Glaucoma is an eye condition resulting from optic nerve damage, caused in most cases by increased eye pressure due to fluid buildup. There are two main kinds of glaucoma: primary open-angle glaucoma, where the only symptom is gradual vision loss, and narrow (closed) angle glaucoma, where symptoms can include severe eye pain accompanied by nausea and vomiting, blurred vision, and eye redness. Glaucoma is the leading cause of blindness in the United States.