[Oxidative tissue damage]

Oxidative stress is defined as an overflow of oxidative metabolites either in the human body or in a compartment of the body. Today this chemical definition has been slightly modified and encompasses an elevation of oxidative metabolites or a relative deficiency of anti-oxidants. Molecular oxygen is the basis of many highly reactive oxidative species which are able to directly damage or lead to the generation of secondary reactions which then initiate oxidative processes. The cell has established numerous mechanisms and strategies to antagonise those oxidative processes at different steps. Many diseases have been shown to be either related to or even be initiated by oxidative processes. The eye is at high risk to be damaged by oxidative mechanisms. One major reason is its permanent exposition to oxidative stimuli. The biochemical composition of ocular structures, especially that of the retina (unsaturated fatty acids), is an important factor making the eye more susceptible as compared to other organs. Ocular ischaemia, ischaemia or hypoxia of the retina, diabetic retinopathy and glaucoma are important disease entities that are initiated or propagated by oxidative processes. Ischaemic processes lead to classical reactions of the oxidative pathway. This is no longer believed to be the case in diabetic retinopathy. Here, advanced glycation end products (AGE's) and related species are able to induce oxidative reactions and the expression of growth factors. In age-related macular degeneration, photodynamic processes that already occur in childhood are believed to be a major factor contributing to the pathogenesis of the disease process. In addition, the expression of growth factors and new vessel growth can be initiated via inflammatory reactions or oxidative metabolites.