Bi-directional corneal accommodation in alert chicks with experimentally-induced astigmatism

This study aimed to characterize corneal accommodation in alert chicks with and without experimentally-induced astigmatism. Refraction and corneal biometry were measured in 16 chicks with experimentally-induced astigmatism (>1.00 D) and 6 age-matched control chicks (astigmatism ⩽ 1.00 D). Corneal accommodation was detected using a Placido-ring based videokeratography system, by measuring changes in corneal curvature from a series of consecutive images acquired from alert chicks. The correlation between the magnitudes of corneal accommodation and astigmatism was analyzed by including data from all 22 chicks. Data from all eyes showed obvious bi-directional changes in corneal accommodation. There was no significant difference in corneal accommodative changes between the fellow eyes of the treated birds, and the right and left eyes of control birds. However, positive accommodation (PA) and maximum magnitude of PA (MPA) were significantly higher in the astigmatic vs. the fellow eyes of treated chicks (mean ± SE: PA=+2.24 ± 0.44 D vs. +1.26 ± 0.20 D; MPA=+7.53 ± 0.81 D vs. +4.38 ± 0.53 D, both p<0.05). This was not the case for negative accommodation (NA) or maximum magnitude of NA (MNA) (NA=-0.46 ± 0.15 D vs. -0.33 ± 0.04 D; MNA=-0.92 ± 0.23 D vs. -0.73 ± 0.12D, respectively, p>0.05). Furthermore, higher PA and MPA were found to be correlated with higher refractive astigmatism (both r=0.34, p<0.05). These results suggest that the presence of astigmatism may interfere with accommodative function in chicks.