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Both disease and eye injury may affect vision. The vision clarity is known as visual acuity, which ranges from total vision to no vision. As acuity decreases, the vision becomes extremely blurred. Acuity is generally measured on a scale that compares an individual’s vision at twenty feet with that of person with full acuity. Thus, the individuals with 20/20 vision can see objects twenty feet away with total clarity, but the individual with 20/200 vision see objects at twenty feet what an individual with acuity sees at 200 feet. Legal blindness occurs when visual acuity worse than 20/200 even after contact lenses or eyeglasses correction. Many legally blind individuals may distinguish shadows and shapes but not normal details. Common causes of blidness include cataract, infection, diabetes, macular degeneration, and glaucoma. Blindness may also happen for any of the following reasons: 1. the brain may not interpret information sent by the eye; 2. Light may not reach the retina; 3. the nerve impulse from the retina is not transmitted normally to the brain; 4. light ray does not focus or retina properly; 5. the retina may not sense light rays normally. Few disorders may result in such problems that lead to blidness. A cataract may block light coming into the eye, so that it is never reaching the retina. Refraction (focusing) error may generally be corrected with lenses, but not always entirely. A hereditary disorder includes retinitis pigmentosa, and detached retina may affect the ability of retina to sense light. Macular degeneration or diabetes may also injure the retina. Nervous system’s disorders include multiple sclerosis or improper blood supply may injure the optic nerve, which carries impulses to the person’s brain. Tumors, including pituitary gland cancer, may injure the nerve. The brain’s areas that are interpreting visual impulses can be damaged by the tumor, stroke, or other disorders.
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