Subjective Visual Vertical (SVV) & Risk of Fall (ROF)

January 03, 2023 by Charles Shearer, OD

Subjective Visual Vertical (SVV): How to measure it and why?

Because of the complex interactions between the visual and vestibular systems, the perception of vertical is often distorted in vestibular imbalances and otolithic deficits. Such errors in visual-spatial perception contribute to errors in postural adjustments, increasing the risk of falls.

Our perception of visual vertical is an important reference for balance, gait, and the effect of gravity on subjects’ orientation with their world. It can cause an individual to lean or even overcompensate for changes in posture and movement, which in turn leads to a greater risk of falling. Age-related changes in the vestibular system and visual-vestibular integration deficits tend to impair these skills.  

A head mounted display (HMD), in this case a virtual reality (VR) headset, can be a valuable tool to assist clinician in evaluating SVV. In addition, as a training tool, HMD technology (with the proper software) can improve SVV performance, potentially reducing risk of fall.

What is Subjective Visual Vertical (SVV) and how is it tested?

The SVV test allows us to measure the difference between the perceived and the actual vertical alignment of the head. This was often referred to as the “bucket test” in days past, as it was measured by having the subject look into a 5-gallon bucket and then turn or twist the bucket until a line inside the bucket appeared to be vertical. Once the subject felt the line was vertical, a clinician would use a protractor and a level to determine the angle of deviation in degrees. This is referred to as static SVV.

Today, we can not only measure the Static SVV much more precisely with a HMD, but we also have the luxury of controlling the type and direction of various backgrounds within the HMD while the subject is setting the line to perceived vertical. The alignment task, when performed with a moving background, is referred to as the Dynamic SVV. The difference between the Static and Dynamic SVV is referred to as Visual Dependence (VD) and is another valuable metric or measure in the aging patient.

The simple bucket test unfortunately cannot give us Dynamic SVV information, and therefore the VD cannot be calculated. In addition, the bucket test is a long and cumbersome test which takes 20-30 minutes to complete, whereas the HMD/SVV test takes half the time and gives more precise information. The HMD also gives us information about the subject's head position, such as pitch and roll, during the testing process.

How are Risk of Fall (ROF) tests related to SVV?

Totilienė4 and her colleagues found that older adults who were identified as “high risk” by the Tinetti Performance Oriented Mobility Test also demonstrated an impaired SVV. The Totilienė study showed that a poor response on the Dynamic SVV test was the most sensitive SVV fall risk predictor, followed by Visual Dependence, and then static SVV.  Although the Totilienė study showed statistically significant correlations between risk of fall and SVV, it did not imply that SVV testing should replace all other testing. SVV is however another tool that many clinicians will add to their battery of tests to help evaluate ROF in “at risk” populations.  

From a clinical perspective, it is not only important to take traditional, or Static SVV measurements, but also to take Dynamic SVV measurements that relate more closely to real-world situations. New HMD technology not only allows us to take both Static and Dynamic SVV measurements, it also enables us to perform rehabilitation tasks in visually realistic simulations.  

The importance of precisely and accurately identifying these unique deficits in multisensory integration cannot be understated. This allows for identification of risk factors, longitudinal tracking across the lifespan of the ailment, and monitoring rehabilitation progression.

Risk of Fall (ROF): How big is the problem?

Falls have received priority attention from both the World Health Organization (WHO) and the Centers for Disease Control (CDC) (Older Adult Falls | Fall Prevention | Injury Center | CDC) as a major public health issue. Even though falls can occur at any age, the major concern is with the elderly and other at-risk patients. The human suffering and economic costs are staggering when you consider these statistics1,2,:

Falls are the second leading cause of unintentional injury and death in the United States.
Over 40% of the population over the age of 70 will experience a significant fall at least once a year.

Falls are the leading cause of death from injury, above the age of 65.
Falls are expensive: $1,049 medical costs per fall event in the U.S., fall prevention strategies could save $120 million annually.

Contributing factors to ROF in an aging population include age-related declines in gait control, musculoskeletal decline, dizziness, balance and disease (e.g. dementia, Type II diabetes, cardiovascular, vestibular, and visual dysfunction) as well as anxiety and depression.

A major emphasis of the WHO, CDC and global fall prevention groups3 is to achieve consensus on clinical practice guidelines for a person-centered approach to fall prevention in those with the highest risk for fall, injury and death. As a corporation specializing in innovative precision instrumentation to assess and treat vision and balance problems, Bertec is devoting significant attention and resources to solving this global public health problem.

Let’s review the difference between static SVV, dynamic SVV, and visual dependence.

Static SVV: the subject is asked to adjust a line that starts in a tilted position, to be straight up and down vertically. There are no other visual clues or references to assist in the alignment as it takes place in a plain visual field with no background or movement, so no orientation clues are present. 

Dynamic SVV: the same task is performed except there is typically a nondescript patterned background that gives no vertical clues for alignment but is dynamic or moving. The dynamic SVV is measured first with the background moving in a clockwise manner and then moving in a counterclockwise manner. This moving background typically adds more confusion and challenge to the task.

Visual Dependence is calculated by subtracting the static SVV value from the dynamic SVV value. As the vestibular system changes with age, most patients become more visually dependent. The vision system changes as well, but typically not as dramatically as the vestibular system.

Clinical Applications of the HMD/SVV system:

An initial SVV assessment is performed to establish a baseline which allows for the monitoring of improvement and progress, which of course is often required to continue therapy.  The initial baseline also aids in selection of the starting point for therapy. If there is a significant deficit in static SVV, it would be wise to start with static SVV training procedures and proceed on to dynamic SVV training only after static SVV has been mastered.  The Bertec HMD system has several types of dynamic SVV training, allowing the clinician to make the therapies incrementally more difficult as the subject exhibits success.  These therapies place a significant emphasis on optic flow patterns which provide peripheral stimulation and promote re-integration of the peripheral and central vision systems. This is extremely useful for those patients suffering from vision motion sensitivity or VMS.

The Bertec HMD is currently available for the evaluation and training of SVV deficits but makes no claims to diagnose or reduce fall risk at this point. This exciting new technology should prove to be another valuable tool to identify patients who are at a greater risk of falling and allow us to reduce that risk by designing precisely targeted interventions. 

We invite you to learn more about the Bertec HMD device and other related products such as the Bertec Vision Trainer (BVT), the computerized dynamic posturography system (CDP), and other precision balance products on Bertec's Website.

About the Author:

Charlie Shearer, OD has been a nationally recognized leader in the areas of vision therapy, visually related learning problems and sports vision enhancement. He has worked with US Olympic athletes and teams, professional and amateur athletes, and coordinated national screening of both Special Olympics and Junior Olympics. Charlie is a leader in the field of instrumentation development as related to his role of Vision Science Consultant for Bertec Corporation, as well as his work with other companies. Charlie was born in Mishawaka, Indiana. He received his undergraduate and optometry degrees from Indiana University, upon graduation he returned to Mishawaka, Indiana, where he is in private practice. He has been a leader and innovator in of areas of pediatric vision, neuro-optometry, vision training and visual perceptual problems. He also has an interest in sports vision and continues to consult with the Colorado Rockies Major League Baseball team.

References:

WHO falls fact sheet: https://www.who.int/news-room/fact-sheets/detail/falls accessed on November 30, 2022.
Barceló, M., Torres, O.H., Mascaró, J. et al. Hip fracture and mortality: study of specific causes of death and risk factors. Arch Osteoporos 16, 15 (2021). https://doi.org/10.1007/s11657-020-00873-7
Montero-Odasso, M., van der Velde, N., Martin, F. C., Petrovic, M., Tan, M. P., Ryg, J., Aguilar-Navarro, S., Alexander, N. B., Becker, C., Blain, H., Bourke, R., Cameron, I. D., Camicioli, R., Clemson, L., Close, J., Delbaere, K., Duan, L., Duque, G., Dyer, S. M., Freiberger, E., … Task Force on Global Guidelines for Falls in Older Adults (2022). World guidelines for falls prevention and management for older adults: a global initiative. Age and ageing, 51(9), afac205. https://doi.org/10.1093/ageing/afac205
Totilienė, M., Uloza, V., Lesauskaitė, V., Damulevičienė, G., Kregždytė, R., Kaski, D., & Ulozienė, I. (2021). Impaired subjective visual vertical and increased visual dependence in older adults with falls. Frontiers in aging neuroscience, 13, 297.