What is vision loss? Vision is our most precious special sense. Nearly half of the human brain is engaged in vision-related activities. Vision loss is any reduction in the ability to see, including blurred vision, cloudy vision, double vision, blind spots, poor night vision, and loss of peripheral vision (tunnel vision). Vision loss may affect one or both eyes, it may occur gradually or suddenly, and it may be partial or complete. Vision changes may originate in the eyes themselves or may be caused by many different conditions that affect the brain or even the whole body. The path of light rays, which form all the images we perceive, begins at the cornea, the clear “window” in the front of the eye. The iris, which is the colored part of the eye, controls the pupil size and allows light rays to enter the eye and pass through the lens. This helps to focus the rays onto the retina, which is the light-sensitive layer on the inner surface of the eye. Damage caused by trauma, infection, inflammation, or other changes in these structures can reduce vision. The shape of the eye is maintained by the pressure of the fluid inside the eye against the sclera, the white part of the eye, and the cornea. Conditions that affect the clarity or pressure of the fluid in the eye can also affect vision. The optic nerve organizes the visual impulses from the retina and transmits them to the brain for interpretation. Damage to the nerve due to inflammation, autoimmune disease, or decreased blood supply can lead to vision loss, as can conditions that affect the brain either generally or in the specific locations of the brain that interpret vision. Some common causes of vision loss include eye trauma, clouding of the lens (cataract), increased eye pressure (glaucoma), retinal damage due to diabetes (diabetic retinopathy), breakdown of the central portion of the retina (age-related macular degeneration), retinal detachment, inflammation of the optic nerve (optic neuritis), and stroke. Some medications can also affect vision. Vision loss can be permanent and may be a symptom of a serious medical condition. Seek prompt medical care for any type of vision loss. Seek immediate medical care (call 911) for sudden vision changes, trauma, eye pain or redness, double vision, partial or complete blindness, or vision loss that occurs like a shade dropping or a curtain closing. Immediate medical care is also necessary if vision loss is accompanied by a severe headache, sudden weakness or numbness on one side of the body, altered level of consciousness, dizziness, difficulty speaking or understanding speech, or loss of sensation. Even temporary vision loss should be treated as an emergency.