One hundred and four healthy subjects (56 men, 48 women) with a median age of 21 years (range, 18–60) took part in the expedition. The tenets of the Declaration of Helsinki were adhered to and the research was approved by the Lothian Research ethics committee. At a pre-expedition meeting, informed consent was gained from subjects. No participant had a history of ocular disease. Measurements of IOP and CCT were always taken in the early afternoon, to minimize the effects of diurnal variation. Measurements were taken in one eye per subject under topical anesthesia with benoxinate. A hand-held tonometer (Tono-Pen XL; Medtronic-Solan, Jacksonville, FL) was used to measure IOP, because of its proven efficacy in field trials.
16 Corneal thickness was measured with an ultrasound pachymeter (SP-2000; Tomey, Nagoya, Japan). Both the tonometer and pachymeter were calibrated to ensure accuracy before each testing session. IOP measurements were repeated three times at each testing session (three averages of three) and CCT measurements were taken nine times at each testing session. Subjects were flown to La Paz (altitude, 3700 m), Bolivia, where they acclimatized for 4 days before being driven over 2 hours to the Cosmic Physics Laboratory at Chacaltaya (5200 m). Testing took place at sea level before and at least 1 month after all subjects had returned to sea level. Testing on the expedition took place on the first, third, and seventh days at the laboratory. After the seventh day, the subjects were driven back to La Paz (the ascent profile is shown in
Fig. 1 ). In La Paz (3700 m), subjects were advised to avoid any strenuous activity and were not allowed to descend below 2000 m. At the laboratory (5200 m), subjects were not formally restricted in their activity but were advised to rest for the first 2 days. Afterward, they were allowed to exercise moderately if they wished but were not allowed to descend below 5000 m. Cigarette smoking and alcohol consumption were prohibited for the duration of the expedition. Acute mountain sickness (AMS) scores were recorded using the Lake Louise scale.
17 Systolic and diastolic blood pressures (BP) were measured by one observer with a mercury sphygmomanometer. The subjects also took part in a double-blind randomized controlled trial to compare the efficacy of sildenafil and antioxidants versus placebo in preventing AMS. No other drugs were allowed unless one of the expedition doctors had given permission. None of the subjects was allowed to take acetazolamide, which is known to lower IOP. Twenty-four subjects did not consent to take part and 4 did not complete any testing sessions at altitude due to severe AMS, which left 76 subjects (40 men and 36 women) with a mean age of 22 years (range, 18–60, SD 5). Seventy-six (100%) completed the first altitude testing session; 70 (92%) completed both the first and second altitude testing sessions, and 63 (83%) completed all three altitude testing sessions. Data in each testing session was collected in only 53 subjects. Reasons for missing testing sessions are given in
Table 1 .