What does the dissociation between the results of video head impulse versus caloric testing reveal about the vestibular dysfunction in Ménière's disease?

CONCLUSION: It is suggested that the different results of rotational (video head impulse - vHIT) and caloric tests in patients with Ménière's disease (MD) may be a consequence of the physical enlargement of the membranous duct in the hydropic labyrinths in MD, causing a reduced response to caloric stimulation.
OBJECTIVES: There have been reports that the results of two tests of semicircular canal function, the caloric response and the responses to vHIT do not agree. This retrospective study at a tertiary referral hospital examined this disagreement.
METHODS: This study reviewed the data of 22 patients who met the AAO-HNS criteria for MD and who had both caloric and vHIT testing.
RESULTS: There was a clear dissociation: patients with MD had a small or absent response to caloric stimulation of their affected ear, whilst their response to vHIT was in the normal range. DISCUSSION: The accepted Gentine model of the mechanism of caloric stimulation could account for this dissociation: the increased diameter of the semicircular duct in hydropic labyrinths resulting in endolymph circulation within the duct itself and so a smaller thermally induced pressure across the cupula. The increased duct diameter will have little effect on responses to rotation.