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R. Genest, N. Chandrashekar, E. L. Irving; The Effect of Intraocular Pressure Variation on Chick Eye Geometry and Its Application to Myopia. Invest. Ophthalmol. Vis. Sci. 2010;51(13):1193. doi: https://doi.org/.
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To investigate the change in geometry of normal and myopic chick eyes with increasing intraocular pressure (IOP).
Sixteen normal and six myopic 7-day old chick eyes were obtained. Myopia was induced in the right eye of six chicks using -15 D lenses for 7 days and the left eye was used as a control. The eyes were cannulated using a standard hypodermic needle and a computer controlled syringe pump was used to inject precise amounts of fluid into the eyes. A digital pressure gauge connected to a syringe was used to measure IOP. Two digital cameras were mounted perpendicular to each other to photograph and measure axial and equatorial deformation of the eye. Fluid was injected in 20 microliters increments between 0 and 100 mmHg, followed by 100 microliters injections until scleral rupture (IOP returns to zero). The experiment was executed rapidly with just enough time between injections to record the IOP and take the pictures so that creep could be minimized. The axial and equatorial deformation with IOP along with the pressure-volume relationship for normal and myopic eyes were obtained and compared. Finally, a finite element model of a normal chick eye was constructed.
Chick eyes deform more in the axial than in the equatorial direction as pressure increases. Axial length increases with increasing IOP and 60 % of the axial deformation occurs when the IOP reaches 100 mmHg. The maximum axial elongation for normal eyes was 1 mm which equals ~ 25 D of myopia. The equatorial diameter initially decreased as IOP increased and the decrease was less pronounced for myopic eyes. As pressure further increases, the equatorial diameter starts to increase and eventually comes back to its original dimension at an IOP of 550 mmHg and 250 mmHg for normal and myopic eyes respectively. The mean IOP at failure was 826 mmHg with a range of 660-1015 mmHg. No difference was found between the pressure-volume curves of normal and myopic eyes. The finite element model demonstrates that the oblate geometry of chick eyes leads to greater deformation in the axial direction and initial contraction in the equatorial direction.
This study showed that elevated IOP is capable of producing axial elongation and therefore myopia in post-mortem enucleated chick eyes. The oblate geometry of the chick eye explains the greater axial deformation and the initial contraction in the equatorial direction. The fact that the pressure-volume behaviour of normal and myopic eyes is similar indicates that the process is more complicated than simple passive stretching.
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