Contrast Sensitivity Testing: An Emerging Standard
Applied before and after refractive surgery, it can improve patient management and add a layer of medical-legal protection.
BY DAVID EVANS, PH.D., MBA

"Quality of vision" has become the "buzzword" for LASIK surgery worldwide. Clinicians use this term to describe the improvements in surgical technology and in surgical outcomes that have occurred during the past few years. Patients use the term to describe their improved vision following surgery. From a practice and patient management standpoint, many new technologies have evolved that combine to provide superior quality of vision.

Contrast sensitivity is an important measure for quantifying quality of vision and can provide a unifying metric for the new technologies that have emerged. In this article, I'll explain how contrast sensitivity testing before and after refractive surgery can benefit both surgeon and patient. In a subsequent article, I'll address the use of contrast sensitivity screening in other eye conditions.

Advances in Treatment and Measurement

During the evolution of assessing quality of vision, measurement of spherical and cylindrical error using corneal topography has given way to the use of wavefront measurement. Wavefront measurement provides not only an accurate indication of the correction needed for spherical and cylindrical refractive errors, but a means to quantify higher-order aberrations. The newest technology excimer lasers are programmed based on these aberrations and induce fewer of them, which appears to contribute substantially to the postoperative quality of vision.

Rosenbaum cards for the measurement of pupil size have been discarded in most LASIK practices in favor of infrared pupilometers. These pupilometers provide a better means to quantify patient pupil size under darkened conditions, which is a critical factor in providing the LASIK patient with better quality of vision. Higher-order aberrations increase dramatically in the peripheral cornea and, as such, are typically much higher in patients with large pupils. Accurate measurement of the pupil size allows a better prediction of the aberrations under low light levels and increases the probability of correcting these aberrations with corneal ablation.

The original excimer lasers were broad beam, allowing very little detail in ablation profiles. This technology has evolved into sophisticated flying-spot lasers that can pinpoint the critical ablation areas and extract specific amounts of corneal tissue with each laser pulse, allowing the laser to create different levels of thickness across the cornea. Without this technology, the ability to customize ablation for different patients would be impossible.

Working on Standards

Visual acuity has been the standard for vision assessment since the mid-1800s. But in advanced practices, contrast sensitivity is replacing visual acuity as the measurement of quality of vision.

Contrast sensitivity testing is an excellent way to determine the visual function outcome of refractive surgery. It's important to keep in mind that visual acuity was originally developed in 1862 as a standardized means to prescribe spectacles. The standard eye chart served that purpose well. The black-on-white letters provided an excellent stimulus target for determining spherical and cylindrical refractive error. But, visual acuity provides little information about the quality of vision because it can't measure how patients see in real-world situations, such as driving at night. Aberrations typically reduce a patient's ability to see in real-world conditions, in many cases without affecting visual acuity. Contrast sensitivity testing helps to augment visual acuity testing by measuring changes in vision associated with the new surgical and wavefront assessment technology that can affect the patient's satisfaction with the outcome.

The U.S. Food and Drug Administration began the evolution from visual acuity to contrast sensitivity for clinical trials in the early 1990s, when Summit and VISX became the first excimer companies to submit for FDA approval.

During that period, much confusion surrounded contrast sensitivity testing. A variety of testing techniques were accepted for documentation in those early FDA clinical trials. However, over the past 3 to 4 years, a move has been afoot to standardize the methods for measuring contrast sensitivity. The FDA aligned with the American National Standards Institute (ANSI) to develop specific standards for clinical trials. The ultimate goal is to have the ANSI standards align with the International Standards Organization (ISO) so that there will be worldwide standards for the measures of visual function in clinical trials. While it isn't necessary for clinicians to incorporate the full FDA protocol to join the evolution to contrast sensitivity from acuity, a routine screening of patient contrast sensitivity can have several benefits for managing LASIK patients.

Patient Communication, Doctor Protection

Many patients are well aware of the new technologies that have evolved for LASIK surgery. They understand the importance of pupil size measurement. They know about wavefront technology. They're knowledgeable about the new types of lasers. They're also becoming aware that contrast sensitivity can be affected by surgery and that these effects may not be detected by the old standard "E-Chart."

Contrast sensitivity testing can aid in communicating with these patients by demonstrating the improvements in contrast sensitivity following surgery. It's also useful for interacting with patients who have trouble seeing at night, or in other low-contrast situations, to better communicate and document the visual function outcome. A patient with good acuity can understand and accept a loss in contrast sensitivity at night if it's communicated properly. But this patient may have difficulty being told he has 20/20 acuity if he doesn't get an explanation for his reduced night vision.

Testing Before Surgery

I have served as a defense expert witness for visual function assessment in numerous medical malpractice cases. In some of these cases, the issue of poor quality of vision after LASIK surgery, as measured by contrast sensitivity testing, has been used as an argument for the plaintiffs to implicate surgeon malpractice. One of the key components of the defense in these cases was the level of the plaintiffs' contrast sensitivity before surgery. Many surgical patients can have poor contrast sensitivity before surgery --- and the documentation of the quality of vision before surgery can provide an additional protection from a medical-legal standpoint. Of course, contrast sensitivity is clearly not a panacea for all legal protection. But, it can provide added documentation and serve as a means to better communicate with patients concerning their expectations for the quality of vision following surgery.

Brian Boxer Wachler, M.D., whose practice specializes in refractive surgery in Beverly Hills, Calif., uses the VectorVision CSV-1000 to screen every patient for contrast sensitivity before and after surgery. He utilizes a screening version that incorporates 20/100 to 20/10 ETDRS acuity and one spatial frequency, 12 cycles per degree, of contrast sensitivity. This test provides a quick screening for contrast sensitivity and allows the testing of ETDRS acuity (to as low as 20/10) under the same lighting conditions. Dr. Boxer Wachler uses the contrast sensitivity results, in combination with wavefront and other measurements, as a means to verify the quality of vision outcome and to better communicate these results to the patient.

In summary, many new technologies have evolved in the LASIK practice to achieve improved quality of vision. Contrast sensitivity screening of patients before and after surgery can aid in managing the quality of vision in the new technology LASIK practices.

Dr. Evans is a consultant faculty member at the University of Alabama Birmingham. He has performed research in visual function since 1978 and has published many scientific articles and book chapters. He's a well-known speaker at United States and European ophthalmology meetings and has served as an expert witness for a variety of cases. He also serves on the FDA ANSI subcommittee for vision standards. He received a Ph.D. in Ocular Physiology from Indiana University, a Masters in Engineering and an MBA from Wright State University, and a B.S. in Engineering from the United States Air Force Academy. Dr. Evans has a financial interest in the VectorVision CSV-1000 contrast-testing instrument.