Chapter 208 Senile Cataracts
Cataracts are the leading cause of impaired vision and blindness in the United States. Approximately 4 million people have some degree of vision-impairing cataract, and at least 40,000 people in the United States are blind because of cataracts. Cataracts are a source of a tremendous financial burden on our society; cataract surgery is the most common major surgical procedure performed in the United States each year (600,000 per year) for persons receiving Medicare benefits.
Cataracts may be classified by location and appearance of the lens opacities, by cause or significant contributing factor, and by age at onset. Many factors may cause or contribute to the progression of lens opacity, including ocular disease, injury, surgery, systemic diseases (e.g., diabetes mellitus, galactosemia), toxins, ultraviolet and near-ultraviolet light, radiation exposure, and hereditary disease.
The crystalline lens is obviously a vital component of the optical system owing to its ability to focus light (via changes in shape) while maintaining optical transparency. Unfortunately, this transparency diminishes with age. The majority of the geriatric population displays some degree of cataract formation. In the normal aging eye there is a progressive increase in size, weight, and density of the lens throughout life.
The etiology of cataract formation is ultimately related to an inability to maintain normal homeostatic concentrations of Na+, K+, and Ca2+ within the lens. These abnormalities are apparently the result of decreased Na+,K+-ATPase activity,1–6 a defect usually due to free radical damage to some of the sulfhydryl proteins in the lens, including Na+,K+-ATPase, which contains a sulfhydryl component.
In cataract formation, the normal protective mechanisms are unable to prevent free radical damage. The lens, like many other tissues of the body, depends on adequate levels and activities of superoxide dismutase (SOD), catalase, and glutathione (GSH) as well as adequate levels of accessory antioxidants such as lutein, vitamins E and C, and selenium to help prevent damage by free radicals. Individuals with higher dietary intakes of vitamin C and E, selenium, and carotenes (especially lutein) have a much lower risk of developing cataracts.7 Several studies have shown that various nutritional supplements—multiple vitamin formulas, vitamins C and E, B vitamins (especially vitamin B12 and folic acid), and vitamin A—also offer significant protection against both nuclear and cortical cataracts.8–11 Studies conducted by the Age-Related Eye Disease Study Research Group and others indicate that a combination of these nutrients will likely produce better results than any single nutrient alone or even limited combinations of three or less nutrients in the prevention of both age-related macular degeneration and cataracts (see Chapter 187 for more information).
Lutein, the yellow-orange carotene that offers significant protection against macular degeneration, also exerts protection against cataract formation.12 Like the macula, the human lens concentrates lutein. In 1992, a prospective cohort study showed that consumption of spinach (high in lutein) was inversely related to the risk of cataracts severe enough to require extraction.13 This initial investigation was followed by three prospective studies showing that intake of lutein was inversely associated with cataract extraction (20% to 50% risk reduction).14–16 In a double-blind intervention trial, 17 patients clinically diagnosed with age-related cataracts were randomly assigned to receive dietary supplementation with lutein (15 mg), α-tocopherol (100 mg), or placebo three times a week for up to 2 years.17 Visual performance (visual acuity and glare sensitivity) improved in the lutein group, whereas there was a trend toward the maintenance of visual acuity with α-tocopherol and a decrease with placebo supplementation.
A high dietary intake of vitamin C from either dietary sources or supplements has been shown to protect against cataract formation.8–1118 In addition to preventing cataracts, antioxidant nutrients like vitamin C may offer some therapeutic benefits. Several clinical studies have demonstrated that vitamin C supplementation can halt cataract progression and in some cases significantly improve vision. For example, in a study conducted in 1939, a total of 450 patients with cataracts were started on a nutritional program that included 1 g/day of vitamin C, resulting in a significant reduction in cataract development.1 Similar patients had previously required surgery within 4 years, but in the vitamin C–treated patients only a small handful needed surgery, and in most there was no evidence that the cataracts had progressed over the 11-year study period.
It appears that the daily dose of vitamin C necessary to increase the vitamin C content of the lens is 1000 mg.2 The lens of the eye and active tissue of the body require higher concentrations of vitamin C. The level of vitamin C in the blood is about 0.5 mg/dL, whereas that in the adrenal and pituitary glands is 100 times that. In the liver, spleen, and lens of the eye, the vitamin C level is increased by at least a factor of 20. In order for these concentrations to be maintained, the body must generate enormous amounts of energy to pull vitamin C out of blood against this tremendous gradient. Keeping blood vitamin C concentrations elevated helps the body concentrate vitamin C into active tissue by reducing the gradient. That is probably why such a high dose is required to raise the vitamin C content of the lens.
In a large double-blind trial, 11,545 apparently healthy U.S. male physicians 50 years or older without a diagnosis of cataract at baseline were randomly assigned to receive 400 IU of vitamin E or placebo on alternate days and 500 mg of vitamin C or placebo daily.19 After 8 years of treatment and follow-up, there was no significant difference in cataract formation in the groups. This study may have failed to show benefit because it was below the threshold of 1 g/day of vitamin C.