The effect of hypoxia on the central light reflex of retinal arteries and veins

Computerized scanning microdensitometry was used on fundus photographs of 40 men (aged 18-23 years) and effects of hypoxia on intensity profiles of the 'light reflex' in 140 arteries and 138 veins were analysed. Changes from sea level to four levels of acute hypoxia of simulated altitude were studied: 8000 feet (ft) (2438 meters) (m), 10,000 ft (3048 m), 12,500 ft (3810 m), and 15,000 ft (4572 m) using a low-pressure chamber. The intensity of the reflex increased at 12,500 ft and 15,000 ft by 24% (P less than 0.05) and 39% (P less than 0.01) in arteries and 50% and 33% in veins (P less than 0.05), respectively. As an overall response from sea level to all four altitudes, the width of the reflex was reduced. The reduction in width was negatively correlated to the diameter of the arteries (P less than 0.0001) and veins (P less than 0.001) at sea level, and the variability of changes was larger in veins (P less than 0.01). Funduscopic light reflected from intravascular blood columns of retinal vessels is thus influenced by hypoxic stress, and 12,500 ft of simulated altitude represents a threshold value of circulatory interest.