Sensorineural hearing loss

Synonyms

1

Overview

Sensorineural hearing loss (SNHL) is a type of hearing loss, or deafness, in which the root cause lies in the inner ear (cochlea and associated structures), vestibulocochlear nerve (cranial nerve VIII), or central auditory processing centers of the brain. SNHL accounts for about 90% of hearing loss reported. A hallmark of such hearing loss is that it is asymmetrically distributed usually toward the high frequency region, or may have a notch at some frequency. SNHL is generally permanent and can be mild, moderate, severe, profound, or total.

Symptoms

Since the inner ear is not directly accessible to instruments, symptomatology is by patient report and audiometric testing. Within all the people who go to their doctor with sensorineural hearing loss, 90% report having diminished hearing, 57% report having plugged feeling in ear, and 49% report having ringing in ear (tinnitis). Itching ears, while reported in only a small percentage of cases, is a hallmark symptom of SNHL not present in other types of hearing loss. About half report vestibular (vertigo) conditions.

For a detailed exposition of symptoms useful for screening, a self-assessment questionnaire was developed by the American Academy of Otolaryngology, called the Hearing Handicap Inventory for Adults (HHIA). It is a 25-question survey of subjective symptoms.

Causes

Sensorineural hearing loss may be genetic, congenital, age-related, or acquired (i.e. disease, noise-induced, trauma, etc).

Genetic

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.

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.

• The most common syndromic forms of hearing impairment include (dominant)Stickler syndrome and Waardenburg syndrome, and (recessive) Pendred syndrome and Usher syndrome.
• MT-TL1 mutations cause hearing loss, along with diabetes and other symptoms.
• Charcot–Marie–Tooth disease[3] an inherited sensorineural disorder with delayed onset that can affect the ears as well as other organs
• 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 rubella (German measles) virus during pregnancy. CRS has been controlled by universal vaccination (MMR or MMRV vaccine).
• Human Cytomegalovirus (HCMV) transmission to a developing fetus during pregnancy (congenital infection) is currently the most common 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. Thus, an estimated 7,000,000 people alive today have suffered hearing loss attributed to HCMV congenital disease. The majority of cases do not have recognisable hearing loss at birth but develop it in the first decade of life.
• toxoplasmosis, a parasitic disease affecting 23% of the population in the U.S. that can cause sensorineural deafness to the fetus in utero.

hypoplastic auditory nerves or abnormalities of the cochlea

Presbycusis

Normal progressive age-related loss of hearing acuity or sensitivity starting 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. Hearing loss that accumulates 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 persons have significant hearing loss by age 65; by age 75, one in two. Age-related hearing loss is not preventable or reversible.

Acquired

Noise

Most people living in modern society suffer from 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 70db (about twice as loud as normal conversation) if there is prolonged (24 hour) or continuous exposure. 125db (loud rock concert ~120db) is the pain level; sounds above this level cause instant and permanent ear damage.

Noise, along with heredity, may be a primary factor in presbycusis, or age-related hearing loss, the most common kind of hearing loss in industrial society.

Noise-related causes of hearing loss are divided into extrinsic (nosocusis) and intrinsic (sociocusis) causes: In the auditory system, the extrinsic component (nosocusis) includes hearing loss due to otologic disease, hazardous noise exposure, acoustic trauma, and ototoxic agents. The intrinsic component (sociocusis) indicates the wear-and-tear effects of exposure to the everyday sounds of normal living. People who live in nonindustrialized regions avoid both nosocusis and sociocusis and demonstrate excellent hearing into old age.

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 6db represents a doubling of the SPL, or energy of the sound wave, and therefore its propensity to cause ear damage. Because our ears hear logarithmically, not linearly, it takes an increase of 10db to produced a sound that is perceived to be twice as loud. Ear damage due to noise is proportional to sound intensity (SPL), not perceived loudness, so it's misleading to rely on our 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 5db for OSHA standard) increase in SPL. For example, the safe daily exposure amount at 85 dB (90db for OSHA) is 8 hours, while the safe exposure at 94 dB(A) (nightclub level) is only 1 hours. 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 65db defines a noise-impacted area.
• Personal audio electronics, such as iPods (iPods often reaching 115 decibels or higher), can produce powerful enough sound to cause significant NIHL.
• Repeated exposure to loud noise (90-95 dB or more, such as rock music) can cause progressive hearing loss. Sound levels at places where live or dance music is performed typically range from 95db to 104db.
• 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 - 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

Tumor

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.
Acoustic neuroma (vestibular schwannoma) - benign neoplasm of Schwann cells afflicting the vestibulocochlear nerve
Meningioma - benign tumour of the pia and arachnoid maters

Ménière's disease

pneumococcal Meningitis may damage the cochlea

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 conductive hearing loss or mixed hearing loss.
Syphilis is commonly transmitted from pregnant women to their fetuses, and about a third of the infected children will eventually become deaf.
Ramsay Hunt syndrome type II (herpes zoster oticus)

Ototoxic and neurotoxic chemicals and drugs

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).

Prolongued 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 gasolne; 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 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 hear 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 tinnitis.

Perinatal conditions

These are much more common in premature babies, particularly those under 1500 gms 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, 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.

Idodine deficiency/ Hypothyroidism

Iodine deficiency and hypothyroidism are associated with hearing loss. Depending on age of onset, may be correctable by iodine or thyroid medication.

Brain stroke

Brain stroke in a region affecting auditory function such as a posterior circulation infarct has been associated with deafness.

Prevention

Presbycucis is the leading cause of SNHL and is progressive and nonpreventable, and at this time, we do not have either somatic or gene therapy to counter heredity-related SNHL. But other causes of acquired SNHL are largely preventable, especially nosocusis type causes. This would involve avoiding environmental noise, and traumatic noise such as rock concerts and nightclubs with loud music. Use of noise attenuation measures like acoustic ear plugs is an alternative.

Diagnosis

Case history

• major concern
• pregnancy and childbirth information
• medical history
• development history
• family history

Otoscopy

Direct examination of the external canal and tympanic membrane (ear drum) with an otoscope, a medical device inserted into the ear canal that uses light to examine the condition of the external ear and tympanic membrane, and middle ear through the semi-translucent membrane.

Differentiel testing

Differential testing is most useful when there is unilateral hearing loss, and distinguishes conductive from sensorineural loss. These are conducted with a low frequency tuning fork, usually 512hz, and contrast measures of air and bone conducted sound transmission.

• 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.
• Rinne test, which tests air conduction vs. bone conduction is positive, because both bone and air conduction are reduced equally.
• less common Bing and Schwabach variants of the Rinne test.
• absolute bone conduction (ABC) test.

Tympanometry

A tympanogram is the result of a hearing test with a tympanometer. It tests the function of the middle ear and mobility of the eardrum. It can distinguish conductive hearing loss from other kinds of hearing loss including SNHL.

Audiometry

An audiogram is the result of a hearing test called pure tone audiometry, the most common type of hearing test. It charts the thresholds of hearing sensitivity at a selection of standard frequencies between 250 and 8000hz. There is also high frequency pure tone audiometry which tests frequencies from 8000-20,000hz. It can be used to differentiate between conductive hearing loss, sensorineural hearing loss, auditory processing hearing loss, and mixed hearing loss. Hearing loss which shows up as a relatively flat (but lowered) line on the audiogram is called a "flat loss". It means that thresholds are higher (sounds are harder to hear) at all frequencies. A flat loss is relatively uncommon and is often caused by a conductive problem rather than a sensorineural problem. Sensorineural hearing loss is characterized by a notch or notches in the audiogram, or asymmetrical audiogram with progressively greater loss with rising frequency. A relatively new refinement of pure tone audiometry is Bekesy audiometry.

There are also other kinds of audiometry designed to test hearing acuity rather that sensitivity (speech audiometry), audio processing and nerve transmission (evoked response audiometry).

Magnetic resonance imaging

To refine clinical findings and after audiometry, a contrast-enhanced MRI may reveal clinically unsuspected inflammatory, auto-immune or tumoural disease.

Treatment

Treatment modalities fall into three categories: pharmacological, surgical, and management. As SNHL is a physiologic degradation and considered permanent, there are as of this time, no approved or recommended treatments.

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 and clinically unproven. Such pharmaceutical treatments as are employed are palliative rather than curative, and addressed to the underlying cause if one can be identified, in order to avert progressive damage.

Profound or total hearing loss may be amenable to management by cochlear implants, which stimulate cochlear nerve endings directly. A cochlear implant is surgical implantation of a battery powered electronic medical device in the inner ear. Unlike hearing aids, which make sounds louder, cochlear implants do the work of damaged parts of the inner ear (cochlea) to provide sound signals to the brain. These consist of both internal implanted electrodes and magnets and external components. The quality of sound is different than natural hearing but may enable the the recipient to better recognize speech and environmental sounds. Because of risk and expense, such surgery is reserved for cases of severe and disabling hearing impairment

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 tuned to the individual hearing loss to give maximum benefit.