A: Glaucoma is an eye disease, resulting from optic nerve damage, which left untreated causes irreversible vision loss. Often this damage results from inadequate fluid drainage, causing your eye’s internal pressure to rise above normal levels. The most common type of glaucoma is what is known as primary open angle glaucoma, which occurs when drainage canals become blocked. Less often, angle closure glaucoma may occur, which happens when the entrance to the drainage canal is too narrow. Although, even people with regular eye pressure can sometimes exhibit optic nerve damage, which is called normal tension glaucoma.