The eye at altitude.
High altitude retinopathy (HAR) was first described in 1969 as engorgement of retinal veins with occasional papilloedema and vitreous hemorrhage. Since then various studies have attempted to define the incidence, etiology and significance of this phenomenon, usually with small numbers of subjects. Recently studies on relatively large groups of subjects in Nepal, Bolivia and Tibet have confirmed that the retinal vasculature becomes engorged and tortuous in all lowlanders ascending above 2500m. Sometimes this leads to hemorrhages, cotton wool spots and papilloedema, which is the pathological state better known as high altitude retinopathy. These studies have also shown a significant change in both corneal thickness and intraocular pressure at altitude. The retinal blood vessels are the only directly observable vascular system in the human body and also supply some of the most oxygen-demanding tissue, the photoreceptors of the retina. New techniques are being applied in both hypobaric chamber and field expeditions to observe changes in retinal function during conditions of hypobaric hypoxia. This work allows better advice to be given to lowlanders traveling to altitude either if they have pre-existing ocular conditions or if they suffer from visual problems whilst at altitude. This especially applies to the effect of altitude on refractive eye surgery and results of recent studies will be discussed so that physicians can advise their patients using the latest evidence. Retinal hypoxia at sea level accounts for the developed world's largest cause of blindness, diabetic retinopathy. The investigation of retinal response to hypobaric hypoxia in healthy subjects may open new avenues for treatment of this debilitating disease.