When the pressure is too high, damage occurs to the optic nerve. The optic nerve is made up of a bundle of nerve fibers which sends signals to the brain. Damage to the optic nerve can initially cause blind spots at the outer edges of the field of vision called peripheral or side vision. This is the main sign of glaucoma. As damage to the optic nerve gets worse, the visual field can shrink leading to tunnel vision or even loss of central vision affecting a patient’s ability to read. Fortunately, this occurs only in patients with very severe disease.