Comprehensive - October 2003

EyeNet Magazine >> Comprehensive

Comprehensive

From Snellen to Sinewaves:
Pros and Cons of Measuring Visual Function

by Laura J. Rongé, Contributing Writer

Vision is the raison d’être of ophthalmology, but how do we measure it? “Not well,” said Lawrence F. Jindra, MD. “The first thing an eye doctor does when he or she opens an office is to put up a Snellen visual acuity chart,” said Samuel Masket, MD.

The quick and easy Snellen test has long been the gold standard. Even so, it measures the function of only one subset of retinal ganglion cells, said Dr. Jindra, assistant clinical professor of ophthalmology at Columbia University and in private practice in Floral Park, N.Y. “Testing how someone can see in a darkened room with stationary high-contrast optotypes is no substitute for [testing] how somebody really sees.”

So why does Snellen testing still lead the pack? “Because old habits die hard,” said Dr. Masket, clinical professor of ophthalmology at the University of California, Los Angeles, and in private practice in Century City, Calif.

Is Sinewave Superior?
The best way to assess visual function is by spatial-temporal contrast sensitivity testing with sinewave grating patterns generated by a computerized device, according to Dr. Jindra.

Contrast sensitivity testing measures a broad range of visual function, he explained. On a contrast sensitivity function plot, which is an inverted U-shape, visual acuity is only one point. The whole function shifts up or down, left or right, but the point where the x axis intercept occurs at a contrast of 1, or 100 percent, is the Snellen analogue of visual acuity.

“If you compare Snellen acuity to audiometry, it is like hearing only one middle C note of Beethoven’s Ninth Symphony. You can’t take one isolated piece of information, i.e., Snellen visual acuity, and extrapolate it to the whole range of visual function,” he said.

Spatial contrast sensitivity testing measures a patient’s response to patterns that differ in their spatial content. Temporal contrast sensitivity measures the response to patterns that change in time, that flicker quickly or slowly, he said. Some current systems for testing contrast sensitivity with electronic sinewave grating patterns include the B-Vat (Mentor), CTS-5000 (Cadwell), NIC Optronics CS-2000 (Nicolet) and the Venus (Neuro Scientific).

The idea of contrast sensitivity testing was first published in the 1950s. In the late 1980s, “contrast sensitivity” became a buzzword, Dr. Jindra noted, and quite a few practical papers were published on it. “But no major manufacturer has stepped up and made a test that is clinically useful, clinically significant and reimbursable by Medicare.”

This is unfortunate, he noted, because contrast sensitivity can more sensitively assess visual function in a wide variety of disease states. “In fact, contrast sensitivity testing would be helpful for every patient, but we need a test that is reliable and quick,” Dr. Jindra said. “If it takes 30 minutes, nobody will do it.”

Those who promote the concept of sinewave grating have science on their side, Dr. Masket agreed. The problem is one of practicality.

“I have a viewbox sinewave contrast sensitivity test, but it is cumbersome. You have to move the patient in front of it [and] adjust the settings. It is much easier for office technicians to have the patient stay in the examining chair and look at distance charts in a wall box,” Dr. Masket said.

Contrast Charts
Given these limitations, many clinicians rely on the use of contrast charts. Pelli-Robson, Terry or Regan charts, for instance, add progressive change in contrast to familiar letter optotypes. The log MAR (minimum angle of resolution) Regan charts, for example, exist in contrast from 96 percent down to 4 percent with logarithmic progression of reduction of letter sizes, Dr. Masket explained.

Stationary contrast charts are good, but limited, Dr. Jindra said. They measure contrast sensitivity in a rudimentary forced-choice way. Like the Snellen charts, they test only a specific subset of retinal ganglion cells—the parvocellular projecting retinal ganglion cells, or p cells, which are more apt to fire for a stationary, high-contrast target, he said. “If the patient has deficits in the magnocellular pathway, which is affected first and most by glaucoma, contrast chart tests might not pick it up.”

Adding Glare
When looking at visual function, it is also important to assess the effect of glare, which usually arises when light bounces off of inhomogeneities in the visual pathway.

One can simulate a problem with general background glare, such as sunlight, or use peripheral spots to simulate nighttime driving glare, Dr. Masket explained.

In the Miller-Nadler test, for example, the light of the tabletop slide projector provides a diffuse background glare, while a series of 20/400-size Landolt C rings appears on the screen. “It isn’t a great tester of contrast sensitivity, because the size of the Landolt objects is the same,” Dr. Masket said, “but there is a constant brightness gradation of the Landolt C with the background on which it sits. This is a very sensitive test for glare disability.”

With the Brightness Acuity Tester, a patient views a distance contrast chart while holding the BAT light source in front of his or her eye. “Because the light source is so close to the patient’s eye, it can dazzle vision on its highest setting, even in a normal eye,” Dr. Masket noted, “and it also induces miosis. Miosis can improve visual function in certain eyes—like a pinhole effect. In other eyes, it can make vision worse than it might be under other glare testing circumstances.”

Nonetheless, using the BAT with Regan charts became a standard in some FDA protocols, he said.

While glare tests are useful, they are neither standardized nor reimbursable, according to Dr. Jindra. “A reliable way to test glare would be wonderful. I just don’t know if we have that way yet.”

Looking Ahead
In the 1980s and ’90s, cataract surgeons were asked to justify surgery in patients who had good Snellen acuity. “These patients couldn’t function, couldn’t drive at night, but they could read the chart,” Dr. Masket explained. “Doctors accused of doing unnecessary surgery had to defend themselves, and functional vision tests became popular.”

Lack of practical objective tests, industry standards and Medicare reimbursement nipped this trend in the bud. Nonetheless, an adjunct to Snellen visual acuity is clearly needed.

“We as clinicians must tell third party payers and governmental agencies that Snellen acuity is meaningless in terms of evaluating patients’ visual complaints,” Dr. Masket said.

If equipping an office today, Dr. Masket would choose a vision testing system that includes a glare source and sinewave contrast sensitivity testing. In time, other up-and-coming technologies may be key. “Wavefront analysis, for example, provides a whole new way of assessing the optical system,” he said.

Another new objective electrophysiologic test, called sweep VEP (visual evoked potential), can rapidly run through a series of spatial and temporal frequencies to give a broad range of visual functioning. “It is like an electrocardiogram,” Dr. Jindra said. “The patient just sits there while electrodes record the data.” However, this technology has not been commercially developed and is largely limited to use among researchers.

A practical and comprehensive office test for visual function may be a long time coming. In the meantime, “Snellen charts are not the be-all and end-all,” Dr. Jindra said. “A patient may test at 20/25, but if he says that his vision is changing, listen to the patient.”

______________________________
Where Snellen Fails

The following problems exist with Snellen charts:

There is no standard lighting.
There is no logarithmic reduction in letter size.
There is no accounting for differences in contrast. “Patients tell us that they have difficulty driving or they can’t see the golf ball. Snellen acuity alone gives an inaccurate measure of contrast sensitivity,” Dr. Masket said.
Patients are familiar with the optotypes. “If I sense that a patient is not really seeing as well as they read the Snellen chart, I’ll put up number charts or use a Landolt C ring, and I find that the patient’s vision is not as good as it seems,” Dr. Masket explained.

Overall, Snellen visual acuity is quantifiable, albeit limited. “It is better than asking a patient, ‘Do you see better or worse than six months ago?’” Dr. Jindra pointed out.

______________________________

Drs. Jindra and Masket have no related financial interests.