Chapter 187 Macular Degeneration
The macula, the area of the retina where most images focus, is the portion of the eye responsible for fine vision. Degeneration of the macula is the leading cause of severe visual loss in the United States and Europe in persons 55 years and older and is second only to cataracts as a cause of decreased vision in persons older than 65 years. It is estimated that more than 150,000 Americans are legally blind from age-related macular degeneration (ARMD), with 20,000 new cases occurring each year.1,2
The two most common types of ARMD are the atrophic (dry) form, by far the more common, and the neovascular (wet) form. In either form, patients may experience blurred vision. The patient may note that straight objects appear distorted or bent; the presence of a dark spot near or around the center of the visual field; and, while reading, that parts of words are missing.
The primary lesions are atrophic changes in the retinal pigmented epithelium (RPE), which constitutes the innermost layer of the retina. Apparently, beginning in early life and continuing throughout life, cells of the RPE gradually accumulate sacs of cellular debris (lipofuscin). The lipofuscin bodies are either remnants of incompletely degraded abnormal molecules from damaged RPE cells or derivatives of phagocytized rod and cone membranes. Progressive engorgement of the RPE cells with lipofuscin is associated with the extrusion of tissue components like hyaline, sialomucin, and cerebroside. The hallmark excrescences beneath the RPE apparent on ophthalmoscopic examination are referred to as drusen. The formation of drusen is a result of free radical damage similar to the type of damage that induces cataracts (see Chapter 208). However, decreases in nutrient, blood, and oxygen supply to the retina are the harbinger of the development of drusen and subsequent macular degeneration.
The disease progresses slowly and only central vision is lost; peripheral vision remains intact. It is rare for anyone to become totally blind from dry ARMD. Currently, there is no standard medical treatment for this common form of ARMD, although the use of nutritional supplements designed to address the underlying oxidative damage is becoming the “unofficial” standard of care.
Wet ARMD is also known as the neovascular form or advanced ARMD. It affects 5% to 20% of persons with ARMD. Wet ARMD is characterized by the growth of abnormal blood vessels. Because the disease can rapidly progress to a point at which surgery cannot be used, treatment should be performed as soon as possible. A common early symptom of wet ARMD is that straight lines appear wavy.
Wet ARMD can be treated quite effectively in the early stages with laser photocoagulation therapy. Photodynamic therapy using photosensitive drugs (verteporfin) and a low-powered laser3 or low-dose radiation therapy are also alternatives.4 Antiangiogenics or anti−vascular endothelial growth factor (anti-VEGF) agents are also used. These drugs can cause regression of the abnormal blood vessels and improvement of vision when injected directly into the vitreous humor of the eye. The injections have to be repeated on a monthly or bimonthly basis. Examples of these agents include ranibizumab (Lucentis), bevacizumab (Avastin), and pegaptanib (Macugen).1,2
The primary treatment goal of the dry form and prevention of the wet form of ARMD involves the use of antioxidants and natural substances that correct the underlying pathophysiology—free radical damage and poor oxygenation of the macula. This can be accomplished by reducing the risk factors for ARMD by not smoking, focusing on preventive factors against atherosclerosis, increasing dietary intake of fresh fruits and vegetables, and supplementing with nutritional and botanical antioxidants.
In particular, tobacco smoking greatly increases the risk of ARMD.5 Someone who smokes a pack of cigarettes a day for any significant length of time increases his or her risk for ARMD by two to three times that of someone who has never smoked.5,6 This risk does not return to the control rate until after a person has stopped smoking for 15 years.6
There is also a strong genetic component to consider. Although a number of genetic markers have been identified, a family history may be the most convenient screening method. The lifetime risk of developing late-stage macular degeneration is 50% for people who have a relative with macular degeneration versus 12% for people who do not—a fourfold higher risk. Although genomic family scans of 364 families have revealed a region of chromosome 10 as possibly containing an association with ARMD,7 diversity in phenotype and the late onset of disease complicate the feasibility of linkage studies. Interestingly, higher birth weight and lower head circumference/birth weight ratio are associated with significantly higher risk.8
Although atherosclerosis is now a well-accepted risk factor for macular degeneration, this association was unconfirmed until 1995.9 In the confirmation study, 104 subjects with and 1324 subjects without macular degeneration were evaluated for atherosclerosis via ultrasonographic determination of the intimal-medial thickness, assessed for the presence of atherosclerotic plaque in the carotid arteries, and measured for the ankle/arm systolic blood pressure ratio (an indicator of peripheral atherosclerosis). The results indicated that in subjects younger than 85 years, plaques in the carotid bifurcation were associated with a 4.7 times higher prevalence of macular degeneration. Lower extremity atherosclerosis was associated with a 2.5 times greater risk. These results indicated that measures designed to reduce the risk of atherosclerosis are of great significance in the prevention and treatment of macular degeneration (see Chapter 148 for a comprehensive program for prevention and reversal).
A number of dietary factors are important. Not surprisingly, these mirror those dietary factors that prevent other chronic degenerative diseases, including atherosclerosis. A diet rich in fruits and vegetables is associated with a lower risk for ARMD. Presumably this protection is the result of a greater intake of antioxidant vitamins and minerals.10–12 However, various “nonessential” food components—such as flavonoids and the non–provitamin A carotenes lutein, zeaxanthin, and lycopene—are proving to be even more significant in protecting against ARMD than traditional nutritional antioxidants (e.g., vitamins C and E, zinc, and selenium). The macula, especially its central portion, the fovea, owes its yellow color to its high concentration of lutein and zeaxanthin. These yellow carotenoids function in preventing oxidative damage to the area of the retina responsible for fine vision and obviously have a central role in protecting against the development of macular degeneration.13,14
The carotene lycopene, a component of tomatoes and other red fruits and vegetables, is also protective. In one study, individuals with lycopene content in the lowest quintile were twice as likely to have ARMD.12
Moderate wine consumption is also associated with a decreased risk of ARMD.15 Red wine contains anthocyanins, powerful antioxidants that are probably responsible for the protective effect. It is important to note that beer consumption increased the accumulation of drusen and the risk of exudative macular disease16 and therefore should be avoided.
Just as in atherosclerosis, the type of dietary fat appears to play a role in ARMD. A cohort study of 261 individuals with early or intermediate stages of ARMD revealed a twofold increased risk of progression with a high intake of animal fats and processed baked goods. In contrast, higher fish and nut intakes were associated with a lower risk of ARMD progression.17 The intake of long-chain omega-3 fatty acids reduces the incidence of ARMD and was shown to be inversely associated with the 12-year progression to ARMD.18–20