Macular Degeneration
What is it?
There are many disorders that affect the eyes. Some are localized problems while others are a sign of disease elsewhere in the body. Eye diseases are particularly common in elderly people. Cataracts are the most prevalent eye disease in the world and are a major cause of visual loss in developing and developed countries. Other common eye diseases include age-related macular degeneration, glaucoma, and diabetic retinopathy. There are a number of risk factors for these diseases; including age, high blood pressure, prolonged exposure to sunlight, and nutritional deficiencies.
Macular degeneration is a condition in which the macula, the central part of the retina, degenerates. This is where most of the images formed by the lens are sent and it is much more sensitive to detail than the peripheral retina. The macula provides the central or close up vision necessary for detailed activities such as reading. Yellowish deposits called drusen accumulate beneath the retina of the eye. Loss of vision can start at the edge of the visual field and work its way towards the center or it may start in the center and work its way outwards.
There are two kinds of macular degeneration: dry and wet. Dry macular degeneration accounts for 85 to 90 per cent of cases and wet macular degeneration accounts for approximately 10 per cent. In dry macular degeneration, drusen is deposited in the macula without any evidence of scars, blood, or other fluid leakage. In wet macular degeneration, leaked material forms a mound, often surrounded by small hemorrhages. Eventually the mound contracts, leaving a scar. Both forms of macular degeneration usually affect both eyes at the same time. The dry form develops slowly and usually causes mild vision loss. The wet form is a much greater threat to vision and is indicated by a large blind spot in the middle of the visual field.
Age-related macular degeneration is the most common cause of blindness in developed countries and affects about 6 per cent of Americans between the ages of 65 and 75 and almost 20 per cent of those aged over 75.
Nutrition and the eye
Nutrient deficiencies can either directly affect eye function or increase the susceptibility to degenerative problems such as cataracts and macular degeneration.
Vitamin A
Vitamin A deficiency is the leading preventable cause of blindness in the world. Vitamin A plays a role in maintaining the cornea and enhancing night vision through a compound known as visual purple. One of the first symptoms of vitamin A deficiency is night blindness, and prolonged deficiency leads to xerophthalmia, a condition in which eyes become dry, ulcers appear on the cornea, and the eyelids become swollen and sticky. This condition eventually leads to blindness.
B vitamins
Several B vitamin deficiencies can lead to abnormal eye function. Riboflavin deficiency can cause aversion to bright light, dimness of vision and a burning and itching in the eyes. Low intake must occur for several months before these symptoms appear. A low level of riboflavin may also contribute to cataracts and macular degeneration. Low levels of vitamin B12 can also lead to poor vision.
Antioxidants
As oxidative damage is believed to play a role in the development of eye diseases, and in particular cataract and macular degeneration, many research studies have investigated the ability of antioxidant vitamins and trace minerals to prevent the onset or progression of the disorders. Basic research studies have shown that antioxidants can protect against the cumulative effects of oxidative stress in animal models of cataract and macular degeneration. Epidemiological evidence in humans suggests that people with comparatively higher intakes or blood concentrations of antioxidant vitamins are at a reduced risk of cataract and macular degeneration.
Symptoms of macular degeneration
Macular degeneration leads to a painless loss of vision. Symptoms include night blindness, blurry or fuzzy vision, straight lines appearing as wavy ones, a dark or empty area in the center of vision, narrowed blood vessels in the retina and gradual development of poor vision that can lead to total blindness.
Causes of macular degeneration
It is unclear exactly why macular degeneration develops and there is currently no cure. Risk factors for the disorder include high blood pressure, cardiovascular disease, nutritional deficiencies, exposure to certain chemicals, and heredity. Light eye pigmentation and cigarette smoking may also be risk factors. Some experts believe that long-term exposure to light causes free radical damage to the retina, and that this contributes to the development of macular degeneration.
Treatment of macular degeneration
Some forms of macular degeneration can be treated with laser surgery and low vision aids can be useful in allowing people to continue normal activities. Sunglasses may be useful in preventing light damage.
Macular Degeneration and diet
Several studies suggest that increasing the consumption of foods rich in vitamin A, carotenes and other antioxidants; in particular dark green, leafy vegetables; may decrease the risk of developing advanced macular degeneration.
As part of the Eye Disease Case-Control Study, researchers compared blood levels of carotenes, vitamins C and E, and selenium in 421 patients with macular degeneration and 615 people without the disorder. The results showed that people with high carotenoid intakes had a lower risk of macular degeneration. Results also showed that those with high intakes of all the antioxidants had significantly reduced risk of the disorder.8
Carotenes
The carotenes, lutein and zeaxanthin, may also have protective effects. Lutein and zeaxanthin are constituents of the pigment of the eye. A low density of this pigment in the macula of the eye may increase the risk of the disorder, macular degeneration, possibly because it permits greater blue light damage.
In a study published in 1997, researchers at Florida International University in Miami tested the effects of 30 mg of lutein on eye pigment in two people for a period of 140 days. The results showed that 20 to 40 days after the people started taking the lutein supplement, the density of the pigment in their eyes started to increase. This amount of blue light reaching the vulnerable eye tissues that are damaged in macular degeneration was reduced by around 30 to 40 per cent.9
Zinc
Zinc is highly concentrated in the eye, particularly in the retina and tissues surrounding the macula. Zinc deficiency can lead to loss of eye function as several zinc-dependent enzymes play important roles in eye function. Levels of these enzymes decline with age. Zinc deficiency may contribute to macular degeneration of the central part of the retina. Results from the Beaver Dam Eye Study, published in 1996, suggest a link between low zinc intakes and risk of macular degeneration.10
Selenium
As part of the enzyme, glutathione peroxidase, selenium may be important in protecting the eye from oxidative damage. A recent study found an association between macular degeneration and low selenium levels.11