 The main cause of damage to the optic nerve is intraocular pressure (IOP), excessive fluid pressure within the eye, which can be due to various reasons including blockage of drainage ducts, and narrowing or closure of the angle between the iris and cornea.The primary division in categorizing different types of glaucoma is open-angle and closed-angle (or angle-closure) glaucoma.In open angle glaucoma, the iris meets the cornea normally, allowing the fluid from inside the eye to drain, thus relieving the internal pressure.