Over 90% of glaucoma is open angle, meaning the drainage system is open and not blocked by other structures. Within this category, the most common form is primary open-angle glaucoma. It is not uncommon for the eye pressure to be elevated which is thought to cause the damage to the optic nerve. However, one may have glaucoma and damage to the optic nerve despite having seemingly low eye pressures. There are several secondary glaucomas such as pseudoexfoliation or pigmentary glaucoma which are treated very similarly to primary glaucoma.