Abstract
Abstract: :
Purpose: Two observations in chick are relevant to this study 1. the ocular dimensions of normal eyes show diurnal rhythms, 2. diurnal rhythms can be altered by visual manipulations that alter eye growth rates (Nickla et al, 1998). Specifically, axial length was greatest in the afternoon, when choroidal thickness was thinnest, in both normal and form–deprived (FD) eyes although the axial length rhythm in FD eyes tended to be phase advanced. To obtain more insight into whether lens–induced and FD myopia involve the same or different mechanisms, we measured ocular rhythms for both conditions. Methods: 5 day–old White Leghorn chicks were fitted with either no lens, white diffusers (WD), –5D or –30D lenses and reared in a 12/12 hr light/dark cycle (n=6 per group). Axial dimensions were measured by high frequency A–scan ultrasonography on day 0, 2, 4, 9, and 11, on each occasion at 6 hr intervals over 24 hr (10am, 1 hour after lights on; 4pm; 10pm, 1 hour after light off; and 4am next day). Results: Diurnal rhythms were found in most of the ocular components and there were only subtle treatment–induced and time–dependent differences: (1) The vitreous chamber (VC) showed similar diurnal fluctuations across the study period, reaching peak length at 4pm, and minimum length at 4am; the four time points are different from each other for control eyes (p<0.0001), but the 10 pm and 4am data were not different for treated eyes. (2) The choroid was thickest at 4am for all 4 groups (p<0.0003), with treated eyes showing a more distinct trough (thinning), during the day (p<0.0001), than control eyes. (3) Anterior chamber depth was greatest at 4am (p<0.0001), with the –30D group showing the greatest increase (P=0.0002). (4) Lens thickness peaked at 4am, and was thinnest at 10am (p<0.0001). (5) Both the retina and sclera showed thinning during the day and thickening at night (p<0.0001 for each). (6) Optical axial length (cornea to front of retina), peaked at the same time as the VC (p<0.0001). Conclusions: The ocular rhythms documented in the current study are similar to the patterns described previously by Nickla et al., with the choroidal and axial length rhythms being approximately in anti–phase in all cases. Thus observed differences between lens– and FD–induced myopic eyes require another explanation.
Keywords: myopia • circadian rhythms • emmetropization