| Abstract |
Purpose:
The purpose of this study is the evaluation of accommodative response and the concurrent changes in higher order aberrations before and after refractive surgery for myopic refractive correction using PRK or LASIK.
Method:
The data were colected from 14 patricipants of young age (6 men and 8 women, mean age 26,3 ± 5,6 years). Their pre-operative refractive error was at maximum 4,75D myopic sphere (mean: -3,29 ± 1,06D) and 1,5D myopic astigmatism (mean -0,64 ± 0,58D). All operations were done using the Wavelight Eye-Q Excimer Laser, with 400Hz irradiation frequency. Both PRK & LASIK photo excision was done in a zone of 6,5mm and a transition zone up to 9mm.
A wavefront analysis system (COAS-Wavefront Sciences Ltd), based in Shack-Hartmann principle, was used for measuring the wavefront aberrations of the eye. With the use of a purpose-developed Badal optometer coupled in COAS sensor the aberrations of the accommodating eye were recorded for a range of accommodative vergences (from optical infinity to approximately 8D). The accommodation was stimulated using a high contrast (≥80%) letter E as a target, with an optical angle of 1,750 (equivalent to 6/126 in Snellen scale). Fifty continuous measurements were taken.
Accommodative response was evaluated pre-operatively and 1 and 3 months post-operatively. Accommodation was corrected pre-operatively for effectivity (for these measurements the subjects were wearing lesnes with their sphero-cylindrical correction). Wavefront data were analysed using the Zernike coefficients (up to 4th order).
Results:
A lead of accommodation is observed for all eyes for distant objects, while the accommodation is less than the required (lag of accommodation) for closer objects. This was evident before and after the operation.
No difference was observed in thei response-stimulus slope of accommodative response following refractive surgery. However, the error of accommodation for the highest vergence is higher (approx. 0,5D) one month post-operatively, with the effect being modest 3 months after the operation.
The pupil diameter during the accommodation is decreased smoothly having a rate of 0,2mm/D on average. This was not affected after the refractive surgery.
Upon accommodation, spherical aberration C40 is moving to more negative values, following a linear correlation. After the refractive surgery no significant change in the slope of the corresponding regression lines (spherical aberration vs. accommodation) is observed. Spherical aberration changes by approximately 0,02 μm for every 1D of accommodation (for a pupil diameter: 4mm). However, as expected, a significant increase (a change to more positive values) in spherical aberration is found one month and three months post-operatively. That happens because after the operation, the central optic zone of the cornea becomes flatter, while corneal curvature remains unchanged in the periphery.
An increase in the RMS for higher order aberrations when accommodating to distant targets is found, which is slightly decreasing at 3rd months post-op. For near targets the high order RMS is found better for some subjects and worse for others, depending on their spherical aberration for distant targets.
The third order aberrations, eg vertical and horizontal coma show large variety in change. The horizontal coma does not change with the accommodation before or after the operation. The vertical coma changes more than the horizontal with the accommodation (there is a slightly increasing trend). Following refractive surgery, a wider distribution is observed between subjects compared to pre-operative values
Conclusions:
It is evident that accommodative response following refractive surgery remains unaffected, although an increase in higher order aberrations, and specifically spherical aberration is observed. One month post-operatively, an increase error in accommodative response is found which decreases to pre-
operative values three months post-operatively. The erros at one month maybe due to the delay in post-op healing folloeing PRK.
It is possible that the participants adapt to the changes in aberrations structure caused by refractive surgery, particularly following a period of three months. Moreover, it is possible that after refractive surgery a different set of aberrations is used as an accommodating cue with the resulting accommodative response being unaffected. Important information will derive from studies evaluating the magnitude of accommodating fluctuations following refractive surgery.
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Μέτρηση και αξιολόγηση της δυναμικής προσαρμογής του οφθαλμού πριν και μετά από διαθλαστικές επεμβάσεις διόρθωσης μυωπίας με excimer laser
