Glaucoma is not just one eye disease, but a group of eye conditions resulting in optic nerve damage, which causes loss of vision. Abnormally high pressure inside your eye (intraocular pressure) usually, but not always, causes this damage. Glaucoma is the second leading cause of blindness. Sometimes called the silent thief of sight, it can damage your vision so gradually you may not notice any loss of vision until the disease is at an advanced stage. The most common type, primary open-angle glaucoma, has no noticeable signs or symptoms except gradual vision loss. Early diagnosis and treatment can minimize or prevent optic nerve damage and limit glaucoma-related vision loss. It’s important to get your eyes examined regularly, and make sure your eye doctor measures your intraocular pressure.