves or abnormalities of the cochlea. Abnormal development of the inner ear can occur in some genetic syndromes such as LAMM syndrome (labyrinthine aplasia, microtia, and microdontia),
Pendred syndrome,
branchio-oto-renal syndrome,
CHARGE syndrome •
GATA2 deficiency, a grouping of several disorders caused by a common defect, viz., familial or sporadic
inactivating mutations in one of the two parental
GATA2 genes. These
autosomal dominant mutations cause a reduction, i.e., a
haploinsufficiency, in the cellular levels of the gene's product,
GATA2. The GATA2
protein is a
transcription factor critical for the
embryonic development, maintenance, and functionality of
blood-forming,
lympathic-forming, and other tissue-forming
stem cells. In consequence of these mutations, cellular levels of GATA2 are deficient, and individuals develop over time hematological, immunological, lymphatic, and/or other disorders. GATA2 deficiency-induced abnormalities in the lymphatic system are proposed to be responsible for a failure in generating the
perilymphatic space around the inner ear's
semicircular canals, which in turn underlies the development of sensorineural hearing loss.
Presbycusis Progressive age-related loss of hearing acuity or sensitivity can start as early as age 18, primarily affecting the high frequencies, and men more than women. Such losses may not become apparent until much later in life.
Presbycusis is by far the dominant cause of sensorineural hearing loss in industrialized societies. A study conducted in Sudan, with a population free from loud noise exposures, found significantly fewer cases of hearing loss when compared with age-matched cases from an industrialized country. Similar findings were reported by a study conducted of a population from Easter island, which reported worse hearing among those that spent time in industrialized countries when compared with those that never left the island. Researchers have argued that factors other than differences in noise exposure, such as genetic make up, might also have contributed to the findings. Hearing loss that worsens with age but is caused by factors other than normal aging, such as noise-induced hearing loss, is not presbycusis, although differentiating the individual effects of multiple causes of hearing loss can be difficult. One in three people has significant hearing loss by age 65; by age 75, one in two. Age-related hearing loss is neither preventable nor reversible.
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. 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. 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 is 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. 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. •
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 A recent study found that hearing loss is twice as common in people with diabetes as it is in those who do not 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 hearing loss and neuropathy (nerve damage). •
Tumor • Cerebellopontine angle tumour (junction of the
pons and the
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 overdiagnosed. • 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 bilateral. •
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 heavy use of
hydrocodone (Vicodin) 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 their 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 a 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, and 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. 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. ==Pathophysiology==