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Corneal and Lenticular Components of Total Astigmatism in a Preschool Sample

SHANKAR, SUNITA PhD; BOBIER, WILLIAM R. OD, PhD, FAAO, MBC

Optometry and Vision Science: July 2004 - Volume 81 - Issue 7 - p 536-542
Articles: Original Article

Purpose. To examine the contribution of corneal and lenticular components to total astigmatism in preschool low and high astigmats to determine whether there was any compensation for high astigmatism by the lenticular component.

Methods. Cycloplegic refractive and keratometric measures using the Retinomax K-Plus (Nikon Inc., Melville, NY) were conducted on 129 children (mean age, 51.1 ± 8.4 mo) in Oxford County, Canada. We divided the sample into high astigmats (total cylinder ≥1 D; mean, 1.38 ± 0.43 D; n = 29) and normal astigmats (total cylinder ≤ 0.75 D; mean, 0.22 ± 0.20 D; n = 100). Measures of total and corneal cylinder were transposed into J 0 and J 45 components, where positive and negative J 0 values quantified with-the-rule (WTR) and against-the-rule astigmatisms, respectively, and J 45 quantified oblique astigmatism.

Results. WTR astigmatism was dominant in both the high and normal astigmatic group. J 0 and J 45 components of corneal astigmatism were highly correlated with total astigmatism in high astigmats, whereas only J 0 was significantly correlated with total astigmatism in normal astigmats. Although the magnitude of total and corneal cylinder was significantly greater in high astigmats, overall lenticular cylinder was similar in both groups. However, the Fourier transforms showed high astigmats to have significantly lower lenticular J 0 and higher lenticular J 45 than the normal astigmats.

Conclusions. Astigmatism in 3- to 5-year-old children is primarily corneal. In preschool children, the lens does not vary in response to high amounts of corneal WTR astigmatism, and in fact, it increases the oblique astigmatism component when the corneal component is high. In high astigmats, lenticular astigmatism contributes to both J 0 and J 45 components, whereas the corneal contribution is primarily J 0.

School of Optometry, University of Waterloo, Waterloo, Ontario, Canada

Received December 31, 2002; accepted March 21, 2004.

Sunita Shankar

School of Optometry, University of Waterloo, 200 University Avenue West, Waterloo, Ontario N2L 3G1, Canada e-mail: sshankar@uwaterloo.ca

© 2004 American Academy of Optometry