Rotational Test, Off-vertical Axis Rotation

Abstract

Since our visual sense performs analysis best when images remain steady on the retina, the eyes need to move. To move the eyes, optokinetic and vestibular reflexes have evolved to stabilize images on the retina during head movements. The vestibular and optokinetic systems work together to maintain clear vision during head movements. Because natural head movements are of high frequency, the visual system is impeded by relatively slow retinal processing (about 70 msec), and cannot act rapidly enough to produce compensatory eye movements that can hold images steady on the retina. In contrast, the semicircular-ocular reflex (ScOR) has a latency of less than 16 msec. The ScOR promptly produces slow phase eye movements to compensate for head rotations. To evaluate the ScOR, caloric and rotational tests are routinely used. Barany first described the velocity step test in 1907. In 1948, Van Egmond et al. described an elaboration of this test, which they called cupulometry. These tests are rarely performed today. They have proved to be highly unreliable and not sensitive enough in the detection of lesions, because of the difficulty in producing repeatable stimuli and making accurate response measurements. The modern era of rotational testing began in the 1960s. Today all aspects of rotational testing including stimulus generation, response measurement, and data analysis are controlled by computer. The vestibular system also contains otolithic receptors that respond to linear accelerations of the head. The otolith-ocular reflex (OOR) becomes important when head translations cause a slip in the image on the retina. Off-vertical axis rotation (OVAR) is a stimulus wherein persons are rotated while the axis about which they are rotating is tilted with respect to gravity. Thus the OVAR can be used to assess the OOR. In this article, the principle and clinical application of rotational tests are introduced.