Evaluation of Effects of the Human Immunodeficiency Virus on the Vestibular System by Using the Video Head Impulse Test.

OBJECTIVE: Some studies have suggested that the human immunodeficiency virus causes dizziness and other balance problems; however, the exact effects on the vestibular system in acute and chronic phases of the disease are not clear. In this study, we aimed to evaluate the effect of the human immunodeficiency virus on semicircular canals using a video head impulse test. MATERIALS AND METHODS: Seventy-two cases were included in the study. Twenty-six of the cases had positive human immunodeficiency virus RNA (group A) and 22 had negative human immunodeficiency virus RNA with positive anti-human immunodeficiency virus (group B) laboratory results. Twenty-four of the cases were healthy individuals (group C). The vestibular system was evaluated with a video head impulse test in all cases. RESULTS: In the evaluation of overt/covert saccades, a statistically significant difference was detected for the left posterior semicircular canal between group B and the other 2 groups. However, this was considered an incidental finding and not a clinically significant result. There was no other significant difference in the catch- up saccades for other canals. In addition, there was no statistically significant difference between the groups for the vestibulo-ocular reflex gain. CONCLUSION: Although the human immunodeficiency virus has been reported to be vestibulotoxic in previ- ous studies, we found that the video head impulse test findings were not affected in our patient groups. Because the video head impulse test is considered a high-frequency test of vestibulo-ocular reflex, it is pos- sible that vestibular effects of the human immunodeficiency virus can be confined to low frequencies. It is also possible that HIV affects the central structures while sparing the peripheral vestibular pathways.

Effect of Gaze Angle During the Vertical Video Head Impulse Test Across Two Devices in Healthy Adults and Subjects With Vestibular Loss.

OBJECTIVE: To evaluate the effect of gaze angle on vertical vestibulo-ocular reflex (VOR) gain using two different video head impulse (vHIT) devices in healthy adults and subjects with bilateral vestibular loss (BVL). STUDY DESIGN: Prospective study. SETTING: Hospital research laboratory. SUBJECTS: Twenty-four healthy adults (mean [standard deviation {SD}] age = 32 [4.8]; 23-42; 8 men) and four subjects with previously diagnosed BVL (mean age [SD] = 32 [8.2]; 21-40; 3 men) participated. INTERVENTION: Vertical canal vHIT was administered with two different devices using three gaze angles (-45 degrees, 0 degree, +45 degrees). These devices have different gain calculation algorithms and different head and gaze angle protocols. MAIN OUTCOME MEASURES: Vertical canal gain and presence or absence of reset saccades. RESULTS: A significant stepwise reduction in vHIT gain was noted as gaze moved away from the plane of the canals stimulated (from -45 degrees to 0 degree, to +45 degrees) for both healthy adults and subjects with BVL. vHIT gain was able to separate the two groups using gaze angles -45 degrees and 0 degree. CONCLUSIONS: In spite of their differences in gain algorithm and recommended head position and gaze angle, each device was able to appropriately separate healthy adults from subjects with BVL with high sensitivity/specificity.

Estimating loss of canal function in the video head impulse test (vHIT).

BACKGROUND: The vestibulo-ocular reflex (VOR) gain is the primary parameter for quantifying and interpreting the video head impulse test (vHIT). Yet, the relationship between the VOR gain and the extent of canal function is not clear. OBJECTIVE: The goal of this paper was to determine if the loss of canal function in vHIT can be estimated from the VOR gain. METHODS: A model of the VOR was developed that included linear components for the cupula and the velocity storage mechanism as well as nonlinear components for the vestibular nerve and the vestibular nuclei. Multiple simulations were carried out as the level of function for the right and left VOR pathways was varied systematically over their entire range. RESULTS: Simulation results were similar to the typical findings in normal individuals as well as in patients with unilateral and bilateral loss of canal function. Based on these simulations, a relationship between the canal function and the VOR gains was established. This relationship was surprisingly independent of most model parameters. CONCLUSIONS: The sum of right and left VOR gains (or 2 times the mean of VOR gains) at a given head velocity is an estimate of the total function of the involved canals. This simple formula can estimate the loss of canal function in purely unilateral lesions. For bilateral lesions, the same formula can estimate the total loss of bilateral function but contributions from individual canals cannot be determined without additional information.

Impact of Target Distance, Target Size, and Visual Acuity on the Video Head Impulse Test.

The video head impulse test (vHIT) assesses the vestibulo-ocular reflex. Few have evaluated whether environmental factors or visual acuity influence the vHIT. The purpose of this study was to evaluate the influence of target distance, target size, and visual acuity on vHIT outcomes. Thirty-eight normal controls and 8 subjects with vestibular loss (VL) participated. vHIT was completed at 3 distances and with 3 target sizes. Normal controls were subdivided on the basis of visual acuity. Corrective saccade frequency, corrective saccade amplitude, and gain were tabulated. In the normal control group, there were no significant effects of target size or visual acuity for any vHIT outcome parameters; however, gain increased as target distance decreased. The VL group demonstrated higher corrective saccade frequency and amplitude and lower gain as compared with controls. In conclusion, decreasing target distance increases gain for normal controls but not subjects with VL. Preliminarily, visual acuity does not affect vHIT outcomes.

Video Head Impulse Test (vHIT): The Role of Corrective Saccades in Identifying Patients With Vestibular Loss.

OBJECTIVE: 1) Characterize corrective saccades (CS) in normal controls, and 2) examine the sensitivity of the video head impulse test (vHIT) for identifying vestibular loss using both gain and CS. STUDY DESIGN: Prospective combined with retrospective review. SETTING: Tertiary referral center. PATIENTS: Seventy subjects with normal vestibular function served as controls (mean age, 44.1 yr; range, 10-78) and data from 49 patients with unilateral and bilateral vestibular loss was retrospectively reviewed (mean age, 50; range, 7-81). INTERVENTION: vHIT; individual horizontal head impulses were then analyzed in MATLAB. MAIN OUTCOME MEASURES: Horizontal vHIT gain, CS peak velocity, frequency, and latency. RESULTS: There was not an age effect for CS velocity or latency, and only a weak relationship between CS frequency and age in the control group. Gain and CS latency were the only parameters affected by impulse side, demonstrating higher gain and longer latency on the right. The group with vestibular loss had significantly lower mean vHIT gain, higher mean CS frequency, higher mean CS velocity, earlier CS latency, and smaller mean CS standard deviations of the latency compared with the control group.When all factors were analyzed separately by logistic regression, vHIT gain provided the best classification (83.8%), closely followed by CS frequency (83.1%). Using a two variable approach (both gain and CS frequency) yielded the best diagnostic accuracy (overall classification = 84.6%). CONCLUSIONS: Along with gain, incorporating CS frequency in interpreting vHIT improves diagnostic accuracy. A repeatable CS (>81.89%) and/or low gain (<0.78) indicate vestibular loss.

Can the Video Head Impulse Test Define Severity of Bilateral Vestibular Hypofunction?

OBJECTIVE: The objective of the study was to compare rotary chair and video head impulse test (vHIT) findings in patients with bilateral vestibular hypofunction (BVH) to determine whether vHIT can: 1) define severity of BVH and 2) accurately predict rotary chair findings in patients with BVH. STUDY DESIGN: Retrospective chart review. SETTING: Research hospital. PATIENTS: Twenty subjects with bilateral vestibular hypofunction as assessed by rotary chair. INTERVENTION: Rotary chair and vHIT. MAIN OUTCOME MEASURES: The main outcome measures were rotary chair phase, gain, and symmetry and vHIT vestibulo-ocular reflex (VOR) gain. Rotary chair and vHIT results were assessed and subjects were stratified into groups according to the severity of their vestibular hypofunction. For rotary chair, subjects were classified as mild, moderate, or severe BVH. For vHIT, subjects were classified as normal, unilateral, or bilateral. RESULTS: Average lateral canal vHIT VOR gain: 1) significantly increased as severity of BVH decreased, and 2) demonstrated a significant and positive, linear relationship with rotary chair gains. vHIT was in disagreement with rotary chair in the classification of five subjects, which could be due to right-left asymmetry of BVH. CONCLUSION: vHIT can serve as an initial tool for identifying patients with BVH. Lower vHIT gains are consistent with having severe BVH. There was disagreement between vHIT and rotary chair, though not for any patients with severe BVH. Compared with rotary chair, the clinical gold standard for identifying BVH, vHIT possesses 100% sensitivity for excluding severe BVH when average vHIT gains are greater than 0.46.

Dizziness in the elderly.

Vertigo, unsteadiness, and other balance-related symptoms are common among older adults. These complaints should be taken seriously because they can lead to falls, injuries, loss of independence, and even death. This article provides a review of the underlying causes for the increased prevalence of dizziness with age, and discusses how specific test procedures may need to be modified for older individuals. Issues related to the management of these symptoms in the aging population are also considered.

Dynamic stability of spine using stability-based optimization and muscle spindle reflex.

A computational method for simulation of 3-D movement of the trunk under the control of 48 anatomically oriented muscle actions was developed. Neural excitation of muscles was set based on inverse dynamics approach along with the stability-based optimization. The effect of muscle spindle reflex response on the trunk movement stability was evaluated upon the application of a perturbation moment. The method was used to simulate the trunk movement from the upright standing to 60 degrees of flexion. Incorporation of the stability condition as an additional constraint in the optimization resulted in an increase in antagonistic activities demonstrating that the antagonistic co-activation acts to increase the trunk stability in response to self-induced postural internal perturbation. In presence of a 30 Nm flexion perturbation moment, muscle spindles decreased the induced deviation of the position and velocity profiles from the desired ones. The stability-generated co-activation decreased the reflexive response of muscle spindles to the perturbation demonstrating that the rise in muscle co-activation can ameliorate the corruption of afferent neural sensory system at the expense of higher loading of the spine.

Quantitative analysis of the ankle strategy under translational platform disturbance.

The ankle strategy is one of the postural adjustment maneuvers humans utilize when the support platform is disturbed. This paper presents a quantitative analysis of the ankle strategy. A three-link sagittal biped model is considered. The first link represents the two legs locked together. The second link represents the two thighs locked together. The third link represents the hip, the torso, the upper limbs, the neck, and the head. The dynamics, control, and stability of the three-link biped, under platform translation, are considered. The disturbance of the platform is represented as an input and the effect of the muscular system is reduced to a set of torques applied to the joints and across the joints. Two digital computer simulations are presented to demonstrate the behavior of the biped under backward or forward platform disturbance. The simulations are compared with experimental measurements of humans subjected to postural disturbances. It is shown that the effect of a horizontal disturbance at the ankle appears to be about 40 times that of the effect of the disturbance at the knees and at least a few hundred times larger than the effect of a disturbance at the hip. This means that, under translational platform disturbance, the ankle angle is subjected to the largest excursion. The knee and the hip angle excursions are relatively minor. Consequently, the biped, as a whole, appears to move as a single inverted pendulum. Major postural corrections are initiated by the ankle excursion. Further, when the available ankle torque is limited or nonexistent, the stability requires resorting to the knee or hip strategies.

Dynamics, stability, and control of stepping.

The dynamics, stability, and control of stepping are considered. The role of internal models is elaborated. The main objective of the paper is to provide a better understanding of the machinery and processing in the central nervous system (CNS) that relates to stepping. The role of the vestibular system in balance and balance recovery is described. Balance and balance recovery are essential in stepping, and guarantee the stability of the system before, during, and after stepping. In sagittal standing, humans use two distinct sets of control strategies to maintain their postural stability in response to external disturbance. In one set of strategies, the configuration of the base of support, namely, the position of the feet, remains unchanged. The ankle and hip strategies are examples of postural adjustments where the feet do not move. When the disturbances are large, and move the center of mass or pressure outside the support boundaries, stepping strategies are required. A simple control strategy is proposed for illustrative purposes. Its effectiveness is verified by computer simulation of a seven-link two-dimensional sagittal biped. The applications of the model in assessing trauma and injury are discussed.

Rhythmic Movement of a Pair of One-Link Arms: Coordination by Intermittent Control.

This paper considers the coordination and control of periodic movements of a pair of one-link arms. The system consists of two one-link arms each controlled by two muscle-like actuators. The muscle-like actuators are activated by simulated neural inputs. The model is simple enough to analyze, yet it embodies many aspects of human arms. Three attributes of the rhythmic coordinated movement of two arms, namely frequency, magnitude, and relative phase, are the only inputs to the controller. The controller uses mild co-activation and primarily activates the agonist. The effects of transmission delays, present in the reflex loop of physiological systems, also are modeled. The results of this research indicate the feasibility of controlling oscillatory body movements with short periods of activation. The result of many simulations, by varying the frequency or amplitude of the movement, indicate that the apparent lack of a simple relationship between neural control and desired behavior of the system should not be mistaken as evidence for the absence of a causal relationship between the activation patterns of the muscles and the desired behavior. Simulations of this system show stable oscillations at different frequencies and magnitudes even with additive noise and changes in the system mass.