Glycerol‐induced fluid shift attenuates the vestibulosympathetic reflex in humans

Abstract

Classically, the glycerol dehydration test (GDT) has been used to test for the presence of Ménière's disease and can cause acute alterations in vestibular reflexes in both normal and pathological states. The vestibulosympathetic reflex (VSR) elicits increases in muscle sympathetic nerve activity (MSNA) and peripheral vasoconstriction. We hypothesized that the GDT would attenuate the VSR through an acute fluid shift of the inner ear. Nine male subjects (27±1 years) performed head‐down rotation (HDR), which engages the VSR, before and after administration of either the GDT or saline. MSNA (microneurography), arterial blood pressure, and leg blood flow (venous occlusion plethysmography) were measured during HDR. Subjects performed 80‐degree head‐up tilt for up to 30 minutes after the post‐tests. Before drug administration, HDR significantly increased MSNA in burst frequency (Δ5±1 bursts/min, Δ8±1 bursts/min; p<0.01) and total activity (Δ44±13% Δ77±17%; p<0.01), and decreased calf vascular conductance (Δ15±6%, Δ20±3%; p<0.01), in both the saline and glycerol trials, respectively. Post‐saline injection, HDR still significantly increased MSNA (Δ6±2 bursts/min, Δ83±20% total activity; p<0.01) and decreased calf vascular conductance (Δ21±4%, p<0.01), which was not significantly different from pretesting. In contrast, post‐GDT resulted in an attenuation of MSNA (Δ3±1 bursts/min, Δ22±3% total activity) and reduction in calf vascular conductance (Δ7±4%) during HDR. These results suggest that a fluid shift of the inner ear via glycerol dehydration attenuates the VSR. These data provide support that dynamic fluid shifts can have a significant effect on the VSR.Research supported by NIH DC006459, HL077670, & M01RR10732 and NSBRI CA00404