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S Shankar, WR Bobier; Corneal and Lenticular Components of Total Astigmatism in a Pre-school Sample . Invest. Ophthalmol. Vis. Sci. 2002;43(13):1500.
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Purpose: Astigmatism is primarily found to be corneal in nature with a small lenticular component of .50D against-the-rule (AGR). We examined whether this relationship would be found in a highly astigmatic pre-school sample. Methods: Cycloplegic refractive and keratometric measures using retinoscopy and the Retinomax-K Plus (Nikon Co, Japan) were conducted on 129 pre-school children (Ages 4.3±0.7 years) in Oxford County, Canada. Previously we found the normal (95%) limits of astigmatism in this population to be at 1.25D1. Accordingly, the group was divided into high astigmats (total cyl ≥ 1D; M= 1.38 ± 0.43; N=29) and normal astigmats (total cyl ≤ 0.75D; M= 0.22 ± 0.20; N=129). Data from one eye was used for analyses. Measures of Total and Corneal cylinder were transposed into J0 and J45 components2, where positive and negative J0 values quantify with-and against-the-rule astigmatisms respectively, and J45 quantifies the oblique components. Results: 1) The components of corneal astigmatism were highly correlated with total astigmatism in high astigmats (J0, r = 0.90, p< .01; J45 r = 0.73, p< .01) while only J0 was significantly correlated with total astigmatism in normals (J0, r= 0.38, p<.01). 2) While the magnitude of corneal cylinder is significantly greater in high astigmats (High-Normal Mean diff = -0.72D, p<.01), overall lenticular cylinder was similar in both groups (High ; M = -0.37 D x 62.47 and Normal, M= -0.38 x 83.11). However, the Fourier transforms show high astigmats to have significantly lower lenticular J0 and higher lenticular J45 than the normal astigmats. Conclusion: Astigmatism in 3-5 year old children is primarily corneal in nature, as was found by others 3,4. In the developing eye, the lens does not appear to vary in response to high amounts of corneal WTR astigmatism and it acts to increase the small oblique astigmatic component found in the highly astigmatic group. In high astigmats lenticular astigmatism contributes to both J0 and J45 components, while the corneal contribution is primarily J0. 1. Cowen, L, Bobier, W.R. Submitted to IOVS, 2001; 2. Thibos, LN, Wheeler, W, Horner, D. Optom Vis Sci. 1997, 74:349-76. 3. Dobson,VD, Miller,JM, Harvey,EM. Optom Vis Sci. 1999, 76:855-60. 4. Howland, HC, Sayles, N. Vision Res, 1985, 25:73-81.
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