Gray-tone appearances on 4-hour delayed gadolinium-enhanced magnetic resonance imaging indicate severe inner ear pathology and symptoms in sudden sensorineural hearing loss.

Objective: Hybrid of reversed image of positive endolymph signal and negative image of perilymph signal (HYDROPS) in delayed gadolinium-enhanced magnetic resonance imaging (MRI) typically depicts normal inner ear as "white-tone" and endolymphatic hydrops as "black-transparent" appearances, whereas ears with auditory and vestibular disorders are occasionally depicted as "gray-tone." This study aimed to investigate the pathological basis of sudden sensorineural hearing loss (SSNHL) patients with "gray-tone" appearances on HYDROPS. Methods: Delayed gadolinium-enhanced MRI examinations were conducted on 29 subjects with unilateral SSNHL. We mainly analyzed positive perilymph image (PPI) and positive endolymph image (PEI), which were components HYDROPS. Results: On PPI, signal intensity (SI) values extracted from the cochlear and vestibular region of interest (ROI) were higher in the SSNHL ears with dizziness/vertigo symptom at the first visit compared to the healthy ear. Additionally, the PPI/PEI enhancement pattern in the vestibule was associated with a high prevalence of hearing and vestibular deteriorations at the first visit and poor hearing improvement after treatment. Conclusion: Enhancement on PPI/PEI may result from leakage of gadolinium into the inner ear following breakdown of the blood-labyrinth barrier, with high SI being correlated with the amount of leakage. Particularly, a significant leakage into the endolymphatic space, defined as PPI+/PEI+, indicates severe inner ear pathology. Ultimately, we emphasize that the "gray-tone" appearance in the inner ear on HYDROPS comprises enhancements on both PPI and PEI and propose a new classification for evaluating SSNHL Peri- and Endolymphatic image Enhancement pattern in Delayed gadolinium-enhanced MRI (SPEED). Level of Evidence: 4.