CONTRAST SENSITIVITY AND LASIK

EVALUATING THE QUALITY OF VISION FOLLOWING REFRACTIVE SURGERY:

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 Can Affect the 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 are unrelated to eye focus. These factors can cause the quality of vision to vary greatly, even when a patient has 20/20 or better acuity. Contrast sensitivity is the best method for quantifying the quality of vision.

Immediately after LASIK refractive surgery, contrast sensitivity falls such that the patient's quality of vision is well below the normal range. After several weeks, or in some cases several months, the quality of vision and contrast sensitivity recover to normal levels. For PRK refractive surgery, the amount of time required to recover completely to normal levels may be longer, up to six months. It is still unknown which procedure, LASIK or PRK, provides the best overall quality of vision in the latter months after surgery, that is, after patients have reached their best post-surgical vision.

Examples of the Change in Contrast Sensitivity with LASIK Refractive Surgery:

The graphs show the pretreatment level of contrast sensitivity (with the patient wearing his or her glasses) and the contrast sensitivity after 1 day, 1 week and 1 month of surgery (with the patient wearing the best correction).

In some cases, the patient contrast sensitivity does not recover to the presurgery levels. The following graph shows an example of this response in vision quality.

Several surgeons have noted that some patients with high levels of myopia (> - 9 diopters) actually improve contrast sensitivity after LASIK surgery. The graph below shows an example. The reason for this improvement in the quality of vision is not completely understood, but it is thought to be due to the poor quality of vision that some patients have through their glasses or contact lenses before LASIK.

Pupil Size and the Quality of Vision after Refractive Surgery:
Pupil size is very important when quantifying the quality of vision. The quality of vision is typically much worse for patients in darkened environments than in bright conditions. This is because the larger pupil allows more light into the eye from the edge of the corneal zone where the laser has ablated tissue (ablation zone). Studies by Boxer Wachler and others have shown that the size of the ablation zone is related to the quality of vision. If the pupil size is large and overlaps the ablation zone, the quality of vision suffers.

The example below shows the difference between the contrast sensitivity of a patient in bright light and of the same patient in a darkened room. Note that the contrast sensitivity is much lower for the patient when the pupil size is large.

Low Contrast Acuity After LASIK:

Low contrast acuity can also be used to evaluate the quality of vision following refractive surgery. The graph below shows data from Dr Jack Holliday and his colleagues. The high contrast standard acuity recovers to normal levels within 1 week after LASIK eye surgery, but low contrast acuity does not recover to the same levels. This data was taken for patients in a darkened room. This data suggests that low contrast acuity is more sensitive to subtle changes in the quality of vision than standard acuity. In well-lit conditions, both high and low contrast acuity typically recover to levels very similar to pretreatment (within half an acuity line of the pretreatment level).

Glare and Contrast Sensitivity Evaluation for Refractive Surgery:
Many patients complain of glare after refractive surgery and it is tempting to test these patients with a glare test in order to assess the quality of vision. However, glare testing, which shines a bright light in the patient's eye, reduces pupil size and typically causes vision to improve. The graph below shows the results of a patient tested for contrast sensitivity in darkness and then again with glare. Note that the glare condition dramatically improves the contrast sensitivity. It is not recommended that LASIK patients be tested with a high-luminance glare test to quantify the quality of vision.

Recommended Test Conditions to Evaluate Vision Quality Following LASIK:
Patients should be tested for contrast sensitivity, both in a well-lit room and in a darkened room. Test the patient with the CSV-1000 (either using the contrast sensitivity test face, the CSV-1000E, or one of the low contrast acuity test faces, or CSV-1000ETDRS10%.) in a room with all of the lights on. Then retest the patient with all of lights turned off in the room. The change in contrast sensitivity from the light to the dark condition provides information concerning the true quality of vision.

Research Studies for the Quality of Vision:
For research studies, the dark 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.)

The Cost of LASIK

Most patients are surprised to learn that they have numerous re-payment options that make laser eye surgery affordable. LASIK eye surgery cost is no longer a burden with options such as financing and numerous insurance companies providing assistance. When considering LASIK surgery, including the newest procedures such as Custom LASIK, LASEK and epi-LASIK, speak with your chosen eye care specialists to learn about their customized patient payment plans.

Selected References:

• Perez-Santonja JJ, Skalka HF, Alio JL. Contrast sensitivity after laser in situ keratomileusis. J Cataract Refract Surg. 1998; 24:183-189.
  
  
• Holliday JT, Dedeja DR, Chang J. Functional vision and corneal changes after laser in situ keratomileusis determined by contrast sensitivity, glare testing and corneal topography. J Cataract Refract Surg. 1999; 25: 664-669.
  
  
• Boxer-Wachler BS, Durrie DS, Assil KK, Krueger RR. Role of clearance and treatment zones in contrast sensitivity: Significance in refractive surgery. J Cataract Refract Surg. 1999; 25: 16-23.
  
  

VectorVision CSV-1000 instrument can be used with contrast sensitivity tests, glare tests or ETDRS tests.
 
VectorVision – World Leader in Standardized Vision Testing