 By the time symptoms start to appear, this sneak thief of sight, as glaucoma is often referred to, has already started to permanently damage the eye. If left undiagnosed or poorly controlled, glaucoma can cause irreversible damage and ultimately the loss of sight. People with a glaucoma often start to notice the loss of their peripheral vision or the sight around the edges of their field of vision, but by then it's often too late. Once vision is lost to glaucoma, it cannot be restored.