The most common form of glaucoma is called primary open-angle glaucoma. It occurs when the drainage system of the eye called the trabecular meshwork gradually becomes less efficient at draining fluid. As this happens, your eye pressure, called intraocular pressure (IOP), rises. Raised eye pressure leads to damage of the optic nerve. Damage to the optic nerve can occur at different eye pressures among different patients. Your ophthalmologist establishes a target eye pressure for you that he or she predicts will protect your optic nerve from further damage. Different patients have different target pressures.