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From the editor

Measuring vision in refractive surgery

Kohnen, Thomas MD

Journal of Cataract & Refractive Surgery: December 2001 - Volume 27 - Issue 12 - p 1897-1898
doi: 10.1016/S0886-3350(01)01260-3
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After years of inventing, evaluating, and perfecting refractive surgical procedures, the future focus will be on the “quality of vision” after surgical interventions. The number of surgical procedures to correct refractive errors is steadily increasing, older procedures are being replaced by newer ones, the complication rate is decreasing, and the results of each established procedure are improving with more experience, better technology, and scientific evaluation.

The success or failure of refractive procedures, defined by criteria such as safety, efficacy, stability, and predictability,1 is based on Snellen acuity. However, some patients who present with anatomically perfect results and excellent visual outcome with respect to these criteria complain of visual disturbances such as decreased contrast, unusual color perception, glare, halos, or simply “bad vision.” In some cases, we can explain the problem; eg, residual astigmatism or a discrepancy between the scotopic pupil diameter and the corneal ablation zone in myopic excimer surgery. In some cases, we cannot find an answer. And in some cases, we see retrospectively that there should have been problems (6.0 mm ablation zone for laser in situ keratomileusis [LASIK] and a 7.0 mm scotopic pupil size) that fortunately did not occur.

Determining the outcome is a complex process. Why do only some patients complain? Are some complaints associated with residual refractive error? Do we increase aberrations with our interventions? Is avoiding eyes with large pupils a guarantee for avoiding visual disturbances? Which refractive procedure is best suited to a specific patient and a specific refractive error regarding quality of vision? Snellen acuity alone will not be sufficient to determine the quality of vision for refractive procedures, as emphasized in a recent editorial.2

How should we determine vision in refractive surgery?

Equal Conditions

Visual testing, particularly in myopic eyes, must be performed under the same conditions preoperatively und postoperatively. Only the best preoperative measurements (often better with contact lenses than with spectacles) should be compared with the postoperative outcome because the patient will always compare the result with the optimal preoperative situation.

Quality of Vision

Quantitative measures are needed to assess this somewhat vague concept. Typical recurring complaints and their effect on daily life can be catalogued. There are many questionnaires about vision and daily activities—some for cataract patients, some recently created to assess refractive procedures. These questionnaires are the link between the patients' sensations and all other measurements.3 This correlation is crucial because only the patient decides whether the quality of vision is “good” or “bad.” Questionnaires are needed to objectify the individual impressions in a reproducible and comparable way.

Clinical Testing

Clinical tests have to be established to “measure” subjective phenomena reproducibly. Until now, the main outcome measure in assessing refractive surgical procedures has been Snellen acuity, the angular visual acuity determined by high-contrast optotypes. As many activities in daily life do not take place under optimal lighting conditions, Snellen acuity reflects only 1 element of visual function and gives only partial insight into the quality of vision. Thus, contrast sensitivity and low-contrast visual acuity become parameters in determining the quality of vision. Some studies after photorefractive keratectomy or LASIK show little decrease in contrast sensitivity; however, we know that some patients experience a loss of contrast sensitivity. Therefore, further studies are needed to correlate subjective impressions with clinical measurements to establish the parameters that are sensitive enough to uncover subtle changes in the quality of vision. Contrast sensitivity and/or low-contrast visual acuity should be determined under different illumination conditions with comparable tests. Other measurements may include color vision testing or tests such as the Van den Berg stray light meter,4 which enables measuring of forward scatter by a psychophysical test, or software programs to objectify halo perception.5

Standardization

How do we measure visual acuity and contrast sensitivity? There are only a few standards for a common procedure such as testing visual acuity, mostly applied in cases of medicolegal issues. No guidelines for testing Snellen acuity in refractive surgery have been established. Different studies involve different investigators and devices that may produce biased results, making the results less comparable. Testing visual acuity or contrast sensitivity means determining psychophysical thresholds. All these thresholds should be tested before and after surgery under comparable and reproducible conditions; eg, similar optotypes and lighting conditions, forced-choice testing, no feedback by the investigator, and low probability of guessing.6 When the threshold is defined as the steepest point of the psychometric function and rigorous forced-choice is applied, acuity values can be around 20/10 even without “super normal correction.”

Correlation

These measurements should be correlated with variable parameters such as sphere, astigmatism, spherical equivalent, pupil diameter (photopic, mesopic, and/or scotopic) or optical zone size, and ablation depth. Objective findings with corneal topography, macromorphological and micromorphological changes (eg, with confocal microscopy), or investigation of aberrations with wavefront measurements will also be included in this big “vision” picture.

Many measurements will be reserved for scientific studies, as the testing will be time consuming and, in most cases, will require centers that are organized to conduct clinical trials. But I predict that in the near future, outcomes of refractive surgery will not be measured by Snellen acuity alone. Additional measurements will be used to determine the outcome. Ultimately, it will be the patient's satisfaction that determines the success of a refractive surgical procedure. Nevertheless, sophisticated and improved testing will help us document and achieve our goal of quality vision after the surgical correction of refractive errors.

References

1. Koch DD, Kohnen T, Obstbaum SA, Rosen ES. Format for reporting refractive surgical data (editorial). J Cataract Refract Surg 1998; 24:285-287
2. Koch DD. Measuring outcomes after refractive surgery (editorial). J Cataract Refract Surg 2001; 27:645-646
3. McLeod SD. Beyond Snellen acuity; the assessment of visual function after refractive surgery (editorial). Arch Ophthalmol 2001; 119:1371-1373
4. Van den Berg TJTP, Ijspeert JK. Clinical assessment of intraocular stray light. Appl Optics 1992; 31:3694-3696
5. Lohmann CP, Fitzke FW, O'Brart D, et al. Halos—a problem for all myopes? A comparison between spectacles, contact lenses, and photorefractive keratectomy. Refract Corneal Surg 1993; 9(suppl):S72-S75
6. Bach M. The Freiburg visual acuity test—automatic measurement of visual acuity. Optom Vis Sci 1996; 73:49-53
© 2001 by Lippincott Williams & Wilkins, Inc.