Page 1
Auditory sensitivity is usually assessed in the clinic by means of pure-tone audiometry, which measures the lowest detectable sound levels at different frequencies. Thus, this measurement may reflect the loss of sensitivity to weak sounds, making it impossible to distinguish between outer and inner hair cell dysfunction or changes in the auditory nerve, even though damage to these structures is known to occur without affecting audiometric thresholds.
Speech recognition is one of the most important aspects of human auditory function, since it enables individuals to communicate efficiently, which is fundamental for their social integration, and this understanding depends on the integrity of the auditory nervous system. It has already been described that listeners with normal audiometric thresholds may report difficulties in understanding speech in noisy environments. In an epidemiological study, which investigated the prevalence of hearing difficulties, 26% of adults interviewed reported hearing difficulties in a noisy environment, whereas only 16% had audiometric threshold changes.
It has also been reported that patients with tinnitus, but without hearing loss, often complain of speech comprehension difficulties, especially in noisy environments; in addition to that, they perform worse in speech tests in noise when compared to individuals without tinnitus.Tinnitus can be defined as an auditory illusion, or sound sensation unrelated to the external source of stimulation, i.e. a perception of sounds in the absence of a physical sound source. It is frequently related to hearing loss, but it is also known to be present in individuals without apparent hearing loss.
In a study investigating brainstem auditory evoked potential responses (BAERs) in individuals with normal hearing, with and without a complaint of tinnitus, a significant reduction in Wave I amplitude (generated by primary auditory fibers) was observed in the normal amplitudes of wave V. According to the authors, these results provide physiological evidence for possible deafferentation (in this case, a synaptopathy), which manifests as a reduced cochleoneural response (wave I), with consequent normalization of the magnitude of the neuronal response within the brainstem (wave V). The clinical manifestations of this process could be perceived as poorer performance in psychoacoustic tasks, such as speech recognition in noise, tasks for discriminating temporal and intensity aspects.