Glaucoma is a group of eye diseases that slowly steal sight, often with no immediately recognizable symptoms. Most often, vision loss is the result of increased pressure inside the eye, which causes damage to the optic nerve. As glaucoma progresses, peripheral vision becomes more and more compromised as the area of effective vision continues to shrink. How does pressure cause loss of sight? Think of the eye as a balloon filled with air. As more air is pumped in, the balloon expands. The eye, however, is too strong to expand. As the pressure increases, the eye gives at its weakest point, where the optic nerve leaves the eye.