Discussion
According to the 2015 diagnostic criteria, hearing loss is a necessary
component for definitive clinical diagnosis of MD, highlighting its
pivotal role in the pathogenesis of this condition. However, the
underlying mechanisms leading to hearing loss remain poorly understood.
After intravenous administration of gadolinium, the 3D-FLAIR sequence
offers distinct advantages in visualizing EH and plays a pivotal role in
diagnosing MD11, 12. In this study, a 3D-FLAIR scan
was conducted four hours after double-dose gadobutrol injection,
revealing varying degrees of EH among all patients. However, our study
also revealed a statistically significant enhancement in signal
intensity at the IAC bottom on the affected side of the patient compared
to the unaffected side. This suggests that the contrast agent permeates
more deeply into IAC bottom of the affected than the unaffected side,
potentially due to compromised barrier integrity on the affected side.
So, does it affect hearing? Therefore, further investigation is
necessary to determine its impact on hearing.
In this study, we further investigated the correlation between the
signal intensity ratio of the IAC and hearing levels. Our analysis
revealed a significant correlation between signal intensity at the IAC
bottom and low, middle, and high-tone hearing loss. The IAC contains
vital anatomical structures such as nerves and blood vessels, whose
integrity is crucial for optimal hearing function. Disruption of the IAC
barrier can result in nerve damage, leading to hearing loss; the
severity of hearing loss is directly proportional to the extent of
damage to the IAC barrier. Similar to our study, in patients with
neurogenic deafness, the signal intensity at the IAC bottom was observed
to be higher on the affected side than on the unaffected
side9, which is believed to be associated with hearing
loss on the affected side; Labyrinthitis is a prominent etiology of
sensorineural hearing loss13, and certain
investigations have demonstrated that patients with labyrinthitis
exhibit conspicuous enhancement of the labyrinth and IAC bottom on
enhanced MRI, implying an association with hearing
impairment14, 15. Therefore, we hypothesize that a
barrier exists at the IAC bottom, and its disruption results in hearing
impairment in the affected ear.
Studies indicate that EH can result in significant loss of spiral
ganglion cells located at the cochlea apex, and this loss is closely
associated with the severity of EH6; Other studies
have demonstrated a consistent pattern of spiral ganglion cell loss
progressing from the cochlear apex to the base, which aligns with the
progression of EH16; Currently, numerous studies have
demonstrated a positive correlation between the degree of EH and hearing
loss in patients diagnosed with MD10, 17, 18.
Therefore, it is our belief that EH may serve as a contributing factor
to hearing loss among individuals afflicted with this condition.
We observed a positive correlation between the signal intensity of the
inner auditory canal bottom and hearing loss on the affected side in
patients with MD. However, there was no significant correlation found
between this signal intensity and the degree of cochlear and vestibular
hydrops. Therefore, we hypothesize that the changes in signal intensity
at the affected side’s IAC bottom are not directly associated with EH,
but rather caused by destruction of the barrier within the canal. In
conclusion, it is postulated that the disruption of the IAC barrier in
MD patients may contribute to hearing impairment. The destruction of
this barrier can potentially harm the auditory nerve traversing through
it, ultimately leading to hearing loss.