Audiometry and Occupational Hearing Loss

A. H. Mehrparvar, MDOccupational Medicine departmentYazd 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): 20-

20000 Hz Some examples:

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

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

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 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

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 LossSensorineural 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

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

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

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. high-

pitched) Frequently accompanied by tinnitus

NIHL Differential diagnosis: Presbycusis Atrophy of the hair cells or central auditory

pathways Gradual, symmetric, progressive high-

frequency 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 controlsAdministrative controls

Periodic audiometric evaluationsWorker education

HPDs

Noise monitoring Sound level meter Noise dosimeter

Engineering controls 3 important variables: Source

enclosure Path

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/day100dB: 2h exposure/day105dB: 1h exposure/dayCeiling: 115db: 15m. Exposure/day

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.

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:

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