Audiometry and Occupational Hearing Loss

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Audiometry and Occupational Hearing Loss

• A. H. Mehrparvar, MD
• Occupational Medicine department
• Yazd 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 gt 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
• gt---gt right BC
• lt---lt 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 Evaluation

• Normal hearing
• Hearing by ACBC 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 ACBC 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 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. welders 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 gt15dB 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
• Noisegt 85dB
• Hearing conservation program (HCP)
• Noise monitoring
• Engineering controls
• Administrative controls
• Periodic audiometric evaluations
• Worker 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 dBgt 90 dB exposure time is halved
• 95dB 4h exposure/day
• 100dB 2h exposure/day
• 105dB 1h exposure/day
• Ceiling 115db 15m. Exposure/day

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Audiometric evaluations

• Pre-employment.
• Prior to initial assignment in a hearing
  hazardous work area.
• Annually as long as the employee is assigned to a
  noisy job (gt85 dBA)
• At the time of reassignment out of a hearing
  hazardous job.
• 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)
• gt10 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 gt 25dB in each ear
• Difference between two ears at 500, 1000,
  2000gt15dB or at 3000, 4000, and 6000gt 30dB
• Periodic
• gt15dB decline from baseline at 500, 1000, 2000
• gt20 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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