Abstract

PURPOSE. To investigate further the emmetropization process in young chicks by studying the diurnal fluctuations and developmental changes in the ocular dimensions and optical aberrations, including refractive errors, of normal eyes and eyes that had the ciliary nerve sectioned (CNX). METHODS. The ocular dimensions and aberrations in both eyes of eight CNX (surgery on right eyes only) and eight normal chicks were measured with high-frequency A-scan ultrasonography and aberrometry, respectively, four times a day on five different days from posthatching day 13 to 35. A fixed pupil size of 2 mm was used to analyze aberration data. Repeatedmeasures ANOVA was applied to examine the effects of age, time of day, and surgery. RESULTS. Refractive errors and most higher-order aberrations decreased with development in both normal and CNX eyes. However, although normal eyes showed a positive shift in spherical aberration with age, changing from negative spherical aberration initially, CNX eyes consistently exhibited positive spherical aberration. Anterior chamber depth, lens thickness, vitreous chamber depth, and thus optical axial length all increased with development. Many of these ocular parameters also underwent diurnal changes, and mostly these dynamic characteristics showed no age dependency and no effect of CNX. Anterior chamber depth, vitreous chamber depth, and optical axial length were all greater in the evening than in the morning, whereas the choroids were thinner in the evening. Paradoxically, eyes were more hyperopic in the evening, when they were longest. Although CNX eyes, having enlarged pupils, were exposed to larger higher-order aberrations, their growth pattern was similar to that of normal eyes. CONCLUSIONS. Young chicks that are still emmetropizing, show significant diurnal fluctuations in ocular dimensions and some optical aberrations, superimposed on overall increases in the former and developmental decreases in the latter, even when accommodation is prevented. The possibility that these diurnal fluctuations are used to decode the eye's refractive error status for emmetropization warrants investigation. That eyes undergoing ciliary nerve section have more higher-order aberrations but do not become myopic implies a threshold for retinal image degradation below which the emmetropization process is not affected. I t has become increasingly apparent that the eye cannot be viewed as a static system, optically, anatomically, or physiologically. A number of ocular parameters, including refractive errors, ocular dimensions and intraocular pressure (IOP), undergo dynamic changes on both short (seconds) and longer time scales. One interesting manifestation of these dynamics is the various diurnal rhythms that have been reported in conjunction with ocular function. For example, rhythms in melatonin production, IOP, pupil size, and corneal epithelial thickness have been reported. 13,14 The cues used to decode the sign of defocus during emmetropization are not known. Plausibly, the eye could use odd-error cues from astigmatism and higher-order aberrations to decode the sign of defocus (Hunter J, et al. IOVS 2003;44: ARVO E-Abstract 4341). 15 Drawing on an analogy with accommodation in humans where accommodative microfluctuations play a role in decoding the sign of defocus, 16 diurnal fluctuations in refractive errors and/or higher order optical aberrations could play a similar role in emmetropization. Short-term fluctuations in higher-order aberrations 17 as well as changes on the scale of days, weeks, and months have been reported in young adult humans.