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Audiometry and Occupational Hearing Loss
A. H. Mehrparvar, MDOccupational Medicine departmentYazd University of Medical Sciences
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Hearing Sound External ear canal Tympanic membrane Ossicles and muscles Oval window Cochlea Sensory hair cells Sensory nerve fibers 8th. Cranial nerve
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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)
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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): 20-
20000 Hz Some examples:
Conversation: 60-70 dB Some industrial machinery: 80-100 dB Chainsaw: 110 dB
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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
Difficulty with soft speech Moderate Hearing Loss: 40 - 55 dB HL
Difficulty with normal speech Moderately Severe Hearing Loss: 55 - 70 dB HL
Difficulty with loud speech Severe Hearing Loss: 70 - 90 dB HL
Can only understand shouting Profound Hearing Loss: > 90 dB HL
Cannot understand even amplified speech
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Audiometric testing Pure tone audipmetry (PTA) Speech audiometry (SRT,SDS) Impedance audiometry Auditory Brainstem Response (ABR) Otoacoustic Emissions
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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
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Standard signs in audiometry O---O: right AC ×---×: left AC >--->: right BC <---<: left BC [----[ : right BC with masking ]----] : left BC with masking
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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)
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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%
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Impedance audiometry Tympanometry: Measure the impedance of eardrum and
ossicular chain: Type An: normal
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Type As (Reduced compliance): otosclerosis, tympanosclerosis
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Type C: auditory tube dysfunction
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Type B (no compliance): TM perforation or effusion
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Type Ad (Increased compliance): laxity of TM or disruption of ossicular chain
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ABR Evoked potentials in response to clicking
noise Localizing retrococlear lesions Five waves:
8th cranial nerve to inferior colliculus)
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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
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Principles of Hearing Principles of Hearing EvaluationEvaluation 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
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Conductive hearing loss
Hearing loss due to impairment of conducting sound down ear canal to inner ear.
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Conductive hearing loss Otosclerosis Tympanosclerosis TM perforation Middle ear effusion Laxity of TM Disruption of ossicular chain
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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)
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Sensorineural Hearing LossSensorineural Hearing Loss
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Sensorineural Hearing Loss Presbycusis Metabolic disorders Infectious hearing loss CNS disease Meniere diseae Noise-induced hearing loss
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Mixed hearing loss
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Occupational hearing loss Conductive Sensorineural Mixed
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Occupational hearing loss Acute acoustic trauma Ototoxic hearing loss Hearing loss due to workplace injuries Noise-induced hearing loss (NIHL)
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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
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Ototoxic hearing loss Exposure to substances that injure the
cochlea Non-occupational (Drugs):
Aminoglycosides (gentamicin) Loop diuretics (furosemide) Antineoplastic agents (cisplatin) Salicylates (aspirin)
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Occupational: Heavy metals
As Co Pb Hg
Cyanide Benzene Propylene glycol CS2 Styrene
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Ototoxic hearing loss Bilateral high-frequency sensorineural
hearing loss
Importance:Exposure to ototoxic substances makes the
worker more suceptible to NIHL
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Workplace injuries Conductive
Blunt head trauma Longitudinal temporal bone Fx Burns (e.g. welder’s slag) barotrauma
Sensorineural Blunt head trauma (labyrinth concussion,…) Transerve temporal bone Fx
Mixed Blunt head trauma Temporal bone Fx
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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)
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NIHL 5% of individuals exposed to 80dB noise
levels develop a significant hearing loss.
5-10% for 85dB exposure
15-25% for 90dB exposure
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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
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NIHL First asymptomatic Gradual deterioration in hearing esp. in
the presence of background noise Vowels better than consonants Distortion of speech sounds (esp. high-
pitched) Frequently accompanied by tinnitus
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NIHL Differential diagnosis: Presbycusis Atrophy of the hair cells or central auditory
pathways Gradual, symmetric, progressive high-
frequency sensorineural hearing loss
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CNS pathologies (cerebellopontine tumors):
Unilateral, sensorineural hearing loss
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Meniere disease Fluctuating low-frequency or flat unilateral
sensorineural hearing loss
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Functional (non-organic) hearing loss Poor correlation between SRT and average
threshold (SRT >15dB better than PTA) Test-retest variability
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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 controlsAdministrative controls
Periodic audiometric evaluationsWorker education
HPDs
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Noise monitoring Sound level meter Noise dosimeter
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Engineering controls 3 important variables: Source
enclosure Path
barriers Reciever
Increasing distance
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Administrative controls Purchasing suitable equipment Reducing the exposure time:
Each 5 dB> 90 dB exposure time is halved
95dB: 4h exposure/day100dB: 2h exposure/day105dB: 1h exposure/dayCeiling: 115db: 15m. Exposure/day
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Audiometric evaluations1. Pre-employment.2. Prior to initial assignment in a hearing
hazardous work area.3. Annually as long as the employee is
assigned to a noisy job (>85 dBA)4. At the time of reassignment out of a hearing
hazardous job.5. At the Termination of employment.
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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
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Referring criteria Baseline:
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
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HPDs Ear plug (aural) Canal cap (semiaural) Ear muff (circumaural)
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Ear muff
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