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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: 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 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 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 Sensorineural 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: 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