Positional Nystagmus/Vertigo with Specific Gravity Differential Between Cupula and Endolymph (Buoyancy Hypothesis)

Abstract

Transient positional nystagmus has been repeatedly described following the ingestion of water-soluble molecules with differing specific gravities, such as alcohol or heavy water. The semicircular canals selectively transduce angular velocity and acceleration, and under normal circumstances are insensitive to gravitational orientation and linear acceleration. A major reason for this lack of sensitivity is that the cupula and endolymph have the same specific gravity (the sensory haircells are embedded in the cupula which is housed in the ampulla of the canals). The neutral buoyancy of the cupula in the endolymph prevents any out-of-balance forces when linear accelerations are apphed. If a considerable specific gravity differential occurs between cupula and endolymph then the semicircular canals should become sensitive to changes in head position within the gravitational field, resulting in positional rotatory vertigo and nystagmus. The direction of nystagmus and vertigo should be dependent on the particular head position (according to the different planes of the horizontal and vertical semicircular canals) and on whether the specific gravity of the cupula is greater or less than that of the endolymph. Thus, nystagmus should be direction-changing with either head position right lateral or left lateral, beating towards the undermost ear with the cupula lighter than endolymph and beating towards the uppermost ear with the cupula heavier than endolymph.