The Relationship between Anterior Corneal Aberrations and Contrast Sensitivity in Conventional LASIK

LASIK is linked to corneal aberrations and contrast sensitivity

While common optical imperfections in the eye excluding traditional refractive errors have been noticed for more than a century, it is only in the last few years that they have been considered from a clinical perspective. In the language of eye medicine, such imperfections are known as optical aberrations. The basic goal and objective of a typical corneal refractive surgery performed to correct myopia such as LASIK is to flatten the central corneal area which results in the reduction of its refractive power. However, even after LASIK, some adverse events have been reported in the eyes and large optical aberrations are some of those significant side effects. Therefore, there has been a heart-felt need to study and evaluate modifications or variations of anterior corneal aberrations before and after laser in situ keratomileusis (LASIK) which could also help assess the correlation between contrast sensitivity and anterior corneal aberrations in such patients. Quite recently, the implementation of techniques to precisely measure the optical wave aberration pattern before and after LASIK surgery has given rise to dramatic excitement among specialists in refractive surgery.

What is an aberration?
Aberration is the difference between a real image and an ideal image occurring when plane wave front in accordance with Snell's law. Ideally, parallel light focuses on one point. However, the eye is not a perfect circle for focusing incoming light on one point, and eventually an aberration occurs. In simple meanings, aberrations are imperfections, distortions, crude flaws or noise that blurs the retinal image in the eye, especially at high spatial frequencies.

It is important to note that when these asymmetries or imperfections occur on the corneal surface to affect visual acuity, they are known as corneal aberrations.

What is contrast sensitivity?
It is a physical dimension referring to the light–dark transition at a border or an edge of an image that delineates the existence of a pattern or object.

Contrast is defined as the ratio of the difference in the luminance of these two adjacent areas to the lower or higher of these luminance values. In simple words, contrast sensitivity refers to a measure of how much contrast a person requires to see a target. Contrast-sensitivity measurements differ from acuity measurements; acuity is a measure of the spatial resolving ability of the visual system under conditions of very high contrast, whereas contrast sensitivity is a measure of the threshold contrast for seeing a target. Therefore contrast-sensitivity can provide useful information by revealing in some conditions visual loss not identifiable through visual acuity tests, by providing another method of monitoring treatments, and by providing a better understanding of visual performance problems faced by persons with vision impairment.

Scientific evidence showing the effects of LASIK on corneal aberrations & contrast sensitivity
Previous studies show high correlations between corneal aberrations (wave front variance) and visual performance (area under contrast sensitivity function). The aim of this study was to assess the optical outcome of the Lasik procedure on 51 post-LASIK Right patients (6 males and 45 females) during the period of about 3 years (from the year 2000 to 2003). Corneal topography method was used to calculate and analyze anterior corneal aberrations before and after LASIK in these patients. At the same time, contrast sensitivity was also measured after LASIK with under daytime and nighttime settings and with glare conditions. At the end of the study, two interesting findings were observed:

  1. Significant increase in the number of various types of corneal aberrations were noticed in majority of the patients
  2. While contrast sensitivity at daytime and nighttime was reduced at each spatial frequency after LASIK, interestingly enough, there were no significant correlations between the changes of anterior corneal aberrations and the changes of contrast sensitivity at daytime and nighttime in such patients, with and without glare.

Conclusion
Laser in situ keratomileusis (LASIK) has become a popular surgical alternative for the correction of myopia, and a rapidly increasing number of LASIK procedures are performed daily worldwide. As LASIK surgery induces variations and changes in the anterior corneal surface, most changes in the total aberration pattern can be attributed to changes in the anterior corneal aberrations. In fact, studies have shown that both corneal and total aberrations increase after LASIK surgery for myopia; the higher the preoperative myopia, the higher the increase. However, it must also be noted that none of these changes in anterior corneal correlates with the changes of contrast sensitivity at daytime and nighttime, regardless of the presence or absence of glare.

Learn more about how decreased contrast sensitivity and other LASIK complications on our forums!