Two types of direction-changing positional nystagmus with neutral points.

OBJECTIVES

We encountered patients who had static direction-changing positional nystagmus (DCPN) canceled at about 20-30° yaw head rotation from the supine position. This nystagmus was also canceled when the head was rotated 180° from this position. We termed these head positions neutral points. The positional nystagmus observed (except at the neutral points) was thought to occur due to a "heavy cupula" or "light cupula". The purpose of this study was to examine DCPN with neutral points as well as the pathomechanism of this condition.

METHODS

Retrospective case review of patients attending two hospitals. Sixteen patients who exhibited DCPN with neutral points were examined using an infrared camera (installed in goggles). Using this system, the vestibulo-ocular reflex (VOR) was recorded, and VOR gain was obtained. Vestibular function and the affected side were determined. In addition, the angle between the supine position and neutral point was measured in each patient. We also examined other positional nystagmus occurring at other times.

RESULTS

In the heavy cupula type group, we noted positional nystagmus for which repositioning maneuvers were successful, whereas, in the light cupula type group, repositioning maneuvers were not effective. The angle between supine position and neutral point was 26.5 ± 11.6°.

CONCLUSIONS

Heavy cupula type may occur as a result of otoconia while light cupula type may be due to the specific gravity of the endolymph. The VOR gain and side of the benign paroxysmal positional vertigo (BPPV) observed suggested that the affected side was that to which the neutral point was deviated.