1. Noise Induced Hearing Loss
Dr. Vishal Sharma
Topic of discussion

2. Definitions
Noise = wrong sound, in wrong place, at wrong time (Park & Park)
Acoustic Trauma = sudden, permanent sensori-neural deafness due to single exposure to an intense sound ( dB) of < 0.2 sec
Chronic Noise Induced Hearing Loss = gradual SNHL due to years of exposure to noise

3. 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

4. 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).

5. Classification of noise
A. Based on duration: B. Frequency based:
1. Continuous: cotton spinning High: sawing
2. Interrupted: traffic Low: grinder
3. Transient (< 0.2 sec) White noise: boiler
a. Impulse: explosion
b. Impact: metal to metal collision

6. 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

7. Common Occupational Noise
Agriculture
Mining
Construction
Manufacturing
Public Utilities
Transportation
Military

8. Factory boilers / trains
Volcano eruption
190 dB
Jet plane dB
Thunder
120 dB
Factory boilers / trains
110 dB
Cars & bikes
90 dB
Loud radio music
85 dB
Grand piano
Children crying
80 dB
Loud conversation
70 dB
Dog barking

10. Sound Thermometer

11. 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

12. 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

13. 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.

14. Characteristics of chronic NIHL (Dobie, 1990)

15. 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 years of exposure & decreases later as hearing worsens. Hearing loss does not progress after noise exposure is discontinued.

16. Commonest cause of asymmetric NIHL is exposure to firearms
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.

17. Acoustic head shadow

18. 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)

19. Temporary Threshold Shift
Exposure to loud noise for seconds to hours may cause SNHL that recovers within hours.
Magnitude of TTS depends on:
More intense sounds lead to larger shifts.
Speech frequencies ( Hz) are most susceptible to TTS.
Interrupted exposures cause less TTS than continuous exposures.

20. 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

21. Calculation of N.I.P.T.S.

22. Age related hearing loss

23. Progress of N.I.P.T.S.

24. Progress of N.I.P.T.S.

27. Male vs. female N.I.P.T.S.

28. Non-auditory effects of noise
Insomnia
Anxiety & nervousness
Coronary heart disease
Lack of concentration
Fatigue
Irritability & impatience
Indigestion
Peptic ulcer
Speech interference
Hypertension
ed heart rate
ed intra-cranial tension
ed breathing rate
ed sweating
ed color perception
ed night vision

29. Factors influencing N.I.H.L.

30. 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

31. 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

32. 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

33. 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

34. Patho-physiology NIHL damage is explained by: Macro-mechanical theory
Micro-mechanical theory: produced by high intensities
Biochemical theory: produced by moderate intensities

35. 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.

36. Organ of Corti

37. 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

39. 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

40. 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

41. Audiogram in Acoustic Trauma

42. Audiogram in Chronic N.I.H.L.

43. 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.

44. Reasons for 4 kHz notch
Natural resonance of EAC being 2-3 kHz, 150% of this produces maximum damage between 3-5 kHz
Protective effect of acoustic reflex below 2 kHz
Intermittent noise is more damaging for 3-5 kHz
Outer hair cells of 3-5 kHz region are more prone to oxidative stress, have reduced vascularity (?) & ed oxygen consumption (?)

45. Acoustic gain: middle ear & pinna

46. 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.

47. Normal Transient evoked OAE

48. Normal Transient evoked OAE
Reproducibility should be > 75 %

49. Early detection of N.I.H.L.

50. Early stage N.I.H.L.

51. Advanced stage N.I.H.L.

52. Malingering of N.I.H.L.

53. Auditory Evoked Potentials
Auditory Brainstem Response: ms post stimulus; originates in 8th cranial nerve (waves I & II) up to lateral lemniscus & inferior colliculus (wave V)
Middle Latency Response (MLR): ms post stimulus; arises in upper brainstem & auditory cortex
Slow Cortical Response: ms post stimulus; originating in auditory cortex

54. Auditory Evoked Potentials

55. 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

56. 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 

57. Multiple Auditory Steady-state Evoked Response audiometry

58. Photo-micrograph of Cochlea

59. Scanning electron micrographs

61. Cyto-cochleogram

62. Noise Exposure Evaluation

63. 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

64. Weighted Audiometry scales

65. Subjective Evaluation
Voice level
Distance of listener from speaker
Noise level (dBA)
Loud voice
6 feet 85
4 feet 90
2 feet 95
Shout
100
2 ft
105
Impossible to hear even when close
> 110

66. Sound Level Meter Hand-held instrument
Real-time display of sound level (dBA) coming from noise sources

67. Noise Dosimeter
Worn by employees to determine TWA exposure

68. Sound meter

69. 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.

70. Occupational Safety & Health Administration regulations
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

71. 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

72. 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

73. 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

76. 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

77. Domestic Noise Control
Reduce volume of:
Music systems
Home theatres
Generators
Alarms
Power tools
Personal stereos
Cell phones

78. 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

79. Ear plugs

80. Proper use of Ear Plug Roll earplug up into a small, thin "snake".
Pull your ear up & back with other hand & slide in rolled-up earplug.
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.
Check fitting of earplug. Most of foam body of earplug should be within ear canal.
NIOSH Roll, Pull, Hold (Check) method
See web site

81. Proper use of Ear Plug

82. Proper fitting of Ear Plug
IMPROPER

83. Canal cap protectors

84. Ear muffs

85. Dual Protection (plug + muff)

86. 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

87. Electronic Earmuffs

88. Efficacy of earplugs & ear muffs

89. 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
Performance [NRR is a guideline not actual performance]
Add 5dB to best protector for dual protection

90. 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
Introduce MSHA requirements for hearing protection based on exposure level via (D) Noise Dose or (TWA8) Time-Weighted Average 8-hour sound level

91. 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

92. 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

93. Accepted noise levels Residential 25 - 40 dB Commercial 35 - 45 dB
Industrial dB
Educational dB
Hospital dB

94. Punishment for offenders

95. Hearing Impairment
Monaural hearing impairment calculated from four-frequency (500, 1000, 2000, 3000 Hz) Pure Tone Average
Monaural hearing impairment (MHI) in % = (Pure Tone Average - 25) %
Binaural hearing impairment (BHI) in % =
5 (MHI better ear) + 1 (MHI worse ear)
_______________________________ 6

96. 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.)

97. Health Education

101. 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

102. 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

103. Vibrating wrist watch & alarm clock

104. CO2 & smoke alarm with strobe light

105. Amplified & captioned telephone

106. T.V. & F.M. amplifiers

107. Personal & multi-user amplifier

108. Alerting Devices

109. Amplified Stethoscope

110. Hearing Aids

112. References
Scott-Brown: 6th edition; volume 2; chapter th edition; volume 3; chapter 238b
Cummings: 3rd edition; volume 4; chapter 162
Ballenger: 16th edition; chapter 15
Byron Bailey: 4th edition; chapter 147
Paparella: 3rd edition; volume 2; chapter 45
Ludman: 6th edition; chapter 35
Park’s textbook of PSM: 19th edition; p
Mathur NN, Roland P. Inner ear NIHL. emedicine
Ganguly SN, Reddy NS. JCOMS. 2008; 5:1 p 9-11

113. Miners should be concerned about preserving their hearing