Download Occupational hearing loss

Audiometry and
Occupational Hearing
Loss
A. H. Mehrparvar, MD
Occupational Medicine department
Yazd University of Medical Sciences
Hearing
Sound
 External ear canal
 Tympanic membrane
 Ossicles and muscles
 Oval window
 Cochlea
 Sensory hair cells
 Sensory nerve fibers
 8th. Cranial nerve

Introduction
Sound: small, rapid, local fluctuations in
atmospheric pressure
 SPL (sound pressure level): amplitude or
loudness of sound




Dynes/cm2
dB (a logarithmic scale)
Frequency (Hz)
Introduction (cont.)
0 dB (zero reference level): the faintest
sound the average normal young humans
can hear
 Human range of hearing (SPL): 0-120dB
 Human range of hearing (frequency): 2020000 Hz
 Some examples:


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Conversation: 60-70 dB
Some industrial machinery: 80-100 dB
Chainsaw: 110 dB
Introduction (cont.)
Hearing threshold: minimum SPL that
sound is first recognized for a given
frequency
Sound types:



Pure tone: regular at a single frequency
complex

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Normal threshold: -0.5 – 25 dB
Mild Hearing Loss: 25 - 40 dB HL


Moderate Hearing Loss: 40 - 55 dB HL


Difficulty with loud speech
Severe Hearing Loss: 70 - 90 dB HL


Difficulty with normal speech
Moderately Severe Hearing Loss: 55 - 70 dB HL


Difficulty with soft speech
Can only understand shouting
Profound Hearing Loss: > 90 dB HL

Cannot understand even amplified speech
Audiometric testing
Pure tone audipmetry (PTA)
 Speech audiometry (SRT,SDS)
 Impedance audiometry
 Auditory Brainstem Response (ABR)
 Otoacoustic Emissions

Pure Tone Audiometry




Most common test
Threshold of hearing in different frequencies
Comparing hearing threshold with zero reference
level
Two kinds:


Air conduction assesses entire system
Bone conduction assesses cochlea onwards



BC with and without masking
A graph showing HTL as a function of frequency
Frequencies: 125, 500, 1000, 2000, 4000,
8000Hz and 3000, 6000 Hz
Standard signs in audiometry
O---O: right AC
 ×---×: left AC
 >--->: right BC
 <---<: left BC
 [----[ : right BC with masking
 ]----] : left BC with masking

Speech audiometry
SRT (speech reception threshold)
 Balanced two-syllable words (spondee
words)
 Intensity at which listener can repeat 50%
of words
 Close agreement with average hearing
threshold (500-3000 Hz)

Speech audiometry (cont.)
SDS (speech discrimination score)
 Phonetically balanced one-syllable words
 Intensity: SRT + 25-40 dB
 Percentage of words correctly repeated
 Normal: 88-100%

Impedance audiometry
Tympanometry:
 Measure the impedance of eardrum and
ossicular chain:
 Type An: normal


Type As (Reduced compliance):
otosclerosis, tympanosclerosis

Type C: auditory tube dysfunction

Type B (no compliance): TM perforation or
effusion

Type Ad (Increased compliance): laxity of
TM or disruption of ossicular chain
ABR
Evoked potentials in response to clicking
noise
 Localizing retrococlear lesions
 Five waves:



8th cranial nerve to
inferior colliculus)
OAE
A test for non-organic pathology
 35-40 dB hearing threshold produce OAE
 Hair cell damage
 Sensory hearing loss
 In conductive hearing loss OAE can not be
performed

Principles of Hearing Evaluation

Normal hearing
 Hearing by AC=BC and both are within normal limits

Conductive hearing loss
 Hearing by AC is poorer than hearing by BC and BC is within
normal limits

Sensorineural hearing loss
 Hearing by AC=BC and both are impaired to the same degree

Mixed hearing loss
 Hearing by AC is poorer than hearing by BC and both are
impaired
Conductive hearing loss

Hearing loss due to
impairment of
conducting sound
down ear canal to
inner ear.
Conductive hearing loss
Otosclerosis
 Tympanosclerosis
 TM perforation
 Middle ear effusion
 Laxity of TM
 Disruption of ossicular chain

Sensorineural Hearing Loss

Hearing loss due to loss of function, from cochlea
onwards

Cochlea (inner ear), auditory nerve (from cochlea
to brain), and auditory cortex (brain)
Sensorineural Hearing Loss
Sensorineural Hearing Loss
Presbycusis
 Metabolic disorders
 Infectious hearing loss
 CNS disease
 Meniere diseae
 Noise-induced hearing loss

Mixed hearing loss
Occupational hearing loss
Conductive
 Sensorineural
 Mixed

Occupational hearing loss
Acute acoustic trauma
 Ototoxic hearing loss
 Hearing loss due to workplace injuries
 Noise-induced hearing loss (NIHL)

Acute acoustic trauma
Brief exposure to extremely loud noise
(120-140 dB) or due to blast injuries
 Conductive, sensorineural or mixed
 Temporary or permanent
 Vertigo, tinnitus and pain
 Unilateral or bilateral
 Follow-up for 4-6 months

Ototoxic hearing loss


Exposure to substances that injure the
cochlea
Non-occupational (Drugs):
 Aminoglycosides (gentamicin)
 Loop diuretics (furosemide)
 Antineoplastic agents (cisplatin)
 Salicylates (aspirin)

Occupational:

Heavy metals

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As
Co
Pb
Hg
Cyanide
Benzene
Propylene glycol
CS2
Styrene
Ototoxic hearing loss

Bilateral high-frequency sensorineural
hearing loss
Importance:
Exposure to ototoxic substances makes the
worker more suceptible to NIHL

Workplace injuries

Conductive



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Sensorineural

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Blunt head trauma
Longitudinal temporal bone Fx
Burns (e.g. welder’s slag)
barotrauma
Blunt head trauma (labyrinth concussion,…)
Transerve temporal bone Fx
Mixed


Blunt head trauma
Temporal bone Fx
NIHL
Noise: the most pervasive hazardous
agent in the workplace
 NIHL: second most common acquired
hearing loss after presbycusis
 Mechanism: trauma to the sensory
cochlear epithelium (esp. hair cells) due to
exposure to noise
 TTS (temporary threshold shift)
 PTS (permanent threshold shift)

NIHL

5% of individuals exposed to 80dB noise
levels develop a significant hearing loss.

5-10% for 85dB exposure

15-25% for 90dB exposure
NIHL
A sensorineural hearing loss
 Mostly high-frequency
 Most severe around 4000 Hz (notch)
 Mostly bilateral (may be unilateral)
 Related to intensity and duration of
exposure

NIHL
First asymptomatic
 Gradual deterioration in hearing esp. in
the presence of background noise
 Vowels better than consonants
 Distortion of speech sounds (esp. highpitched)
 Frequently accompanied by tinnitus

NIHL
Differential diagnosis:
 Presbycusis
 Atrophy of the hair cells or central
auditory pathways
 Gradual, symmetric, progressive highfrequency sensorineural hearing loss

CNS pathologies (cerebellopontine
tumors):
 Unilateral, sensorineural hearing loss

Meniere disease
 Fluctuating low-frequency or flat unilateral
sensorineural hearing loss

Functional (non-organic) hearing loss
 Poor correlation between SRT and average
threshold (SRT >15dB better than PTA)
 Test-retest variability

Prevention of hearing loss in
workplace



OSHA TLV-TWA for exposure to noise: 90db
NIOSH exposure limit: 85dB
Noise> 85dB
Hearing conservation program (HCP):
Noise monitoring
Engineering controls
Administrative controls
Periodic audiometric evaluations
Worker education
HPDs
Noise monitoring
Sound level meter
 Noise dosimeter

Engineering controls
3 important variables:
 Source



Path


enclosure
barriers
Reciever

Increasing distance
Administrative controls
Purchasing suitable equipment
 Reducing the exposure time:

Each 5 dB> 90 dB
exposure time is halved
95dB: 4h exposure/day
100dB: 2h exposure/day
105dB: 1h exposure/day
Ceiling: 115db: 15m. Exposure/day
Audiometric evaluations
1.
2.
3.
4.
5.
Pre-employment.
Prior to initial assignment in a hearing hazardous
work area.
Annually as long as the employee is assigned to a
noisy job (>85 dBA)
At the time of reassignment out of a hearing
hazardous job.
At the Termination of employment.
Audiometric evaluations
Baseline (after 16 h. away from exposure)
 Periodic (do not require to be away from
exposure)
 STS (standard threshold shift):
>10 dB decline from baseline, in average
2000, 3000, and 4000 Hz.

Retest 30 days later
Referring criteria

Baseline:

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Average 500,1000,2000 and 3000 > 25dB in
each ear
Difference between two ears at 500, 1000,
2000>15dB or at 3000, 4000, and 6000>
30dB
Periodic:


>15dB decline from baseline at 500, 1000,
2000
>20 dB decline from baseline at 3000, 4000,
6000
HPDs
Ear plug (aural)
 Canal cap (semiaural)
 Ear muff (circumaural)

Ear muff