Quantitative descriptors in myopic eyes

Resumen

Myopia prevalence has demonstrated to be greater among the population with the highest education. The time spent in near work activities is thought to have a role in such association between myopia and the educational level achieved. Indeed, the time spent in near work and its intensity, especially continuous reading, have shown to increase the myopia odds in children. Students at university constitute a young population group exposed to high prolonged near work which is at special risk of myopia development and progression during their studies. The purpose of this research was to characterise the myopic eye by means of quantitative ocular descriptors and to analyse the change of them over time in connection with myopia progression in a sample of young university students. A prospective longitudinal study was designed to evaluate myopic and emmetropic university students with a one-year follow-up. Myopic eyes exhibited differences in some of the quantitative descriptors evaluated at baseline. For ocular biometry, the myopic group had deeper anterior chamber depth (ACD), longer vitreous chamber depth (VCD) and tended to have slightly thinner lens thickness (LT). Further, deeper ACD and thinner LT associated with longer VCD, however, this linear relationship was not maintained when the VCD elongation exceeded 20 mm. Greater corneal curvature and astigmatism, particularly in central cornea, was found in the myopic group as well as steeper elevation for anterior and posterior corneal surface. Both corneal curvature and elevation showed to flatten until the VCD reached 19-19.5 mm, point from which cornea could no longer compensate the eye enlargement and it even steepened. Therefore, higher AL/CR ratio was seen in myopes than in emmetropes, where myopes were mostly above 3.00 in this ratio. Accordingly, multiple linear regression proved that the spherical equivalent (SE) was better predicted when the corneal curvature was included with the ocular biometry (91.5%). Emmetropes obtained better compensation of the WTR/ATR astigmatism Z(2,2) so that myopic manifested more negative values in the ocular wavefront. In contrast, there was more proportion of partial compensation in myopic eyes for spherical aberration Z(4,0), resulting in lower ocular spherical aberration root mean square (RMS). Besides, ocular vertical coma Z(3,-1) was more positive in myopic eyes and was related to more myopic sphere and longer VCD. Moreover, multiple regression analysis manifested that the SE and VCD were predicted including all Zernike coefficients but no significant model resulted including only high order aberrations (HOA). After the follow-up, the refractive error experienced a negative shift in some part of the initial myopic students, though small, demonstrating that myopia may keep progressing during this academic stage. The VCD, ACD and LT changed significantly but only the VCD change was related to the refractive changes. Myopic students showed higher VCD elongation agreeing with their greater negative SE shift compared to the emmetropes. The curvature of the flat meridian as well as the anterior BFS experienced a decrease that was greater in emmetropes and not related to the SE change. These corneal changes may indicate that axial elongation still may be compensated by corneal flattening in young adults, particularly among emmetropic eyes. The change of the AL/CR ratio differed among refractive groups where the myopic group experienced an increase whereas the emmetropic a reduction, however, AL/CR changes were not related to the SE changes. As expected, ocular defocus Z(2,0) had a greater increase in the myopic group and was related to the VCD elongation. WTR/ATR astigmatism Z(2,2) increased negatively in myopes whereas it remained stable in emmetropes. Moreover, myopic eyes tended to increase positively the ocular horizontal coma Z(3,1) and reduce the positive values of ocular trefoil Z(3,3). Ocular spherical aberration Z(4,0) underwent a slight reduction in the myopic group but this was not related with the refractive nor biometric changes. The VCD increment was the main responsible for the SE changes in both myopia and emmetropia. Furthermore, only a small part of the variance (26.8%) of the SE change was explained by HOA. Thus, negative SE shift was related to a negative change of trefoil Z(3,-3) and Z(3,3) while a positive change of horizontal coma Z(3,1).