New insights into positional alcohol nystagmus using three-dimensional eye-movement analysis

The semicircular canals selectively transduce angular velocity and are norm ally insensitive to gravity and linear acceleration. In acute alcohol intox ication, however, the cupula becomes lighter than the endolymph, rendering it sensitive to gravity (buoyancy hypothesis). This results in positional a lcohol nystagmus (PAN) and rotatory vertigo. We evaluated PAN in 8 normal s ubjects by means of three-dimensional eye-movement analysis in an attempt t o clarify if the buoyancy mechanism is sufficient to explain PAN. Forty min utes after intake of 0.8 g of alcohol/kg of body weight, the subjects were positioned such that the lateral canals were earth vertical. They were then rotated in the plane of the lateral canals about an earth-horizontal axis to either 459 or 90 degrees, right or left ear down, and eye movements were recorded for 40 seconds in each position. The spatial analysis of the resp onses showed that in addition to the nystagmus induced by the buoyancy of a ll six cupulae, alcohol intoxication also causes a vertical velocity offset (in all subjects, slow phase down) that is independent of the orientation of the subject in space. The offset may represent a toxic effect on central vestibular pathways, producing a tone imbalance of the vertical vestibule- ocular reflex.