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HARP. Case Notes
Noise Induced Hearing Loss (NIHL)
Alternative Names:
 Occupational noise induced hearing loss refers to a hearing loss caused
by loud sounds experienced in a work place (Mathur & Roland, 2009).
 Acoustic Trauma refers to a sensorineural hearing loss that is caused by a
single impulse of sound above 130 dB (Rosen & Vrabec, 2001). This can
cause trauma to the ear drum, middle ear and the cochlear (Gelfand,
2009) but it is not the focus of this report.
Definition:
A hearing loss caused by exposure to loud sounds at 85 dB or over through a
prolonged period of time (Rabinowitz, 2000). NIHL is most likely to occur in an
occupational setting where sounds are either presented in a continuous
manner or impulsively (Axelsson, 1979).
Pathophysiology:
Prolonged exposure to loud sounds is most likely to occur in occupational
settings, for example in woodworking shops or factories (NIDCD), where
noise is constantly being emitted. This differs from an impulsive sound when
the noise is sudden, such as those produced from an explosion (NIDCD), like
a firework or a gun (Gelfand, 2009).
Other factors that could cause NIHL are ototoxic drugs such as gentamian
(Gelfand, 2009) and certain ototoxic chemicals such as those found in metals
and pesticides (Morata, 2007). These drugs and chemicals have been found
to damage the ear and related auditory systems as well as interacting with
sound (Morata, 2007). A study by Toppilia et al (2001) found that more than
two confounders such as blood pressure, smoking and serum cholesterol
could be the overriding reason for a hearing loss rather than sound exposure.
Toppilia et al. (2001) found the age could affect NIHL because the inner ears
of the elderly are more susceptible to noise.
NIHL can be permanent or temporary. Temporary Threshold Shift (TTS) is a
brief hearing loss after noise exposure but there is a full recovery after a rest
period. This normally disappears between 16 and 48 hours after exposure to
loud sounds (NIDCD). Permanent threshold shift (PTS) occurs after repeat
noise exposure which causes an irreversible increase to the hearing threshold
(Gelfand, 2009).
NIHL damages the stereocilia in the cochlea affecting the outer hair cells
more than inner hair cells (Gelfand, 2009). Depending on the frequency and
intensity of the noise exposure the stereocillia can be affected in different
ways (Lynch & Kil, 2005). It is thought that TTS is related to stereocillia losing
their rigidity (Mathur &Roland, 2009). PTS could be due to the loss of
stereocillia and stereocillia fusing together (Mathur & Roland, 2009). Loss of
stereocillia is due to extended metabolic activity when the hair cells respond
to intense sounds. The increase in metabolic activity means the hair cells are
more susceptible to the formation of oxygen and nitrogen free radicals,
leading to antioxidant mechanisms being inundated (Lynch & Kil, 2005). This
combination causes metabolic exhaustion leading to cell death (Mathur &
Roland, 2009)
HARP. Case Notes
Symptoms
 Difficulty understand speech because sounds are indistinct and
misinterpreted (NIDCD).
 Tinnitus accompanies TTS and PTS (Mathur & Roland, 2009)
Signs
 Sensorineural hearing loss with higher frequencies deteriorating first
(Rabinowitz, 2000).

Temporary tinnitus and temporary hearing loss was reported after noise
exposure in a study involving teenagers. 70% commented on having
temporary tinnitus and 44% reported temporary threshold shift (Jokitulppo
et al, 1997). This study does not include individuals over the age of 18
however the NIDCD also states that tinnitus is usually present especially in
TTS, suggesting these symptoms are experienced throughout different
ages. NIDCD also states that the longer the exposure to loud sounds the
longer the tinnitus will last until it becomes permanent.
Epidemiology:
NIHL can affect anybody at any age, male or female although it has been
shown that males are more affected than males (Rabinowitz, 2000). It has
also emerged that up to 10% of younger people are more at risk of developing
NIHL (Borchgrevink, 2003, cited in Hodgetts, et. al, 2009). Race may also be
an influencial factor with “Caucasian males" more likely to have poorer
audiometric thresholds than "African American females" (Morata, 2007).
The prevalence of NIHL can be hard to estimate because it is not definitive
that noise exposure is the sole cause of a hearing loss, other factors such as
genetics, ototoxicity could play a part. NICDC predicted that 15% of 20-69
Americans have a high frequency hearing loss thought to have been caused
by noise exposure at work or in social activities. Estimates from National
Institute of Health (1990, cited in Morata, 2007) suggest 1/3 of hearing losses
are caused in some part due to noise exposure and Henderson (2001)
estimated 35% to 51% have a hearing loss due to noise trauma when looking
at occupational noise exposed populations.
Management:
The US Occupational Safety and Health Administration (OSHA) have
imposed regulations in noisy occupational places. For each 5 dB increase in
sound above the average noise level (85 dB) the exposure time for individuals
with unprotected ears decreases by half (MedicineNet. com). Ear defenders
or ear plugs should be worn when sounds will exceed 85 dB.
Otoscopy
Figure 1: Normal otoscopy results that are present in NIHL
Right
Left
HARP. Case Notes
Notes: Otoscopy will reveal no abnormalities because the outer ear is not
damaged. Noise induced hearing loss (NIHL) affects the cochlear hair cells
making the hearing loss sensorineural (Gelfand, 2009). In this case the NIHL
is bilateral.
Problems associated with testing: Otoscopy is not very useful in this case
because it will not help to identify the condition because it does not affect the
structure of the tympanic membrane.
Tuning forks
Figure 2: Tuning fork results
Weber:
Rinne:
Right Ear
positive
positive
Notes: NIHL produces a sensorineural hearing loss (Gelfand, 2009). If there is
no conductive element then the Weber test will lateralise to the side that has
the smallest sensorineural loss, in this case the right ear is the better hearing
ear. A positive rinne result suggests that there could be a sensorineural
element to the hearing loss or that the hearing is within normal limits (British
society of audiology, 1987 &Rabinowitz et al, 2000)
Reliability of test: Tuning forks only provide an indication of what type of
hearing loss could be present. As shown above there are several options for
the same result. Therefore other hearing test must be performed to clarify
these results, for example PTA.
Pure tone audiometry and uncomfortable loudness levels
Figure 3: Common configuration of NIHL
HARP. Case Notes
RIGHT
LEFT
-10
0
0
10
10
20
20
30
30
Hearing level (dB)
Hearing level (dB)
-10
40
50
60
70
80
40
50
60
70
80
90
90
100
100
110
110
120
120
130
130
140
140
125
250
500 1000 2000
Frequency (Hz)
4000
8000
125
250
500 1000 2000
Frequency (Hz)
4000
8000
Notes: The most common configuration of NIHL is a bilateral, symmetrical
sensorineural hearing loss that rarely exceeds 75 dB (Mathur & Roland, 2009)
shown above in figure 1. No masking is needed for air conduction or bone
conduction. This distinguishing feature is a notch that is present at 4000 Hz.
It is unclear why but theories include the cochlea’s structure is more sensitive
to damage at this frequency or the outer and middle ear resonance boosts the
noise between2 KHz and 4 KHz (Gelfand, 2009).
Figure 4: Other possible configuration of NIHL
The longer the exposure to noise the notch at 4 KHz widens and deepens
(Mathur & Roland, 2009) to include the lower frequencies. In extreme cases, if
continual noise exposure continues then the audiogram can resemble a steep
sloping hearing loss (Rabinowitz, 2000).
Alternative Tests: Research has suggested that the use of extended
frequency audiometry can be used to identify NIHL at earlier stages. However
this is dependent on age and may only be useful in identifying younger
patients (Hallmo et al, 1995 & Somma et al, 2008).
Tympanometry
Figure 5: Tympanogram results
HARP. Case Notes
Notes: Tympanometry results will be within normal limits (Jerger et al, 1981)
displaying a type A tympanogram because NIHL is a cochlear disorder and
does not affect the middle ear.
Acoustic reflexes
Figure 6: Acoustic Reflex Thresholds for NIHL with thresholds below 50 dB
Threshold
Right
contralateral
ipsilateral
85 dB
85 dB
500 Hz
ipsilateral
85 dB
Left
contralateral
80
80
80
1000 Hz
80
85
60
60
BBN
65
65
Notes: For a sensorineural hearing loss that is below 50 dB HL the acoustic
reflex thresholds will be within the normative range (Gelfand, 2009 &
Glasscock & Gulya, 2003). For pure tones this is between 75 – 90 dB HL and
for broadband noise this is between 60 – 75 dB HL(reference ppt).
Figure 7: Acoustic reflex thresholds for NIHL with thresholds above 50 dB
Threshold
Right
contralateral
ipsilateral
85 dB
85 dB
500 Hz
Left
ipsilateral
contralateral
85 dB
90
90
85
1000 Hz
90
90
95
95
2000 Hz
95
95
>100
>100
4000 Hz
>100
>100
85
80
BBN
85
80
If the NIHL exceeds 50 dB then the acoustic reflex thresholds will elevate
according to the degree of loss (Gelfand, 2009 & Glasscock & Gulya, 2003).
Depending on the shape of the audiogram the lower frequencies are more
likely to have an acoustic reflex threshold because the hearing loss is not as
severe. As the acoustic reflexes are tested at higher frequencies the degree
of hearing loss gets progressively worse so acoustic reflex thresholds are
going to be elevated or absent. Where >100 dB HL is stated it means that the
tests has been stopped to prevent any risk of injury to the ear drum.
Broad band noise (BBN) is to be used if the patients suffers from severe
tinnitus because it allows the reflex to be obtained using lower frequencies so
that the tinnitus is less likely to be aggravated.
HARP. Case Notes
Otoacoustic emissions
Transient Evoked Otoacoustic emissions (TEOAE’s) testing was carried out
between 500 Hz to 4 kHz. ILO 292 measurement system was used. A
standard non-linear stimulus paradigm was used at a rate of 50 clicks per
second in order to minimise the risk of stimulus artefacts. Definition of a
present TEOAE was based on the reproducibility percentage with values
above considered to be a possible TEOAE, above 75% a definite TEOAE and
below 50% the procedure was repeated once and then deemed to be too
noisy to obtain an accurate measurement. It was also based on the dispersion
of the TEOAE waveform with high frequency components appearing first,
followed by low frequency components. The overall shape of the OAE was
also taken into account. This test is particularly useful in children, as it does
not require patient response (OAE Guide, 2009).
Figure 7: TEOAE results for a typical audiogram configuration for NIHL
Definitely present
Right

Left

Possibly present
Absent
Notes: For a NIHL with an audiogram similar to figure 3 the TEOAE’s are
most likely to be present because the hearing loss does not exceed 40 dB in
all frequencies. A TEOAE may not be present at 4 KHz if it exceed 40 dB.
Figure 8: TEOAE results for a progressive NIHL
Definitely present
Possibly present
Right

Left

Absent
As the NIHL progresses and the notch widens OAE’s may be absent because
the lower frequencies are most likely to exceed 30 dB causing the TEOAE’s to
be absent (Gelfand, 2009)
Figure 9: TEOAE results for a severe case of NIHL
HARP. Case Notes
Definitely present
Possibly present
Absent
Right

Left

In severe cases the OAE’s will be absent because the sloping hearing loss
will include the lower frequencies threshold being below 30 dB (Gelfand,
2009).
General comments: Then diagnosis of NIHL should only be made following a
battery of audiological tests, such as those indicated above. Differential
diagnosis should be considered with conditions that give similar results.
Medical concerns should be referred to ENT.
References
Axelsson, Aif, (1979), Diagnosis and treatment of noise induced hearing loss,
Acta Oto-laryngologica, [online], 86:360, pp 86-87. Available from:
http://informahealthcare.com/doi/pdf/10.3109/00016487809123481 [Accessed
3rd May 2010]
Gelfand, S.A, (2009), Essentials of Audiology, 3rd Ed., New York, Thiemes
Medical Publication, pg 184, pg 351 & pg 228
Glasscock, Michael E, Guyla, Aina J (2003) Surgery of the Ear 5th Ed.
Ontario, BC Decker Inc.
Hallmo, Petter, Borchgrevink, Hans M, & Mair, Iain WS, (1995), Extended
High-frequency Thresholds in Noise-induced Hearing Loss, Scandinavian
Audiology, 24: 1, pp 47 -52, Available from:
http://pdfserve.informaworld.com/90745_731421667_789451982.pdf
[Accessed: 3rd May 2010]
Henderson, Don, Prasher, Deepak, Kapke, Richard, Salvi, Richard &
Hamernik Roger, (2001), Noise Induced Hearing Loss: Basic Mechanisms,
Prevention and Control, London, NRN Publications
Hodgetts, W, Szarko, R, & Rieger, E, (2009) What is the influence of
background noise and exercise on the listening levels of iPod users,
International Journal of Audiology, 48, pp 825-832
Jerger, Susan., Jerger, James, (1981), Auditory Disorders A manual for
clinical evaluation, Boston, Little, Brown and Company
Jokitulppo, Jaana S, Bjorki, Erkki A, Akaan-Penttia, Eero, (1997), Estimated
Leisure Noise Exposure and Hearing Symptoms in Finnish Teenagers,
International Journal of Audiology, [online], 26:4, pp 257-262, Available from:
http://informahealthcare.com/doi/pdfplus/10.3109/14992029709048017
[Accessed 2nd May 2010]
HARP. Case Notes
Lych, Eric D, Kil, Jonathon, (2005), Compounds for the prevention and
treatment of noise induced hearing loss, Drug Discovery Today, [online],
10:19, pp 1291-1298, Available from:
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31st January 2010]
Morata, T.C, (2007), Promoting hearing health and the combined risk of
noise-induced hearing loss and ototoxicity, Audiological Medicine, 5, pp 33-40
NIDCD, National Institute of Deafness and Other Communication Disorders,
Bethesda MD, USA, Available from:
http://www.nidcd.nih.gov/health/hearing/noise.asp [Accessed 12th November
2009]
Rabinowitz, Peter M, (2000), American Family Physician, Connnecticut,
American Academy of Family Physicians, Available from:
http://www.aafp.org/afp/20000501/2749.html [Accessed: 31st January 2010]
Rosen, ElizabethJ, Vrabec, Jeffery T, (2001), Noise Induced Hearing Loss,
Available from: http://www.utmb.edu/otoref/grnds/Hear-Loss-Noise000110/Hear-Loss-Noise-slides.pdf [Accessed 3rd May 2010]
Somma, Giuseppina, Pietroiusti, Antonio, Magrini, Andrea, Coppeta, Luca,
Ancona, Carla, Gardi, Stefano, Messina, Marco, Bergamaschi, Antonio,
(2008,) Extended High-Frequency Audiometry and Noise Induced Hearing
Loss in Cement Workers, American Journal of Industrial, 51, pp 452–462,
Available from: http://www3.interscience.wiley.com/cgibin/fulltext/117954009/PDFSTART [Accessed 3rd May 2010]
Toppila, Esko , Pyykkö, Ilmari andStarck, Jukka, (2001), Age and noiseinduced hearing loss, Scandinavian Audiology, [online], 30: 4, pp 236 – 244,
Available from:
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(1996) MedicineNet.com, San Clemente California, Available from:
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(1987) 'Recommended procedure for Rinne and Weber tuning-fork tests',
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[Accessed: 18th March 2010]
HARP. Case Notes