Subjective Visual Vertical (SVV) Patterns Obtained Using Different Translation Times during Unilateral Centrifugation (UC) Testing

Background: Unilateral centrifugation (UC) is a test of utricular function that involves the eccentric displacement of a patient while undergoing high velocity rotation. These off-center placements permit evaluation of utricles as only one organ is subjected to linear stimulation. Currently, several different protocols are used for translation times between positions (e.g., Clarke et al, 1996; Wuyts et al, 2003). Purpose: This study examined the effects of different translation times between eccentric and centric positions during UC on an individual’s perceived subjective visual vertical (SVV). Research Design: A prospective repeated measures design was used in this study. Study Sample: Forty-three young adults with no history or complaints of vestibular or neurological disorders participated in this study. Data Collection: All testing was performed on a Neuro Kinetics Inc. (Pittsburgh, PA) Neuro-Otologic Test Center (NOTC) rotational chair at the Bloomsburg University of Pennsylvania Vestibular Clinic. Each participant completed six SVV trials in the static condition (sSVV) as a baseline of function. In addition, each participant completed UC trials using a rotational velocity of 300°/sec with 4 cm eccentric lateral displacements that were maintained for 60 sec per position. Translations of 5, 10, 15, 20, 25, and 30 sec were randomized. The maximum velocity was maintained for 90 sec prior to the eccentric placement. Multiple estimations of dynamic subjective visual vertical (dSVV) were performed for each position. Results: dSVV data yielded several trends. At 5 sec translations, dSVV values had greater variability and required longer for participant responses to stabilize. Regardless of UC translation time, initial trial values were often quite different from the remaining trials. Shorter UC translation times were associated with more eccentric dSVV values outside of the clinical norms. While not statistically significant, more variance was associated with the shortest UC translation times, while 15 and 25 sec translation data demonstrated the best correlations and lowest variances. Discussion: Findings suggest that incorporating sSVV data to normalize dSVV data should be considered to reduce the influence of the underlying baseline static otolithic function on the UC results. The presence of a clear tendency for shorter UC translation times to be associated with larger numbers of clinically abnormal findings may indicate that very short UC translation times may not be ideal for clinical applications. This is supported by our finding that longer UC translation times elicited more consistent SVV results and less response variability. While some of these findings were not statistically significant, the results do suggest that clinical UC testing may benefit from optimization of translation time as well as inclusion of sSVV in response interpretation.

Unilateral examination of utricle and saccule function

Attention is directed towards the recently developed unilateral tests of saccular and utricular function. Together with the now widely used head-thrust test and the standard caloric test for semicircular canal function, these provide for a more comprehensive unilateral examination of labyrinth function. The efficacy of vestibular evoked myogenic potentials (VEMP) as a direct unilateral test of saccular function is currently being demonstrated in an increasing number of reports. Furthermore, the relevant neuronal pathways have been delineated in animal studies, so that all evidence points to the validity of the VEMP as a saccule-mediated response. Concerning utricular function, considerable headway has been made using the unilateral centrifugation paradigm. Testing is performed with a variable radius rotary chair with constant velocity rotation about the earth-vertical axis. Displacing the head by 3.5–4 cm from the rotation axis, the eccentrically positioned utricle is stimulated unilaterally by the resultant centrifugal force. This paradigm can be employed to elicit a utriculo-ocular response (UOR) or to permit measurement of the subjective visual vertical (SVV). More recently, it has also been demonstrated that testing during normal, on-centre yaw axis rotation is often sufficient to localise peripheral otolith dysfunction by means of SVV estimation. This test mode can be easily integrated into routine clinical testing. To illustrate the efficacy of such differential testing, the findings from two patients are presented that demonstrate for the first time an isolated unilateral utricular dysfunction.

Utricular sensitivity and preponderance assessed by the unilateral centrifugation test

Utricular sensitivity and preponderance of the right or left utricle can be assessed by means of the unilateral centrifugation test. In this test, subjects are rotated about an earth vertical axis at a velocity of 400 degrees per second. During the ongoing rotation, the subject is gradually translated 4 cm first to the right, and then to the left, along an interaural axis, to a position at which one utricle becomes aligned with the axis of rotation, and at this point is subjected only to gravitational forces. At this eccentric position, the contralateral utricle is exposed to the combination of gravity and a centrifugal acceleration of 0.4g, corresponding to an apparent roll-tilt of 21.7 degrees. This stimulus induces ocular counterrolling (OCR), which is measured on-line using three-dimensional video-oculography (VOG). We observed that ocular counterrolling appears as a linear function of the gravito-inertial acceleration tilt of the head centre (GIA H C ) during the lateral translation. We present a theoretical model for this linear relationship that contains two parameters: 1) the slope of the linear regression is a measure for the utricular sensitivity and 2) the intercept of the linear regression is a measure of the preponderance of the right or left utricle. The strength of the model is supported by data obtained from 28 healthy subjects and 14 patients with unilateral vestibular deafferentiation (UVD) due to acoustic neuroma surgery.

The interutricular distance determined from external landmarks

Knowledge of the exact distance between the utricles is important in new vestibular tests, such as the unilateral centrifugation (UC) test for the unilateral examination of the utricles. During this test, subjects are rotated at constant velocity and simultaneously laterally displaced along an interaural axis so that one labyrinth becomes aligned with the axis of rotation. When the axis of rotation crosses precisely through one labyrinth, only the opposite labyrinth is stimulated. To achieve this setup, precise knowledge of the interutricular distance is needed. The purpose of this study is to investigate the correlation between the interutricular distance (IUD), measured on T2-weighted magnetic resonance images, and specific external measures of head dimensions such as distance nasion-inion, intermastoid distance (IMD), distances between the temporomandibular joints and between the lateral margins of the orbits. Data have been collected in a series of 50 subjects (25 men and 25 women). On MR images we found a mean IUD of 7.22 cm ( S D = 0.42 cm). There was a strong correlation between the IUD measured on MR images and the intermastoid distance. A linear combination of the IMD, nasion-inion distance and height of the subjects could predict the IUD very satisfactory (R=0.85, adjusted R 2 = 0.723). We also determined a measure of eccentricity of the vestibular labyrinths. The 95% prediction interval for the asymmetry appeared to be less than 4.3%.