When a Contact Lens Is The Healthier Choice
Learn about recent advances in ultraviolet radiation research and clinical applications.
By Jan P.G. Bergmanson, OD, PhD; James E. Walsh, MSc, PhD; Laura Vrazel Koehler, BS; Judy Harmey, PhD
Contact lens practitioners should not be shy about stressing the advantages of the products we prescribe. The contact lens has a number of beneficial attributes, and too often, we fail to point out these attributes to our patients.
As an example, recent research at the Texas Eye Research and Technology Center (TERTC), part of the University of Houston College of Optometry, has provided excellent evidence for the ultraviolet (UV) radiation protection offered by a contact lens when the lens material is formulated with a UV radiation blocker. What�s more, research has provided evidence that a thinner cornea, one that has undergone laser refractive surgery, for example, is a less effective UV filter.
In this article, we will discuss new scientific data on UV radiation and its clinical application.
First, let�s review why UV protection has become necessary to maintain ocular health and why it is an especially important consideration for eyecare practitioners.
Over the years, chemicals released by man, especially chlorofluorocarbons, have led to thinning of the ozone layer, a natural UV radiation filter. As the ozone thins, a greater proportion of UVA and UVB radiation can penetrate it. When a hole develops in the ozone layer, highly toxic UVC radiation has free access to the earth�s surface. What takes man only a short time to harm in our environment will take nature many years to fully recover. Scientists have projected a half century will pass before this occurs, if it ever does.1
There is little doubt UV radiation is one of our most serious and farreaching health challenges. Today, skin cancer is the most common cancer in the southern United States.2,3 At the University of Houston College of Optometry, we assessed ambient UV radiation intensities on the roof of our facility during two consecutive summers. Our findings revealed that radiation levels were unhealthy nine days out of 10.4
This man-made environmental change has the potential to increase the incidence of all UV-related eye diseases unless we become more vigilant in our preventive care. In rural regions with high UV radiation intensities where, as a result, the lifetime UV radiation dose is high, pterygia and cataracts are the most common ophthalmic diseases.5 A careful review of the literature shows that the only scientifically established risk factor associated with pterygium formation is UV radiation.
The combined epidemiology of UV-related ocular conditions is quite astounding when you consider the prevalence of pterygia, which may affect more than 20% of the population in some places on earth, with cataract statistics not far behind.5 Since this environmental health threat will be around for a very long time, we should consider changing our prescribing habits.
Inadequate Natural Response
When considering this toxic environment, we must examine human behavior and society�s awareness of the need to wear ocular UV-radiation protection. When the eye is exposed to the sun�s direct rays, the natural protection response is to squint. However, when we�re not directly facing the sun or when there is light cloud cover, the squint mechanism is reduced. Most people are unaware that up to 40% of ambient UV radiation is contained in the diffuse solar component. In addition, a laterally incident direct solar beam can be focused onto the nasal limbus without stimulating the squint mechanism. Not surprisingly, the nasal limbus is a common site of UV-related ocular diseases, such as pterygium and pinguecula.
Although helpful, UV-filtering sunglasses should not be our sole response to this ocular hazard because certain styles reduce the natural squint mechanism while allowing laterally focused UV radiation onto the nasal limbus. A UV-blocking contact lens will more efficiently reduce UV radiation and will do so at all times if worn all day, as is the case with most successful contact lens wearers. Unfortunately, refractive surgery patients, while no longer requiring contact lenses, may have a reduced natural UV radiation filter, a thinner cornea.
Ocular Protection Front and Back
The degree of protection offered by UV-blocking eyewear is not just a function of their design or the fact that they are placed over the eye. The spectral nature of the UV blocker they employ is also important. Solar UV radiation can be subdivided into three distinct wavebands, each with its own unique considerations.
� UVC (100-280 nm) is the most toxic waveband, but it is filtered by the ozone in the earth�s atmosphere, so if the ozone layer is not excessively damaged, UVC filtering does not have to be considered in eyewear design.
� UVB (280-315 nm) is present at toxic levels on the earth�s surface, and it is vital that this waveband is fully eliminated by eyewear UV blockers.
� UVA (315-400 nm) used to be considered safe for tanning salons. So safe, in fact, that it wasn�t considered in the development of skin protection and tanning lotions. This is no longer true. A good UV-blocking eyewear filter should eliminate all UVA radiation and possibly some of the blue light region, which is also thought to have a toxic contribution, especially to retinal receptor cells.
The principal measure of protection is the UV-visible transmission spectrum and related protection factor (see Figures 1, 2 and 3). The ideal transmission spectrum has its cutoff wavelength as close to the visible blue region as possible. The protection factor is essentially the inverse of the transmission summed up in the waveband of interest. It is similar to those calculated for skin-related sunblock, but the requirement is to maximize the value because it is undesirable to have any ocular UV radiation exposure. The eye cannot see UV radiation so, consequently, this radiation is useless to vision.
Just as people are aware of the relative protection factors of UV-blocking sunscreens, they also should be aware of the blocking efficacy of their eyewear, particularly when they live in tropical or subtropical zones. With most diseases, protection is the best form of cure, and this is particularly true for certain UV-related ocular conditions, such as pterygium, for which surgical excision is the primary treatment and recurrence and complication rates are high.
In addition to anterior segment protection from UV radiation, in which the cornea provides very limited natural protection, posterior segment protection can be enhanced. One option is contact lenses with a UV-cutoff wavelength closer to the visible blue than the crystalline lens, which is the primary UV-blocking ocular component. The crystalline lens itself also needs protection to prevent cataract formation, which is considered a more serious condition than those that occur on the ocular surface, such as pterygia. The relative importance of various UV-related ocular conditions is not an issue, however, if efficient blocking is provided.
The conjunctiva is as easily harmed by UV radiation as the cornea,7 and in UV keratitis, the sandin-the-eyes symptomology results from conjunctival trauma. As we mentioned previously, the most well-established UV-related conjunctival disease is the pterygium, for which the only scientifically known risk factor is UV radiation. This anomaly becomes more prevalent among people who live closer to the equator. In the southern United States, it may affect more than 10% of the population, 8 and in some high altitude areas of central Mexico, more than 20% will have pterygia.5 This high incidence makes it by far the most common ophthalmic disease.
In most people, a pterygium is a cosmetic problem that can affect quality of life. However, it can grow over the limbus and into the cornea, affecting vision, first by inducing astigmatism, ultimately covering the visual axis, and causing blindness. Surgery to remove a pterygium is not an easy procedure. Complications and recurrence rates are unacceptably high.
Pterygia manifest most often on the nasal side of the limbus along the horizontal meridian. The current theory on the etiology of the pterygium suggests that temporally and tangentially incident rays are focused by the cornea on the opposite side at the limbus. It has been calculated that this corneal focusing ability can concentrate light by a factor of 20. Coroneo and colleagues9 were first to propose this optical effect; and researchers at the TERTC have demonstrated this phenomenon in vivo.10 The effect of UV radiation focused in this manner is to harm vital stem cells in this region. The irradiated stem cells will mutate in the p53 gene, which initiates a cascade of events leading to pterygium growth. The goal of our research is to map the series of changes from the mutation among epithelial cells leading to the growth of mature pterygium, which shares many characteristics of a precancerous growth. Once we�ve achieved a more complete understanding of pterygium formation, we will be better equipped to search for ways to stop its advancement and eventually develop a nonsurgical method of treatment. Currently, prevention remains our best strategy.
The UV-blocking soft contact lens is uniquely able to protect the limbal and conjunctival stem cells by covering the regions populated by these cells. Thus, people who wear UV-blocking contact lenses all day are protected from sunup to sundown, while nonlens wearers typically do not wear sunglasses all day. What�s more, most sunglasses will not protect wearers from the tangentially incident rays, although wraparound sunglasses would offer some protection against peripherally incident radiation. Spectacles have the same limitations as sunglasses.
Although the literature suggests a connection with UV radiation, pingueculae have not been studied extensively and specifically for a UV radiation origin. Interestingly, pingueculae show the same predilection for a nasal presentation as pterygia.
UV-related corneal complications are well-documented. The association between snowblindness and photokeratitis/UV keratitis was uncovered by the Swedish ophthalmologist E.J. Widmark more than 100 years ago.
UV keratitis is an acute response to an abovethreshold dose, which leads to epithelial cell death but spares the nerve fibers.11 The survival of the nerve fibers leads to the significant pain associated with this condition. Cell death may be observed clinically as an area of distinct staining within the palpebral fissure. Erythema of the eyelids and face may be present as well, and a broad-spectrum, topical, prophylactic antibiotic should be prescribed.
Irrigation and patching of the eye also may be indicated, but if the patient wears non-UV-blocking contact lenses, patching is contraindicated.
Chronic exposure of the cornea to UV radiation leads to climatic droplet degeneration, which characteristically manifests within the palpebral aperture and affects people who spend considerable time outdoors. This condition may also be seen in the conjunctiva and may occur together with pterygium and pinguecula. Histopathologically, spheroids are located in the stroma and also involve the epithelium. Their content is proteinacious, but the precise composition is not known.
Literature has shown that the endothelium, which cannot regenerate, is vulnerable to UV radiation. Laboratory experiments have demonstrated the toxicity of UV radiation to the endothelium, and clinical observations have established polymegethism and corneal edema as part of the endothelial response to UV radiation.12-16
Recent research at the TERTC demonstrated that significant corneal UV radiation absorption takes place within the stroma. Previously, it was thought that most, if not all, corneal UV radiation absorption occurs in the epithelium.17 This notion probably stems from the fact that in photokeratitis, or snowblindness, there is a clinically obvious loss of epithelial cells, and there is full recovery with no obvious permanent loss of transparency. Since the corneal stroma is an important UV radiation absorber, it follows that thinning of this tissue will have a negative impact on the cornea�s ability to keep such radiation out of the eye. Conditions that lead to stromal thinning include keratoconus and pellucid marginal degeneration. In these cases, it may be beneficial to prescribe a UV radiation filter, preferably formulated in the matrix of a contact lens.
Perhaps more important, considering the number of patients affected, are individuals who have had LASIK, PRK or PTK. These patients have thinner stromas because the laser removes stromal tissue. These corneas will allow more and shorter UV radiation wavelengths through to the crystalline lens, thus increasing the risk for early cataract development. However, it will take years before we have the clinical data on the incidence of cataracts for patients who had excimer laser surgery. Some may argue that the burden of proof on this issue lies with the laser manufacturers and possibly the surgeons performing these procedures. We have not proven that excimer laser patients will develop cataracts at an earlier age. This is, however, a realistic concern, and we think it is prudent to advise our refractive surgery and PTK patients about the potential increased risk. We should also advise them about UV radiation protection.
Research into the risks of refractive surgery presents an outstanding opportunity for contact lens practitioners to promote some of the virtues of contact lenses. These devices actually protect the eye from UV radiation and potentially prevent early cataracts and other UV-related diseases. In contrast, excimer laser surgery renders a patient more vulnerable to complications caused by radiation.
The crystalline lens is the principal ocular UV radiation filter, and above-threshold UV radiation causes this tissue to lose its transparency, a condition we all know as cataract. If we live long enough, we all will get cataracts. It follows that the primary risk factor for cataract is age, but several other risk factors are worth mentioning. These include UV radiation, diabetes, certain medications, smoking and being female.18,19,20
It makes good sense to reduce the burden on the crystalline lens by minimizing exposure to the factors we can control, including UV radiation. The cortical cataract is the type most associated with UV radiation.21 Delaying cataract surgery is not only a personal decision but also a public health issue of significant proportions.
It has been projected that the ozone will deplete by as much as another 20% over the next 10 or 20 years, which would lead to an increase of cortical cataracts by 1.3% to 6.9%. This increase amounts to an additional 167,000 to 830,000 cases, at an estimated cost to society of $563 million to $2.8 billion.22 Add to this public health challenge the potential for early cataracts to occur in excimer laser patients, and we have a huge expense to cover in the foreseeable future.
The retina and the vitreous chamber are predominantly protected from UV radiation by the blocking power of the crystalline lens, which cuts off the high UVA beyond the cutoff wavelength of any currently available contact lens. However, because UV transmission through the crystalline lens is higher in the younger eye and because this population tends to expose itself to more UV radiation, it may be prudent to provide additional protection beyond that afforded by the crystalline lens. The requirements for a UVblocking contact lens that could achieve this would include a cutoff wavelength as close to or even into the visible blue region. This kind of prophylactic lens would provide nearly complete protection from the cornea to the retina.
The retina is easily harmed by UV radiation but, as mentioned, the crystalline lens normally provides protection. However, in the aphakic eye, this protection must be provided by other means. Typically, the implanted intraocular lens will offer protection, but in cases where the patient has not been given one, we must ensure that the retina is not exposed to UV radiation without adequate protection. A UV-blocking contact lens is the ideal solution for these patients.
Thwart the Threat
In our opinion, we are underselling the contact lens as a UV radiation filter. The most recent UVblocking contact lenses provide as good or better UV radiation protection than spectacle lenses. In the successful wearer, the lenses are on the eye all day and cover the vital limbal and conjunctival stem cells, which are essential to corneal health. Our research has shown that even in some of the most UV radiation-intense climates, the UV-blocking contact lens reduces exposure to safe levels.5
UV radiation causes cataract and is a threat to corneal and conjunctival health. Unfortunately, current literature and scientific and continuing education meetings are a poor reflection of this fact. Instead, it seems there is a preoccupation with grade 1 and 2 corneal staining and ocular redness, and matters relating to comfort. These are important performance measures for contact lenses, but they are hardly serious ocular health concerns. The industry may wish to advertise any advantages products have, but in clinical practice, we must not lose sight of the bigger picture. Stem cell damage, cataracts, pterygia and climatic droplet keratopathy are serious ocular health considerations that should not be ignored or overshadowed by lesser concerns.
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