Some individuals with normal hearing experience difficulties with hearing in adverse listening conditions, such as in noisy or reverberant environments or when listening to someone talking rapidly. In recent years, the term “hidden hearing loss” has been used to describe this type of hearing difficulty among individuals with hearing thresholds within normal limits (less than 25 dB HL). One physical cause of hidden hearing loss is thought to be “cochlear synaptopathy,” which refers to the damage of synaptic connections between inner hair cells and auditory nerve (AN) fibers. Research in animals has revealed evidence of cochlear synaptopathy following temporary hearing loss while outer hair cell (OHC) function remained intact. There is some debate in the literature on whether cochlear synaptopathy does occur in humans, and if it does, what its prevalence and consequences in terms of hearing in noise are. This is partly due to the difficulty in obtaining direct evidence of cochlear synaptopathy in living humans given that histological examination of fixated tissue is required. Importantly, clinical research on whether cochlear synaptopathy occurs in humans has been inconclusive. In addition to cochlear synaptopathy, research in humans and animals suggests that OHC function may also contribute to speech understanding in the presence of background noise.
A recent study by Parker (“Identifying Three Otopathologies in Humans.” Under review.) investigated the relative contributions of AN and OHC function to hearing-in-noise in a clinical population. The study involved 218 adult individuals with normal hearing or sensorineural hearing loss (SNHL) no greater than a moderate degree. Participants were divided into three groups based on pure-tone averages (PTA): the normal hearing (PTA < 15 dB HL), minimal SNHL (15 ≤ PTA ≥ 24 dB dB HL), and mild to moderate SNHL (PTA > 25 dB HL) groups. The function of peripheral auditory nerves was assessed by measuring the amplitude of the Compound Action Potential (CAP) of the eighth cranial nerve obtained from electrocochleography; OHC function was assessed using DPOAE signal-to-noise ratios (SNRs); and speech recognition in adverse conditions using QuickSIN, Word Recognition Score (WRS) in noise, and time-compressed WRS in reverberation.
OUTER HEARING CELL FUNCTION
The study found a strong and significant correlation between OHC function and hearing thresholds even among individuals with hearing thresholds at or below 25 dB HL, which suggests that OHC dysfunction may be another cause of hidden hearing loss. The study showed that people with minimal SNHL exhibited diminished DPAOE SNRs compared with those with audiometric thresholds of less than 15 dB HL, and that DPOAE SNRs were further diminished as SNHL progressed from mild to moderate. The study also revealed a significant correlation between OHC function and performance in all three hearing-in-noise tests with the lower DPOAE SNR correlating to poorer hearing-in-noise performance. These findings not only indicate that OHC dysfunction occurs in adults with minimal SNHL (15 ≤ PTA ≥ 24 dB HL), which clinically applies to the pediatric population only but should also raise awareness that use of this minimal hearing loss category for adults may better reflect the functional hearing ability in adverse conditions among this population.
AUDITORY NERVE FUNCTION
With regard to auditory nerve function, the study revealed that CAP amplitudes were not significantly correlated with hearing in noisy conditions. Interestingly, the study found that 30 percent of people with hearing thresholds lower than 15 dB HL exhibited operational synaptopathy, which is defined as having low CAP amplitudes and normal OHC function. This suggests that cochlear synaptopathy may be a relatively common occurrence in our society. However, the study further showed that operational synaptopathy does not correlate with hearing-in-noise performance.
This study provides several valuable insights into the realm of hidden hearing loss. Although further research is necessary to clarify the clinical effects of operational synaptopathy contribution, the results from this study indicate that hearing in noise is likely governed by OHC function. Finally, this paper argues for the utilization of the minimal (or slight) hearing loss category, which encompasses thresholds between 15-24 dB HL, as a clinically significant category of hearing impairment. Currently, this category only applies to the pediatric population. However, this study provides evidence that an adult who complains of difficulty hearing in noise and has thresholds between 15 and 24 dB HL may exhibit an underlying OHC dysfunction that is causing the hearing impairment.