Auditory neuropathy/dys-synchrony and its perceptual consequences

Abstract

Auditory neuropathy/dys-synchrony is a form of hearing impairment in which cochlear outer hair cell function is spared but neural transmission in the auditory pathway is disordered. This condition, or group of conditions with a common physiologic profile, accounts for approximately 7% of permanent childhood hearing loss and a significant (but as yet undetermined) proportion of adult impairment. This paper presents an overview of the mechanisms underlying auditory neuropathy/dys-synchrony-type hearing loss and the clinical profile for affected patients. In particular it examines the perceptual consequences of auditory neuropathy/dys-synchrony, which are quite different from those associated with sensorineural hearing loss, and considers currently available, and future management options.

Figure 1.

ABR recordings for a 3-year-old child with AN/AD type hearing loss. The dotted line represents the point at which the stimulus reached the cochlea. The top tracings show no repeatable potentials to alternating clicks presented at 100 dBnHL. The middle tracing pairs show repeatable cochlear microphonic responses but absent brain stem response waveforms to unipolar stimuli at 80 dBnHL. The asterisks indicate the positive peaks in the cochlear microphonic waveform. The final tracings, in which only the stimulus artefact is evident, were obtained to rarefacting clicks presented with the tubephone clamped.

Figure 2.

The distribution of behavioral hearing thresholds (3-frequency average) for 38 ears with auditory neuropathy (Rance et al., 1999).

Figure 3.

Audiometric results for a 5-year-old child with auditory neuropathy/dys-synchrony type hearing loss. The five assessments were carried out over a 6-month period. Results obtained were considered to be an accurate reflection of the child's acuity for that day (Rance et al., 1999). Reproduced with permission of Lippincott, Williams Wilkins Publishing Group.

Figure 4.

Grand mean cortical event-related potential waveforms in response to tones (left panel) and to speech (right panel) for children with normal hearing (top traces), sensorineural (SN) hearing loss (440 Hz: N = 17; /dæd/: N = 15, middle traces), and auditory neuropathy (AN) (N = 11, bottom traces). Reproduced with permission of Lippincott, Williams Wilkins Publishing Group.

Figure 5.

Open-set word/average hearing level comparisons for 46 children with auditory neuropathy/dys-synchrony type hearing loss. The dashed line represents the minimum expected score for ears with sensorineural hearing loss (Yellin et al., 1989). Contributing studies are listed with the number of ears for each. [Table: see text]

Figure 6.

Open-set speech perception results (aided/unaided) for children with auditory neuropathy/dys-synchrony. Study A: Deltenre et al. (1999); Study B: Rance et al. (1999); Study C: Lee et al. (2001); Study D: Rance et al. (2002).

Figure 7.

PBK phoneme scores in the aided test condition are shown as a function of 3-frequency average hearing level for children with sensorineural (open circles) and auditory neuropathy/dys-synchrony type hearing loss (filled circles).

Figure 8.

White/notched noise masker level differences for each subject. SN, sensorineural; AN/AD, auditory neuropathy/dys-synchrony (Rance et al., 2004).

Figure 9.

Amplitude modulation detection thresholds in auditory neuropathy/dys-synchrony (AN/AD) subjects. Closed circles represent the findings for children in the AN/AD group with speech perception scores > 60%. Open circles represent the children in the AN/AD group with speech scores <30%. Open triangles show the findings for children in the AN/AD <30% group unable to detect a modulation depth of 0 dB (100%). The enclosed area shows the mean ±2 SD range for the normal group (Rance et al., 2004).

Figure 10.

Amplitude modulation detection threshold (10 Hz MF) plotted as a function of consonant-nucleus-consonant (CNC) phoneme score for auditory neuropathy/dys-synchrony subjects (Rance et al., 2004).

Figure 11.

Graph shows 500-Hz frequency discrimination limens for each subject plotted as a function of consonant-nucleus-consonant (CNC) phoneme score. SN, sensorineural; AN/AD, auditory neuropathy/dys-synchrony (Rance et al., 2004).