PURPOSE. To assess longitudinal change in refractive, keratometric, and internal astigmatism in a sample of students from a population with a high prevalence of with-the-rule (WTR) astigmatism and to determine the optical origins of changes in refractive astigmatism. METHODS. A retrospective analysis of longitudinal measurements of right eye refractive and keratometric astigmatism in Tohono O'odham Native American children was conducted. Changes in refractive and keratometric astigmatism per year were compared in a younger cohort (n ¼ 1594, 3 to <11 years old) and an older cohort (n ¼ 648, 11 to <19 years old). Data were analyzed in clinical notation (Cyl) and vector notation (J0, J45). RESULTS. On average, refractive astigmatism (means: 1.19 diopters [D] Cyl, þ0.54 J0, þ0.03 J45) resulted primarily from WTR corneal astigmatism (means: þ0.85 J0, À0.02 J45) and against-the-rule (ATR) internal astigmatism (means: À0.31 J0, þ0.05 J45). Mean longitudinal changes in astigmatism were statistically significant (younger cohort À0.02 D/y Cyl; older cohort þ0.06 D/y Cyl). In the younger cohort, astigmatism decreased with age in low and moderate astigmats (<3.00 D) and increased with age in high astigmats ( ‡3.00 D). In the older cohort, astigmatism increased with age across all levels of astigmatism. Longitudinal changes in keratometric and internal astigmatism were negatively correlated in both cohorts. CONCLUSIONS. Cross-sectional data suggest the presence of a constant ATR contribution from internal astigmatism (0.60 D Cyl) that is close to the 0.50 D ATR constant reported by Javal and others. Highly astigmatic 3-to <11-year-old children and children older than age 11 years show a small (not clinically significant) increase in astigmatism with age. A negative correlation between changes in keratometric astigmatism and internal astigmatism suggests an active compensation that may contribute to the stability of astigmatism in Tohono O'odham children. Keywords: astigmatism, refractive error development, children, Native American R esearch conducted over the past 15 years has yielded a large longitudinal database on refractive development in Tohono O'odham children. There is a high prevalence of astigmatism among elementary school children who are members of this Native American tribe. 1-4 The high prevalence of refractive astigmatism in Tohono O'odham children is present in infancy and persists at least through elementary school. 1,2,4-7 A longitudinal study of corneal astigmatism in 960 Tohono O'odham children aged 6 months through 7 years showed that in early development (6 months to <3 years), astigmatism decreased in children with high astigmatism (À0.37 diopters [D]/y) and remained stable in children with little or no astigmatism (þ0.05 D/y). 7 From age 3 through 7 years, astigmatism decreased in children with both high astigmatism (À0.11 D/y) and low/no astigmatism (À0.03 D/y). 7 A detailed account of changes in astigmatism beyond age 7 years in this population has not yet been reported. However, a recent report on change in spherical equivalent (M) in a longitudinal sample of Tohono O'odham students ages 3 to 18 years showed that rate of change in M with age was significantly greater for children who were hyperopic (M ‡ þ2.00) prior to age 5.5 years, consistent with continued emmetropization into the school years. In addition, higher levels of refractive astigmatism were associated with predicted myopia onset (M À0.75 D) by age 18 years. 9 This relation, specifically the relation between keratometric and refractive (total) astigmatism, was originally described by Javal in 1890 10,11 : Refractive Astigmatism ¼ 1.25(Keratometric Astigmatism) À 0.50 D 3 90. A more recent empirically based modification of Javal's rule suggests that WTR and ATR refractive astigmatism can be predicted by subtracting an ATR constant (0.50 D) from the magnitude of keratometric astigmatism, with little or no adjustment based on magnitude of keratometric astigmatism. 10 Little is known about how the relation between keratometric and internal astigmatism changes with development in childhood. For example, it is not known if magnitude of AT
