Download NIHL - The Medical Post

Noise Induced
Hearing Loss
Dr. Vishal Sharma
Definitions
• Noise = wrong sound, in wrong place, at wrong
time (Park & Park)
• Acoustic Trauma = sudden, permanent sensorineural deafness due to single exposure to an
intense sound (130-140dB) of < 0.2 sec
• Chronic Noise Induced Hearing Loss = gradual
SNHL due to years of exposure to noise
History
• Ramazzini (1713): reported NIHL in copper workers
• NIHL recognized in US, Germany & UK in 1870s
• Thomas Barr of Glasgow (1886): did first survey
• Habermann (1890): histology of NIHL in cochlea
• Fowler (1929): first to comment on 4 kHz dip
• Bunch (1939): audiometric features of NIHL
Epidemiology
• 30 million adults in United States are exposed to
hazardous occupational sound levels (National
Institute for Occupational Safety & Health, 2000).
• Among these 30 million, 1 in 4 will acquire a
permanent hearing loss (American Academy of
Audiology, 2003).
• 50 % of male miners have hearing loss by age of
50 & 70 % by age of 60 (NIOSH, 2001).
Classification of noise
A. Based on duration:
B. Frequency based:
1. Continuous: cotton spinning
1. High: sawing
2. Interrupted: traffic
2. Low: grinder
3. Transient (< 0.2 sec)
3. White noise: boiler
a. Impulse: explosion
b. Impact: metal to metal collision
Sources of noise
• Industrial noise
• Road traffic noise
• Aircrafts & Railways noise
• Entertainment noise: clubs, discos
• Residential noise: alarms, music systems, home
theatre, air conditioners, generators
• Personal noise: personal stereos, mobile phones
• Firearms & bomb blast noise
Common Occupational Noise
• Agriculture
• Mining
• Construction
• Manufacturing
• Public Utilities
• Transportation
• Military
Volcano eruption
190 dB
Jet plane
120-150 dB
Thunder
120 dB
Factory boilers / trains
110 dB
Cars & bikes
90 dB
Loud radio music
85 dB
Grand piano
85 dB
Children crying
80 dB
Loud conversation
70 dB
Dog barking
70 dB
Sound Thermometer
Clinical Features of N.I.H.L.
• Similar to early ototoxicity & presbycusis
• Early NIHL limited to high frequency: no symptom
• First symptom: trouble in speech comprehension
with loud background noise
• As NIHL progresses, patients have difficulty in
understanding high-pitched voices (women &
children) even in quiet conversational situations
Clinical Features
• Telephonic conversation is unimpaired because
telephones don’t use frequencies > 3000 Hz
• Many patients experience tinnitus & hyperacusis
• Post-exposure tinnitus & temporary deafness are
warning signs of impending permanent NIHL
Impact of noise on children
• Household noise retards cognitive development of
children b/w 7-24 months. Brains of children cope
with loud noise by sound filtering which also
includes human speech. This leads to retardation
of intellectual skills.
• Foetus also may suffer from effects of noise.
Characteristics of
chronic NIHL
(Dobie, 1990)
• Always sensorineural, symmetric & bilateral.
• Greater SNHL present at 3, 4, & 6 kHz, with
recovery at 8 kHz ***. Usually greatest at 4 kHz.
***Presbyacusis has no recovery at 8 kHz
• Isolated NIHL is never > 75 dB in high
frequencies or > 40 dB in lower frequencies.
• Rate of hearing loss in chronic NIHL is greatest
during first 10-15 years of exposure & decreases
later as hearing worsens. Hearing loss does not
progress after noise exposure is discontinued.
• Commonest cause of asymmetric NIHL is
exposure to firearms. Right-handed shooters have
more severe left ear deafness (left ear faces barrel
while right ear is in acoustic shadow of head).
• Similarly, Tractor operators look over their right
shoulder, exposing their left ear to noise of prime
mover + exhaust & their right ear gets shielded.
Acoustic head shadow
Auditory Effects of Noise
• Per-stimulation fatigue or Adaptation:
Noise exposure of > 90 dB elevates hearing
threshold temporarily, especially at 4 kHz
• Post-stimulatory fatigue:
• Temporary Threshold Shift (reversible)
• Permanent Threshold Shift (irreversible)
Temporary Threshold Shift
• Exposure to loud noise for seconds to hours may
cause SNHL that recovers within 16-24 hours.
• Magnitude of TTS depends on:
• More intense sounds lead to larger shifts.
• Speech frequencies (500-3000 Hz) are most
susceptible to TTS.
• Interrupted exposures cause less TTS than
continuous exposures.
Permanent Threshold Shift
• Repeated TTS over weeks, months & years fail to
recover completely & become noise-induced
permanent threshold shift (NIPTS)
• NIPTS is measured by subtracting amount of
hearing loss to be expected due to aging from
hearing threshold level
• Amount of NIPTS & frequencies involved depend
primarily on same parameters, as for TTS
Calculation of N.I.P.T.S.
Age related hearing loss
Progress of N.I.P.T.S.
Progress of N.I.P.T.S.
Male vs. female N.I.P.T.S.
Non-auditory effects of noise
• Insomnia
• Speech interference
• Anxiety & nervousness
• Hypertension
• Coronary heart disease • ed heart rate
• Lack of concentration
• ed intra-cranial tension
• Fatigue
• ed breathing rate
• Irritability & impatience
• ed sweating
• Indigestion
• ed color perception
• Peptic ulcer
• ed night vision
Factors influencing
N.I.H.L.
1. Frequency: noise b/w 2-3 kHz  more damage
2. Intensity & duration of exposure: noise > 85 dBA
for 8 hr time weighted average is unsafe
3. Intermittence: intermittent noise with quiet
intervals is safer than steady noise
4. Age: children & elderly are more prone to NIHL.
Presbyacusis has additive effect not synergistic.
5. Sex: males are more prone than females to NIHL
6. Individual Susceptibility: Tender ears are more
prone to NIHL than tough ears due to difference in:
 Genetic: Ahl gene
 E.A.C. skin elasticity
 area ratio of TM:stapes
 mass of ossicles
 tension of stapedius
 outer hair cell density
 basilar membrane stiffness  cochlear vascularity
 endolymph composition
 psychological
7. Experience: Green ears have bigger NIPTS than
those exposed to noise before (ripe ears). NIPTS
can be reduced by adding prior exposure at lower
levels (toughening or conditioning of ear).
8. Conductive deafness: gives protection (?) against
NIHL as less sound gets conducted to inner ear
9. Ototoxic drugs: enhance NIHL
10. Vibration: Raynaud phenomenon es NIHL
11. Absent Stapedial reflex: es NIHL for lower
frequency
12. Melanization: Albinos are more prone to NIHL
13. Diabetes mellitus, Hyper-cholesterolemia,
Cardiovascular disease: increase risk of NIHL (?)
14. Smoking: increase risk of NIHL (?)
15. Deep sea divers: are more prone to NIHL
Patho-physiology
NIHL damage is explained by:
• Macro-mechanical theory
• Micro-mechanical theory: produced by high
intensities
• Biochemical theory: produced by moderate
intensities
Macro-mechanical theory
• Traveling sound wave produces movement of
basilar membrane
• Central part of basilar membrane undergoes
maximum rocking vibration. This part houses
outer hair cells (especially inner row) hence they
are subjected to maximum damage.
Organ of Corti
Micro-mechanical Damage
Temporary Threshold Shift: moderate swelling of
hair cells + shortening of rootlets of hair cells +
small vacuoles in supporting cells
Permanent Threshold Shift: marked swelling of hair
cells + fracture of rootlets of hair cells + large
vacuoles in supporting cells + damage to outer
hair cells + damage to inner hair cells
Biochemical Damage
• Micro-mechanical damage causes interruption of
normal chemical gradient of cochlea which leads
to ionic poisoning
• Noise activity induced vaso-constriction leads to
cochlear ischaemia
• Metabolic exhaustion of activated hair cells
Investigations
A. Diagnostic
B. Research only
1. Subjective
 Low power Microscopy
 Pure Tone Audiogram
 Electron Microscopy
2. Objective
 Cyto-cochleogram
 Oto-acoustic emissions
 Cortical Evoked Response Audiometry
 Multiple Auditory Steady-state Evoked Response
Audiogram in Acoustic Trauma
Audiogram in Chronic N.I.H.L.
4 kHz (Boilermaker’s) notch, C5 dip
• characteristic audiometric
pattern of early NIHL.
• If exposure is continued,
notch gradually deepens
& widens.
• 4 kHz notch also seen in
head injury, barotrauma
or even in absence of any
explanatory history.
Reasons for 4 kHz notch
1. Natural resonance of EAC being 2-3 kHz, 150% of
this produces maximum damage between 3-5 kHz
2. Protective effect of acoustic reflex below 2 kHz
3. Intermittent noise is more damaging for 3-5 kHz
4. Outer hair cells of 3-5 kHz region are more prone
to oxidative stress, have reduced vascularity (?)
& ed oxygen consumption (?)
Acoustic gain: middle ear & pinna
Otoacoustic Emissions (OAE)
• Spontaneous OAE: Sounds emitted without
stimulus. Presence indicates hearing < 25 dB HL.
• Transient evoked OAE: Sounds emitted in response
to short duration click stimulus. Presence = < 35 dB HL
• Distortion product OAE: Sounds emitted in
response to 2 tones of different frequency & intensity.
Presence = < 50 dB HL. Good for higher frequencies.
Normal Transient evoked OAE
Normal Transient evoked OAE
Reproducibility should be > 75 %
Early detection of N.I.H.L.
Early stage N.I.H.L.
Advanced stage N.I.H.L.
Malingering of N.I.H.L.
Auditory Evoked Potentials
• Auditory Brainstem Response: 1.5-10 ms post
stimulus; originates in 8th cranial nerve (waves I & II)
up to lateral lemniscus & inferior colliculus (wave V)
• Middle Latency Response (MLR): 25-50 ms post
stimulus; arises in upper brainstem & auditory cortex
• Slow Cortical Response: 50-200 ms post stimulus;
originating in auditory cortex
Auditory Evoked Potentials
Cortical Evoked Response Audiometry
(CERA) or P1-N1-P2 response
• good specificity over speech frequency range
• recorded from higher auditory level than BERA, so
less subject to organic neurological disorders
• CERA must be done to evaluate accurate hearing
threshold in pt with flat audiogram & hearing
threshold of > 25 dB at 500 Hz
Multiple Auditory Steady-state
Evoked Response audiometry
• Are responses to rapid stimuli where brain response to
one stimulus overlaps with responses to other stimuli
• Gives rapid, frequency specific & objective hearing
assessment by giving 4 continuous tones to each ear
• Slow rate responses (<20 Hz) arise in cortex & faster
rate responses (>70 Hz) originate in brainstem
Multiple Auditory Steady-state
Evoked Response audiometry
Photo-micrograph of Cochlea
Scanning electron micrographs
Cyto-cochleogram
Noise Exposure Evaluation
Definitions
• A scale (dBA): gives more weight to frequencies
b/w 1 to 5 kHz & less weight to other frequencies
• Time Weighted Average (TWA8): noise level (dB) if
kept constant for 8 hours would have same risk of
NIHL as briefer noise exposure in question
• Noise dose (D): Percentage of maximum allowed
noise exposure throughout the working day
Weighted Audiometry scales
Subjective Evaluation
Voice level
Distance of listener
from speaker
Noise level
(dBA)
Loud voice
6 feet
85
Loud voice
4 feet
90
Loud voice
2 feet
95
Shout
4 feet
100
Shout
2 ft
105
Impossible to hear even when close
> 110
Sound Level Meter
• Hand-held instrument
• Real-time display of
sound level (dBA)
coming from noise
sources
Noise Dosimeter
Worn by employees to determine TWA exposure
Sound meter
• Sound power doubles for each 3-dB increment
• Frequent intermittency in occupational noise
exposures, reduces risk of NIHL. Thus:
• O.S.H.A. reduces Permissible Exposure Level
(PEL) by 50% for every 5-dB increment
• 8-hour exposure at 90 dBA, 4-hour exposure at 95
dBA, 2-hour exposure at 100 dBA are considered
equally hazardous.
Occupational Safety & Health
Administration regulations
Sound
Intensity
Permitted
exposure
Sound
Intensity
Permitted
exposure
85 dBA
16 hours
105 dBA
60 minutes
90 dBA
8 hours
110 dBA
30 minutes
95 dBA
4 hours
115 dBA
15 minutes
100 dBA
2 hours
> 115 dBA
< 1 second
Methods for noise control
• Reduction of noise production
• Reduction of noise transmission
• Protection of people exposed to noise
• Suitable legislation against noise pollution
• Worker’s rights to claim compensation
• Health education
General Noise Control
• Separation of Industrial & Transport areas
• Separation of residential areas from main streets
• House fronts should be > 15 m from street
• Intervening space planted with thick trees
• Acoustic insulation of buildings
• Use of public microphones with noise limiters
• Ban on bursting loud firecrackers
Industrial Noise Control
• Substituting new quieter equipment for old
• Isolate vibrating & noisy equipments
• Enclose noisy source or working personnel
• Attenuate noise energy at source via insulation
• Line surfaces with sound absorbing material
• Shield workers with sound barriers
• Use remote control systems for noisy operations
Traffic Noise Control
• Heavy vehicles should not enter in narrow streets
• Restricted vehicular traffic on residential streets
• Prohibition on indiscriminate blowing of horns
• Banning use of pressure horns
• Encourage mass transportation & cut down on
personal vehicles
• Noisy vehicle testing program
Domestic Noise Control
Reduce volume of:
• Music systems
• Home theatres
• Generators
• Alarms
• Power tools
• Personal stereos
• Cell phones
Personal Treatment
• Temporary Threshold Shift is reversible
• No treatment or recovery is expected once
Permanent Threshold Shift occurs
• Deterrence is only accepted management method
• Treat NIHL exacerbating conditions: smoking,
cardiovascular disease, diabetes mellitus,
hyperlipidemia, exposure to ototoxic drugs
Ear plugs
Proper use of Ear Plug
1. Roll earplug up into a small, thin "snake".
2. Pull your ear up & back with other hand & slide
in rolled-up earplug.
3. Count to 20 loudly while waiting for earplug to
expand & fill ear canal. Your voice will sound
muffled when earplug has made a good seal.
4. Check fitting of earplug. Most of foam body of
earplug should be within ear canal.
Proper use of Ear Plug
Proper fitting of Ear Plug
PROPER
IMPROPER
Canal cap protectors
Ear muffs
Dual Protection (plug + muff)
Electronic Earmuffs
• Earmuffs permitting normal hearing in absence of
loud noise are now available
• They detect loud noise & attenuate it before it
reaches subject’s ear
• They permit normal hearing except in loud noise
• They allow wearer to hear environmental sounds
Electronic Earmuffs
Efficacy of earplugs & ear muffs
Performance of hearing protectors
• Hearing protectors are labeled with noise
reduction rating (NRR)
• Noise exposure in dBA – {(NRR-7) x .5} =
employee dose while wearing hearing protector
• Employee dose where exposure is 95 dBA, while
wearing earplugs with 29 NRR
= 95 – {(29-7) x .5} = 84 dBA dose
When to wear ear protection?
TWA8 ≥ 85 dBA or D ≥ 50%
• Worker’s option to wear hearing protection
TWA8 > 90 dBA or D > 100%
• Worker must wear either ear plug or ear muff
TWA8 > 105 dBA or D > 800%
• Worker must wear both earplug & earmuff
Hearing Conservation Program
• Baseline PTA within 6 months of onset of exposure
• Audiogram done when employee has been noise free
for at least 48 hours, (3 month for blast injury) - SB
Workers exposed to TWA of > 85 dBA, must have:
• Annual audiometric testing, annual training about
effects of noise on hearing, purpose of audiometry
testing & hearing protective devices
Medical care (?)
• Hyperbaric oxygenation + corticosteroid therapy
• Intratympanic infusion of: JNK ligand, AM-111
• Neural stem cells injection into scala tympani
• Anti-oxidants (prevent oxidative stress of
cochlea): Acetyl-L-carnitine, Carbamathione, D-
methionine, N -acetyl-l-cysteine, alpha tocopherol
• Dexamethasone infusion into perilymphatic space
Accepted noise levels
Residential
25 - 40 dB
Commercial
35 - 45 dB
Industrial
40 - 60 dB
Educational
30 - 40 dB
Hospital
20 - 40 dB
Punishment for offenders
Hearing Impairment
• Monaural hearing impairment calculated from
four-frequency (500, 1000, 2000, 3000 Hz) Pure
Tone Average
• Monaural hearing impairment (MHI) in % =
1.5 (Pure Tone Average - 25) %
• Binaural hearing impairment (BHI) in % =
5 (MHI better ear) + 1 (MHI worse ear)
_______________________________
6
• Hearing Impairment: Structural, functional or
psychological damage to hearing
• Hearing Disability: Hearing impairment affects
subject’s ability to perform normal body functions
• Hearing Handicap: Hearing disability prevents pt
from performing duties towards society
• Pure Tone Average (500, 1000, 2000, 3000 Hz) > 25
dB is considered as hearing handicap (A.A.O.)
Health Education
Hearing Rehabilitation
“I would choose blindness
over deafness, because
blindness just separates
me from things, while
deafness cuts me off
from people, & exchange
of ideas about today,
tomorrow & yesterday.”
- Helen Keller
Assisted Listening Devices
• They are NOT hearing aids
• They are NOT used instead of hearing aids
• Help pt with hearing loss to function better in
communication situations to overcome distance,
background noise, or poor room acoustics
• Can be used with or without hearing aids
Vibrating wrist watch & alarm clock
CO2 & smoke alarm with strobe light
Amplified & captioned telephone
T.V. & F.M. amplifiers
Personal & multi-user amplifier
Alerting Devices
Amplified Stethoscope
Hearing Aids
References
1. Scott-Brown: 6th edition; volume 2; chapter 11
7th edition; volume 3; chapter 238b
2. Cummings: 3rd edition; volume 4; chapter 162
3. Ballenger: 16th edition; chapter 15
4. Byron Bailey: 4th edition; chapter 147
5. Paparella: 3rd edition; volume 2; chapter 45
6. Ludman: 6th edition; chapter 35
7. Park’s textbook of PSM: 19th edition; p 598-600
8. Mathur NN, Roland P. Inner ear NIHL. emedicine
9. Ganguly SN, Reddy NS. JCOMS. 2008; 5:1 p 9-11