DYNAMIC VISUAL ACUITY TEST (DVA)
Description
DVA Report
Functional Implications
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Description
The DVA test assesses impairments in a patient's ability to perceive objects accurately while actively moving the head. In normal individuals, losses in visual acuity are minimized during head movements by the vestibular ocular reflex (VOR) system that maintains the direction of gaze on an external target by driving the eyes in the opposite direction of the head movement. When the VOR system is impaired, visual acuity degrades during head movements.
To quantify losses in visual acuity during head movement, the first step is to determine the patient's head fixed (static) visual acuity. This is accomplished by seating the patient a prescribed distance before the computer monitor and displaying sequences of the optotype "E" of predetermined size and in one of four possible random orientations. When the patient correctly identifies the orientation of at least 3 of 5 successive "E" presentations of a given size, the optotype size is reduced and the process repeated until the orientation of the optotype can no longer be reliably determined. Static visual acuity is then based on the smallest "E" that can be identified accurately.
The patient's dynamic visual acuity can be measured separately in each of three movement axes. For each movement axis, the static visual acuity protocol described above is repeated with the following modifications: The patient wears a sensor that continuously measures the position and velocity of head movements and the optotype "E" appears ONLY while the head is moving in the prescribed direction and minimum velocity. The patient is allowed to practice triggering appearances of the optotype "E" by moving their head in the presecribed direction and at the minimum velocity. To prevent the patient from predicting the direction and timing of the optotype appearnce, trials involving the two directions are randomly intermixed.
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DVA Comprehensive Report
- Visual Acuity Difference Differences between static and dynamic visual acuity are displayed in separate graphs for each movement axis. For a given axis of movement, each graph displays differences between static and dynamic visual acuity in the two directions that comprise that axis (left and right in the case of the yaw axis, for example). Differences are expressed in LogMAR, a unit describing the apparent size of an image based on a ratio of its absolute size to distance from the eye.
- % Left/Right Symmetry The difference in the visual acuity differences for the two head movement directions is expressed as a percentage.
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Functional Implications
The DVA is an impairment test that quantifies the impact of VOR system pathology on a patient's ability to maintain visual acuity while moving. Information provided by the DVA is complementary to and not a substitute for physiological tests of the VOR system. The DVA quantifies the combined influences of the underlying VOR pathology and the patient's adaptive response to pathology. Because patients with VOR deficits can improve their dynamic acuity by performing rapid "catch-up" saccadic eye movements and/or predictive saccades, the degree of VOR impairment will vary between patients with similar pathology.
Research has shown that the DVA symmetry measure provides information relative to the probable side of lesion in patients with suspected peripheral vestibular deficits.1
In patients with history of VOR system pathology, DVA information helps identify patients likely to benefit from vestibular rehabilitation and is helpful in designing their rehabilitation exercises.1 For example, the DVA identifies which head movement axes and directions are most in need of training.
As a direct indication of impairment, the DVA is an excellent objective measure of outcome, providing precise indication of the patient's ability to see objects clearly while performing daily life movement tasks.2
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References
- Herdman SJ, et al (1998). Computerized Dynamic Visual Acuity Test in the Assessment of Vestibular Deficits. American J Otology, 19:790-796.
- Hall CD, Schubert MC, Herdman SJ (2003). Prediction of Fall Risk Reduction in Individuals with Unilateral Vestibular Hypofunction. Otology & Neurotology, pending publication.