Tuesday, February 8, 2011

Anatomical Changes in the Eyes as We Age

People have debated for generations about the most valuable of the five senses for the human being. The vision is definitely a very important, if not the most important human sense we have. Blindness affects approximately three million people older than 40. A progressive loss of vision is normal as we age. It is normal to not be able to see as well as we used to as we grow older, but a significant loss of vision may be abnormal and require special attention.

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The various internal and external structures of the eyes, which all work together to help people see clearly at various distances and under different lighting conditions, begin to wear down as people get older. Common age-related vision complaints include:
  • "I can't see as clearly as I used to."
  • "I have difficulty seeing objects close up."
  • "Colors don't seem as vivid."
  • "It's getting more difficult to see in the dark."
  • "I'm less able to adapt to glare."
  • "I need more light to see."

The visual system changes in many ways as the human body ages. Many visual changes are considered normal while others are considered pathogenic, or disease-related. While many normal age-related changes occur in all healthy eyes, many people also suffer from disease-related changes that further impair vision. In general, older eyes are more susceptible to common age-related diseases, such as macular degeneration, cataracts, and glaucoma. Health problems may also make the elderly even more at risk for eye disease, such as the case of diabetics who develop diabetic retinopathy.

Unfortunately, most of the elderly population cannot distinguish between normal vision loss and disease-related vision loss, so many do not seek professional care. We will start our review with normal age-related anatomical changes that occur within the visual system.

Anatomical changes in the older visual system include changes in the lenses, the pupil, the vitreous humor, the retina, and the retinal pigment epithelium. These structures are labeled in the diagram below. Although pictured in red, the vitreous humor is a clear fluid.

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The fixed lens, or the cornea, is the first lens that focuses the light and, as the most powerful lens in the eye, is responsible for focusing most of the incoming light. The fixed lens does not change shape like the variable lens, but it does go through changes with age similar to the variable lens. The cornea thickens over time and adds to the amount of extra light scatter already inside the aging eye. This thickening can also influence the focusing power of the fixed lens.

The variable lens (simply called the lens in the diagram above) is a second lens within the eye and can adjust its shape to further focus incoming light. The variable lens begins to change at birth. As we age, the center of this variable lens loses flexibility and becomes more rigid. Over time, the variable lens loses so much flexibility that it can no longer focus on detailed objects at close range. Although this change occurs throughout the lifespan, adults most often notice it around the age of forty when the lens can no longer focus on average sized print at a normal reading distance. The chemical composition of the lens also changes with time as proteins are produced in different proportions. These chemical changes cause the lens to yellow over time. This yellowing of the lens reduces its transparency, causing it to become more opaque with time.

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The pupil is the opening in the iris inside the eye that allows light to enter. As we get older, the pupil gets smaller in diameter. This, along with the change in variable lens transparency, reduces the amount of light that the older adult eye receives. Because of these changes, people in their 60s need three times more ambient light for comfortable reading than those in their 20s. Also, seniors are more likely to be dazzled by bright sunlight and glare when emerging from a dimly lit building such as a movie theater.

Changes also occur in the vitreous humor, which is the fluid that fills the back of the eye. In young and healthy eyes, the vitreous is a clear gel-like substance with the consistency of egg whites. With age, the vitreous becomes thinner and more water-like. Pockets of liquid vitreous develop within the eye. This liquefied vitreous makes floaters more visible to older individuals. Floaters are clumps of cellular debris that accumulate within the eye and settle at the bottom of the eye in normal eyes. When the fluid thins, the floaters do not settle and, while seeing the floaters does not significantly impair vision, they can be a source of irritation to seniors already dealing with changing vision. Another fairly common change that occurs within the vitreous is referred to as posterior vitreous detachment. In older individuals with this condition, the thinning vitreous begins to pull away from the retina at the back of the eye. This detachment does not cause any serious impairment alone, but the symptoms can impact vision. Symptoms include flashes of light, distorted and blurred images, and increased floaters.

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The retinal pigment epithelium is a layer of darkly pigmented tissue behind the retina. This layer provides the retina with rich nutrients as well as serving to absorb excess light and prevent light scatter within the eye. In the older human eye, the cells of the pigment epithelium become irregular. This change means that the pigment epithelium is less able to absorb excess light and less able to help control light scatter in the elderly eye. Controlling light scatter in the elderly eye is important because light scatter causes glare.

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