The optic nerve head (ONH), located in the back of the eye, is a critical site in understanding the pathophysiology of glaucoma. However, longitudinal changes of the ONH as disease develops have not been well characterized. Our goal was to develop an improved tool to quantify these changes in an in vivo monkey model of glaucoma.
Longitudinal spectral-domain optical coherence tomography (OCT) imaging of the ONH was performed every other week under manometric intraocular pressure (IOP) control (10 mmHg) in a monkey during baseline and after induction of unilateral experimental glaucoma. We developed a computational pipeline that applied digital volume correlation (DVC) to measure the 3D ONH deformations. The chronic changes, akin to stretch, compression and shear strain were computed from OCT scans acquired in vivo at multiple stages of experimental glaucoma. Custom programs were developed to verify the robustness of the DVC algorithm and calculate a confidence map. Two regions of the ONH were segmented to focus the DVC analysis: the lamina cribrosa (LC), which plays an important role in glaucoma, and a region of the peripapillary retina, which is expected to thin through glaucoma progression.
We successfully developed a set of programs to calculate chronic tissue changes from OCT scans. We use classic DVC terminology and refer to them as displacements and strains. However, this is not exactly the case because these are long-term changes that could include deformation, and other changes such as shrinkage, growth and remodeling. The verification results of the displacement map demonstrated high robustness of the DVC algorithm. The computed strain map suggested that chronic elevated IOP and glaucoma progression caused deformations of the ONH. The maximum chronic stretch, compression, and shear strains did not always colocalize. The LC tended to be more sensitive to chronic IOP elevation compared to the peripheral retinal nerve fiber layer. The ONH deformations did not necessarily follow the trend of chronic IOP elevation in glaucoma.
To the best of our knowledge, this is the first study to analyze the longitudinal and in vivo ONH deformations in glaucoma. Results from this study can help clarify the pathophysiology of glaucoma
