Introduction
Cataract is an ocular condition in which there is affectation of the lens whereby it becomes covered in an opaque film subsequently affecting the transmission of light rays that is meant to be focused on retina for visual perception. The development of this condition is usually multifactorial in origin but this article will discuss the role of the ultraviolet radiation in the pathogenesis of cataract.
UVR and Cataract
Several researches has indicated an increase in prevalence of cataract in tropics especially those region with very high amount of annual and daily sunlight, where there is lowered ambient temperature or regions with low latitude (Coleman et al 1988). This increased prevalence of the condition in those locations as been attributed to the high level of ultraviolet and infrared radiation and visible light, which are all important in the pathogenesis of cataract.
Data from several experiments has shown the relationship of the UV radiation with the development of cataract. Both ecological and epidemiological factors has shown that increased exposure to the UV increase the risk of developing cataract while experimental studies conducted on some animals also supported this conclusion. A research conducted by Paul Dolin (1994) concluded that there is strong association between exposure to UV and the development of cortical opacities.
Though, the actual mechanism by which the ultraviolet radiation causes opacification of lens is yet to be made clear, but there are several theories and possible suggested mechanism by which ultraviolet radiation especially the UVR A & B types take part in the pathogenesis of cataract. UVR-B type has been found to cause tryptophan degradation and damage to the epithelial lining of the lens has been suggested. Tryptophan degradation is thought to cause a decreased level of ATPase enzyme and increase level of free radicals while the damage to the epithelial lining has been attributed to result in problem with the calcium balance in the membrane (Dillon 1995).
Increased the level of free radicals being produced by tryptophan degradation has been found to cause lipid peroxidation and protein modification (Kuijk 1991). This changes results in alteration of membrane permeability which cause cells to malfunction and eventually die. Another major product of the lipid peroxidation is the aldydic products which has also been found to be very reactive and combine with the amino and sulphydril groups of protein, thereby causing retinal damage (Kuijk 1991).
UVR-A has been found to transverse cornea more effectively than the UVR-B and this was considered to be a contributing factor, which will favour the role of UVR-A in the pathogenesis of cataract (Azzam, Levanon & Dovrat 2003). This type of ultraviolet radiation has been found to cause more damage compare to the UVR-B but on the other hand, small amount of the UVR-B is actually needed to cause more damage simply because it’s more energetic compare to the former.
UVR-A has been favoured to cause an increase level of some brownish pigments which in turn fascilitate the absorption of more radiation subsequently resulting in more damage due to increase photooxidation and destruction of various amino acids that are meant to prevent the aging processes occurring in the lens(Azzam, Levanon & Dovrat 2003). This type of radiation also cause swelling and vacuolation in the anterior suture system and increased in the interfibrillar distances of the structural proteins in the lens(Azzam, Levanon & Dovrat 2003)