Contrast sensitivity can detect very small changes in the quality of vision, even when patients have 20/20 or better visual acuity. Acuity measures a patient’s ability to see black and white objects. It does not test how well a patient sees under low contrast conditions, such as driving at night or playing tennis under the lights.
Why Refractive Surgery Affects Quality of Vision
In certain persons, due to the eye’s shape, visual images are not naturally in focus on the back of the eye. For these people, glasses or contact lenses are needed to see properly. Refractive surgery techniques are designed to change the eye’s focus and reduce or eliminate the need for glasses or contact lenses. However, there are other factors in the eye, which affect vision quality, that cannot be measured by simply evaluating the eye’s focus.
These factors can cause the quality of vision to vary greatly, even when a patient has 20/20 or better acuity. In some cases these factors are corrected during surgery and improve the quality of vision and in a few cases the quality of vision may diminish after refractive surgery.
Use of CS for Quantifying Quality of Vision
20/20 Versus 20/Happy
Contrast sensitivity is the best method for quantifying the quality of vision related to refractive surgery. The device can be used to evaluate patients who are “20/20 versus 20/Happy” in an objective way. The test can identify those patients who really do have some issue with quality of vision, those patients that may be good candidates for post-enhancement and those who improve with the enhancements. In many cases, patients in these situations have 20/20 or better acuity and it is difficult to distinguish objectively the patient vision.
The CSV-1000E has been used in more than 50 published research studies measuring the quality of vision with refractive surgery patients.
Contrast Sensitivity for Wavefront Custom LASIK
Laser refractive surgery was first approved for use in the US in 1995. Initially, PRK was the only approved procedure. For early refractive surgery techniques and laser technologies, the quality of vision as measured by contrast sensitivity was typically lower after surgery. The introduction of customized wave-front LASIK greatly improved the accuracy of the laser surgery techniques resulting in tremendous improvements in the quality of vision. Data published by Ashley Tuan and others in 2006 showed an improvement in contrast sensitivity in a significant percentage of patients following custom LASIK. This image below provides the pre and post results for photopic, mesopic and mesopic with glare contrast sensitivity data. Their data showed that 40% of patients improved contrast sensitivity following surgery, demonstrating the usefulness of contrast sensitivity testing to identify clinically those patients who benefited the most from surgery.
Enhancements After Surgery
In some cases, patients are able to obtain excellent acuity following surgery, but have vague complaints about the quality of their vision. Contrast sensitivity can be very useful for objectively identifying and quantifying vision losses in these patients that cannot be detected by standard acuity, and more importantly, demonstrating the improvement in the quality of vision following enhancement. Data below from Dr Kanellopoulos shows results from 17 patients who underwent enhancements following LASIK surgery. After the enhancement, as shown in the bar graph below, contrast sensitivity at 12 cycles per degree (Row C on the CSV-1000E) jumped dramatically in these patients with no change in acuity, ie the patients had 20/20 or better acuity before and after the enhancement treatments.
Measuring and Correcting Vague Visual Complaints
Below is case report from Prof Dan Reinstein of London, UK demonstrating the dramatic improvement in contrast sensitivity for a patient who had relatively good acuity, 20/25, yet had substantial complaints about the quality of vision. The CSV-1000E contrast sensitivity test detected a substantial vision loss, much greater than would be suggested by standard visual acuity. Prof Reinstein more closely evaluated the corneal curvature and surface. The patient had had 3 previous refractive surgeries, yet Prof Reinstein determined that the topography showed no indication of an irregular corneal curvature. He did note that the patient appeared to have an irregular stromal surface, apparently induced by the previous surgeries, and he felt this may have been causing the dramatic reduction in CS. Prof Reinstein performed PTK surgery to smooth the stromal surface resulting in a significant improvement in CS. Contrast sensitivity testing provided a direct measure of the improvement in vision related to the enhancement surgery.
The graph on the right shows the large improvement in vision post PTK as measured on the CSV-1000E. Contrast sensitivity improved across the full range, with the largest improvements, 5 to 6 contrast levels, at 6 and 12 cycles per degree (Rows B and C on the CSV-1000E).
Research has demonstrated that the middle portion of the CS curve, 6 and 12 cpd, is most closely associated with real-world visual performance, such as for drivers and pilots. Such reduction in CS and subsequent improvement following PTK objectively explains the patient’s difficulty seeing in the real-world prior to PTK and the benefit to the patient’s life-style after the enhancement.
Research Studies for the Quality of Vision
For research studies, the test can also include the patient wearing a pair of neutral density glasses. A level of 1.5 neutral density provides an excellent test condition for evaluating the patient for mesopic contrast sensitivity. (Contact Brian Wilson at VectorVision for specific information concerning the neutral density glasses.)