The next level of the retina to be described is the photoreceptor layer itself. The hundreds of millions of photoreceptors are generally organized with the rods or night-vision cells surrounding a 1-2 millimeter visual center composed of color-vision cells called cones. The cones are located throughout the retina but are concentrated in a depression called fovea.
Human, retinal photoreceptors, whether rods or cones, are lined with insulative phospholipids or fatty acids, primarily docosahexaenoic acid (DHA) which is dietary derived from omega-3 fatty acids. (Readers take note: DHA is NOT DHEA, the adrenal hormone that has received much attention in the news media recently.) Rhodopsin is the visual pigment responsible for night vision and each rhodopsin molecule is surrounded by approximately 60 phospholipid molecules, most composed of DHA. Far too much attention has been given to vitamin A in RP rather than the fatty acids like DHA. The current rationale for vitamin A supplementation among RP patients is based upon improvement in electroretinogram tests. Recently DHA was tested in animals and found to account for 21-35% of the variability in an electroretinogram. DHA deficiency is directly linked to the function of rhodopsin, the night vision pigment. However, too much DHA, or omega 3 fatty acids that are rich in DHA, may produce diminishing returns and even increase the oxidation of these fats (Lipids 1996; 31: 65-70).
It is interesting to cite the 1988 report of two RP patients with a skin disorder (Dariers disease) who experienced diminished night vision. Skin problems can occur if there is a deficiency, malabsorption or poor metabolism of vitamin A. However, these patients also exhibited brittle nails, a classic sign of fatty acid deficiency. Vitamin A and beta-carotene supplements failed to produce any improvement in visual function among these two subjects. The researchers surmised that there may be abnormalities in binding proteins for vitamin A among RP patients, but never explored the possibility of poor metabolism ore deficiencies of fatty acids (Brit J Derm 1988; 119: 397-402).
DHA levels are nearly absent among children with Zellweger syndrome. When DHA supplements were given to five Zellweger-syndrome children who were virtually blind before treatment, with in months liver function normalized and dramatic improvement in visual function was documented (Lipids 1996 31: S145-52). In one family with x-linked variety of RP, only the family members who had low blood levels of DHA were affected by the disease (Trans Oph Soc UK 1983; 103: 508-12). Humans who are deprived of fatty acids, or who do not adequately absorb ore metabolize fatty acids, exhibit skin problems (dryness), decreased visual acuity, electroretinogram abnormalities and frequent thirst and urination (Nut Rev 1992; 50: 21-29).
Regardless of the cause, a decreased supply of omega-3 fatty acids to the eye may contribute to retinal dysfunction, and perhaps to photoreceptor cell degeneration, if the interruption is prolonged (Retinal Degenerations, CRC Press, 1991, p. 157). The fact that DHA has been used to improve adaptation to night vision without vitamin A among dyslexic individuals increases the interest in supplemental fatty acids for all retinal disorders (The Lancet 1995; 346: letters section). Rodents intentionally fed a diet deficient in DHA exhibit impaired visual acuity in dim light. The rodent retinas had only 1% DHA in their retinal fats compared to 14% among animals fed a normal diet. The livers of the DHA-deficient rodents were also found to be devoid of vitamin A (Biochem J 1965; 95: 279-84).
Fatty acids known as prostaglandins (E1) have been administered intravenously and intramuscularly among macular disease patients with positive results (Klin Mbl Augenheilk 1989; 194: 282-84; Klin Mbl Augenheilk 1983: 182- 570-75).
Omega-3 fatty acids, particularly DHA within moega-3 oils, are also known to reduce inflammation caused by autoimmune disease (Health Effects of Fatty Acids, American Oil Chemists Society, 1991, 203-08). For RP patients with symptoms of rheumatoid arthritis, omega 3 fatty acids were recently found to effectively reduce inflammation in place of non-steroidal anti inflammatory drugs which may have serious side effects (Arth Rheum 1995; 38: 1107-1114). Since autoimmune antibodies have been found to be prevalent among RP patients (Oph 1991; 98: 602-07), the inclusion of omega-3 fatty acids in the diet should be emphasized for RP patients. The British Nutrition foundation estimated that the average intake of DHA is 800mg. Per day (Unsaturated Fatty acids, British Nut Found, Chapman & Hull, London, 1992, pp.152-63). The typical American diet is very low in DHA (less than 100-mg./per day).
If DHA-rich omega-3 fatty acids are lacking in the diet retinal cells will recycle these fatty acids, attempting to prevent their depletion. Omega-6 fatty acids may be substituted for omega-3 fatty acids in retinal cells (Inv Oph 1994; 35: 794-803), which is undesirable. The American diet is rich in corn oil and safflower oil which provide omega-6 fatty acids. Animals fed corn oil rich diets experience loss of DHA in the outer segments of their photoreceptors (Curr Eye Res 1984; 3: 1221-24). Man made fats are also a potential problem, namely the trans fats or hydrogenated vegetable oils.
It is interesting to note that fatty acid deficiencies among young animals and humans result in scaly, dry skin and diminished visual acuity. It has long been known that a lack of omega-3 fatty acids, which contain or convert to DHA, can stunt the development of the visual system among pre-term infants (Am J Clin Nut 1996; 63: 687-97). It has now been shown that the visual system of full-term infants who are given a formula that includes omega-3 fatty acids exhibit visual improvement when compared to infants who take a standard infant formula (Lipids 1996; 31: 115-19). Mothers milk contains DHA while infant formulas do not (Nutr & MD 1994; 20: 1-2). It is during the last trimester of pregnancy that the developing infant eye begins to acquire large amounts of fatty acids. Mothers are urged to eat cold-water fish, or if not available, take omega-3 fatty acids during pregnancy and while nursing their children or to provide formula-fed infants with omega-3 fatty acids. The importance of breast feeding and omega-3 fatty acids is underscored for families of RP patients. The consumption of fatty acid supplements should be accompanied by protective antioxidants.
Fish obtain DHA from phytoplankton they eat. Recommended daily consumption of DHA is not specified in the medical literature, but 400-1,000 milligrams per day seems reasonable based upon available dietary consumption data.
Fatty acids in the retina are subject to oxidation when exposed to light. Research studies suggest increasing antioxidants when consuming more fatty acids in order to protect vulnerable photoreceptors from oxidation.