Glaucoma, a leading cause of irreversible blindness in the world, is characterized by progressive degeneration of the retinal ganglion cells (RGCs), the retinal nerve fiber layer, and the optic nerve. Glaucoma is often associated with elevated intraocular pressure (IOP), and because the increased IOP exerts a compressing force on the blood vessels in the eye, it has long been hypothesized that the RGC damage is caused by mild, but chronic, reduction of basal blood flow and/or blood-flow dysregulation.