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.