Noise-induced hearing loss (NIHL) characteristically produces a 4 kHz notch on audiometry. The primary mechanism for this selective frequency vulnerability is:
- A The resonance frequency of the external ear canal (~3-4 kHz) provides acoustic amplification, focusing maximum energy at the 4 kHz cochlear region ✓
- B The 4 kHz cochlear region receives maximum mechanical energy because it is closest to the oval window
- C The stapedial reflex selectively inhibits protection at 4 kHz due to higher reflex thresholds at this frequency
- D Outer hair cells at 4 kHz have the lowest density of connexin-26 gap junctions, making them less resistant to excitotoxicity
Explanation
The external ear canal has a resonant frequency of approximately 3-4 kHz, amplifying sounds in this frequency range by 10-15 dB. This means the organ of Corti at the cochlear region corresponding to 4 kHz receives maximum acoustic energy during noise exposure, making it most vulnerable to excitotoxic and metabolic injury. The stapedial acoustic reflex (which protects against sustained noise) also has a latency of 25-150 ms, rendering it ineffective against impulse noise. Together, these factors explain the characteristic 4 kHz notch. Recovery of a temporary threshold shift at 4 kHz is the basis for industrial audiometric monitoring.
Reference: Dhingra Diseases of Ear, Nose and Throat, 7th ed.
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Written and medically reviewed by the StethoPrep medical team.