Natural Therapies for Ocular Disorders, Part One: Diseases of the Retina
 
   

Natural Therapies for
Ocular Disorders, Part One:
Diseases of the Retina

This section is compiled by Frank M. Painter, D.C.
Send all comments or additions to:
   Frankp@chiro.org
 
   

FROM: Alternative Medicine Review 1999 (Oct);   4 (5):   342359 ~ FULL TEXT

Kathleen A. Head, ND


Introduction

During the past few decades numerous studies have been published on the efficacy of nutritional and botanical medicines in the prevention and treatment of ocular diseases, including macular degeneration, diabetic retinopathy, retinitis pigmentosa, cataracts, glaucoma, and others. Part One of this review will explore the research on diseases of the retina, including macular degeneration, retinopathy, and retinitis pigmentosa. Part Two (to be published in a future issue) will include a review of the literature on cataracts, glaucoma, and other non-retinal disorders.

Macular Degeneration

Age-related macular degeneration (AMD) is the leading cause of visual impairment and blindness in older Americans. Laser photocoagulation therapy is effective in only a small percent of late-stage cases--those with neovascularization and exudates. Therefore, prevention is of prime importance in reducing the health impact on this growing elderly population.

Pathophysiology

Macular degeneration is characterized by atrophy of the macular disk. Tissues most effected are the photoreceptors and the retinal pigmented epithelium (RPE) (see Figure 1). The photoreceptors most sensitive to damage are the rods and the blue-light sensitive cones. Two types of AMD have been identified: an atrophic form, which involves pigmentary changes in the macula without hemorrhage or scar formation, and disciform macular degeneration, characterized by an exudative mound and sub- and intraretinal hemorrhage. [1] In both types the retinal pigment cells degenerate, with a resulting loss of rods and cones. Risk factors include family history of macular disease, cigarette smoking, [2] light exposure, [3] light iris pigmentation, [4] chemical exposure, history of cardiovascular disease, decreased hand grip strength, and hyperopia. [5]

Antioxidants and AMD

While the etiology of macular degeneration is not fully understood, evidence from animal studies points to the role of free radical damage from light exposure as a potential contributing factor. The photoreceptors of the eye are high in polyunsaturated fatty acids (PUFAs), particularly docosahexaenoic acid (DHA), especially in their outer-segment membranes. DHA is readily oxidized in the oxygen-rich environment of the retina. [6] Several of the known risk factors for AMD, including cigarette smoking and light exposure, appear to be at least partially related to oxidative stress.

Animal studies have demonstrated exposure to UV light, ionizing radiation, or visible light can lead to free radical formation and subsequent lipid peroxidation of the photoreceptor membranes. The resulting retinal damage is similar to that noted in humans with macular degeneration. [7] Antioxidants including vitamins C and E, and the carotenoids lutein and zeaxanthin, are found in high concentrations in the retina. Vitamin C is found in aqueous portions of cells throughout the retina, vitamin E in the photoreceptor outer segment membranes, and the carotenoids throughout the retina, but especially concentrated in the macula. [7]

Antioxidants enjoy a symbiotic, mutual recycling relationship in the retina (see Figure 2). Vitamin E, the major lipid-soluble antioxidant in the retina, scavenges peroxyl radicals, yielding phenoxyl radicals which are then reduced by ascorbate, recycling vitamin E in the retina. Glutathione appears to be unsuccessful at recycling vitamin E in the absence of ascorbate. Dihydrolipoic acid enhances ascorbate's protective effect by regenerating it from dehydroascorbate. [8]

Although cigarette smoking has been linked to a higher-than-normal risk of developing AMD, a Finnish study, which examined the effect of vitamin E (50 mg/day), beta carotene (20 mg/day), or the combination in male smokers found no decreased incidence in AMD after 5-8 years. [9]

Epidemiological data suggests the importance of antioxidants in the prevention of macular degeneration. Individuals with low plasma concentrations of carotenoids and antioxidant vitamins were found to have an increased risk for AMD. [10] A study of 976 subjects found a high antioxidant index achieved by vitamins C and E and beta carotene was protective for AMD. While this study found high serum levels of these substances to offer protection, supplementation of vitamins did not seem to provide protection. [11]

Vitamin E

The study cited above on 976 subjects also found high serum levels of vitamin E (as a-tocopherol) alone was protective for AMD. [11] The Physician's Health Study evaluated 21,120 individuals over a period of 12.5 years, none of whom had a diagnosis of AMD at the beginning of the study. Supplementation with vitamin E resulted in a 13-percent reduced risk of AMD (which was statistically insignificant). [12] Users of a multivitamin demonstrated a statistically insignificant 10-percent reduced risk.

Rat retinal studies have found low levels of vitamin A coupled with low levels of vitamin E appear to contribute to retinal damage similar to that seen in macular degeneration. Depressed levels of vitamin A in the presence of a vitamin E-free diet led to a five-fold increase in lipofuscin granule deposits in the pigment epithelial cells, marked disruption of photoreceptor outer segment membranes, and significant loss of photoreceptor cells (rods and cones equally). When vitamin A levels were higher, the same retinal damage without the loss of photoreceptor cells was in evidence. The conclusion which can be drawn from this study is that vitamin A status plays a significant role in the extent of retinal damage caused by a vitamin E deficiency. [13] Retinal degeneration, with lesions such as photoreceoptor outer segment degeneration similar to that seen in human macular degeneration, has been reported in dogs with low serum vitamin E levels who were fed homemade diets deficient in vitamin E. [14]

Vitamin C

Ascorbic acid given to rats prior to exposure to intense light was found to protect against degeneration of retinal pigment and photoreceptors. Light contributed to a five-to-six-fold increase in phagosome density, a measure of light damage. Animals who received prior treatment with ascorbate did not demonstrate an increase in phagosome density. [15] Therefore, vitamin C appears to protect against some of the damaging effects of light exposure. However, the catch-22 is that vitamin C levels have been found to decrease in retinas exposed to light. [16] Another animal study found vitamin C slowed the light-induced loss of photoreceptor cells. [17]

Vitamin C appears to protect the eye from light damage via its antioxidant effects. Rats exposed to bright light experienced deterioration of the rod outer segments (an effect similar to that which occurs in human AMD). Supplementation with ascorbate prevented loss of rhodopsin and preserved DHA, preventing its oxidation. [18] In this study, the protective effect of vitamin C occurred only if given prior to light exposure.

Dietary Effects on AMD

The Beaver Dam Eye Study evaluated, via a food frequency questionnaire, the diets of 2,003 individuals age 43-84. The study found significant inverse associations between dietary intakes of carotenoids and vitamin E and the development of macular lesions ­ large drusen (subretinal pigment epithelial deposits) and other pigmentary abnormalities ­ consistent with future development of macular degeneration. [19] Other studies have also found intake of dietary carotenoids to be protective for AMD. Goldberg and associates at the University of Illinois, in analyzing the data from the first National Health and Nutrition Examination Survey (NHANES-1), found a negative correlation between intake of fruits and vegetables high in beta carotene and the development of AMD. [20] Seddon et al found a higher intake of spinach or collard greens was associated with a substantially lower risk for AMD. [21] Green leafy vegetables, as well as many other fruits and vegetables, are high in the carotenoid lutein which, as previously mentioned, is found in high concentrations in the retina, especially in the region of the macula.

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