Glaucoma is not just one eye disease, but a group of eye conditions resulting in optic nerve damage, which may cause loss of vision and blindness. In glaucoma, damage to the optic nerve is usually caused by elevated intraocular pressure (IOP). IOP rises when passages that allow fluid to drain become blocked. However, one person may develop nerve damage at a relatively low pressure, while another person may have high pressure for years and yet never develop damage. Although IOP is clearly a risk factor, other risk factors are present. In the early stages of the disease, there may be no symptoms. It is estimated that 50% of people who have glaucoma are not being treated as they are not aware that they have the disease. 

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