The Dynamic Visual Acuity 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. This test can be administered using a traditional Snellen Chart or by new computerized diagnostic methods like shown in the pciture to the right.

A Snellen chart is an eye chart used by eye care professionals and others to measure visual acuity. Snellen charts are named after the Dutch ophthalmologist Hermann Snellen who developed the chart in 1862.

The traditional Snellen chart is printed with eleven lines of block letters. The first line consists of one very large letter, an "E." Subsequent rows have increasing numbers of letters that decrease in size. Patients taking the test cover one eye, and read aloud the letters on each row, beginning at the top. The smallest row that can be read accurately indicates the patient's visual acuity in that eye.

The symbols on an acuity chart are formally known as "optotypes." In the case of the traditional Snellen chart, the optotypes have the appearance of block letters, and are intended to be seen and read as letters. They are not, however, letters from any ordinary typographer's font. They have a particular, simple geometry in which:

• the thickness of the lines equals the thickness of the white spaces between lines and the thickness of the gap in the letter "C"
• the height and width of the optotype (letter) is five times the thickness of the line.

Only the nine letters C, D, E, F, L, O, P, T, Z are used in the traditional Snellen chart.

Wall-mounted Snellen charts are inexpensive and are sometimes used for rough assessment of vision, e.g. in a primary-care physician's office. Whenever acuity must be assessed carefully (as in an eye doctor's examination), or where there is a possibility that the examinee might attempt to deceive the examiner (as in a motor vehicle license office), equipment is used that can present the letters in a variety of randomized patterns.

Snellen Chart

Computerized DVA Testing

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. 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. 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.