No abstract

Oxygen is essential for the metabolism of retinal cells and thus for their functioning. Considered metabolically as a very active tissue, the retina is sensitive to hypoxic conditions. Even in health, the functional reserve of oxygen in the human retina lasts for only seconds.The function of the blood is transportation of nutrients and oxygen to the tissue; that function will be compromised in vascular pathology. Blood flow measurement is an interesting tool for assessment of haemodynamic alterations, although at present few techniques are applied in the clinical setting. Even more challenging for progress in ophthalmology research is direct evaluation of the metabolic supply and needs of the ocular tissue. Apart from the importance of oxygen for retinal metabolism, a direct pathophysiological role of oxygen in ocular pathology also deserves attention. Retinal hypoxia has been implicated in the formation of pathological new vessels. This neovascularisation is a major cause of visual loss in ischaemic retinal diseases, of which diabetic retinopathy is the most common.There is no current clinical method to quantify ocular hypoxia. Most investigations of the role of oxygen in the eye have been performed in animals, predominantly in the retina. Based on such data, distribution and consumption of oxygen in retinal tissue have been estimated using mathematical models of diffusion. In humans, only indirect evidence of the presence of retinal hypoxia has been obtained so far, either by measuring the size of intercapillary areas or by measuring oxygen tension during intraocular surgery.The recent developments in techniques to measure ocular oxygen justify an overview of this subject and of their contribution to our knowledge of the oxygenation in the normal eye. In addition, a link between oxygenation and disease is proposed at the end of this review, with the main focus on diabetic retinopathy, in which a pathophysiological role for oxygen seems more and more evident. Variables and techniques in oxygen studies MEASURED VARIABLESResearch on oxygenation can be classified in several ways.