Sunglasses and Lens Tints: It’s All about Protecting Your Eyes
Understanding and Avoiding Contact Lens Complications: Polymegathism, Neovascularization and the Endothelial Pump
The sun gives us light and supports all life on earth, but some of its rays pose threats to us and our vision. The primary danger from sunlight is Ultraviolet (UV) rays, which come in three types, UV-A, UV-B and UV-C. The last group, UV-C, are screened by the ozone layer, which is one reason we should all be concerned about our atmosphere; however, UV-A and UV-B rays pose a threat to our eyes in several different ways.
Even small doses of sun exposure over a period of many years is an important risk factor in the development of macular degeneration, cataracts, pterygium growth on the front of the eye and cancers of the conjunctiva and the skin of the eyelids and surrounding tissues.
Several other types of lens tints and coatings can provide different types of protection; which lenses recommended will vary depending on the conditions where they will be used and the type of protection desired. Some types of protection can be combined with others as well, as in polarized lenses with UV blockers; even some types of contact lenses can be very helpful in protection from sun damage.
Polarized lenses, sun tints and photochromic lenses that change in the light all reduce glare and make it easier to see in bright sunlight, but the most important factor in sun protection is blocking UV rays from entering the eye; even short periods of sun exposure can cause damage over a long period of time.
Some degree of UV protection is provided by all types of spectacle lenses, whether they are made of glass or optical plastic, with plastic being somewhat better than glass without additional coatings.
UV rays can contribute to several eye conditions that have the potential to affect vision.
Prevention of UV Damage
The best way to protect the eyes from damaging UV rays is to wear sunglasses, which should block 99% to 100% of UV-A and UV-B radiation.
If possible, choose a wrap-around style of sunglasses, which protect more of the eye. These also provide more protection from wind and dust. If you participate in active sports, choose sunglasses with polycarbonate or other high-impact resistant material.
Also, experts advise wearing a wide-brimmed hat to block sunlight from directly overhead. Use artificial tears if needed for extra lubrication and comfort.
Grey tints provide good colour discrimination by wearers, while some tints (like yellow-tinted shooting glasses, for example) distort colours significantly, possibly even making it difficult to tell the difference between traffic lights.
Most people are aware that prolonged sun exposure causes premature aging of the skin and risk of skin cancers, but the sun can also cause more short term effects like sunburn from a day at the beach without adequate sunblock. Short term exposure to excessive sunlight can also cause a condition called “photokeratitis”, sometimes called corneal sunburn. Just like we would protect our skin, we should also protect our eyes with UV-blocking sunglasses and a wide-brimmed hat. This is especially important for children, because they usually spend more time outdoors than adults do.
Photokeratitis is an inflammation of the outer covering of the cornea which is made up of specialized skin cells called the corneal epithelium. Symptoms include painful foreign body sensation, or a gritty feeling in the eyes, redness, light sensitivity and excess tearing. In photokeratitis, these symptoms will be about equal in both eyes and will resolve on its own, usually overnight, although there may be some residual effects the following day. If any of these symptoms appear in only one eye, or last longer without improvement, see an eyecare practitioner right away.
UV-A rays, over many years, can contribute to macular degeneration, which affects the center of the visual field and is one of the leading causes of functional blindness in people over age 65.
To help prevent macular degeneration, use good-quality sunglasses and a wide-brimmed hat should be worn outdoors. For more information about macular degeneration, see the articles on Age-Related Macular Degeneration (AMD or ARMD).
Treatment of macular degeneration includes injections of substances that reduce the growth of new blood vessels in and around the macula, as well as laser surgery to close off blood vessels already growing. However, once vision is lost, the vision loss is permanent; because AMD is usually noticed in one eye before the other, prevention of vision loss in the second eye becomes extremely important.
UV-B rays are harmful to tissues in the front part of the eye, one of which is the crystalline lens, located just behind the iris, which helps the eye change focus from far away to close up. As we age, this inner lens becomes thicker and more inflexible, making it hard to focus on close work, a condition called presbyopia.
As we age, exposure to the sun and its UV-B rays can cause the inner lens to become cloudy, making vision blurry and reducing colour vision and contrast sensitivity. This cloudiness in the lens is a cataract.
Cataracts are treated by removing the inner lens surgically and replacing it with an artificial implant known as an intraocular lens. Fortunately, this type of surgery is considered to be one of the safest of procedures being performed currently, with a very high success rate.
Pinguecula and Pterygium
The thin, transparent skin that covers the front of the eye and lines the insides of the eyelids is known as the conjunctiva. The conjunctiva covering the white part of the eye known as the sclera is susceptible to sun damage because it is the tissue encountered first by light entering the eye.
Prolonged exposure to UV-B rays can cause the conjunctiva to form swellings known as “pingueculas”, (pin-gwek-you-luh) which appear as small lumps of yellowed and/or thickened tissue in either or both corners of the eyes. (See Figure 1) Pingueculas are not harmful to vision, although they may affect comfort because they can interfere somewhat with the tears as they lubricate the eye.
A more serious inflammation of the conjunctival tissue located in the corners of the eyes are called “pterygiums” (tur-ij-ee-ums), which can affect vision because they grow and can cover the corneal tissue, which can cause it to become distorted. Pterigiums usually begin as small lumps of tissue, and sometimes begin to grow from an earlier pinguecula. They will usually develop a blood supply, and grow over time. Most pterygiums appear in the nasal area of the white of the eye, although they can be located in the outer corner as well.
People in tropical climates are especially susceptible to the development of pterygiums because they have more UV exposure than people living further north or south of the equator. Anyone, even children, can develop pingueculas or pterygiums if they are outdoors frequently without protecting sunglasses and a wide-brimmed hat.
Treatment for a pterygium is to use lubricating eyedrops or ointments, with occasional use of vasoconstrictor drops (that “get the red out”) like Naphcon A or Visine. Short term use of steroid drops to calm the inflammation are sometimes helpful as well.
If the pterygium continues to grow, surgical removal may be required; the most successful type of surgery uses a graft of other conjunctival tissue to cover the area of the eye where the pterygium was removed. Without such a graft, the surgery has a recurrence rate of almost 50%, but using the graft reduces this to 5 to 10%. Almost all recurrences happen within the first year after surgical removal, so patients are followed carefully during those twelve months.
Lenses that change with the light are known as photochromic lenses, and are available in either glass or plastic versions. These lenses are perfect for those who want the convenience of prescription sunglasses combined with their clear glasses they use indoors. The lenses contain millions of light-absorbing molecules that react to the UV rays from the sun, to darken them when they come into the light and return to transparency when indoors. The lenses will darken within about 30 seconds of being outside and will lighten when brought indoors over about 5 minutes.
One drawback of photochromic lenses, however, is that most modern automobiles have UV blocking coatings on their windshields and windows, so photochromic lenses don’t work well when inside a car. Even with the windows rolled down, the amount of UV light getting to the lenses and triggering them to darken is reduced. Additionally, if an indoor area is lighted with full-spectrum bulbs, these may cause the lenses to darken.
Mirrored coatings, made by bonding a thin layer of vaporized metal to the surface of the lens, reduces the amount of visible light that reaches the eye, but does not block UV. These coatings do reduce glare significantly, but do not block UV. They are available in a wide variety of fashion colours.
Plastic spectacle lenses may be dyed with various colours, but the lens dyes do not block UV light either, although they will reduce glare for increased comfort in bright conditions. For appearance purposes, light tints may provide desirable effects; as an example, a light pink or brown tint may appear to warm skin tones, while light blue or grey may give a slightly cooler feeling to observers.
Fashion colors may be combined with UV blockers for sun protection.
One of the most popular lens treatments is polarizing, which allows only vertically-oriented light rays to enter the eye, dramatically reducing glare from horizontal surface reflections such as seen in boating, skiing, golfing, bicycling or fishing.
The sun’s rays enter the atmosphere and are reflected off particles in the air so they travel in all directions and are reflected off surfaces in the same way. Light striking flat surfaces such as paving or a lake are largely reflected in a horizontal or sideways orientation, which causes glare and discomfort.
A chemical film applied to the lenses is the most common form of polarizing; the film is applied during the manufacturing process. Polarization is available in combination with single-vision and multifocal lenses, either flat-top styles or progressives.
It is important to note that polarizing itself does not protect the eyes from UV radiation, but can be combined with lens types that do provide that protection.