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Audiometry and Occupational Hearing Loss

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

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Audiometry and Occupational Hearing Loss

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  1. Audiometry and Occupational Hearing Loss A. H. Mehrparvar, MD Occupational Medicine department Yazd University of Medical Sciences

  2. Hearing • Sound • External ear canal • Tympanic membrane • Ossicles and muscles • Oval window • Cochlea • Sensory hair cells • Sensory nerve fibers • 8th. Cranial nerve

  3. 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)

  4. 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

  5. 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

  6. 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

  7. Audiometric testing • Pure tone audipmetry (PTA) • Speech audiometry (SRT,SDS) • Impedance audiometry • Auditory Brainstem Response (ABR) • Otoacoustic Emissions

  8. 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

  9. Standard signs in audiometry • O---O: right AC • ×---×: left AC • >--->: right BC • <---<:left BC • [----[: right BC with masking • ]----]: left BC with masking

  10. 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)

  11. 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%

  12. Impedance audiometry • Tympanometry: • Measure the impedance of eardrum and ossicular chain: • Type An: normal

  13. Type As (Reduced compliance): otosclerosis, tympanosclerosis

  14. Type C: auditory tube dysfunction

  15. Type B (no compliance): TM perforation or effusion

  16. Type Ad (Increased compliance): laxity of TM or disruption of ossicular chain

  17. ABR • Evoked potentials in response to clicking noise • Localizing retrococlear lesions • Five waves: • 8th cranial nerve to • inferior colliculus)

  18. 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

  19. 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

  20. Conductive hearing loss • Hearing loss due to impairment of conducting sound down ear canal to inner ear.

  21. Conductive hearing loss • Otosclerosis • Tympanosclerosis • TM perforation • Middle ear effusion • Laxity of TM • Disruption of ossicular chain

  22. 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)

  23. Sensorineural Hearing Loss

  24. Sensorineural Hearing Loss • Presbycusis • Metabolic disorders • Infectious hearing loss • CNS disease • Meniere diseae • Noise-induced hearing loss

  25. Mixed hearing loss

  26. Occupational hearing loss • Conductive • Sensorineural • Mixed

  27. Occupational hearing loss • Acute acoustic trauma • Ototoxic hearing loss • Hearing loss due to workplace injuries • Noise-induced hearing loss (NIHL)

  28. 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

  29. Ototoxic hearing loss • Exposure to substances that injure the cochlea • Non-occupational (Drugs): • Aminoglycosides (gentamicin) • Loop diuretics (furosemide) • Antineoplastic agents (cisplatin) • Salicylates (aspirin)

  30. Occupational: • Heavy metals • As • Co • Pb • Hg • Cyanide • Benzene • Propylene glycol • CS2 • Styrene

  31. Ototoxic hearing loss • Bilateral high-frequency sensorineural hearing loss • Importance: Exposure to ototoxic substances makes the worker more suceptible to NIHL

  32. 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

  33. 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)

  34. NIHL • 5% of individuals exposed to 80dB noise levels develop a significant hearing loss. • 5-10% for 85dB exposure • 15-25% for 90dB exposure

  35. 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

  36. 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

  37. NIHL • Differential diagnosis: • Presbycusis • Atrophy of the hair cells or central auditory pathways • Gradual, symmetric, progressive high-frequency sensorineural hearing loss

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

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