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T Li, C Wahl, HC Howland; Intraocular Pressure in Chicks Raised in Constant Light . Invest. Ophthalmol. Vis. Sci. 2002;43(13):197.
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Purpose: Raising chicks (Gallus Gallus domesticus) in constant light is known to alter their refractions, corneal radii anterior and posterior chamber depths, and shape of the eye. We wished to see if these changes were correlated with changes in intraocular pressure. Methods: Groups of chicks were raised under normal lighting conditions (12 hours light, 12 hours darkness per day = N or 12/12) and under constant light (= CL) for up to 7 weeks of age. A third group, RE (for "recovery") was raised under CL for 3 weeks and then placed in 12/12 for the remainder of the experiment. We measured intraocular pressures (IOPs) using a "Tonopen" twice a day at noon and midnight at ages 3,5,8,16 and 32 days in N and CL birds. Using "A" scan ultrasound and retinoscopy, we also monitored those aspects of the eye known to be affected or possibly affected by CL, namely: corneal radius of curvature, anterior chamber depth, refraction, lens thickness and vitreous chamber depths at the same intervals. We also measured intraocular pressures in RE birds 1 and 3 weeks after returning them to 12/12. Results: IOPs in normal birds averaged approx. 21.5 mm during the day and approx 17.5 mm Hg at night, and were always significantly different by about 4 mm Hg. day and night (p<0.0001). Day and night IOPs in CL birds were only significantly different from each other on day 3. Thereafter they averaged approx 17.5 mm Hg night and day. The RE group which had been exposed to CL for 3 weeks still showed no significant difference in day and night IOPs after 1 week in 12/12 conditions but returned to normal day and night differences after 4 weeks in 12/12 conditions (p<0.001). A similar pattern of recovery was seen in their ocular dimensions. Ocular dimensions and refractions in 12/12 and CL raised chicks were comparable to those found in earlier experiments. Conclusion: The correlation between changes in ocular morphology and intraocular pressures in birds raised in constant light, together with the fact that daytime IOP is lowered in CL birds, introduces the possibility that these morphological changes may be in part caused by the growing tissues' responses reduced IOP.
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