The eye is a made up of numerous parts that work together to make vision possible. Here is a brief introduction to the anatomy of the eye and an explanation of four common eye diseases that afflict many of us.

How does the eye work? When you look at something, you are able to see it because the surface of the object reflects light rays. These rays pass sequentially through the cornea, the pupil, and the lens of your eye. The cornea and the lens focus the light rays onto a precise point on the retina (the fovea). Light-sensitive cells within the retina translate the patterns of light into chemical signals that are conveyed to the brain via impulses in the optic nerve. In essence, the eye works in much the same way that the lens of a camera focuses light onto film. The sclera, the conjunctiva, and the choroid are tissues that protect and nourish the eye. Diseases of the eye can perturb this process at many different points to interfere with vision. The four most common causes of blindness and vision loss are:

• Eye Condition 1: Cataract. A cloudy or opaque area in the lens can interfere with its ability to transmit light rays from the front of the eye to the back of the eye. This can cause cloudy or filmy vision, the appearance of halos around lights, and loss of contrast or brilliance of colors.

• Eye Condition 2: Glaucoma. Buildup of aqueous humor, a fluid that occupies the space between the cornea and the lens, can damage the optic nerve by increasing pressure within the eye, often because its normal drainage is blocked. The result is loss of side vision and, eventually, of vision directly in the center, with poor visual clarity and, if the problem is not treated effectively, gradual blindness.

• Eye Condition 3: Age-related macular degeneration (AMD). The macula, the central part of the retina, is responsible for central (as opposed to peripheral) vision and for perception of detail and colors. In the less common and more severe type of AMD, called neovascular or “wet” AMD, abnormal blood vessels from the choroid grow and may bleed and leak fluid under and within the macula. As they heal, they typically leave scars that prevent the macula from functioning well. More common is the non-neovascular or “dry” form of AMD, in which deposits called drusen collect under the retina and may lead to thinning or loss of retinal cells. This can result in blank spots in the central vision.

• Eye Condition 4: Diabetic retinopathy. Some people with diabetes gradually develop damage to small blood vessels in the retina, which can lead to leakage of fluid and swelling of the macula. This itself can blur vision in the eye. In advanced stages, fragile blood vessels grow from the retina into the vitreous humor in the back of the eye. These may bleed and form scar tissue, which can eventually cause the retina to detach, leading to blindness. Adequate blood sugar control can minimize this damage and may prevent its progression.

Posted in Vision on April 18, 2008 Reviewed July 2009 Medical Disclaimer: This information is not intended to substitute for the advice of a physician. Click here for additional information: Johns Hopkins Health Alerts Disclaimer