While there are many causes of glaucoma, most cases are associated with increased intraocular pressure. Loss of vision is usually characterized by a gradual reduction in peripheral vision, which can lead to a tunnel vision effect. Glaucoma affects approximately 100 million people globally and is one of the leading causes of blindness in the world today. An estimated three million Americans have this sight-threatening disease. Because it is painless and advances gradually, many people who have glaucoma or elevated IOP have not been diagnosed. If detected and treated early, vision can usually be preserved.