Birefringence of the Lamina Cribrosa and Posterior Eye Structures

University Of Houston, Houston TX

Investigators

Abstract

PROJECT SUMMARY Glaucoma is the leading cause of irreversible blindness worldwide, and a majority of cases are undiagnosed. Loss of vision due to glaucoma cannot be reversed, only slowed or halted. Therefore, it is imperative that damage is detected and glaucoma is diagnosed early, so it can be managed and vision can be preserved. The most common form of glaucoma presents with an elevated increase in intraocular pressure (IOP) with no known cause. This elevated pressure causes gross anatomical changes to the posterior eye over time, such as optic disc cupping. However, once the optic disc, or other structures in the posterior eye, has changed its structure, damage has already occurred with the elevated IOP driving the increase in stress and structural alteration. Current techniques for diagnosing glaucoma are woefully lacking in their sensitivity to minute structural changes in the posterior eye. Optical coherence tomography (OCT) was developed to perform depth-resolved imaging of the layers of the retina and optic nerve head with micrometer-scale resolution. However, the changes in the gross anatomical structure and thickness of retinal layers imaged by OCT may also only be detectable after damage has already occurred. It has been postulated that the difference between intracranial pressure (ICP) and IOP may play a significant role in determining whether damage will occur in cases of elevated IOP. According to this hypothesis, there should be a change in the stress distribution across the lamina cribrosa, signaling that stress is too high and damage may be imminent. In cases where the translaminar pressure difference, and hence imposed stress, is large, there may be a great risk of developing glaucoma. One method to detect small changes in stress relies on the photoelastic effect, where the optical properties of a material, e.g., index of refraction and birefringence, can change due to applied stress. The tissues in the posterior eye have notable birefringence, i.e., different optical properties corresponding to different orientations of light polarization and direction of propagation, and OCT is capable of imaging the birefringence properties of tissues with its polarization-sensitive functional extension. Therefore, this project proposes the development and application of a polarization-sensitive swept source OCT (PS-SSOCT) system for 3D mapping of the birefringent properties of the posterior eye in a non-human primate model. The PS-SSOCT system will be capable of micrometer-scale structural imaging and mapping any minute changes to the tissue stress, driving changes in its birefringent properties induced by changes in the IOP. Here, we propose exploratory in vivo non-human primate studies where the IOP is carefully controlled and gradually stepped up and down, during which PS-SSOCT imaging will be performed. The local distribution of the birefringence will be quantified in the retina, optic nervehead, and lamina cribrosa. The birefringence will be correlated with the IOP and well-established metrics of the geometry of the posterior eye based on the OCT structural image. The outcome of this research will lead to further in-depth research, including human studies and additional animal studies correlating tissue damage to the IOP-ICP pressure difference, which has significant implications for glaucoma diagnosis, therapy development, and disease management.