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closeScatter or higher-order aberration?
Posted by yamaguchi614-335 on 05 Aug 2015 at 04:28 GMT
The authors presented the effect of scatter on visual acuity after DSAEK. I think this topic is very important in lamellar corneal transplantation.
I think the results of OQAS double-pass instrument can be influenced by "higher-order aberration (HOA) and scatter", especially in eyes with high amount of HOA.
As the authors mentioned in the method section, "the manifest refractive error of the subjects was fully corrected during these measurements; the spherical error was automatically corrected by the double-pass system, and the cylindrical error was corrected with an external lens,because the uncorrected refractive error directly affects the optical outcome of the system." the results of OQAS is affected by defocus, regular and irregular astigmatism. So, the authors corrected astigmatism when they measured OQAS. I wonder why the authors measured OQAS only with the correction of regular astigmatism and without correcting irregular astigmatism (HOA). I think the authors had to correct HOA as they corrected astigmatism if they would like to regard the results of OQAS as scatter. As have been reported in the past, the amount of HOA after DSAEK is usually not small.
One of the other concerns is that the "corneal HOA" in this article is "the HOA of the corneal anterior surface" not the HOA of the cornea; sum of anterior and posterior surfaces". As in the previous reports, the amount of HOA of posteiror surface is relatively large, copmared with normal subjects and can be not negligible. I would like to ask the authors to rephrase the corneal HOA as "HOA of the corneal anterior surface".
And the authors have already gotten the data of HOA of the total eye, measured by Hartmann-Shack system. The data of HOA of the total eye and its correlation with logMAR would be valuabe information to understand the optics and visual function after lamellar corneal surgery.
I believe these three points have to be addressed to avoid the misinterpretation of this clincial results.
Kind regards,
Takefumi Yamaguchi, MD
Tokyo Dental College, Chiba, JAPAN
RE: Scatter or higher-order aberration?
kamiyak replied to yamaguchi614-335 on 13 Aug 2015 at 00:00 GMT
We appreciate the insightful comments of Dr Yamaguchi on our published article. Since the OQAS is one of point spread function (PSF) meters using the double-pass method, this measurement can be affected not only by intraocular scattering, but also by ocular HOAs. However, as mentioned in the Methods section, we used the OSI for an objective evaluation of intraocular scattered light, and this index is calculated by evaluating the amount of light outside the double-pass retinal intensity PSF image in relation to the amount of light on the center, based on the concepts that the ocular HOAs influence the light distribution at the central region of the image, but that the intraocular scattering cause an increment of the intensity further away from the central maximum of the retinal image. In our preliminary data, we found no significant association between the OSI and ocular HOAs in eyes with various diseases, indicating that the OSI is less influenced by the amount of ocular HOAs. Although we accept that it is impossible to accurately differentiate scattering and HOAs in daily practice, we believe that the value of the OSI mainly reflects intraocular forward scattering in this series.
We measured anterior corneal HOAs using the Hartmann-Shack sensor in this study, since the amount of anterior corneal HOAs is far (approximately 3 to 4-fold) larger than that of posterior corneal HOAs (approximately 0.1 to 0.2 µm for a 4-mm pupil) even in post-DSAEK eyes. It is unlikely that the small amount of posterior corneal HOAs (approximately 0.1 to 0.2 µm) alone explains for the mean BSCVA of 20/40 3 months after DSAEK. Although the morphology of the posterior surface of the donor cornea may play some role in visual performance after DSAEK, we assume that the contribution of posterior corneal HOAs on visual acuity is far smaller than anterior corneal HOAs even in post-DSAEK eyes. Moreover, BSCVA was gradually increased with time in a clinical setting, whereas corneal HOAs remained almost unchanged after DSAEK. In our preliminary data, the OSI was also gradually decreased with time, in association with the recovery of BSCVA. Time-course changes in the OSI, corneal HOAs, and BSCVA, also support our view that the intraocular scattering, possibly derived from the interface haze between the donor and the recipient corneas and/or anterior stromal opacification, plays a key role in visual performance after DSAEK than anterior and posterior corneal HOAs.
According to our experience, there was a significant correlation of logMAR BSCVA with the OSI, but no significant correlation with corneal HOAs or ocular HOAs in this series. Considering that we included the patients who underwent successful phacoemulsification with intraocular lens implantation before DSAEK, and no concomitant eye diseases, such as posterior capsular opacification, were observed, ocular HOAs were largely derived from anterior and posterior corneal HOAs in this study. These findings also indicate that forward scattering of the eye plays a more vital role in visual performance than corneal HOAs in post-DSAEK eyes.
Kind regards,
Kazutaka Kamiya MD, PhD, Associate Professor
Department of Ophthalmology, Kitasato University Faculty of Medicine, JAPAN