Contrast sensitivity test eliminates potential bias due to optical aberrations

The system offers additional features that contribute to a more accurate measurement of a patient’s quality of vision.

Jack T. Holladay

In part six of a series from the OSN Technology and Equipment Workshop featured at Hawaiian Eye 2010, Jack T. Holladay, MD, MSEE, FACS, presented a new contrast sensitivity testing device that offers potential for greater sensitivity in detecting defects in visual function.

Contrast sensitivity is typically tested with charts that may be inadequate to detect minute fluctuations in a patient’s visual function, largely because they do not allow for a blanking period for the test taker’s retina to relax and reset, nor is a maximal response time dictated. Even current monitor-based tests may be less than ideal because ambient room lighting may be inadequate for measuring contrast in photopic and mesopic conditions.

But according to Dr. Holladay, the most significant shortcoming in current contrast sensitivity testing is the use of vertical and/or horizontal gradings that are biased to a patient’s inherent visual defects.

“Vertical gradings are not a good test because they are biased toward with-the-rule astigmatism, vertical coma and other aberrations that make things look darker vertically,” Dr. Holladay said.

The new device, the Holladay Automated Contrast Sensitivity System (M&S Technologies), employs optotypes with a sinusoidal bull’s eye pattern. This type of pattern is independent of axis-affecting aberrations in that the image will always trigger a response on an unaffected axis point, Dr. Holladay said.

Aberration neutral

The Holladay Automated Contrast Sensitivity System uses an LCD computer monitor that is calibrated to 85 candles/m², which is accepted as the ideal for photopic contrast sensitivity testing. The calibrated monitor eliminates potential luminance problems when room light cannot be adjusted. For mesopic testing, a polycarbonate neutral density filter slides in front of the screen to lower luminance to 3 candles/m².

The filter “allows us to do mesopic testing very accurately and not change any of the color temperatures or the resolution or anything on the screen,” Dr. Holladay said.

The testing system also eliminates the potential for bias due to aberrations in the patient’s visual system. Currently, when an eye with with-the-rule astigmatism sees the vertical meridian darker and an eye with against-the-rule astigmatism sees the horizontal meridian darker, then a test using vertical markings favors the former, whereas horizontal gradings favor the latter.

“The sinusoidal bull’s eye gets rid of that,” Dr. Holladay said. “The horizontal and vertical coma are equal. It is only testing the magnitude, so it doesn’t matter what the direction of your astigmatism or coma or any other aberration is.”

Most current contrast sensitivity tests report results as a standard deviation from the norm, but chart-based tests are too insensitive to detect small but significant deviations, Dr. Holladay said. Instead, he said, contrast sensitivity should be measured in log units, just as visual acuity is, because a 0.1 log unit loss in contrast sensitivity is equivalent to about a 17% reduction in contrast, whereas a 0.3 log unit loss equals about a 50% decrease in contrast.

Contrast sensitivity should be measured with the same sensitivity, down to a tenth of a log unit, as visual acuity, Dr. Holladay said, because a 50% loss in contrast sensitivity is dramatic. “It is a big loss. It is equivalent to a three line loss of visual acuity,” he said.

In a study published in the Journal of Cataract and Refractive Surgery, van Gaalen and colleagues looked at the ability of various contrast sensitivity tests to analyze visual function. In that study, the Holladay system showed a tighter clustering of test results, while chart testing showed a wider range of standard deviation.

According to Dr. Holladay, although the study was designed to illustrate the correlation between contrast sensitivity and spherical aberration, it also showed that the Holladay test was three times more sensitive in measuring contrast sensitivity in healthy eyes under photopic and mesopic conditions when compared with chart tests.

Additional features

The Holladay Automated Contrast Sensitivity System offers additional features that contribute to a more accurate measurement of a patient’s quality of vision compared with chart-based and other monitor-based testing modalities.

For instance, the test automatically displays a blank black screen in between images for 5 seconds, allowing the retina to refresh and avoiding potential false answers after images. Test takers get a maximum of 10 seconds to respond to each presented image, thus eliminating the potential for long focusing.

In addition to presenting the sinusoidal bull’s eye pattern in sequentially decreasing contrast, optotypes are broken up by the presentation of a plain gray disc that patients are triggered to identify. The number of times that a patient correctly identifies the gray disc is calculated to a reliability index so that the interpreter can weigh how much credence to give the test results.

At the end of the 10-minute test, a report is generated with a photopic and mesopic mean curve that can be charted against the mean norm, which is generated from a normative database. Testing is performed at 1.5, 3, 6, 12 and 18 cycles per degree, so the interpreter can view graphically the patient’s performance at any chosen frequency.

However, the test can also be abbreviated if testing is desired only at certain frequencies. “Some people just want to pick up an optic neuritis or just want to do a screening test, so there is the option to select various frequencies,” Dr. Holladay said.

Like for visual field testing, the device can be programmed to recall a patient’s previous tests, starting the new test at 0.2 log units higher than the previous test. The prior test recall both shortens the test and allows the clinician to track relative success of an intervention.

For instance, serial testing can be performed on a patient placed on steroid therapy for treatment of optical complications of multiple sclerosis. If contrast gain is detected, steroids can be tapered or eliminated.

“Same thing if you have somebody that has optic neuritis, diabetes, macular degeneration,” Dr. Holladay said. – by Bryan Bechtel

Reference:

• van Gaalen KW, Jansonius NM, Koopmans SA, Terwee T, Kooijman AC. Relationship between contrast sensitivity and spherical aberration: comparison of 7 contrast sensitivity tests with natural and artificial pupils in healthy eyes. J Cataract Refract Surg. 2009;35(1):47-56.

• Jack T. Holladay, MD, MSEE, FACS, can be reached at holladay@docholladay.com. Dr. Holladay is a consultant to M&S Technologies.