Videonystagmography (VNG) is considered the gold standard for vestibular assessment. Recently, VNG received a face lift and is now being offered with two new tests: Dynamic Visual Acuity (DVA) and Subjective Visual Vertical (SVV). Although DVA and SVV have been part of the vestibular repertoire for some time, they have not routinely been part of the standard VNG assessment given the additional equipment required. Until now, that is.
Individuals who lose function of one peripheral vestibular system often complain of unsteady vision (i.e., oscillopsia) while the head is in motion, resulting in activity avoidance and loss of confidence. Oscillopsia is generally the product of an uncalibrated vestibulo-ocular reflex (VOR). The VOR is an intricate connection of neural fibers between the peripheral vestibular end organs (semicircular canals, utricle, and saccule) and the extra-ocular muscles that work to keep vision stable while the head is in motion. Damage to the peripheral end organs resulting from disease or trauma reduces performance of the VOR and causes images of interest to slip off of the retina, the sensitive portion of the eye that allows for clear vision, resulting in the complaint of unsteady vision during head motion.
Dynamic Visual Acuity Testing
The DVA test gives clinicians a way to evaluate the functional impact of reduced VOR function via an objective measurement of visual acuity during head movements (Herdman, 2010). Visual acuity is first measured with the head still (static visual acuity, SVA) and then with the head in motion (horizontally or vertically) at a fixed speed. With computerized DVA, patients wear a rate sensor to maintain the required head speed, typically 120°/second for horizontal head movements. During head movement, the patient reports the orientation of an optotype E (e.g., Which way do the open legs of the E point—right, left, up, or down?) displayed in the center of a computer screen. The size of the E becomes progressively smaller until the patient identifies the smallest that can be recognized accurately 50% of the time in response to rightward and leftward head movement. These "dynamic" scores are then compared with the SVA and reported on the LogMAR scale (i.e., logarithm of the minimum angle of resolution). The LogMAR score may be reported as a positive or negative number, describing any loss of visual acuity. Positive numbers suggest a loss of visual acuity during head movement; negative numbers suggest better visual acuity (McCaslin, Dundas, & Jacobson, 2008).
When VOR function is normal, SVA and DVA should be equal. When the VOR is compromised, DVA performance is significantly worse than SVA. Again, this result is due to the inability of the VOR to maintain the image of interest on the retina when the head moves toward the affected side(s). For example, if the patient has a unilateral weakness, DVA will be worse during head movements toward the side of the unilateral weakness (≥ 15.6 missed optotypes; logMAR score ≥ 0.28 on the affected side; Herdman et al., 1998). If the patient has a bilateral weakness, DVA will be worse, in comparison to SVA, with head movement to both the right and left (≥ 19.98 missed optotypes; logMAR score ≥ 0.39 to both sides; Herdman et al., 1998). Overall, computerized DVA testing is a sensitive method for detecting VOR impairments (94.5% sensitivity/95.2% specificity; Herdman, 2010; Herdman et al., 1998) and physical therapists frequently use DVA to monitor the effectiveness of vestibular rehabilitation (i.e., the compensation process; Herdman, 2010). Some VNG systems are now equipped with a rate sensor and appropriate software to incorporate DVA testing into the standard vestibular assessment.
Subjective Visual Vertical Testing
The SVV test is quite different, but this assessment also clues clinicians into possible vestibular system impairments. SVV measures the patient's subjective perception of vertical (or horizontal). This test is generally performed in complete darkness and requires the patient to adjust a vertical line (usually via remote control) so the line is perceived to be straight up and down. Individuals with normal peripheral vestibular function can generally set this line within 2–3 degrees to the right and left of true vertical (Bohmer & Mast, 1999; Zwergal et al., 2009). Offsets of the SVV line greater than 3 degrees to either side are considered abnormal, and are generally associated with peripheral vestibular system dysfunction (specifically the utricle) or unilateral brainstem lesions. In individuals with uncompensated loss of unilateral peripheral vestibular system function, the line is often set towards the lesioned side, with offsets of as much as 15–20 degrees (Bohmer & Mast, 1999; Zwergal et al., 2009).
In individuals with brainstem lesions, the direction of SVV offset is dependent on the site of lesion with respect to the pons. Lesions caudal to the pons result in SVV tilts toward the lesion; lesions rostral to the pons result in SVV tilts away from the lesion (Dieterich & Brandt, 1993). It is important to note that SVV offsets are directly tied to vestibular compensation in that SVV offsets are more likely to be detected in the acute stage (within six weeks) and then slowly improve over time. Therefore, in patients with a compensated peripheral vestibular system lesion, a normal SVV does not necessarily indicate normal function. Similarly, some VNG systems are coming equipped with a luminous line and remote control in order to assess SVV during VNG.
Incorporating Tests at the Bedside
If a VNG system is not equipped with either DVA or SVV, these two tests still may be incorporated in a vestibular assessment by using the bedside version of each. The bedside DVA is completed by using a standard eye chart, such as the Snellen or Early Treatment of Diabetic Retinopathy Study (ETDRS) visual acuity eye chart) . SVA is the lowest line on the eye chart the patient can read with 100% accuracy. DVA is the lowest line on the eye chart the patient can read with 100% accuracy while the clinician shakes the patient's head back and forth at approximately 4 Hz (one head repetition per second). The clinician then compares SVA to DVA. SVA and DVA performance are approximately the same for individuals with normal VOR. Typically, a decrease in acuity of greater than two lines is considered abnormal (Bhansali et al., 1993). For individuals with bilateral vestibular impairment, a decrease in acuity of five to six lines is often noted. Clinicians should monitor their technique when performing the bedside DVA, as any pause in head motion can increase the likelihood of a false-negative response. Also, it is a good idea to randomize the order of line reading or use different versions of the eye chart to minimize memorization (Herdman, 2010).
The SVV test also can be completed quickly and efficiently at the bedside. Zwergal et al. (2009) recently introduced "a bucket test of static vestibular function," demonstrating how SVV can be completed using a bucket with a line in the bottom, a protractor, and a plumb line placed outside on the bottom of the bucket to measure offset of the SVV line. A patient places his or her head in the opening of the bucket (obstructing all peripheral vision) and then instructs the examiner to rotate the bucket right and left until the line is perceived as straight up and down. Deviation of the line is measured by the plumb line and protractor on the bottom of the bucket. Interpretation of the test is similar to that described above. Individuals with normal vestibular acuity can set the line within true vertical by approximately 2–3 degrees; any offsets greater than that are considered abnormal.
Adding these new procedures to the standard VNG provides a comprehensive protocol that expands our site-of-lesion assessment to include utricular involvement (SVV), and the functional impact of an impaired VOR system (DVA). Benefiting our patients with a more efficacious evaluation of the vestibular system leads to improved management strategies and patient care.