People can suffer vision loss in a single eye or in both eyes. It may come on suddenly, or may become progressively worse over the course of many months or years. Additionally, some types of vision loss affect only part of the eye, whereas others impact the entire eye. In some cases, changes in vision may occur as the result of a condition outside of the eye. The eye is made up of the cornea (the transparent “shield” over the surface of the eye), the iris (the colored part of the eye that shrinks and dilates the pupils), the retina (the light-sensitive layer of tissue lining the eye’s inner surface), the sclera (the white part of the eye), the lens (the transparent structure that helps refract light onto the retina), and the optic nerve (the structure that organizes the retina’s visual impulses and transfers them to the brain). Trauma, infection, or inflammation may cause damage to these structures, thus reducing vision. Issues with the fluid in the eye (changes to the clarity or pressure, for example) also can impact vision. Additionally, damage to the optic nerve or the areas of the brain that process visual information can lead to vision loss. Such damage often results from inflammation, autoimmune disease, or reduced blood flow. Some of the most common vision-related problems are cataracts (characterized by clouding of the lens), glaucoma (resulting from increased pressure to the eye), diabetic retinopathy (diabetes-related retinal damage), age-related macular degeneration (breakdown of the center of the retina), detachment of the retina, optic neuritis (inflammation of the optic nerve), and stroke. Certain medications can also cause vision loss.