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Kai Yip Choi, Tsz Wing Leung, Serena Zhe Chuang Li, Wing Yan Yu, Chi-wai Do, Paul Hong Lee, Henry Ho-lung Chan; Peripheral optical anisotropy is associated with axial length to corneal curvature ratio. Invest. Ophthalmol. Vis. Sci. 2018;59(9):3385.
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Off-axis astigmatism, a predominated aberration at the peripheral visual field, limits the visual input to a narrow bandwidth of orientation. This study aimed to investigate the effect of off-axis astigmatism on optical anisotropy, and its association with axial length to corneal curvature ratio in Hong Kong schoolchildren.
A total of 1052 Chinese children were recruited (aged 9.95 ± 0.97y) from 8 primary schools in Hong Kong. Axial length (AL) and corneal curvature (CR) were measured by IOL master. Based on the AL/CR ratios, subjects were categorized into low (2.62 to 3.00), mid (3.00 to 3.11), and high (3.11 to 3.74) groups.Open-field autorefractor was used for non-cycloplegic refraction with fixation targets located at 4m away from the eyes. Off-axis refractions were measured at 10○ eccentricity along the vertical and horizontal visual fields. For a subset of 603 subjects, refractions were also measured at 20○ eccentricity along the horizontal field. Relative off-axis refractive errors were calculated as the differences between the vector components of off- and on-axis refractions.The best-focused meridian (BFM) was defined as the principal meridian of relative off-axis astigmatism with the smaller absolute magnitude. Optical anisotropies were subsequently classified into radial (R: axis 90±30○ for vertical & 180±30○ for horizontal fields), tangential (T: axis 180±30○ for vertical & 90±30○ for horizontal fields), and oblique (O: axis 45±15○ & 135±15○) bias.
The mean relative off-axis astigmatism was 0.68±0.49DC at 10○ and 1.33±0.71DC at 20○ eccentricity. These astigmatic blurs resulted in optical anisotropy that was bias to radial orientation at all locations (X2 test, R: >49.3%, p<0.001) except nasal field (R: 40%, T: 30-40%, O: 20-30%, p>0.16).The orientation bias was associated with AL/CR ratio (X2 test, p<0.001) – Low AL/CR group: R bias for all visual fields (>49%, all p<0.001); Mid AL/CR group: R bias for all (>50%, all p<0.001) except nasal field (p=0.16); High AL/CR group: R bias for vertical field (>57%, p<0.001), T bias for nasal 20○ field (49%, p=0.001), other fields insignificant (p>0.40).
Optical anisotropy by off-axis astigmatism was generally radially-oriented, but the bias was dependent on the AL/CR ratio. Further studies are needed to understand whether the anisotropic visual input would influence the ocular functional and structural development.
This is an abstract that was submitted for the 2018 ARVO Annual Meeting, held in Honolulu, Hawaii, April 29 - May 3, 2018.
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