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Joseph M Miller, Michael w Belin, Erin M Harvey; Posterior corneal shape following gain or loss of keratometric astigmatism. Invest. Ophthalmol. Vis. Sci. 2016;57(12):3980.
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© ARVO (1962-2015); The Authors (2016-present)
To assess anterior and posterior SimK tomography and apical pachymetry in eyes with known history of change in corneal astigmatism.
Subjects were Native American children with an examination that included Retinomax keratometry (Nikon Inc.) and a subsequent examination at least 1.5 years later that included Retinomax keratometry and Pentacam tomography (Oculus GMBH). Right eye keratometry was used to determine change in anterior corneal astigmatism. Pentacam anterior and posterior (a and p) keratometry and apical pachymetry were used to compute a corneal shape descriptor we call the Shell Deviation Ratio (Sdr) derived from the a and p SimK for steeper and flatter (s and f) radii of curvature and corneal thickness (kt). Sdr assumes a and p meridians are aligned. If the cornea is a shell of uniform thickness, ar = pr + kt in all meridians. Typically, the corneal thickens away from the apex (ar > (pr+kt). The shell deviation (Sd) = ar-(kt+pr). The Sd along the s and f meridians are Sds and Sdf, and Sdr = Sds/Sdf, or Sdr = (ars-(kt+prs))/(arf-(kt+prf)). Sdr incorporates sensitivity to mean corneal curvature, corneal thickness, and rotational thickness asymmetry.
The sample included 390 children age 3 to 15 years (mean 6.2) at first exam. The interval from first to second exam averaged 4.9 (range 1.6 to 12.5) years. Keratometric astigmatism exhibited little net change (last minus first measurement mean 0.114 D (stdev 0.56 D), range -1.75 to +2.63 D (with-the-rule). Subjects were classified as having decreasing astigmatism based the location of their change scores in the sample distribution (See Table 1). Sdr was calculated from the Pentcam data, and averaged 1.216 (stdev 1.732) for children with decreasing astigmatism and 1.148 (stdev 0.197) for children with increasing astigmatism (t=2.635, p=0.009 two-tailed test).
Why some children exhibit increasing astigmatism over time, and others do not, remains unclear. In this sample, there was little net change in the population average value, yet some children had change in anterior corneal curvature. The Sdr (Shell Deviation Ratio) differed in these two groups. This finding may provide insight into corneal stability associated with relatively thicker and thinner corneas along the steeper and flatter meridians.
This is an abstract that was submitted for the 2016 ARVO Annual Meeting, held in Seattle, Wash., May 1-5, 2016.
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