Effects of endolymphatic space volume on discrepancy of results between caloric test and video head impulse test

The caloric test (C-test) and video head impulse test (vHIT) are known to occasionally show contradictory results in patients with Meniere's disease (MD). The reasons underlying this discrepancy between the two tests are currently unclear. We aimed to reveal the mechanisms responsible for this discrepancy by performing volumetric evaluation of the endolymphatic space (ELS) by using endoluminal contrast-enhanced inner ear MRI (ieMRI).

We enrolled 136 patients (174 ears) who visited the vertigo/dizziness center of our university and underwent the C-test and vHIT between February 2018 and February 2020. Inner ear MRI was also performed to determine the presence of endolymphatic hydrops (EH). The percentage of patients diagnosed with each vestibular disease was as follows: MD, 23.0%; benign paroxysmal positional vertigo (BPPV), 17.8%; bilateral vestibular disorder (BVD), 9.2%; sudden deafness with vertigo (SD), 8.0%; peripheral dizziness (PD), 7.5%; unilateral vestibular disorder (UVD), 6.9%; vestibular neuritis (VN), 6.3%; delayed endolymphatic hydrops (DEH), 3.4%; central dizziness (CD), 2.9%; Hunt syndrome (Hunt), 1.2%; and other disorders (OD), 13.8%.

Among the ears in the present study, 46.0% (80/174) showed a discrepancy in the results of the C-test and vHIT, and the disease-related distribution of patients showing this discrepancy was as follows; MD, 38.8% (27/80; p = 0.0019); BVD, 13.8% (11/80); UVD, 12.5% (10/80); SD, 7.5% (6/80); BPPV, 6.3% (5/80); PD, 6.3% (5/80); VN, 3.8% (3/80); DEH, 3.8% (3/80); CD, 2.5% (2/80); Hunt, 0.0% (0/80); and OD, 10.0% (8/80). In all cases, the discrepancy presented as a positive C-test result and negative vHIT result. The ELS ratio was measured for the whole inner ear, cochlea, vestibule, and semicircular canal, and the relationships between the rates and the presence of discrepancy was examined. Inner ear ELS ratio was 17.9% ± 10.8% in patients with the discrepancy and 15.2% ± 8.8% in those without the discrepancy (p = 0.036). Cochlear ELS ratio was 14.9% ± 11.3% in patients with the discrepancy and 11.9% ± 10.3% in those without the discrepancy (p = 0.0012). Vestibular ELS ratio was 22.3% ± 16.2% in patients with the discrepancy and 17.2% ± 12.7% in those without the discrepancy (p = 0.032). Semicircular canal ELS ratio was 18.0% ± 11.0% in patients with the discrepancy and 16.5% ± 9.6% in those without the discrepancy (p = 0.442).

The volume of the ELS may affect the discrepancy of results between the C-test and vHIT.