Preschool Children Exhibit Evident Compensatory Role of Internal Astigmatism in Distribution of Astigmatism: The Nanjing Eye Study.

PURPOSE

To determine the prevalence and associated risk factors for total, corneal, and residual astigmatism and to evaluate the relations between components of astigmatism in Chinese preschool children.

METHODS

In the population-based, cross-sectional Nanjing Eye Study, children were measured for noncycloplegic refractive error using an autorefractor and for biometric parameters using an optical low-coherent reflectometry. Data from right eyes were analyzed to calculate the prevalence of astigmatism using various cutpoints (0.5, 1.0, and 1.5 diopters [D]) and for determining risk factors using logistic regression models. Relations between astigmatism components were assessed using Spearman correlation coefficients (ρ).

RESULTS

Of 1817 children (mean ± SD of age: 54.8 ± 3.5 months, 54.2% male), the median (1st and 3rd quartile) of total, corneal, and residual astigmatism (vectorial difference between total and corneal astigmatism) was -0.25 (-0.50, 0), -1.06 (-1.49, -0.72), and -0.92 (-1.23, -0.62) D and their prevalence rate 1.0 D or more was 14.2%, 56.1%, and 44.2%, respectively. With-the-rule was the most common type in total astigmatism (75.2%) and in corneal astigmatism (88.2%) while against-the-rule was predominant in residual astigmatism (75.6%). A negative correlation was found between corneal J0 and internal J0 (ρ = -0.74, P < 0.001) and between corneal J45 and internal J45 (ρ = -0.87, P < 0.001). Based on compensation factor (CF), defined as the minus ratio of internal astigmatism (vectorial difference between total and anterior corneal astigmatism) and anterior corneal astigmatism, internal J0 compensated for total J0 in varying degrees (CF: 0.1-2) in 91.5% cases, while that percentage for J45 component was 77.2%. In univariate logistic regression model, older age was significantly associated with total astigmatism (odds ratio [OR] = 0.96 for per-month increase, P = 0.03), and larger axial length-corneal radius ratio was significantly associated with higher risk of residual astigmatism (OR = 2.28 for per unit increase, P = 0.03).

CONCLUSIONS

The compensatory role of internal astigmatism on reducing corneal astigmatism was prominent in preschool children. Larger axial length-corneal radius ratio was significantly associated with higher risk of residual astigmatism.