Diffusion-Weighted Magnetic Resonance Imaging: A Diagnostic Tool for Auditory (Axonal) Neuropathy

Abstract

Background: Axonal neuropathies are disorders that impair neural transmission, leading to substantial sensory deficits. In the auditory system, axonal degeneration can disrupt auditory processing, causing significant hearing difficulties. Understanding the extent of axonal degeneration and its impact on auditory function is crucial for improving diagnosis and management. This study aims to quantify axonal degeneration in the VIIIth nerve using diffusion-weighted MRI and to correlate these findings with auditory function.

Methods: Fifty-two children and adults participated. A total of, 27 with normal hearing, 7 with cochlear hearing loss and 18 with auditory neuropathy (AN). Hearing thresholds and dMRI data was collected for all participants and the VIIIth nerve was evaluated using the fixel-based analysis metric of Apparent Fibre Density (AFD).

Results: AFD was significantly lower in participants with AN compared to participants with normal hearing and cochlear hearing loss (p < 0.05). 9/18 participants with AN exhibited AFD values ≥ 2 standard deviations below the normal range. Additionally, AFD was strongly correlated with hearing thresholds in participants with no evidence of cochlear dysfunction (r = -0.776, p < 0.001), suggesting reduced auditory nerve fibre density is associated with impaired sound detection.

Conclusions: dMRI-derived AFD is a sensitive marker for axonal degeneration in the VIIIth nerve. This study provides the first in vivo evidence linking VIIIth nerve microstructure with hearing thresholds, highlighting the potential of dMRI in diagnosing and monitoring AN. The findings suggest that dMRI could be a valuable tool in clinical settings for assessing auditory nerve health and guiding treatment strategies for individuals with AN.

Keywords: auditory neuropathy; axonopathy; diffusion‐weighted MRI; hearing loss; neuroimaging.

FIGURE 1

(A) Auditory Brainstem Response recordings to 90dBnHL acoustic click stimuli. The top trace was recorded from a normal hearing (NH) control participant and is a typical result obtained from this cohort. The second trace shows the result obtained from participant #7 with hearing loss (HL7) from their better hearing ear. The lower traces were recorded from participant #3 and are representative of the type of results obtained from the participants with auditory neuropathy (AN). In these cases, the main peaks (Wave I, III and V) are absent, however, rarefaction and condensation polarity stimuli can be used to identify the cochlear microphonic (CM) potential (positive peaks of the CM are shown by the asterisks). (B) Pure tone audiometric thresholds averaged across both ears for the participants with auditory neuropathy. Bold black line represents the average of all the individual hearing thresholds shown.

FIGURE 2

(A) T2‐weighted MR image showing an axial slice acquired from a participant. The cochlea and internal auditory canal (IAC) are clearly identifiable. (B) White matter fibre orientation distribution function population tempate, axial slice, with the VIIIth nerves shown. The VIIIth nerves were generated with probabilitic tractography using the dMRI data. (C) Coronal view of the same image shown in (B). The position of the VIIIth nerves are shown.

FIGURE 3

Boxplots showing a comparison of the apparent fibre density (AFD) results obtained from the VIIIth cranial nerve between the groups included in this study: The auditory neuropathy (AN) group, the normal hearing control group (controls), and the cochlear hearing loss group (HL(Cochlear)). The AFD metric is provided AFD values are represented in arbitrary units (a.u.) due to the relative nature of the metric. The boxplots show the median (centre line), the interquartile range (shaded area), and whiskers (range) of the data.

FIGURE 4

Left panel: Scatterplot showing the relationship between VIIIth nerve apparent fibre density [arbitratry units (a.u.)] results and 4‐frequency average hearing level (dBHL) for Group 1 (participants with ‘normal’ cochlear function). Group 1 is comprised of participants with normal hearing (NH) and auditory neuropathy (AN). Right panel: Scatterplot showing the same relationship for Group 2 (participants with hearing loss resulting from cochlear dysfunction).