Retinal Vessel Responses to Exercise and Hypoxia Before and after High Altitude Acclimatisation.

Computerised microdensitometry was used to study diameter changes in 93 arterial and 91 venous vessel sites in the retinas of four mountain climbers before and after spending seven weeks in the Himalaya mountains. The vascular response to short-term strenuous exercise was measured at sea level, to acute hypoxia of simulated altitude of 15,000 ft (4,572 m) at rest, and to strenuous exercise while acutely exposed to the same hypoxic condition. Before the mountain exposure, retinal vessels constricted during exercise -1.9% (arteries); -3.3% (veins) and dilated in acute hypoxia 9.4% (arteries); 8.1% (veins). Superimposed exercise on hypoxic dilated arteries reduced the vasodilation (9.4% to 6%). After acclimatisation, the same physical work load at sea level constricted the arteries more (-5.4% vs -1.9%) but did not constrict the veins (0.2% vs -3.3%). Superimposed exercise on hypoxic dilated vessels, excessively dilated both arteries (8.4% vs 6.0%) and veins (13.7% vs 8.4%), compared to changes seen before the mountain sojourn. This study shows that physical conditioning and long adaptation to hypoxia, significantly change the vascular response of the retina to physical activity both in normal atmospheric conditions and during hypoxic stress. High altitude retinal hemorrhages (HARH) were present in one climber, and the study may indicate why HARH is seen often in young and physical well-trained subjects.