Content area
Full Text
Author for correspondence: Dr R Srinivasa Raghavan, Department of ENT, Royal Surrey Hospital, Guildford, Surrey, GU2 7XX, UK E-mail: [email protected]
Dr R S Raghavan takes responsibility for the integrity of the content of the paper
A report of case one was presented at the British Association of Audiovestibular Physicians Annual Conference, 16–17 March 2017, Chesham, UK.
Introduction
Dissociation between caloric and head impulse test results in patients with unilateral Ménière's disease has been well documented in the literature.1–5 Specifically, caloric testing indicates impairment of vestibulo-ocular reflex on the horizontal semicircular canal plane; however, video head impulse testing shows near-normal, normal or even increased vestibulo-ocular reflex gain on the horizontal semicircular canal plane.
In 2015, McGarvie et al. proposed a novel theoretical model to explain this dissociation.6 In agreement with work by Valli et al.,7 they conclude that the hydrostatic model, as proposed by Gentine et al.,8 is the only viable mechanism to explain caloric responses.
The hydrostatic model suggests that during caloric testing, the temperature variation across the temporal bone results in a difference in density, and therefore buoyancy, in the endolymph, between the medial and lateral arms of the horizontal semicircular canal. This produces a pressure differential across the cupula, which causes it to deflect.
Based on this model, McGarvie et al. proposed that in patients with Ménière's disease, the hydropic expansion of the horizontal semicircular canal membranous labyrinth allows for local convective flow and a mixing of the lower- and higher-density endolymph.6 This dissipates the buoyancy that produces the pressure differential across the cupula, and therefore results in an impaired or absent response. Regarding the effect of hydropic expansion on angular acceleration testing (i.e. head impulse testing), they explain that as the membranous duct lies along the outer radius of the bony semicircular canal, there will not be a change in the overall diameter of the duct, rather only an increase in the cross-sectional area of the duct.6 They reference modelling by Grieser et al.,9 who hypothesise that the increase in cross-sectional area reduces the duct's natural hydrodynamic flow resistance. This allows greater endolymph flow velocities with an angular rotation, and therefore a larger cupula deflection, resulting in...