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Appendix I:C-2. Sensorineural Hearing Loss
Chronic noise-induced hearing loss is a permanent sensorineural condition that cannot be treated medically. "Sensory"
hearing loss is associated with irreversible damage to the inner ear. The term "neural" suggests a degeneration of the neural
elements of the auditory nerve. It is initially characterized by a declining sensitivity to high-frequency sounds, usually at frequencies
above 2,000 Hertz (Hz).
- Animal studies indicate that after moderate exposures to noise, subtle effects may be noticed, such as:
- Twisting and swelling of hair cells.
- Disarray of the cilia on top of the hair cells.
- Detachment of the tectorial membrane from the cilia.
- Reduction of enzymes and energy sources in the cochlear fluids.
- These are conditions that would reduce the sensitivity of the hair cells to mechanical motion.
- The system at this point is in a state of auditory fatigue.
- In order to initiate neural activity, more acoustic energy must enter the cochlea than did before the noise exposure
- As the severity of the noise exposure increases, the following changes increase in degree and eventually become irreversible:
- Hairs become fused into giant cilia or disappear
- Hair cells and supporting cells disintegrate
- (Ultimately) the nerve fibers disappear
- These damaging effects are increased in acoustic trauma, where a single noise exposure of relatively short duration but very high
intensity occurs, such as an explosion. In this case, the system is vibrated so violently that its elastic limit has been exceeded.
Attachments of the various elements of the organ of Corti are disrupted, hair cells are torn completely from the basilar membrane, and a
temporary rupture of the reticular lamina may occur, allowing intermixture of fluids within the cochlea (poisoning those hair cells that may
have survived the mechanical stress of the explosion).
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