Abstract
Purpose:
To demonstrate that, like spherical refraction, the vector components of astigmatism follow a bimodal Gaussian distribution and to investigate how this distribution is influenced by age-related changes in corneal and lenticular astigmatism.
Methods:
A cohort of 1197 eyes of 1197 healthy Caucasian subjects was measured with an autorefractometer and Scheimpflug imaging (various mutually validated devices) to obtain astigmatic vector components J0 and J45 for the refraction, the anterior cornea and the posterior cornea. Astigmatism of the crystalline lens was estimated by vector subtraction of corneal astigmatism from the refractive astigmatism.
Results:
The distribution of the J0 and J45 components both follow a bimodal Gaussian distribution with a very narrow peak at 0 superimposed on a much broader peak. The shape of this distribution changes considerably with age. In J0 the narrow peak shortens progressively with age and the broad peak shifts from negative towards positive values, corresponding with increased prevalence of astigmatism and a shift from with-the-rule towards against-the-rule astigmatism with age. For J45 a similar age-related shortening of the narrow peak and flattening of the broad peak were seen. The source of these refractive changes is a growing imbalance between the J0 of the cornea and lens, where the average of the former increases, that of the latter decreases, and both distributions broaden progressively with age, especially after the age of 50. In J45 the average of the corneal and lenticular astigmatism both decrease with age and their distributions broaden progressively. This produces an age-related loss of balance in J45 as well.
Conclusions:
The astigmatic vector components J0 and J45 can be accurately described using a bimodal Gaussian distribution, representing eyes with “balanced” and “imbalanced” astigmatism. These distributions change drastically with age due to a combination of corneal and lenticular changes, resulting in a growing imbalance between both. The net effect is the increasing prevalence of against-the-rule astigmatism with age reported frequently in the literature.
Keywords: 676 refraction •
428 astigmatism •
413 aging