Auditory Neuropathy in Adults: Behavioral and lectrophysiologic Findings

Loss of speech perception is a major effect of hearing impairment. In many cases with "auditory neuropathy", speech perception sustains severe impairment and is disproportionate with behavioral pure-tone average. In a primary audiologic evaluation, there is usually no auditory brainstem response (ABR) or acoustic reflex, while the cochlear function is within normal range. Patients with auditory neuropathy exhibit impairments in psychoacoustic evaluations, and their difficulties in sound temporal processing are due to the dys-synchrony of their auditory nerve fibers. This cross-sectional analytical study was performed on 16 young adults with auditory neuropathy and 28 normal hearing subjects. In both auditory neuropathy and control groups, auditory middle latency response (AMLR) and auditory late response (ALR) were recorded and psychoacoustic measurements including "intensity discrimination", "frequency discrimination", and "gap detection" as well as a word discrimination test in silent and in three different signal-to-noise ratios were carried out. There was no agreement between the pure-tone average (PTA) and word discrimination score (WDS). Despite AMLR absence in 6 patients, auditory late response ALR was recorded in all the individuals and there was a significant correlation between some characteristics of these responses and the word discrimination score. The patients revealed remarkable impairment in the discrimination of intensity, frequency, and temporal characteristics of sound. In addition, there was a significant correlation between each of these characteristics and WDS. The WDS showed considerable variability in individuals with auditory neuropathy and decreased in the presence of noise considerably. It seems that neural synchrony had different representations in short (and middle) auditory evoked responses by comparison with late ones. Auditory neuropathy patients’ difficulties in discriminating the intensity, frequency, and temporal characteristics of sound and their relation with WDS showed eight nerve functions in processing these psychoacoustic perceptions and their effects on speech understanding.