That can put pressure on the optic nerve and damage it. If you're also seeing halos around lights, your eyes are very red and hurt a lot, and you feel queasy, you may have a type of glaucoma called acute angle closure glaucoma. It develops very quickly, and you could lose your vision within a day if it's not treated. Open-angle glaucoma is more common, but it doesn't usually affect your vision at first, because it progresses very slowly over time. Once glaucoma affects your vision, there is no way to get that vision back because your optic nerve has been permanently damaged. The best way to prevent damage is to be diagnosed early by seeing your doctor regularly, who will monitor your eye pressure. If it’s too high, they may recommend eye drops or surgery to lower the eye pressure into a safe zone to prevent damage and vision loss.