October 2015

 

INTERNATIONAL

 

International outlook

Stereoacuity: A new measure of LASIK success?


by Matt Young and Gloria D. Gamat EyeWorld Contributing Writers

 
 

Quality of vision is a subjective assessment, and for this reason it is difficult to quantify. We know quality of vision is important, and after safety, it may be the most important factor influencing patient satisfaction. All refractive surgeons know that Snellen acuity does not always predict if a patient will be happy with his or her vision. Stereoacuity has largely been overlooked as a quantitative measure of visual quality following refractive surgery. Could this be another factor to consider?

John A. Vukich, MD, international editor

 

A 3D Delta sign image as seen binocularly on the Distance Randot Stereotest (Stereo Optical, Chicago) Source: Rohit Saxena, MD

As refractive surgeons have fine-tuned the art of performing LASIK throughout the years, the usual markers of a successful LASIK surgery (20/15, 20/20 vision) are now becoming easily achieved, and therefore may no longer be the only measure of the procedures success. Although LASIK has provided unaided visual acuity and better quality of life to patients, the procedure has shown to have an impact on other visual functions as well, including contrast sensitivity and induced astigmatism, among others. In a new study, researchers from India analyzed LASIK results in a much different way: by looking at stereoacuity. While the scientific literature reports that LASIK improves near stereopsis, most of the evidence previously available has been either anecdotal or incomplete.

The importance of stereoacuity in LASIK results

The research team postulated that since the 20/20 vision goal is now achieved with current LASIK technology, the finer functions need evaluation, and if required, future development of this technology may be directed to provide greater quality of vision. Traditionally, LASIK has always been about achieving 20/20 visual acuity, but the effects on other visual functions have been gaining prominence, said Rohit Saxena, MD, additional professor, Dr. Rajendra Prasad Centre for Ophthalmic Sciences, All India Institute of Medical Sciences, New Delhi. The world is increasingly becoming 3D, Dr. Saxena said. In our daily lives [especially in India], there is a huge requirement for binocularity with the advent of 3D movies and games, TVs and holographic projection technology; consequently [stereoacuity] is an important visual function that people may need, and they realize its utility, he explained. Stereoacuity is a fine but essential visual function. Although it is not the primary parameter for LASIK correction, according to Dr. Saxena, the next level for a refractive surgery outcome should be 20 seconds of arc stereoacuity (20/20/20). The quality of the visual experience after LASIK is greatly affected by the level of stereoacuity achieved, he said. Therefore, I think that we need to aim for wavefront-guided and optimized LASIK protocols, keeping in mind a goal of good visual acuity, correction of anisometropia and introduction of minimal aberrations that will ensure the best chance for improved stereoacuity. Affected by various physiologic and pathologic conditions (age, optical blur, intereye differences, strabismus, and amblyopia), stereoacuity is the highest grade of binocular vision and a cortical function that has a significant role in everyday life.

Once we have consistently achieved 20/20 monocular vision, stereoacuity, which depends on many factorsparticularly good monocular visionshould be the next goal to consistently achieve, Dr. Saxena said. Therefore, I [think] stereoacuity should now be considered a better parameter to evaluate outcomes of any refractive surgery.

Clinical evidence

In evaluating changes in near and distance stereoacuity after LASIK, Dr. Saxena and colleagues conducted a prospective interventional study at an apex tertiary care ophthalmology center in India. Their findings, published in a recent issue of Optometry & Vision Science, demonstrated that near and distance stereoacuity show significant improvement after LASIK. While previous studies have not evaluated distance stereoacuity, the research group from India evaluated near and distance stereoacuity and found that both have shown significant improvement after LASIK.

Additionally, distance stereoacuity was found to be more affected in the preoperative period with more cases having subnormal stereoacuity for distance than for near. Further, the researchers emphasized another important observation in this study: stereoacuity is associated with the degree of anisometropia, but the change in stereoacuity did not depend on the amount of refractive error corrected. Vision is a comprehensive function with many aspects, Dr. Saxena said, and stereoacuity is one of them. For example, even with the best optical correction, induced myopia is long known to worsen stereoacuity. However, according to the researchers, this can partly be attributed to induced optical aberrations or optical decentration and the minimizing effect of spectacles. The researchers further noted in their paper: In postoperative cases whereby distance stereoacuity is worse than near stereoacuity, the most possible explanation is that uncorrected myopia in the early age would cause a greater blur for distance than near, and potentially hamper the development of distance stereoacuity despite development of a normal near stereoacuity. Reduction of these optical effects and higher order aberrations by keratorefractive surgery, reported the researchers, explains the postoperative improvement of stereoacuity. Another possible reason for improvement of stereoacuity after LASIK, they noted, is the elimination of any significant preexisting anisometropia, which has been previously established to negatively impact stereoacuity in cases with long-standing surgical monovision.

Although good stereoacuity cant be achieved in the absence of good visual acuity, it involves other factors including higher cortical functions, and it is imperative to achieve optimal monocular vision before thinking in terms of stereopsis, Dr. Saxena said. Patients who have undergone LASIK have often expressed a better experience of vision, which goes beyond just achieving a 20/20 outcome: This is due to better contrast, lower optical aberrations, lower minimizing effect of glasses and better stereoacuity. According to Dr. Saxena, it is imperative for ophthalmologists to insist that LASIK goes beyond a cosmetic surgery and results in a measurable improved functional outcome in the form of gained stereoacuity. These need to be recognized and acknowledged as a benefit of this procedure; this improvement in stereoacuity will result in better handling of the 3D world we live in and the patients improved quality of life, he said.

Reference

Singh D, Saxena R, et al. Stereoacuity changes after laser in situ keratomileusis. Optom Vis Sci. 2015;92(2)196200.

Editors note: Dr. Saxena has no financial interest related to this article.

Contact information

Saxena: rohitsaxena80@yahoo.com

Stereoacuity: A new measure of LASIK success? Stereoacuity: A new measure of LASIK success?
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