Monochromatic Optical Aberrations In Myopic And Astigmatic Anisometropia

Abstract

Purpose: : Comparing high and low order monochromatic aberrations of the more myopic and less myopic eyes of 25 myopic and astigmatic anisometropic patients.

Methods: : Twenty five anisometropic subjects with at least 2.00 D spherical equivalent difference between eyes were enrolled. All had spherical equivalent refractive error between +3.25D and -21.75 D and refractive astigmatism of less than -3.00 D. High (up to 5^th order) and low order monochromatic aberrations were measured for 5mm pupils using the OPD Scan II (Nidek). Many biometric and aberrometry parameters were compared between the more myopic and less myopic eye of the patients using Student paired t test.

Results: : The spherical equivalents and axial lengths of the more myopic eyes were significantly larger than the less myopic eyes (p< 0.0001 and p < 0.0001) but the cylindrical difference was not significant (p>0.005) as for the corneal curvatures (p = 0.108) and the magnitude of corneal astigmatism between the 2 groups. The more myopic eyes showed significantly higher root mean square (RMS) values of corneal fourth-order (p< 0.0001) and corneal spherical aberration (p=0.015) than the less myopic eyes. The RMS of high order aberrations: total, internal and corneal (except spherical and 4^th order aberrations) along with the RMS of coma didn’t show any significant difference between the more myopic and less myopic eyes. Corneal asphericity was less negative for the more myopic eyes (less prolate) but this difference was not significant (p=0.541).

Conclusions: : The presence of a less negative asphericity in myopes and therefore a tendency for less rapid flattening in the corneal periphery (Carney et al; 1997) may explain the more positive spherical aberrations (0.13microns) in the more myopic eyes than in the less myopic eyes (0.103microns) (p=0.015). This may be also associated to ocular growth. Our results are analogous to those of He et al., 2002 and Carkeet et al., 2002 but unlike those of Collins et al.,(1995) and Kwan et al.,(2009). Our results showed no relationship between the degree of myopia and the presence of a corneal or total astigmatism.However, we found a relationship between positive spherical aberrations and myopia contributing to retinal image degradation in myopes but the cause and effect issue resides.

Keywords: cornea: clinical science • aberrations • refraction