Subjective visual vertical: A novel approach to otolith assessment
What is the subjective visual vertical (SVV) test?
The subjective visual vertical (SVV) represents a method to test an individual’s ability to adjust a luminous laser presented line to be parallel with true vertical in the absence of any other visual cues.
This is done in the Orion Auto-Traverse Chair.
The sensory information required to perform this task is provided predominantly by the vestibular organs of the inner ear, and in particular, by the utricles (Clarke et al, 2003).
Figure 1: SVV test in the Orion Auto-Traverse Chair.
The roles of the vestibular organs
The vestibular organs consist of the three semicircular canals, which transduce angular acceleration, and the two otolith organs, the utricle and saccule.
The otolith organs transduce linear acceleration, including gravity, with respect to the head.
Of critical importance, the information from the otolith organs facilitates correct perception of the orientation of the head with respect to gravity.
The otolith organs function so that any linear acceleration displaces the otoconial mass and applies a shear force on the embedded sensory hair bundles against the otolith maculae.
This results in a potential change in the sensory cell and consequently a change in the discharge rate of the primary afferent vestibular nerve.
Since the earth’s gravity constitutes a constant linear acceleration, the orientation of the head relative to gravity is constantly signaled from the otolith organs to the central nervous system (Clarke, 2002).
As a result, the individual can accurately estimate the so-called SVV.
Any dysfunction of the otolith apparatus is usually accompanied by incorrect spatial orientation and postural instability.
The estimation of the SVV thus serves as a diagnostic indicator of otolith (utricular) function (Clarke et al, 2001).
How to perform the subjective visual vertical (SVV) test
The SVV is determined by presenting the patient with a luminous line in otherwise total darkness and requesting that he/she rotate the line to be in a perceived vertical alignment (Clarke et al, 2003).
The test can be performed as either a static or dynamic protocol.
In the dynamic SVV test, the patient is spinning at 300 deg/sec, typically at 4 cm off axis in order to place the axis of rotation in line with the utricle.
Centripetal acceleration applied to either the right or left labyrinth allows for independent utricular evaluation without influence from the other ear.
Once the patient has been rotating at a constant velocity for approximately one minute and the per-rotary nystagmus has subsided, the SVV trials are initiated.
It is possible to also perform static SVV tests without off axis centripetal acceleration.
However, static SVV testing provides simultaneous stimulation of the otolith organs.
Dynamic SVV testing completed during unilateral centripetal acceleration permits exclusive stimulation to the right or to the left utricle for improved specificity (Clarke et al, 2001) and enhances the diagnosis, particularly for peripheral vestibular disorders (Schonfeld & Clarke, 2011).
Numerous studies have documented that SVV may also be sensitive to central pathologies, such as brainstem or cerebellar lesions (Akin et al, 2011).
Findings in previous static SVV studies
For the static SVV, normal individuals can adjust the luminous line to within a few degrees of the gravitational vertical.
Patients with unilateral otolith abnormalities will deflect the top of the line (SVV) toward the dysfunctional side.
However, patients with a compensated unilateral peripheral impairment will often have static SVV estimates that appear within normal limits.
The published data shows ±2.0° as the normal tolerance (Akin & Murnane, 2009).
In the acute stage of a peripheral vestibular lesion such as vestibular neuritis, the static SVV shows an increase in deviation (>2°) toward the side of the lesion (Byun et al, 2010).
The normal tolerance of ±2.0 does not change for on-axis rotation (Akin et al, 2011).
There is no gender effect and no significant change in static SVV with aging (Kobayashi et al, 2002).
Findings in previous dynamic SVV studies
The normal tolerance for off-axis dynamic SVV increases relative to the static test.
As one may anticipate, normal individuals will present with larger SVV estimates as the axis of rotation deviates from center during unilateral centripetal acceleration.
Normal individuals will have SVV estimates tilted to the right during off-axis left dynamic testing and estimates tilted to the left during off-axis right dynamic testing (Akin et al, 2011).
Akin et al reported off-axis rotation SVV angles as great as 11.0° for normal individuals, with normative values of –3.0° (SD=2.1°) for chair offset to the right, and 5.0° (SD=2.6°) for chair offset to the left.
Because of the variability within the normal population for dynamic off-axis SVV testing, the authors recommend using <11.0° as the cutoff value for normal and suggest that using a difference angle (on-axis value – off-axis value) will reduce variability and bias when using the test for diagnostic purposes.
Concluding remarks on the use of the SVV test
The SVV test is recommended for acute vertigo, brainstem infarcts, and oculomotor disorders and the test can be used to monitor recovery/compensation of acute lesions.
It is considered a novel approach to assessment of otolith function that can be easily incorporated into the clinical test battery (Clarke et al, 2001).
About the authors
Cammy Bahner is the Director of Audiology-Balance Division for Interacoustics U.S. In her role, she provides clinical training, education and support for a variety of products, with a primary emphasis in vestibular and electrophysiologic assessment. She provides clinical input for the development of vestibular products. She received her Master’s Degree from the University of Nebraska-Lincoln in 1999 and her Au.D. from A.T. Still University, AZ School of Health Sciences in 2016. She has served as a guest speaker at state, national and international conventions and is a contributing author on published articles in hearing industry journals. Cammy is currently a member of AAA and ASHA.
Michelle Petrak, Ph.D., is the Director of Clinical Audiology and Vestibular Research for Interacoustics. Her primary role is development and clinical validation of new technologies in the vestibular and balance areas. She is located in Chicago where she is a licensed private practice clinical audiologist at Northwest Speech and Hearing (NWSPH). Dr. Petrak received her doctorates in Electrophysiology (1992) and Biomolecular Electronics (1994) from Wayne State University and her Masters in Audiology in 1989. Her special areas of expertise include vestibular and balance assessments and management of the dizzy patient. Dr. Petrak is involved with new innovative product developments, clinical evaluations of new protocols, and publishing, teaching and training on the management of patients with dizziness. She continues to lecture extensively nationally and internationally, and she has numerous articles published in the hearing industry journals. She also participates on the committees for several doctoral students as support for the research projects.
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