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To do this, you must have Adobe Acrobat® Reader (version 4 or higher) installed on your computer. This medical discussion paper will be useful to those seeking general information about the medical issue involved. Each medical discussion paper is written by a recognized expert in the field, who has been recommended by the Tribunala€™s medical counsellors. A mixed hearing loss occurs when both a sensorineural and conductive hearing loss are present at the same time.
According to the 1990 Noise and Hearing Loss Consensus Conference, a€?Noise Induced Hearing Loss (NIHL) results from damage to the ear from sounds of sufficient intensity and duration that a temporary or permanent sensorineural hearing loss is produced. PTS refers to a permanent loss of sensorineural hearing which is the direct result of irreparable injury to the organ of Corti.
The 4000 Hz a€?notcha€? or a€?dipa€? in sensorineural hearing has been a classic finding in NIHL over the years. Why the 4000 Hz frequency appears more affected than other frequencies continues to generate some controversy. Individuals vary in their susceptibility to noise and the damage it may cause to the cochlea. Of interest, one universal finding is that on average hearing threshold levels (HTLs) in the right ear are better than the left ear by about 1dB.
Whether one ear is more resilient to noise in the same individual (the so- called a€?tough vs. Please remember that when an asymmetric sensorineural hearing loss exists steps often need to be taken to exclude pathologic causes (i.e. The following statements tend to reflect what is agreed upon by the majority of scientists and physicians who deal with NIHL. Noise induced hearing loss (NIHL) may produce a temporary threshold shift (TTS), permanent threshold shift (PTS) or a combination of both.
A PTS is caused by destruction of certain inner ear structures that cannot be replaced or repaired. The amount of hearing loss produced from a given noise exposure varies from person to person.
NIHL initially affects higher frequency hearing (3-6 kHz range, for nearly all occupational exposures) than those frequencies essential for communication (i.e. Daily noise exposure (8hrs) > 90 dB for over 5 years (or equivalent) causes varying degrees of hearing loss in susceptible individuals. NIHL is a decelerating process; the largest changes occur in the early years with progressively smaller changes in the later years (so-called Corso's theorem). NIHL first affects hearing in the 3-6 kHz range, for nearly all occupational exposures; the lower frequencies are less affected.
Once the exposure to noise is discontinued, there is no substantial further worsening of hearing as a result of noise unless other causes occur.
Continuous noise exposure over the years is more damaging than interrupted exposure to noise which permits the ear to have a rest period. The pathologic basis for presbycusis appears to be one of gradual devascularization of the cochlea and loss of functioning hair cells. Clinically hearing loss from presbycusis appears to be an accelerating process unlike hearing loss in NIHL.
Unfortunately, there is no specific treatment available that will prevent age related hearing loss at present.
In the adjudication process of an occupational NIHL claim it is often difficult to separate the total amount of hearing loss from noise and age related change.
For example, not everyone as they age will experience age-related presbycutic change (changes from presbycusis are variable with some individuals experiencing greater degrees of age related change than others).
Moreover, exposure to high level noise early on may produce hearing loss more rapidly than aging such that the aging process has a negligible effect (i.e.
The effects of noise exposure and aging on hearing when not combined are reasonably well-understood. Although it seems logical to a€?subtracta€? the age-related effects from the total hearing loss in order to quantitate the amount of hearing loss due to noise, this is really quite simplistic when one considers that aging effects and noise exposure effects can at times be practically indistinguishable for the most part audiometrically. Because compensation claims have required some consideration of presbycusis and its role in the total hearing loss of an individual various correction factors have been applied. Dobiea€™s theorem states that the total hearing loss from noise and age are essentially additive (this is the theory put into practice when a standard correction factor for age after 60 years is applied in the Province of Ontario). Corsoa€™s theorem on the other hand states that any correction for age should be based on a variable ratio (as individuals age the assumption is that the effects of presbycusis variably accelerate by decade).
Nevertheless the quantification of hearing loss attributable to age when occupational NIHL is present is really quite a complex phenomenon. This is an inner ear disorder characterized by episodes of vertigo (an illusion of movement) lasting minutes to hours, fluctuating hearing loss and tinnitus (unwanted head noise). Usually one ear is involved initially although over time the other ear becomes affected in nearly 50% of cases. Treatment is medical in most cases involving a low salt diet and diuretics and vestibular sedatives. Otosclerosis usually results in a conductive hearing loss from stapes footplate fixation due to new bone growth in this area.
The diagnosis of cochlear otosclerosis is usually made when a family history of otosclerosis exists and other rare causes for a chronic progressive loss can be excluded. Although no medical treatment can ever reverse the sensorineural hearing loss, treatment with oral sodium fluoride may minimize and possibly stabilize an individuala€™s hearing.
Physical injury to the ear is usually the result of blunt trauma in the circumstances of a significant head injury.
When physical trauma is severe enough to cause a temporal bone fracture (the temporal bone is the larger part of the skull bone that houses all the structures of the ear), two types of fractures occur; longitudinal and transverse.
In general terms longitudinal fractures are much more common and tend to result in a fracture line through the roof of the middle ear and ear canal.
Transverse fractures of the temporal bone occur when the fracture lines run directly through the hard bone of the otic capsule. Ototoxicity is defined as the tendency of certain substances to cause functional impairment and cellular damage to the tissues of the inner ear, especially to the cochlea and the vestibular apparatus. Aminoglycoside antibiotics are powerful weapons in the treatment of certain bacterial infections.
Antimalarial drugs such as quinine and chloroquin unfortunately have ototoxicity as a side effect if taken in excess. This is a pathologic misnomer since this tumour is strictly a schwannoma of the vestibular nerve. Although benign this tumour has serious health implications for the patient and is sought for by otolaryngologists when there is an undiagnosed asymmetry in the hearing.
This diagnosis is given to the individual who experiences a sudden hearing loss in one ear. An individual's threshold hearing to pure tones at different frequencies (250 - 8000 Hz) is performed.
Air conduction (AC) thresholds are delivered via headphones and the individual is asked to respond to the sound of the lowest intensity (in dB) they hear at the frequency being tested. The pure tone audiogram forms the basis for assessing if an individual qualifies for Workplace Safety and Insurance Board (WSIB) benefits in the Province of Ontario.
Complex words with equal emphasis on both syllables (so called spondaic words such as a€?hotdoga€?, a€?uptowna€?, a€?baseballa€? etc.) are given an individual at the lowest intensity they can hear.
A list of phonetically balanced single syllable words (these are words commonly found in the English language in everyday speech such as a€?fata€?, a€?asa€?, a€?doora€? etc.) are presented to an individual at 40 dB above their speech reception threshold (SRT) in the ear being tested. In this test a probe is placed into the ear canal that both emits a sound and can vary pressure within the canal which causes the ear drum to move. This tells whether the pressure in the middle ear is within normal limits and provides some indirect measurement of Eustachian tube function.
Stiffening of the ear drum from loud noise occurs when the stapedius muscle contracts in the middle ear. Absent stapedial reflexes are usually seen in middle ear pathology such as otitis media with effusion or otosclerosis.
If one measures how the ear drum moves when pressure is changed in the external ear canal from negative to positive a series of curves can be attained. The ability to measure minute electrical potentials following sound stimulation of the cochlea (i.e. Electrocochleography (ECoG) - This test measures electrical activity in the cochlea during the first 2 msec of cochlear stimulation. Auditory Brainstem Response (ABR) - This term is synonymous with the term brainstem evoked potential (BEP) or the brainstem evoked response audiogram (BERA). Threshold Evoked Potentials (TEP) - These electrical waveforms are usually identified between 50 - 200 msec following cochlear stimulation and are thought to represent cortical pathways of electrical activity. If AC thresholds are greater than BC thresholds this usually implies there is a conductive component to hearing loss from pathology involving the external or middle ear. If BC thresholds are greater than AC thresholds this would typically suggest an exaggerated hearing loss or malingering. In its classic presentation a notched sensorineural hearing loss is noted at 4000 Hz that should be relatively symmetric. Age related hearing loss typically presents with a bilateral symmetrical high frequency sensorineural hearing loss. There are often numerous discrepancies noted such as the presence of acoustic reflexes at levels below volunteered pure tone thresholds, discrepancies between volunteered pure tone average and speech reception thresholds etc.
Subjective tinnitus can only be appreciated by the affected individual, is usually associated with a sensorineural hearing loss of some type and is the most common type noted.
Objective tinnitus, on the other hand, is quite rare but by definition is a noise that can be appreciated by an observer.
Regardless of cause tinnitus can be quite disturbing for many individuals and can certainly affect their well-being. Treatment for tinnitus includes the use of more pleasurable competing background noise (i.e. The individual affected has undergone treatments to try to alleviate their perceived unwanted head noise (i.e.
Evidence to support a personality change or a sleep disorder as a result of the tinnitus. Hyperacusis is a curious phenomenon where an individual describes an uncomfortable, sometimes painful hypersensitivity to sounds that would be normally well tolerated in daily life.
Treatment usually requires the gradual introduction of increasing sound intensity that becomes better tolerated with time.
American Academy of Otolaryngology-Head and Neck Surgery Foundation Subcommittee on the Medical Aspects of Noise. American Academy of Otolaryngology-Head and Neck Surgery Foundation: Guide for Conservation of Hearing in Noise. Noise Induced Hearing Loss by Lonsbury-Martin BL, Martin GK and Telischi FF, Volume 4, Chapter 126 in Otolaryngology-Head and Neck Surgery 3rd Edition.
Permanent Impairment of Hearing - When any anatomic or functional abnormality produces a permanent reduced hearing sensitivity. Permanent Handicap for Hearing - The disadvantage imposed by an impairment sufficient to affect the individuala€™s efficiency in the activities of daily living (ADL). Permanent Disability - A person is permanently disabled or under permanent disability when the actual presumed ability to engage in gainful activity is reduced because of handicap and no appreciable improvement can be expected.
Acoustic Neuroma (Vestibular Schwannoma) - A benign brain tumour that arises on the vestibular nerve usually within the internal auditory canal (IAC). Menierea€™s Disease - A classic inner ear disorder associated fluctuant sensorineural hearing loss, tinnitus and episodic attacks of vertigo lasting minutes-hours. Otitis Media with Effusion - An encompassing term that describes the presence of fluid behind the TM. Otosclerosis- Genetically inherited condition (approximately 1 in 20 people carry the otosclerosis gene) associated with the development of new, immature bone which primarily involves the stapes footplate leading to a progressive conductive hearing loss. Tympanosclerosis - Calcification of the middle layer of the eardrum that looks like a patch of white chalk. Mastoidectomy - Procedure designed to exteriorize disease in the mastoid air cells and adjoining middle ear. Ossiculoplasty - Procedure where the ossicular chain is repaired in order to try and improve hearing.
Tympanoplasty - When used in its most simplistic context it means the repair of a previous TM perforation.
Ventilation (myringotomy) Tube - The placement of an open ended tube or a€?grommeta€? into the TM acts like an artificial Eustachian tube which helps ventilate the middle ear space.
Decibel (dB) - A decibel is an accepted measure of sound pressure level used to describe sound intensity.
Evoked Response Audiometry - Measures electrical responses within the inner ear, cochlear nerve and central nervous system that are generated by loud repetitive clicks. This test is typically performed as one of the tests to confirm or exclude whether an exaggerated hearing loss (malingering) is present. Otoacoustic Emissions (OAEs) - Evoked test that measures minute responses to sound thought to arise from the contractile elements of the outer hair cells.
Recruitment - Perceived abnormal progressive loudness of noise in an individual with a sensorineural hearing loss. Speech Discrimination Score (SDS) - The percentage score an individual correctly identifies when presented with a list of phonetically balanced (PB) words usually tested at 40dB above their hearing threshold.
Tympanometry - Formal tracing of how the TM moves when pressure is altered in the external auditory canal. Acoustic Trauma - Damage to the ear caused by a single exposure to an impulse nose of high density. Continuous Noise - A noise that remains relatively constant in sound level for at least 0.2 seconds. Dosimeter - An instrument that integrates a function of constant varying sound pressure over a specified time period in such a manner that it directly indicates a noise dose and estimates the hazard of an entire exposure period. Permanent Threshold Shift (PTS) - The permanent depression in hearing sensitivity resulting from exposure to noise at damaging intensities and durations. Temporary Threshold Shift (TTS) - The temporary (usually several hours) depression in hearing sensitivity resulting from exposure to noise at damaging intensities and durations. Time Weighted Average (TWA) - The sound level or noise equivalent that, if constant over an eight hour day would result in the same exposure as the noise level in question (assuming a 5 dB doubling rule).
The ear is a complex organ that has dual responsibility for hearing and balance (vestibular) function.

The ear canal (external auditory canal) originates from the concha (so-called bowl) of the auricle and extends medially to the tympanic membrane.
The ear canala€™s primary function is to provide a conduit for airborne sound waves to reach the tympanic membrane.
The middle ear cleft is a more extensive designation of the middle ear that has three main components which include the middle ear cavity, mastoid air cell system and the Eustachian tube. The mastoid air cell system consists of a series of small mucosal lined air cells interconnected with each other and the middle ear proper. Important structures of the middle ear include the tympanic membrane and the ossicular chain. The ossicular chain consists of the three small interconnected mobile bones known as the malleus (hammer), incus (anvil) and stapes (stirrup). The inner ear is the sensory end organ within the hardest bone of the body known as the petrous portion of the temporal bone.
The cochlea specifically refers to the sensory portion of the inner ear responsible for hearing.
The vestibular portion of the inner ear consists of three fluid lined semicircular (superior, lateral and posterior) canals and the area called the vestibule which houses the otolithic macular endorgans. Sound vibrations are picked up by the pinna and transmitted down the external auditory canal where they strike the TM causing it to vibrate.
When the mechanical vibrations of the stapes footplate reach the inner ear they create traveling waves in the cochlea. Although the vast majority of the cochlear nerve fibres are termed afferent (ie nerves that carry electrical activity from the inner ear to the brain), within the cochlear nerve itself we have a small number of nerve fibres designated as efferent (nerves that carry electrical activity from the brain to the inner ear). Of interest when the outer hair cells become injured or affected by pathology they also lose their ability to a€?fine tunea€? the electrical responses arising from the inner ear. Although an individual may have apparently normal hearing it does not necessarily mean the cochlear nerve is undamaged.
Sound is the propagation of pressure waves through a medium such as air and water for example. This is a very relevant question that is based on the definition of how hearing loss causes a handicap for the affected individual in their ability to identify spoken words or sentences under everyday conditions of normal living.
Although the higher frequencies are initially affected in NIHL an individual will really only begin to demonstrate a significant handicap to hearing to everyday speech in a quiet surrounding when the average of the hearing thresholds at 500, 1000, 2000 ( + 3000) Hz is above 25 dB.
The inclusion of the 3000 Hz threshold value for NIHL awards is not always required depending on the jurisdiction.
In the province of Ontario the Workplace Safety and Insurance Board (WSIB)a€™s Operational Policy Manual (OPM) Document No.
A presbycusis (aging) correction factor of 0.5 dB is deducted from the measured hearing loss (averaged over the 500, 1000, 2000 and 3000 Hz frequencies) for every year the worker is over the age of 60 at the time of the audiogram. It is important to also appreciate that decisions can also be made on an exceptional basis which the WSIB has recognized.
Since individual susceptibility to noise varies, if the evidence of noise exposure does not meet the above exposure criteria, claims will be adjudicated on the real merits and justice of the case, having regard to the nature of the occupation, the extent of the exposure, and any other factors peculiar to the individual case.
A comprehensive list of US comparisons for NIHL and benefits can be found in Table 35.1 pages 797-799 in the Chapter concerning State and Federal Formulae Differences in Occupational Hearing Loss, 3rd Edition. It is likely that the two effects are not additive (Corsoa€™s theorem) but from a practical point of view this is how they are usually viewed with regards to compensation claims (Dobiea€™s theorem). At any given age for frequencies above 1000 Hz men will have more age related hearing loss than women. Age related hearing loss affects all frequencies although the higher frequencies are usually more affected. Age related hearing loss is an accelerating process where the rate of change increases with age. One can speculate that this might be one of the reasons an individual early on in their exposures may not be aware of a hearing loss, only to appreciate a hearing loss later in life when other factors such as presbycutic change occur. Although we think of presbycusis as an age related event not all individuals will develop this condition.
Future genetic studies may provide us with further information concerning those at greater risk for progressive hearing loss from presbycusis in future. A TTS is considered to represent a pathological metabolically induced fatigue of the hair cells or other structures within the Organ of Corti.
In a PTS the destruction and eventual cochlear hair cell loss is thought to arise from direct mechanical destruction from high-intensity sound and from metabolic decompensation with subsequent degeneration of sensory elements. While one would normally expect full recovery of hearing function after a TTS there is one important consideration that needs to be taken into account.
The position in the world literature continues to support that any progressive hearing loss from acute trauma ceases unless the individual is re-exposed to more acute acoustic trauma or some other factor (s) occurs ( i.e.
The classic studies of acute acoustic trauma by Segal et al (1988) and Kellerhals (1991) are often used to support this position. In the cohort study by Segal et al, 841 individuals from either the regular US army or military reserve sustained a permanent sensorineural hearing loss from acute acoustic trauma without subsequent re-exposure. The importance of the Segal study is that after 1 year one would not expect further worsening of hearing in subjects who ceased to have further noise exposure. Kellerhalsa€™ study reviewed a much smaller series of individuals who were exposed to acute acoustic trauma but looked at them over a 20 year duration. It is intended to provide a broad and general overview of a medical topic that is frequently considered in Tribunal appeals. Each author is asked to present a balanced view of the current medical knowledge on the topic. A vice-chair or panel may consider and rely on the medical information provided in the discussion paper, but the Tribunal is not bound by an opinion expressed in a discussion paper in any particular case.
This is the type of hearing loss that is found in routine unprotected daily exposure to loud noise potentially injurious to hearing in the occupational work force. This type of loss might be found in an individual, for example, with a large tympanic membrane (TM) perforation where mechanical vibrations along the ossicular chain are dampened.
For example an individual with a large TM perforation who received topical antibiotic ear drops for the treatment of a middle ear infection that caused inadvertent toxicity to the inner ear in addition (i.e.
The hearing loss may range from mild to profound, may result in tinnitus (unwanted head noise) and is cumulative over a lifetimea€?.
Noise induced deafness generally affects hearing between 3000-6000 Hz with maximal injury centering around 4000 Hz initially, an important point to remember. There is good pathologic evidence however that demonstrates maximal cochlear hair cell loss in the tonal areas where the 4000 Hz hair cells normally reside in both animals and humans. In susceptible individuals the early effects of presbycusis are occasionally seen around age 40.
Secondary to hair cell loss one can often see progressive neuronal dropout along the cochlear nerve. In this regard the effects of aging in the absence of other factors cause a loss of hearing at all frequencies whose rate of growth becomes more rapid as age increased (especially after 60 years): an important point to remember in this context. To a large degree hearing loss with age is genetically primed; in other words the hearing your parents had as they aged is often passed on to you. When the two processes are combined the resultant pathology and its effects upon hearing are not as well understood. This certainly generates a more complicated mathematical model but probably more closely approaches what is happening physiologically. The hearing loss is typically a low-frequency sensorineural loss that fluctuates initially often reverting close to normal between attacks in the early stages.
The presence of a pink-flamingo hue to the middle ear on otoscopy (so-called Schwartzea€™s sign), absent stapedial reflexes on audiometry and bone density changes involving the surrounding bone of the inner ear best appreciated on high-resolution CT scanning also helps in the diagnosis.
Most patients suffering deafness by this mechanism will have had at least transient unconsciousness and have been admitted to hospital. A fracture through this bone (which is the hardest bone in the body) implies the force of the injury was severe and often incompatible with survival. Unfortunately these antibiotics can cause varying degrees of cochlear, vestibular and renal toxicity. The Auditory Brainstem Response (ABR) is used as a screening test while a gadolinium enhanced Magnetic Resonance Imaging (MRI) study is the gold standard investigation. The extent of the loss varies from a partial loss at one frequency right up to a profound loss at all frequencies with a profound discrimination loss. For the purposes of compensation, the most reliable audiograms will likely be performed by an audiologist who typically has a mastersa€™ degree or doctorate in audiology. The audiometer used to test hearing has been especially calibrated so that each frequency of sound tested for hearing has been normalized where 0 dB represents the lowest intensity of sound that the majority of people with normal hearing can hear from large population studies. When a conductive hearing loss is suspected the ear canal and inner ear mechanisms for transmission of sound energy to the inner ear are bypassed by placing a bone vibrator over the mastoid which directly stimulates the inner ear. A weighted pure tone average from frequencies at 500, 1000, 2000 and 3000 Hz is required for this determination from AC thresholds if a pure sensorineural hearing loss is present or from BC thresholds if any conductive element to hearing loss is present. As a general rule the SRT value should roughly equal the pure tone average in the speech frequencies at 500, 1000 and 2000 Hz. Most individuals with normal sensorineural hearing should get over 80% of the words correct at this level. Most individuals with normal sensorineural hearing will exhibit a reflex at 70-80 dB above their hearing threshold level at the frequency being tested. When reflexes occur at less than a difference than 70-80 dB above hearing threshold at the frequency tested this is indirect evidence of a phenomenon called recruitment which is usually seen in cochlear pathology (i.e. ECoGa€™s chief value lies in its ability to demonstrate wave 1 of the auditory brainstem response (ABR) and whether waveform morphology is suggestive of changes thought to occur in endolymphatic hydrops, the pathophysiologic substrate of MeniA?rea€™s disease.
This test measures electrical waveforms obtained in the first 10 msec from cochlear stimulation.
Middle ear pressures should be normal, stapedial reflexes present and a normal tympanogram noted. In some instances it can be quite disabling for the affected individual who tends to withdraw from any exposure to uncomfortable noise.
Cognitive behavioural therapy (CBT) has also demonstrated promise in a recent randomized controlled study. The Relative Contributions of Occupational Noise and Aging in Individual Cases of Hearing Loss. Specialized treatment based on cognitive behavior therapy versus usual care for tinnitus: a randomized controlled trial. Diabetes and hearing impairment in the United States: Audiometric evidence from the National Health and Nutrition Examination Survey (1999-2004). The pathology is thought to arise from excess fluid in the inner ear leading to membrane ruptures (so-called endolymphatic hydrops). Certain antibiotics (especially the aminoglycoside class), antimalarials, chemotherapeutic agents and even excessive doses of ASA can be toxic to the inner ear. Although changes may take place as early as age 40, hearing loss due to aging usually starts to accelerate around ages 55-60 yrs and continues with age. Auditory Brainstem Response (ABR) - Synonymous with the term BERA (brainstem evoked response audiometry) or auditory BEPa€™s (brainstem evoked potentials). Cortical Evoked Response Audiometry - Synonymous with Threshold Evoked Potential (TEP) testing. Depending on the frequency of the sound certain parts of the cochlea will emit an a€?echoa€? in response.
This occurs to a loud noise typically 70-80 dB above an individuala€™s hearing level at the frequency tested which causes stiffness of the ossicular chain and tympanic membrane Absent stapedial reflexes are usually seen in otosclerosis due to stapes footplate fixation for example.
Conceptually it has three distinct anatomical components which include the external, middle and inner ear.
The auricle is a flattened, funnel shaped appendage that contains a cartilage framework covered by skin. The anatomy favours the transmission of sound waves typically between 500-8,000 Hz which interestingly represents the common speech frequencies of most individuals. Proper function of the Eustachian tube which connects the middle ear cavity to the back of the nose (nasopharynx) is important for the normal middle ear aeration and function. The tympanic membrane or ear drum is an ovoid, pale gray, semi-transparent membrane positioned obliquely at the medial end of the ear canal.
The tensor tympani and stapes muscles help stabilize the bones along with suspensory ligaments of connective tissue inside the middle ear.
Covered by an otic capsule its membranous portion (or labyrinth) contains a delicate system of fluid-filled canals lined with sensory neuroepithelium necessary for hearing and balance perception.
It is a three chambered organ containing anatomical spaces known as the scala media, scala vestibuli and scala tympani. The scala vestibuli and scala tympani (T) by comparison contain fluid rich in sodium called perilymph.
Within each semicircular canal is an area called the ampulla which contains hair cells sensitive to low frequency bidirectional fluid displacement that occurs with angular acceleration type movements of the head. The sound vibrations are then transmitted across the air-filled middle ear space by the 3 tiny linked bones of the ossciular chain: the malleus, incus and stapes . The hair cells change these mechanical vibrations from the waves into electrochemical impulses that can interpreted by the central nervous system (CNS). It can be a simple sound commonly known as a pure tone or it may be complex when we think of speech, music and noise.
The prediction of a handicap did not appear to be improved by the inclusion of other frequencies > 4000 Hz. What do other jurisdictions consider to be disabling hearing loss (provincial, national and international)? The hearing loss that remains after the presbycusis adjustment is then used to determine entitlement to benefits.
It is generally thought that if an ear has suffered a permanent threshold shift from a noise induce etiology further noise exposure will cause less damage than would occur in a normal ear to a similar exposure.
As previously noted the effects of noise exposure and aging on hearing when not combined are reasonably well understood. One however has to qualify this by saying that there is a significant amount of redundancy within the inner ear as it pertains to hearing. Moreover we really dona€™t have a lot of good prospective longterm studies over 4-5 decades that can ultimately answer this question completely.
Can moderate workplace noise exposure causing a TTS and repeated exposures later cause a PTS?
Repeated noise exposure that causes a temporal threshold shift (TTS) can ultimately lead to a permanent threshold shift (PTS) with repeated exposures. Some of this is based on the redundancy principle within the inner ear: not all hairs cells possibly recover following a TTS but enough do as to prevent hearing loss.

Following acute acoustic trauma what is the natural history of the subsequent hearing loss?
Audiometric evaluations were performed on these individuals at 6 months, 1, 2 and 4 years after initial exposure. In his study cohort only 1% of individuals following acute acoustic trauma demonstrated deterioration of hearing of more than 20 dB in at least 1 frequency. Occupational NIHL and presbycusis (degenerative hearing from aging change) represent the two most common causes of sensorineural hearing loss in society today. For this reason workers ideally should be out of noise for at least 24 hours if not 48 hours prior to audiometric testing to avoid the effects of TTS on hearing. Noise exposure from chipping machines and jackhammers characteristically damage the higher frequencies severely before affecting the lower frequencies. Nevertheless some individuals exposed to noise not infrequently demonstrate some asymmetry to their hearing loss.
The majority of changes histopathologically are noted in the basal turn of the cochlea where the high frequency hair cells and their corresponding cochlear nerve neurons are found. When the distended membranes rupture the resulting admixture of inner (endolymph) and outer (perilymph) fluids causes electrolyte disturbances (i.e.
When the foci primarily affect the cochlea the patient may present with a chronic progressive sensorineural hearing loss in one or both ears.
As otosclerosis is a lifelong condition, the sensorineural hearing loss from cochlear otosclerosis is often superimposed on hearing loss from advancing age (i.e. The hearing loss noted is usually conductive and usually arises from discontinuity of the ossicular chain although any combination of conductive and sensorineural hearing loss can occur.
If the individual survives there is usually complete loss of hearing and vestibular function on the involved side. Persistence of a conductive loss after the TM has healed would suggest continued problems with the ossicular chain. Careful and regular monitoring of auditory function (especially in the ultra high frequencies > 8000 Hz) and serum antibiotic levels may help the physician predict when ototoxic effects are occurring. Pressure on or devascularization of the acoustic nerve or inner ear causes hearing loss, usually a high-frequency loss with reduced speech discrimination.
Patients can sometimes identify the moment of occurrence or wake with it or appreciate it only when they go to use the phone. If there is a significant discrepancy this could imply an exaggerated hearing loss is present as well. When speech discrimination scores are especially poor this implies that there may be a lesion involving the cochlear nerve (i.e. In general terms if the Eustachian tube is functioning normally pressure on both sides of the ear drum should be similar (i.e. Waveform morphology is thought to arise from the cochlear nerve and the various relay stations in the brainstem the electrical response has to travel through.
The test is often indicated in an individual if there are concerns regarding an exaggerated hearing loss. Tinnitus maskers are noise generators worn like a hearing aid that produces a sound of similar frequency to an individuala€™s tinnitus.
The problem with tinnitus however is that it is difficult to quantify objectivity and to fully appreciate how it affects an individuala€™s well-being. This test measure electrical activity along the cochlear nerve and at the various relay stations in the brainstem between 1-10 msec of stimulation. This test looks at electrical waveforms in the cortical areas of the brain between 50-200 msec after cochlear stimulation. In humans the auricle has rudimentary functions for gathering sounds and in sound localization which is much more important in animals. The first part of the ear canal is cartilaginous and is lined by thick, hair bearing skin that contains numerous sebaceous and ceruminous glands. Malfunction of the Eustachian tube remains the leading cause for most middle ear disorders.
The air cell system is typically underdeveloped or even absent (sclerotic) in individuals with a past history of recurrent ear disease.
It is a three layered structure whose outer epithelial layer is contiguous with the skin of the external auditory canal, its middle layer is fibrous and its inner layer is composed of the same mucosal lining found throughout the middle ear cleft. Clinically the malleus is attached by its handle to the tympanic membrane and is readily visible in most individuals on otoscopic examination. The separate but intimately related perilymphatic and endolymphatic compartments of the inner ear and their ionic constituents are efficiently controlled by the stria vascularis of the cochlea and the dark cells of the vestibular apparatus. This ionic difference between these chambers forms the basis of the endocochlear electrical potential which is necessary for the conversion of mechanical activity from fluid movement within the cochlea to electrical activity arising from the hair cells. The hair cells of the otolithic endorgans (utricle and saccule) are covered by otoliths ( literally a€?little stonesa€? or crystals of calcium carbonate ). The mechanical vibrations are transmitted to the inner ear via the vibrations of the stapes footplate. The tiny cilia (little hairs) on top of the hair cells (both inner and outer) are covered by a gelatinous membrane called the tectorial membrane. This apparent internal a€?feedback loopa€? is thought to be responsible for a€?fine tuninga€? by inhibiting some unwanted electrical impulses and by changing the mechanical properties of the basilar and tectorial membranes. Distortion of certain sounds arises when enough hair cells are damaged such that the hearing threshold is reduced. A cycle of a pure tone is represented by a sine wave appearance with an area of compression followed by rarefaction. Low sounds tend to have a long wavelength relative to higher pitched sounds which have shorter wavelengths by comparison.
Entitlement to health care and rehabilitation benefits is available when the adjusted hearing loss is at least 22.5 dB in each ear. When the two processes are combined, the resultant pathology and its effects upon aging are not as well understood.
In other words many hair cells in a similar region of the cochlea will produce a certain frequency response to sound stimulation. Upon saying this however we can actually demonstrate that many individuals will start to show early changes in hearing as early as age 40 years (in subjects screened to rule out other ear disease and noise exposure).
It is agreed that hearing loss and injury to the ear increases with the noise level, the duration of exposure, the number of exposures and the susceptibility of the individual.
The findings also indicate that the late progression of hearing loss following a single episode of acute acoustic trauma did not exist.
Tribunal adjudicators recognize that It is always open to the parties to an appeal to rely on or to distinguish a medical discussion paper, and to challenge it with alternative evidence: see Kamara v.
Constant or steady state noise by comparison remains relatively constant and lasts longer although fluctuations in sound intensity may occur.
This is however usually related to the fact that one ear receives a greater exposure to the noise than another. Nevertheless the changes seen in presbycusis are typically non-specific and can also be seen in a vast number of pathologies including the effects of noise upon the inner ear. Occasionally both a low-frequency and a high-frequency loss occurs but not usually the type of high frequency loss seen following noise exposure.
When vertigo is incapacitating the balance function of the inner ear may be attenuated or ablated by the trans-tympanic instillation of gentamicin with relative preservation of hearing.
If the footplate as well as the cochlea is involved then a mixed (conductive and sensorineural) loss might result. The pathognomic sign of a longitudinal temporal bone fracture is the so-called a€?step deformitya€? in the deep ear canal. Facial paralysis from an injury to the facial nerve (a nerve that runs in close approximation to the inner ear) typically occurs.
The prolonged use of topical aminoglycoside antibiotics to treat middle ear pathology in the presence of a TM perforation is not without some risk which should always be kept in mind.
In fact some people with a€?superhumana€? hearing can even hear sound intensities less than this relative value of sound pressure. In complex situations (mixed hearing losses) it may also be necessary to a€?maska€? the ear not being tested (to prevent crossover of sound to the other ear). When tinnitus is severe enough to affect an individuala€™s well-being a trial of pharmacological treatment is generally recommended. Investigations such as a magnetic resonance imaging (MRI) are generally required to exclude the remote possibility of central nervous system pathology.
Because of the brains proximity possible life-threatening complications can occur if left untreated (i.e.
This test is typically performed when an asymmetric sensorineural hearing loss is present when there is concern a retrocochlear lesion such as an acoustic neuroma or MS might be present. The presence of waveforms following stimulation with the lowest intensity sound an individuala€™s cochlea hears provides us with a reasonable estimation of an individuala€™s hearing (usually within 5-10 dB of the anticipated threshold hearing at the frequency tested). The deeper part of the ear canal is bony, lacks skin appendages and is lined by thinner skin densely adherent to the periosteal lining of the bone. It is anatomically divided into 2 parts: the pars tensa and the pars flaccida (located in what is called the attic region of the tympanic membrane). Medially the arch (crura) of the stapes attaches to the inner ear via its footplate (which is a thin, flat oval piece of bone approximately 7mm X 3mm in dimension). Series of generation potentials culminate to form action potentials that transmit this electrical activity along the cochlear nerve fibres to the brainstem and onto the higher centers of auditory processing. The hair cells of the vestibular macula respond primarily to gravity and linear accelerations. The presence of active non-muscular contractile elements within the outer hair cells and their effects on movement of the basilar membrane is used to explain the concept of otoacoustic emission (OAE) testing.
When the sound gets loud enough the inability to a€?fine tunea€? sound becomes lost and more nearby hair cells are drawn into the firing needed to create an electrical signal; hence the distortion of a loud sound.
Hair cell loss can continue until a certain critical point is breeched with the individual unaware of a hearing deficit.
Follow-up beyond the 1 year mark did not reveal any further deterioration of hearing in any individual. For example, truck drivers in North America not infrequently have a greater degree of hearing loss in their left ear (when the window is rolled down the left ear would be exposed to more sound from the engine). In the last resort the whole inner ear can be destroyed surgically by a procedure called a labyrinthectomy.
Exploration of the middle ear surgically with correction of the ossicular chain or insertion of a prosthesis is called an ossiculoplasty.
On examination blood is usually seen in the middle ear behind the ear drum (a hemotympanum).
The resultant hearing loss depending on the mechanism can be conductive, sensorineural or a combination thereof. Of interest the first course of cisplatinum often demonstrates which patients are vulnerable to cochlear damage and what may happen in future if continued treatment courses are required.
ANs are overwhelmingly unilateral, but in association with a rare genetic disorder, neurofibromatosis type 2, they may be bilateral. Another concept to appreciate is that the dB scale is a logarithmic (an exponentially increasing) rather than arithmetic (linear increasing) scale which makes it difficult to apply a percentage hearing loss to any individual.
Certain anti- depressants (especially the tricyclic or SSRI classes) and anti-convulsants (medications used for seizure control) have been effective for some individuals.
Mean comfort levels (MCLa€™s) and uncomfortable loudness levels (ULLa€™s) during audiometry provide some information concerning the threshold intensity of sound an individual can tolerate. One general maxim in medicine is that a€?an unexplained unilateral sensorineural hearing loss should be considered an acoustic neuroma until proven otherwisea€?. Continuous migration of epithelium from the deeper to the lateral portion of the ear canal tends to keep the ear clean. The pars tensa or lower four-fifths of the ear drum is conically shaped and has a strong fibrous middle layer which provides strength. The blood supply to the incus can be tenuous compared to other structures of the middle ear. Abnormalities in vestibular end organ function often result in the patient experiencing the subjective complaint of vertigo.
In general terms the greater the frequency the higher the pitch of the sound and the greater the intensity the louder we hear it. Of interest, in a parallel series of control patients who were further exposed to repeated episodes of acute acoustic trauma during the same study period by the end of 4 years 30% of these individuals had demonstrated significant hearing deterioration.
All things being equal most noise in industry however is a combination of primary continuous and superimposed impact type noise. Those who fire guns often demonstrate a greater degree of hearing loss in the ear closest to the barrel (the left ear in a right-handed shooter) because that ear would be closest to the explosion and the other ear would be protected by a a€?head shadowa€?. However the natural history is enigmatic with unpredictable periods of exacerbation and remission.
For example the difference between 10 and 40 dB in sound pressure intensities is not a 4xa€™s increase but actually an increase by a factor of 1,000 or 103 in sound pressure intensities! An experienced tester, typically an audiologist, is required to perform these technically demanding tests. Wax and debris can collect in the ear canal when this migratory mechanism fails or is overloaded from infectious or inflammatory conditions. The pars flaccida or upper one-fifth is less distinct and has a poorly developed middle fibrous layer. When pathology affects its blood supply it can lead to the erosion of its long process which is attached to the stapes.
This makes the pars flaccida especially susceptible to negative middle ear pressure and prone to retractions that can lead to the formation of retraction pockets. Another structure called Reissnera€™ s membrane (R) forms the roof of the scala media (M) which separates this from the scala vestibuli (V). In complex sounds the interaction of its pure tone components forms the basis of its complexity or its psychological counterpart known as timbre. When retraction pockets fail to be self-cleansing they fill with debris and desquamated skin that can lead to secondary infection and bone erosion better known as cholesteatoma.

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03.11.2014 admin

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