Visual acuity for the ‘real world’: use alternative methods of testing

Issue: October 2003

ByJennifer Byrne

Although traditional Snellen charts continue to be the mainstay of visual acuity testing, emerging alternatives offer comprehensive, “real world” assessments of a patient’s acuity, according to practitioners.

“If you take a look at how far we have advanced in our ability to assess every other aspect of patient health, it doesn’t make sense that we’ve been locked into the same Snellen chart since the beginning,” said Howard B. Purcell, OD, FAAO, director of professional affairs for Vistakon. “I would suggest that there is room for improvement.”

Contrast sensitivity

Dr. Purcell said one of the primary objectives in measuring visual acuity is to assess how well the patient’s eyes function in his or her everyday environment, not just in the doctor’s office.

“I think our job is to best determine the patient’s performance in his or her real world,” said Dr. Purcell. “And yet we have a situation where we have a dim room and a perfectly illuminated and contrasted chart. If we base our decisions on that, we are missing some huge opportunities.”

Dr. Purcell said one simple way of improving this situation is to adjust contrast and illumination. “You can test patients’ vision in high/low contrast and in high/low illumination,” he said. “And then you see how much it drops off. This will really give you a much better idea of how the patient is going to perform in the real world. “Unfortunately, our world is not perfectly contrasted and perfectly illuminated.”

Dr. Purcell said testing for contrast sensitivity can be done quickly and inexpensively using a filter. "Most of us have these filters in our trial lens sets; there is no need for any extra equipment," he said. "It doesn't take much time, but it gives you an opportunity to determine whether these changes in the environment, be they lighting changes or contrast changes, will make a dramatic difference."

Dr. Purcell said by placing the filter in front of the projector, a practitioner can reduce the amount of light coming through by 20%, 30% or 50%. "You can then test the patient again," he said. "We're still using letters, still using Snellen acuity, but we're varying the environmental aspects of that."

Dr. Purcell discussed other effective methods of testing for contrast sensitivity. "We can alleviate the crowding phenomenon. We can select letters that are equally easy to recognize, taking some of the bias out of the charts," he said. "And we can even create high-contrast and low-contrast charts and test at near and distance. This may add a small amount of time, but it is in exchange for what we believe is a significant amount of information."

Glare should also be taken into account when testing visual acuity, Dr. Purcell said. “Not only should we test in high/low contrast and high/low luminance, but we need to determine how glare affects visual acuity,” he said. “Everybody drops off in terms of performance, but the question is, how much do they drop off? This can be a very good clue as to how the patient is likely to perform in the real world.”

In addition, Dr. Purcell said testing for high/low contrast, high/low illumination and glare enables practitioners to more effectively evaluate which contact lens or spectacle would be best for the patient.

“When we start to be a little bit critical in the way we assess vision, it enables us to see some of those differences between the lower-quality product and the higher-quality product,” he said.

Dynamic acuity testing

Another alternate method of acuity measurement, which is currently being used in sports vision practices, is dynamic acuity testing.

According to Donald Teig, OD, of the Institute for Sports Vision in Ridgefield, Conn., a patient’s static visual acuity cannot be applied to highly active situations.

“If an athlete is sitting in an examination chair in a doctor’s office, and his or her visual acuity is great, that doesn’t necessarily translate to the playing field,” he said. “In sports, you are moving, and the way your eyes deal with motion can be a different phenomenon than when you’re stationary.”

While working with professional sports teams 25 years ago, Dr. Teig saw the need for equipment that would quantify the patient’s acuity once motion was introduced. “That’s what dynamic visual acuity attempts to measure,” he said. “Usually, we’re trying to figure out how accurate vision is under varying speeds of motion and under various time constraints — in other words, when you’re moving fast and have a fraction of a second to make a decision.”

Dynamic visual acuity equipment tests vision in a horizontal meridian by a rotating disc. Dr. Teig said this method of measuring acuity could be useful to the average patient as well as athletes. “We human beings are often in motion,” he said. “Even when you’re in a car that’s moving, your vision needs to be stable in spite of the motion.”

Dr. Teig said dynamic acuity can be accomplished qu ickly and easily.

"A lot of these devices have rotating motors and timers on them," he said. "They may have an LED that times a person's responses in milliseconds. What you are doing is you're asking the patient to respond to a target, to give an objective answer about what they're seeing, and do this in a short time period."

Dr. Teig said that alternate methods for measuring acuity can provide much-needed information to practitioners. “Contrast sensitivity, too, is taking it beyond just discerning the shapes of letters to being able to appreciate variances in contrast and detail,” he said. “I also do peripheral awareness tests. These types of evaluations all play a role in determining the visual performance in all kinds of situations.”

Photochromic lenses and acuity

According to research sponsored by Transitions Optical Inc. (Pinellas Park, Fla.), overall visual function entails more than visual acuity.

According to Christopher J. Baldy, PhD, ABOC, a research associate specializing in photochromic chemistry for Transitions, visual comfort also plays a role in visual function.

“Because optometrists know well the complexity of what vision comprises, they understand intuitively and anecdotally that more than visual acuity influences the overall visual experience,” Dr. Baldy said in a press release. “They recognize that certain lens options can greatly enhance visual comfort and overall patient satisfaction in the real world.” Dr. Baldy cited a clinical study that provides objective data demonstrating the importance of visual comfort, as well as visual acuity, in overall visual function.

The study found that photochromic lenses can enhance patients’ vision-related quality of life by meeting their visual comfort needs (Evaluation of vision-related quality of life of patients wearing photochromic lenses, CLAO, 2002;28:128-135). The study, funded by an independent grant from Transitions Optical, compared the experiences of patients wearing regular, clear lenses with their experiences wearing Transitions lenses.

“Results showed that the photochromic lenses offered clinically meaningful improvements in vision-related quality of life over regular, clear lenses outdoors,” Dr. Baldy said.

For Your Information:

Howard B. Purcell, OD, FAAO, is director of professional affairs for Vistakon. He can be reached at 7596 Centurion Pkwy., Jacksonville, FL 32256; (904) 443-1019; fax: (904) 443-1252; e-mail:hpurcell@visus.jnj.com.

Donald Teig, OD, can be reached at the Institute for Sports Vision, 96 Danbury Rd., Ridgefield, CT 06877-4053; (203) 438-5855; fax: (203) 431- 0318; e-mail: doc7ct@snet.net.

Christopher J. Baldy, PhD, ABOC, is a research associate specializing in photochromic chemistry for PPG Industries, the majority owner of Transitions Optical Inc. Dr. Baldy can be reached at 440 College Park Dr., Monroeville, PA 15146; (724) 325-5378; fax: (724) 325-5225; e-mail: baldy@ppg.com.