A New Method for Measuring Visual Acuity in People with Extremely Low Vision

The front cover of Optometry and Vision Science

Throughout my career as a vision rehabilitation and low vision therapist, I have often grappled with medical files that provide minimal information about a client’s/patient’s visual acuties.

It’s common to read “counts fingers,” “senses hand motion,” or, worst of all, “unable to test.” Not terribly helpful, I admit, but it was the best many eye doctors could do with the tools at their disposal for patients with very low vision.

Thus, I was excited to read Grating Acuity and Contrast Tests for Clinical Trials of Severe Vision Loss, published in the October 2011 issue of Optometry and Vision Science, the official publication of the American Academy of Optometry. (Translation: How to measure visual acuity in individuals with extremely reduced vision, which may turn out to be very useful during clinical trials for advanced eye disease.)

Optometry and Vision Science publishes current developments in optometry, physiological optics, and vision science, and promotes interdisciplinary exchange among optometrists and vision scientists worldwide. The authors are Ava K. Bittner, OD, PhD, FAAO; Pamela Jeter, PhD; and Gislin Dagnelie, PhD, FAAO, from the Lions Vision Center, Wilmer Eye Institute, Johns Hopkins University, Baltimore, Maryland.

About the Study

Current tests for evaluating vision in patients with very low vision have a limited ability to detect very low levels of remaining vision. Below a certain threshold, vision professionals usually rely on simple, subjective tests — such as counting fingers or detecting hand motion — to estimate the level of the patient’s remaining vision.

The purpose of the study was to evaluate the ability of two new computerized eye tests in assessing visual acuities that are below the limit of detection of standard eye tests currently in use:

Clinical trials to arrest, slow, prevent, or reverse blinding retinal degenerative diseases are on the horizon and are already increasing in number. Given the possible high-risk nature of some therapies, legally blind subjects with advanced eye disease are likely to be the participants in early phases of such trials; thus, these trials require reliable and well-validated measures of visual function that are particularly well suited to measure low levels of vision.

The Two Tests

The two computer-driven tests studied were the Grating Acuity Test (GAT) and the Grating Contrast Sensitivity (GCS) test. The tests were evaluated by 20 legally blind subjects who had either retinitis pigmentosa or another serious retinal disease.

Both tests evaluated the subjects’ ability to see the direction of, and contrast in, a black-and-white or gray-and-white striped (or “grated”) pattern. The patterns were presented on a high-definition liquid crystal display (LCD) screen. The study subjects indicated the direction of the stripes by pushing buttons that were identifiable by touch.

Results and Conclusion

The grating tests were able to detect visual acuities as low as 20/4000, compared to 20/1600 with current low vision eye charts and letter tests. Because the GAT and GCS tests were computerized, the results were measurable and not as subjective as “perceiving hand motion,” for example.

The authors conclude that “Computer-driven grating tests appear to be reliable, capable of evaluating vision that may fall outside of the range of standard clinical tests, and may be useful during clinical trials for advanced eye disease.”

VisionAware thanks the authors for this important research and will provide updates as they become available.