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PUPIL OCULOMOTOR DYNAMICS ARE USED as an indirect measure of neurological function. We previously reported a reduction in pupil aperture change (PAC) and constriction velocity (CV) within 1 hour of arrival at high altitude (3450 m and 4770 m), with these changes reverting to normal over subsequent days (Wilson et al., 2008). We here extend these observations as part of the Caudwell Xtreme Everest expedition (ascent profile and methodology as previously described (Levett et al., 2010).

Pupil dynamics were measured in both eyes using a handheld ForSite digital pupillometer,(Neuroptics, Irvine, USA) at 75m (London), within 2–3 days post arrival, during the morning, at 3500 m (Namche Bazaar) and 5300 m (Everest Base Camp). Of 222 possible subjects, confounding medical conditions or missing measurements left 174 subjects' data being eligible for analysis. No subjects took medication for high altitude illness at the time of, or prior to,