Hearing thresholds with outer and inner hair cell loss

Area of Science:

• Ototoxicity and Auditory Neuroscience
• Experimental Otolaryngology

Background:

• Aminoglycoside antibiotics (e.g., kanamycin, neomycin) and intense sound exposure are known ototoxic agents.
• These agents can induce permanent hearing impairment and associated cochlear histopathologic changes.

Purpose of the Study:

• To evaluate hearing impairment and cochlear histopathology in experimental animals following aminoglycoside treatment or intense noise exposure.
• To correlate hair cell loss with the degree of hearing threshold shifts.

Main Methods:

• Experimental animals (monkeys, guinea pigs, chinchillas) were treated with aminoglycoside antibiotics or exposed to octave bands of intense noise (120 dB SPL).
• Auditory function was assessed through hearing loss measurements.
• Cochlear tissues were examined histopathologically to quantify outer and inner hair cell loss.

Main Results:

• Aminoglycoside treatment resulted in high-frequency hearing loss progressing to lower frequencies.
• Intense noise exposure caused hearing loss related to the noise's spectral characteristics.
• Outer hair cell loss was more pronounced than inner hair cell loss, correlating with threshold shifts ≤50 dB.
• Inner hair cell loss was generally observed with threshold shifts >50 dB.

Conclusions:

• Data support the place principle of hearing and suggest outer hair cells are vital for hearing up to approximately 50 dB Sensation Level (SL).
• Inner hair cells appear to function for hearing above 50 dB SL.
• Combined behavioral and morphological approaches are valuable for understanding the functional roles of different hair cell types.