Peripheral Refraction and Image Blur in Four Meridians in Myopes and Emmetropes

Abstract

Purpose: : Peripheral refractive error has been hypothesized to be a stimulus for the development of central refractive error. The purpose of this study was to investigate the changes in peripheral refractive error across horizontal, vertical and two diagonal meridians in low, medium, high myopic and emmetropic adults.

Methods: : A commercial Shack-Hartmann aberrometer was modified and used to take measurements in 10^o step out to 30^o of eccentricity along the four meridians. Total of 34 adult subjects (8 Emmetropes, between -0.50 and +0.50 D, and 26 Myopes, between -0.70 and -8.60 D) with astigmatism less than 2.50 D participated in this study. The eye’s sphero-cylindrical refractive error in power vector notation (M, J₁₈₀, J₄₅) was computed from second order Zernike terms.

Results: : Emmetropic and low myopic (-0.50 ~ -3.0 D) subjects showed small relative myopic shift (less than 1.0 D) in the periphery in most meridians. Medium and high myopic subjects exhibited relative hyperopic shift in all four meridians. The degree of hyperopic shift in the periphery increased in high myopic group (greater than -6.0 D) compared to medium myopic group (-3.0 ~ -6.0 D). 90 - 180 degree astigmatism J₁₈₀ showed quadratic changes with visual field eccentricities in horizontal and vertical meridians but changed linearly in two diagonal meridians. 45 - 135 degree astigmatism J₄₅ was linearly related to horizontal and vertical field angle but quadratically related to the oblique meridians. Spherocylindrical image blur at 30^o periphery increased along all four meridians in both emmetropic and low myopic groups but decreased in medium and high myopic groups. Inferior-nasal and inferior meridian showed higher amount of spherocylindrical blur in most of subject groups.

Conclusions: : Medium and high myopes tend to have elongated (prolate) ocular shape in all meridians while emmetropes and low myopes have slightly oblate ocular shape. Image blur formed on the retina is field angle dependent and human eyes, in general, are demonstrating higher amounts of blur in the lower visual field.

Keywords: refractive error development • myopia • astigmatism