Positional nystagmus due to alteration of the specific gravity in the labyrinth

Can the nystagmus pattern in patients with a 'light cupula' be reproduced in hemi-labyrinthectomized subjects during positional alcohol nystagmus 1?

CONCLUSION

A positional nystagmus pattern compatible with a condition of a light cupula in the lateral semicircular canal seen in clinical patients could be reproduced only partially in hemi-labyrinthectomized subjects during the stage of positional alcohol nystagmus 1 (PAN 1).

OBJECTIVES

To mimic the condition of a light cupula in the lateral semicircular canal by using unilaterally deafferented subjects during the stage of PAN 1 and compare the results with those of patients with a light cupula.

METHODS

Five hemi-labyrinthectomized subjects were studied during PAN 1 with videonystagmography when they kept their heads straight forward or turned sideways in the prone and supine positions, respectively. A zero zone, indicating a cupula dysfunction, in which the geotropic nystagmus changed direction during slow head turn in the supine position, was also looked for.

RESULTS

When the subjects were examined with their heads turned left or right in the supine or prone positions, the alcohol-induced nystagmus pattern was compatible with that of a light cupula. However, the nystagmus directions at the head straight forward in the prone and supine positions, as well as localization of the zero zones, deviated from the pattern seen in patients with a light cupula.

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

The semicircular canals selectively transduce angular velocity and are normally insensitive to gravity and linear acceleration. In acute alcohol intoxication, however, the cupula becomes lighter than the endolymph, rendering it sensitive to gravity (buoyancy hypothesis). This results in positional alcohol nystagmus (PAN) and rotatory vertigo. We evaluated PAN in 8 normal subjects by means of three-dimensional eye-movement analysis in an attempt to clarify if the buoyancy mechanism is sufficient to explain PAN. Forty minutes 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 45 degrees or 90 degrees , right or left ear down, and eye movements were recorded for 40 seconds in each position. The spatial analysis of the responses showed that in addition to the nystagmus induced by the buoyancy of all 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 vestibulo-ocular reflex.