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Sunlight and Common Eye Diseases- What does the evidence tell us?
Dr Jai Panchapakesan MBBS BSc(Med) Hons, PhD, FRANZCO

With the proliferation of crime and evidence based TV shows such as CSI and Law and Order we can all get a feel of what it is like to put small bits of evidence together to unravel a complex case. Often the most obvious culprit is not always the most guilty. So if we think of sunlight as our defendant I would like to run through the following charge sheet.

Eyelid Related Lesions
There is very strong evidence relating squamous cell carcinoma, basal cell carcinoma, and melanoma to sunlight exposure. Each of these lesions is very treatable in their early stages so it is vital that we take the time to look as part of our routine ophthalmic examination. The case against sunlight as a causative factor in ocular melanoma is less strong however it may just be the very low prevalence of this disease that makes the gathering of evidence difficult.

The overall prevalence of pterygium in Australia is 1.1%. The Blue Mountains Eye Study (BMES) found that this rate increases to 7.3% of all Australians aged over 49 years. Queensland has the highest rate of pterygium removal in Australia and the cost to taxpayers is just under $10M per year.

Epidemiologic studies are the circumstantial evidence of the eye research world. The BMES is an epidemiologic study that found that pterygium was much more common in people whose skin showed evidence of sun damage, while an epidemiologic study in Barbados went the next step to show that amongst those who had high occupational exposure to sunlight, the wearers of sunglasses or prescription spectacles had a lower rate of pterygium, providing evidence for the role optical appliances play in defeating pterygium. The Chesapeake Bay Waterman study is another widely regarded epidemiologic study of the role of UV light in eye disease. This study found that all visible light and blue light exposure were significant risk factors for the development of pterygium.

Although the circumstantial evidence places sunlight at the scene of the crime we still don’t have the equivelant of a murder weapon. In research terms this is a causative mechanism. Laboratory studies have shown that ptreygium epithelial cells, when exposed to UVB, show increased expression of matrix metalloproeinases (MMP,) an enzyme that is being increasingly linked to tissue melting in the cornea. Other studies have shown that epidermal growth factors (EGF) in pterygium are also increased when the cells are exposed to UV light. It may be that the MMPs break down the barrier to passage of the pterygium at the limbus while the EGFs provide the stimulus to growth across the cornea.

The relationship between cataract and sunlight exposure is one of the most widely studied relationships in ophthalmology. The majority of our evidence is, however, strong epidemiologic (circumstantial) evidence.

The BMES and its sister study in the US, the Beaver Dam Eye Study (BDES) found an increased rate of cortical cataract in the inferonasal quadrant of the lens. The inferonasal quadrant of the lens has been shown by researchers at the University of NSW to be the anatomical quadrant of the lens to receive the most reflected light up from the cheek and nose on top of its share of direct or un-reflected light. Thus the quadrant of the lens that is receiving the most light is the one that is most likely to cortical cataract, hmm very suspicious. The Chesapeake Bay study went further to identify the guilty culprit. By assessing individual exposure rather than group exposure and by detailing the exposure to all light, UVA and UVB separately this study was able to further identify UVB rather than all sunlight as the guilty culprit.

All of the above evidence however is circumstantial only, are there any causative mechanisms that may clinch the conviction? There is quite little laboratory evidence looking at the biological mechanisms behind cataract formation. This is largely due to destruction of evidence….by phacoemulsification (removal of the cataract). Since the advent of phacoemulsification, there has been little human lens material available for study. Compounding this is the lack of good animal models for human cataract.

Age-Related Maculopathy (ARM)
ARM is the commonest cause of blindness in the Western world and so any evidence regarding the culprits behind its cause is extremely important.

The BDES found that those subjects who had spent significant time outdoors during the period from their teenage years to their thirties were more likely to develop ARM over a five year follow-up period. Of great importance to us is that the use of sunglasses and hats during this younger period provided a slight protective effect. However when the typical skin and lifestyle markers of sun exposure were examined there was little other evidence to support the association of sunlight with early ARM. The Chesapeake Bay study looked at the individual roles of UVA and UVB in the development of ARM and found no evidence to link either of these with the crime scene.

Are there any other suspects? There are numerous other suspects whose role in the development of ARM is not completely understood. These include the role of anti-oxidants, systemic vascular disease, and local vascular growth factor mediators such as vascular endothelial growth factor (VEGF). It may be that it requires an interaction between different suspects to cause ARM.

There is strong evidence to convict sunlight of causing pterygium, and UVB of causing cortical cataract Although sunlight has not been convicted of causing ARM it is likely that evidence will be found to show it played at least some role in the crime.

I feel there is a strong case to be made for the benefits of wearing sunglasses. Even if sunlight was only found to be responsible for 1/20 cases of ARM this is still a huge number of Australians every year who would retain better sight for longer. The provision of UV protective lenses and now intra-ocular lenses with UV protection are important steps in reducing the burden of not only pterygium, cortical cataract, and PSC, but perhaps more importantly, their widespread use may be making a small but important dent in the rates of ARM in our community.

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