Audiometry and Occupational Hearing Loss

Hearing SoundExternal ear canalTympanic membraneOssicles and musclesOval windowCochleaSensory hair cellsSensory nerve fibers8th. Cranial nerve

IntroductionSound: small, rapid, local fluctuations in atmospheric pressureSPL (sound pressure level): amplitude or loudness of soundDynes/cm2dB (a logarithmic scale)Frequency (Hz)

Introduction (cont.)0 dB (zero reference level): the faintest sound the average normal young humans can hearHuman range of hearing (SPL): 0-120dBHuman range of hearing (frequency): 20-20000 HzSome examples:Conversation: 60-70 dBSome industrial machinery: 80-100 dBChainsaw: 110 dB

Introduction (cont.)Hearing threshold: minimum SPL that sound is first recognized for a given frequencySound types:Pure tone: regular at a single frequencycomplex

Normal threshold: -0.5 25 dBMild Hearing Loss: 25 - 40 dB HLDifficulty with soft speechModerate Hearing Loss: 40 - 55 dB HL Difficulty with normal speechModerately Severe Hearing Loss: 55 - 70 dB HLDifficulty with loud speechSevere Hearing Loss: 70 - 90 dB HLCan only understand shoutingProfound Hearing Loss: > 90 dB HL Cannot understand even amplified speech

Audiometric testingPure tone audipmetry (PTA)Speech audiometry (SRT,SDS)Impedance audiometryAuditory Brainstem Response (ABR)Otoacoustic Emissions

Pure Tone AudiometryMost common testThreshold of hearing in different frequenciesComparing hearing threshold with zero reference levelTwo kinds:Air conduction assesses entire systemBone conduction assesses cochlea onwardsBC with and without maskingA graph showing HTL as a function of frequencyFrequencies: 125, 500, 1000, 2000, 4000, 8000Hz and 3000, 6000 Hz

Standard signs in audiometryO---O: right AC---: left AC>--->: right BC

Speech audiometrySRT (speech reception threshold)Balanced two-syllable words (spondee words)Intensity at which listener can repeat 50% of wordsClose agreement with average hearing threshold (500-3000 Hz)

Speech audiometry (cont.)SDS (speech discrimination score)Phonetically balanced one-syllable wordsIntensity: SRT + 25-40 dBPercentage of words correctly repeatedNormal: 88-100%

Impedance audiometryTympanometry: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

ABREvoked potentials in response to clicking noiseLocalizing retrococlear lesionsFive waves: 8th cranial nerve to inferior colliculus)

OAEA test for non-organic pathology35-40 dB hearing threshold produce OAEHair cell damageSensory hearing lossIn conductive hearing loss OAE can not be performed

Principles of Hearing EvaluationNormal hearingHearing by AC=BC and both are within normal limits

Conductive hearing lossHearing by AC is poorer than hearing by BC and BC is within normal limits

Sensorineural hearing lossHearing by AC=BC and both are impaired to the same degree

Mixed hearing lossHearing by AC is poorer than hearing by BC and both are impaired

Conductive hearing lossHearing loss due to impairment of conducting sound down ear canal to inner ear.

Conductive hearing lossOtosclerosisTympanosclerosis TM perforationMiddle ear effusionLaxity of TMDisruption of ossicular chain

Sensorineural Hearing LossHearing 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 LossPresbycusisMetabolic disordersInfectious hearing lossCNS diseaseMeniere diseaeNoise-induced hearing loss

Mixed hearing loss

Occupational hearing lossConductiveSensorineuralMixed

Occupational hearing lossAcute acoustic traumaOtotoxic hearing lossHearing loss due to workplace injuriesNoise-induced hearing loss (NIHL)

Acute acoustic traumaBrief exposure to extremely loud noise (120-140 dB) or due to blast injuriesConductive, sensorineural or mixedTemporary or permanentVertigo, tinnitus and painUnilateral or bilateralFollow-up for 4-6 months

Ototoxic hearing lossExposure to substances that injure the cochleaNon-occupational (Drugs): Aminoglycosides (gentamicin)Loop diuretics (furosemide)Antineoplastic agents (cisplatin)Salicylates (aspirin)

Occupational:Heavy metalsAsCoPbHgCyanideBenzenePropylene glycolCS2Styrene

Ototoxic hearing lossBilateral high-frequency sensorineural hearing loss

Importance:Exposure to ototoxic substances makes the worker more suceptible to NIHL

Workplace injuriesConductiveBlunt head traumaLongitudinal temporal bone FxBurns (e.g. welders slag)barotraumaSensorineuralBlunt head trauma (labyrinth concussion,)Transerve temporal bone FxMixedBlunt head trauma Temporal bone Fx

NIHLNoise: the most pervasive hazardous agent in the workplaceNIHL: second most common acquired hearing loss after presbycusisMechanism: trauma to the sensory cochlear epithelium (esp. hair cells) due to exposure to noiseTTS (temporary threshold shift)PTS (permanent threshold shift)

NIHL5% of individuals exposed to 80dB noise levels develop a significant hearing loss.

5-10% for 85dB exposure

15-25% for 90dB exposure

NIHLA sensorineural hearing lossMostly high-frequency Most severe around 4000 Hz (notch)Mostly bilateral (may be unilateral)Related to intensity and duration of exposure

NIHLFirst asymptomaticGradual deterioration in hearing esp. in the presence of background noiseVowels better than consonantsDistortion of speech sounds (esp. high-pitched)Frequently accompanied by tinnitus

NIHLDifferential diagnosis:PresbycusisAtrophy of the hair cells or central auditory pathwaysGradual, symmetric, progressive high-frequency sensorineural hearing loss

CNS pathologies (cerebellopontine tumors):Unilateral, sensorineural hearing loss

Meniere diseaseFluctuating low-frequency or flat unilateral sensorineural hearing loss

Functional (non-organic) hearing lossPoor correlation between SRT and average threshold (SRT >15dB better than PTA)Test-retest variability

Prevention of hearing loss in workplaceOSHA TLV-TWA for exposure to noise: 90dbNIOSH exposure limit: 85dBNoise> 85dB Hearing conservation program (HCP):Noise monitoringEngineering controlsAdministrative controlsPeriodic audiometric evaluationsWorker educationHPDs

Noise monitoringSound level meterNoise dosimeter

Engineering controls3 important variables:SourceenclosurePathbarriersReciever Increasing distance

Administrative controlsPurchasing suitable equipmentReducing 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 evaluationsPre-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 evaluationsBaseline (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 criteriaBaseline:Average 500,1000,2000 and 3000 > 25dB in each earDifference between two ears at 500, 1000, 2000>15dB or at 3000, 4000, and 6000> 30dBPeriodic:>15dB decline from baseline at 500, 1000, 2000>20 dB decline from baseline at 3000, 4000, 6000

HPDsEar plug (aural)Canal cap (semiaural)Ear muff (circumaural)

Ear muff

*******************************************************************