Background/aims The aim of this study was to investigate a change in visual acuity and contrast sensitivity (CS) during high altitude exposure in healthy subjects due to the effects of hypobaric hypoxia. This study is related to the Tübingen High Altitude Ophthalmology study.

Methods Visual acuity and Weber CS were tested monocularly using the Freiburger Visual Acuity and Contrast Test under standardised conditions in 14 healthy subjects at high altitude at the Capanna Margherita (4559 m, Italy) and compared with baseline measurements in Tübingen (341 m, Germany). Intraindividual differences between baseline and follow-up examinations were calculated by multivariate analysis of variance for repeated measures. Clinical parameters of peripheral oxygen saturation (SpO₂) and heart rate (HR) as well as scores for acute mountain sickness (AMS) were correlated to psychophysical tests by Pearson’s correlation coefficient.

Results A significant decrease in CS with a mean effect size of −0.13 logCS was found for Weber CS (day 1=−0.16±0.22, p=0.01; day 2=−0.10±0.2, p=0.049; day 3=−0.12±0.19, p=0.03) at high altitude compared with baseline. Visual acuity remained unchanged. Decreased CS correlated with SpO₂ (r=0.53, p=0.046) but not with HR (r=− 0.16, p=0.59) and occurred irrespective of AMS at high altitude.

Conclusion High altitude exposure leads to decreased CS. Changes occur independent of AMS. This finding is of clinical importance to trekkers and mountaineers exposed to high altitude as visual processing in particular under mesopic conditions at dusk and dawn is altered. Furthermore, it provides novel insight into hypoxia related changes in CS function.