Blinded by the Light:
The Effects of Excessive Light Radiation on the Eye
The eye is constantly in danger from atmospheric radiation. It doesn’t matter if it is summer or winter, sunny or rainy; the eye is always being affected by damaging ultraviolet and high energy visible light rays. Unfortunately, the effects of these rays are cumulative and irreversible. If left unprotected, the eye can be permanently damaged in a variety of ways, not the least of which may lead to blindness.
Ultraviolet wavelengths are an invisible part of the light spectrum for the human eye. Shorter than the visible wavelengths of the spectrum, UV vibrates at a frequency higher than the frequencies that people can identify as the color violet. Although there are many ways to classify the UV portion of the spectrum, for general purposes, the main classifications of UV are UVA, UVB, and
UVA are the closest rays to the visible portion of the light spectrum. Its rays range in wavelengths from 320 to 380 nanometers (nm). UVA completely passes through the ozone and atmosphere to reach the earth’s surface. Traditionally believed to be non-damaging to human tissue, new research suggests that it may contribute to skin cancer and eye disease. Dermatological research has determined that UVA indirectly causes cancer by creating free radicals that damage the DNA found in skin and results in melanoma as well as damage to collagen and vitamin A storage. The eyelids have the thinnest skin on the body. Therefore, melanoma of the lids can lead to life threatening cancer very quickly. This research is being utilized by the ophthalmic fields to determine the effects of UVA on the eye. Recent studies have suggested that it is linked to cataract formation and the development of macular degeneration, although this research is not yet conclusive.
UVB is the middle part of the UV spectrum with wavelengths between 290 and 320nm. UVB is the part of the UV spectrum that has the most documented proof of causing serious damage to the skin. UVB effects the skin by causing direct damage to the skin’s DNA. It breaks down the bonds in the double helix strand causing a mutation that can result in cancer. This is important because 10 percent of all skin cancers occur on the delicate skin around the eye. How it affects the eye itself is more debatable. Originally believed to cause cataracts and macular degeneration, it is now unsure if this is the case. It now appears that the cornea absorbs 100 percent of UVB rays. If this is the case, then it is not possible for UVB to pass through the cornea to reach the crystalline lens and retina. However, it does cause keratitis. This is essentially sunburn of the cornea.
UVC is the strongest segment in the UV spectrum with wavelengths shorter than 290nm. Fortunately, it is blocked by the ozone layer and does not usually reach the earth’s surface and therefore, it is not a concern for the eye care professional at this time. Unfortunately, the continued depletion of the ozone could lead to serious problems in the future and have in the past.
High-Energy Visible Radiation
High energy visible radiation, or HEV, is blue, violet and indigo light. It falls within the violet/blue band of the light spectrum with wavelengths from 380 to 530nm. These rays penetrate deep within the eye and cause damage to the retina. HEV damage is cumulative, usually showing up later in life, and is believed to be a primary cause of macular degeneration.
Eye Diseases that May be Related to Radiation
A pterygium is a triangle shaped thickening of the conjunctiva that infiltrates the clear surface of the cornea. The pterygium may or may not grow large enough to interfere with vision. Other symptoms seen with a pterygium are dry eye, irritation, redness, ocular fatigue and a foreign body sensation. Pterygia are most commonly seen in patients who live in dry, windy, sunny climates and are caused by UV radiation. It is believed that constant UV exposure may cause the callus like thickening as a way of protecting the exposed surfaces of the eye. Ptergia are always on the nasal side of the cornea because the side of the nose acts like a concave mirror and focuses the UV light on the nasal conjunctiva. Treatments include eye drops and, if needed, surgery. However, there is a high likelihood that it will grow back after surgery.
The pinguecula is very similar to the pterygium except that it does not infiltrate the cornea. Since it does not affect vision and is usually asymptomatic, pinguecula concerns are usually cosmetic. Usually gray, yellow, white or colorless, pinguecula are usually found on either the nasal or temporal sclera in the 3 o’clock or 9 o’clock position and is a result of UV causing a degeneration of the conjunctival tissue. In rare cases, it may become inflamed or enlarged. In these cases lubricating drops, anti-inflammatory drops or surgery may be an option.
Photokeratitis, also known as snow blindness or welder’s arc, is the result of intense UV exposure to the cornea and sclera. Similar to a sunburn, symptoms include pain, tearing, photophobia, eyelid twitching and pupil constriction. Symptoms usually resolve themselves without treatment within 20 to 72 hours; however, cool compresses, NSAID eye drops, and oral pain medications may ease pain and irritation while the eye heals. Limiting light, removing contact lenses, not rubbing the eyes and wearing sunglasses may be used to reduce the risk of further injury.
A cataract is any opacity of the crystalline lens. A cataract can be classified with a very minor opacity with clear portions of the lens remaining, to a full opacity with swelling of the lens. Early symptoms are a loss of visual acuity and contrast. Colors are less vivid and contours and shadows are more difficult to decipher. Patients may also see halos around lights. If left untreated, cataracts will lead to blindness. Cataracts are typically treated by removing the effected crystalline lens and replacing it with an intraocular implant (IOP). Although the causes of cataracts are multifaceted, excessive exposure to UV rays have been linked to cataract formation. In particular, the brown cataract and cortical cataract are caused by UV exposure.
Macular degeneration affects over 10 million Americans and is the leading cause of blindness in individuals over the age of 55. Macular degeneration, as the name implies, is a deterioration of the macula. The macula is the small central portion of the retina. The retina is the complex, multilayered, light sensitive tissue that is located inside the eye. Although the retina receives and sends all the visual images up the optic nerve to the brain, the highly specialized macula is responsible for clear, sharp, central vision.
Early stages of macular degeneration may be asymptomatic, especially if only one eye is affected. As the disease progresses, central vision becomes slightly blurry at both near and distance. Next, color and fine detail become harder for the patient to decipher and lines may begin to appear wavy. In advanced stages of the disease central vision becomes fuzzier and more shadowed until eventually blind spots may appear. There are many factors that increase the risk for macular degeneration: age, smoking, heredity, obesity, lifestyle, race, and light colored eyes. Prolonged sun exposure may be a contributing factor. At this time there is no cure for macular degeneration although there are treatment options.
Ounce of Prevention
It is important that lenses block 100% of the UV rays that can reach the eye. In order for some lenses to reach this level of prevention, some lenses need to have a UV coating applied to the lens. For example, a CR-39 uncoated lens will only offer 55 percent UV absorption. Therefore it is especially important that this lens have UV absorption applied to the lens. Some lenses however, do not need to have a UV coating added to the lens. Some lenses have inherent UV protection in the material. The ECP needs to be familiar with these lenses so that they can recommend the proper lenses for their patients based on their needs. These lenses include:
Polycarbonate- Polycarbonate is the most impact resistant lens material that was developed by the Gentex Corporation in 1979. A soft material, polycarbonate has a very hard scratch coat applied to both the front and back surfaces of the lens. Due to the safety features of this lens, polycarbonate is ideal for children’s eyewear as well as eyewear tailor made for sports or other dangerous activities.
Trivex- Trivex was developed for the military and introduced by PPG in 2001. It is a lightweight, mid-index material that is highly impact resistant.
Hi-index- The technical definition for high-index lenses is any lens with a refractive index higher than 1.523. Index is a measurement of the lens density which translates to how much light is bent by the lens material. The higher the number, the more the light is bent by the lens. What this means to the eye care professional and the patient, the higher the index, the less material that is needed to bend the light to fill the doctor’s prescription. This makes hi-index an ideal lens material for patients with high prescriptions.
Polarized- Polarized lenses are the best lenses available for sunwear and most outdoor activities. They are composed of a chemical film that is implanted in a clear lens. The chemicals within the film, usually poly-vinyl alcohol crystals, typically align themselves parallel to each other, creating a window blind effect that blocks wavelengths on one plane while allowing wavelengths on a plane 90 degrees away to pass through the lens. By actually filtering the light instead of just diminishing it, polarized lenses create a safer, more comfortable environment for the patient by eliminating glare from horizontal surfaces and also by reducing eyestrain, headaches and fatigue.
Photochromics- Photochromics provides UV protection and visual comfort to the patient. Depending on the manufacturer, a photochromic lens material is made by either having a photochromic dye mixed with a lens substrate prior to casting or imbibed into the lens after casting. The dye is then activated when exposed to UVA and UVB rays.
There are some basic guidelines when helping patients select a pair of sunglasses for UV protection.
To protect against HEV select a lens with copper, bronze or reddish brown coloring
Help the patient select a frame that fits close to the eye and offers as much facial wrap as possible for his or her prescription
If the lens is dark, it is imperative that there is enough UV protection. Darkness causes the pupil to open wider allowing more UV to enter the eye causing more damage if the lens is not properly coated
Inform the patient that it is important that he or she wear sunglasses even when wearing UV protective contacts. Sunglasses offer more protection because they surround the delicate skin around the eye as well as the sclera.
The fear of losing one’s sight is one of the greatest fears that people have. Since patients only have one pair of eyes, it is the duty of the eye care professional to offer them all the protection that is available. One of the easiest and most effective ways to do this is by providing them with UV and HEV protective eyewear. Not only will it make them more comfortable today but it may help protect their vision tomorrow.
With contributions from: Brian A. Thomas, P.h.D, ABOM