Dry Age-Related Macular Degeneration
Approximately 85% to 90% of the cases of macular degeneration are the dry (atrophic) type. Dry age-related macular degeneration does not involve any leakage of blood or serum. Loss of vision may still occur. Patients with this dry form may have good central vision (20/40 or better) but substantial functional limitations, including fluctuating vision, difficulty reading because of their limited area of central vision, limited vision at night or under conditions of reduced illumination.
In the dry type of macular degeneration, the deterioration of the retina is associated with the formation of small yellow deposits, known as drusen, under the macula. This phenomenon leads to a thinning and drying out of the macula, causing the macula to lose its function. The amount of central vision loss is directly related to the location and amount of retinal thinning caused by the drusen.
The early stage of dry age-related macular degeneration is associated with minimal visual impairment and is characterized by large drusen and pigmentary abnormalities in the macula. Drusen are accumulations of acellular, amorphous debris subjacent to the basement membrane of the retinal pigment epithelium. Nearly all people over the age of 50 years have at least one small druse in one or both eyes. Only eyes with large drusen are at risk for late age-related macular degeneration.
This form of macular degeneration is much more common than the “wet” type of macular degeneration, and it tends to progress more slowly than the “wet” type. However, a certain percentage of the “dry” type of macular degeneration turns to “wet” with the passage of time. There is no known cure for the “dry” type of macular degeneration.
Wet Age-Related Macular Degeneration
Approximately 10-15% of the cases of macular degeneration are the “wet” (exudative) type.
In the “wet” type of macular degeneration, abnormal blood vessels (known as choroidal neovascularization or CNV) grow under the retina and macula. These new blood vessels may then bleed and leak fluid, causing the macula to bulge or lift up from its normally flat position, thus distorting or destroying central vision. Under these circumstances, vision loss may be rapid and severe.
With the “wet” type, patients may see a dark spot (or spots) in the center of their vision due to blood or fluid under the macula. Straight lines may look wavy because the macula is no longer smooth. Side or “peripheral” vision is rarely affected. However, some patients do not notice any such changes, despite the onset of neovascularization . Therefore, periodic eye examinations are still very important for patients at high risk.
There are two forms of choroidal neovascularization (CNV) that have been identified, “classic” and “occult.” The classic form is well-defined and usually results in vision that is between 20/250 and 20/400, but it may be worse than 20/800. For eyes with the occult form, the average visual acuity is somewhat better, between 20/80 and 20/200. Occult lesions are not well-delineated and they have less leakage.
Once CNV has developed in one eye, whether there is a visual loss or not, the other eye is at relatively high risk for the same change. When all four risk factors—more than five drusen, large drusen, pigmental clumping, and systemic hypertension—are present, the five-year risk of CNV in the second eye is 87%, whereas if none of these risk factors are present, the risk is 7%.
In addition, CNV may progress rapidly, and any sudden change in central vision therefore requires a prompt examination after dilation of the eyes. The purpose of this exam is to find out whether the sudden loss of vision is due to leakage of blood vessels and which treatment may be appropriate.
Risk Factors for Developing AMD
Age: The number one risk factor is age. One-third of adults over 75 are affected by AMD.
Smoking: Smoking increases a person’s chances of developing AMD by two to five fold. Because the retina has a high rate of oxygen consumption, anything that affects oxygen delivery to the retina may affect vision. Smoking causes oxidative damage, which may contribute to the development and progression of this disease.
Family History of AMD: A person is more likely to develop AMD if someone in his or her immediate family has had it.
Gender: Females are more likely to develop AMD than males. This factor may be because females live longer than males, and thus have more time to develop the disease.
Race: Caucasians are more likely to develop AMD than other races. This factor may be related to differences in genetic background or pigmentation.
Prolonged Sun Exposure: Although the evidence is not conclusive, some studies suggest an association between AMD and cumulative eye damage from ultraviolet (UV) and other light. This light may damage the retina and increase the risk of AMD.
Diet: People with diets that are elevated in fat, cholesterol and high glycemic index foods, and low in antioxidants and green leafy vegetables may be more likely to develop AMD. High-glycemic index foods, such as white rice, bread and pasta raise blood sugar rapidly, whereas low-glycemic foods, such as whole grain breads or oatmeal can lower the risk of AMD by stabilizing blood sugar levels.
Obesity: A person with a BMI (body mass index, a measure of body fat) of greater than 30 is 2.5 times more likely to develop the disease than a person with a lower BMI.
High Blood Pressure: High blood pressure, like smoking, leads to a constriction (narrowing) of the blood vessels that nourish the retina, restricting oxygen flow.
Eye Color: People with light-colored eyes are more likely to develop the dry type of AMD. This factor may be because light-pigmented eyes offer less protection from damaging UV light.
Inactivity: In dry AMD, the retina does not receive adequate oxygen, leading to the death of cells in the macula. Exercise improves cardiovascular health and might help prevent AMD.
Presence of AMD in One Eye: If a person has AMD in one eye, he or she is more likely to develop it in the other eye.