The effect of intraocular pressure on chick eye geometry and its application to myopia.

Myopia is characterized by an increase in axial length of the eye, but the reasons for the axial elongation are still unknown. Higher intraocular pressure (IOP) has been associated with myopia and could be involved in eye enlargement. The purpose of this study was to investigate the effect of intraocular pressure on the geometry of the chick eye and to investigate whether an increase in IOP could cause the elongation of the eye. The IOP of ten 7-day old chick eyes was raised by injecting fluid into the eyes and the resulting deformation of the eyes was measured using digital cameras. In-vitro pressure-volume curves were obtained. The axial and equatorial strains (deformation normalized to the original dimension) were calculated. Our results showed that IOP increased exponentially with increasing injected volume. About 25 D myopia could be induced by the axial elongation created with an increase in IOP by 100 mmHg. As pressure increased from 0 to 140 mmHg, the chick eye elongated in the axial direction and initially contracted in the equatorial direction. The natural tendency of chick eyes is to elongate as IOP increases and this suggests that from a mechanical perspective IOP could play a role in myopia onset and progression. The results also suggest that oblate eyes might have higher risk of developing myopia.