Purpose:
Whether chicks could compensate for optically imposed astigmatism has been studied previously but the results are equivocal. This study aimed to determine the effect of a lower magnitude of astigmatism on emmetropization in chicks.
Methods:
Thirty-two White Leghorn chicks (Gallus gallus domestica) were randomly assigned to experience astigmatism, optically imposed by cross-cylindrical lenses (+4.00DS/-8.00DC), with axis oriented at one of four directions (0, 45, 90 and 135 deg, n=8 in each group) from day 5 to day 12. Refractive error was measured by a modified Hartinger refractometer on days 5 and 12, whereas corneal astigmatism was measured by a custom-made corneal topographer on day 12. The treatment effects were expressed as interocular difference (IOD) in spherical-equivalent refractive error (SE), J0 and J45 astigmatic components.
Results:
At the beginning of the experiment, there were no significant differences across the four groups of birds in SE, J0, and J45 components (one-way ANOVA, all p≥0.58). After one-week of treatment, significant differences in IOD of refractive SE, J0, and J45 were observed across the four groups of birds (one-way ANOVA, all p≤0.003; see table for details). In addition, although there was no significant difference in mean corneal curvature, significant differences in corneal J0 and J45 astigmatic components were found across the four groups of birds (one-way ANOVA, both p≤0.001). Pearson’s correlation analyses also showed significant correlations between refractive and corneal astigmatic components (Pearson’s r: Cyl=0.73; J0=0.90; J45=0.84; all p≤0.001). Most importantly, the induced astigmatisms appeared to compensate for the axis of imposed astigmatic errors.
Conclusions:
Our results showed that the axis orientation of imposed astigmatic errors has significant effects on both refractive and corneal changes, supporting the presence of an active regulatory mechanism.
Keywords: astigmatism • emmetropization • refractive error development