Effects of refractive surgery in extreme altitude or space.

The stability and function of a normal healthy visual system can be influenced by circumstances such as great depth or high altitude. As refractive surgery changes the properties of the cornea and the lens, concerns about refractive or intraocular lens (IOL) procedures in humans exposed to extreme situations have been raised. Early in the 1990s, it was reported that incisional refractive surgery could influence vision in higher altitudes, eg, when radial keratotomy (RK) had been performed in mountain climbers or hikers. An acute hyperopic shift in subjects who had radial keratotomy (RK) and then experienced altitude exposure has been reported and observed at altitudes as low as 2744 m (9000 feet). A dramatic example of this phenomenon was the experience of Dr. BeckWeathers in the Everest tragedy ofMay 1996, in which 8 climbers also lost their lives. Dr. Weathers had bilateral RK years before the expedition. He noted a decrease in vision starting early during his ascent. Author Jon Krakauer wrote that “.as he was ascending from Camp Three to Camp Four, Beck later confessed tome, ‘myvision had gotten so bad that I couldn't see more than a few feet.’” This decrease in vision forced Dr. Weathers to abandon his quest for the summit shortly after leaving Camp Four and nearly resulted in his death. Another report describes 2 expert climbers who experienced hyperopic shifts of 3.0 diopters or more during altitude exposures of 5000 m (16 400 feet) or higher on Mount McKinley and Mount Everest. One report noted no refractive change after 6 hours in post-RK eyes at a simulated altitude of 3659 m, suggesting that the hyperopic shift requires more than 6 hours to develop. There is strong evidence that the effect of altitude exposure on post-RK eyes is caused by hypoxia rather than hypobarism and that breathing a normoxic inspired gas mix will not protect against the development of hypoxic corneal changes. There is compelling evidence for myopic mountaineers that excimer laser sugery instead of RK is their refractive surgical procedure of choice. Individuals who have had RK and plan to undertake an altitude exposure of 2744 m (9000 feet) or higher while mountaineering should bring multiple spectacles with increasing plus lens power. However, excimer surgery behaves differently. Mader et al. have shown that surface ablation with the excimer laser (photorefractive keratectomy [PRK]) did not lead to corneal flattening