Identifying three otopathologies in humans.

Area of Science:

• Auditory Neuroscience
• Otoacoustic Emissions
• Audiology

Background:

• Hearing-in-noise (HIN) is a common complaint, yet the underlying causes involving auditory nerve (AN) and outer hair cell (OHC) function are not fully understood.
• Cochlear (auditory) synaptopathy, a loss of synaptic contact between hair cells and the AN, is a potential contributor to HIN difficulties, but its clinical significance in humans remains debated.
• This study operationally defines cochlear synaptopathy based on electrophysiological measures (CAP amplitude, DPOAE levels) rather than noise exposure history.

Purpose of the Study:

• To determine if the standard audiogram can detect AN and OHC dysfunction.
• To investigate whether hearing-in-noise performance is primarily dependent on AN function, OHC function, or both.
• To explore the prevalence and impact of 'hidden hearing loss' conditions like cochlear synaptopathy and OHC dysfunction.

Main Methods:

• Recruited adult subjects and collected data on pure tone audiometry, subjective HIN perception, and speech recognition tests (QuickSIN, WR).
• Measured OHC function using distortion-product otoacoustic emission (DPOAE) signal-to-noise ratio (SNR).
• Assessed AN function via compound action potential (CAP) peak amplitude and ratio to summating potential using electrocochleography.

Main Results:

• Operational cochlear synaptopathy may affect up to 30% of individuals with normal hearing.
• Normal hearing individuals can exhibit HIN difficulties due to undetected OHC dysfunction or cochlear synaptopathy.
• AN untuning, linked to OHC dysfunction, is observed in mild-moderate sensorineural hearing loss (SNHL).
• Hearing-in-noise performance strongly correlates with OHC function, with AN function playing a smaller, though significant, role.

Conclusions:

• Hearing difficulties in noise can occur even with normal audiometric thresholds.
• Standard audiograms may fail to detect underlying OHC and AN dysfunction.
• OHC function is the primary determinant of hearing-in-noise ability, while AN function contributes to a lesser extent.