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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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