Lawrence J. White

Refractive Changes at Extreme Altitude After Radial Keratotomy

PURPOSE We studied the effects of altitude on four corneas that had undergone radial keratotomy and four normal corneas exposed to increasing elevation during a high-altitude excursion. METHODS We measured visual acuity, cycloplegic refraction, keratometry, and intraocular pressure at sea level and after 24-hour exposure to 12,000 and 17,000 ft. RESULTS We observed a significant increase in spherical equivalence (hyperopic shift) in radial keratotomy eyes exposed to altitude as compared to controls (P < .0001). The average change in spherical equivalent cycloplegic refraction from sea level to 12,000 ft was 1.03 +/- 0.16 diopters and from sea level to 17,000 ft was 1.94 +/- 0.26 diopters. We also observed a significant decrease in keratometry values at altitude as compared with control corneas (P < .0001). The average change in keratometry from sea level to 12,000 ft was 0.59 +/- 0.19 diopter and from sea level to 17,000 ft was 1.75 +/- 0.27 diopters. CONCLUSIONS Although the specific origin of these changes is open to question, we hypothesize that hypoxic corneal expansion in the area of the radial keratotomy incisions may lead to central corneal flattening and a hyperopic shift in refractive error. The cornea that has undergone radial keratotomy appears to adjust constantly to changing environmental oxygen concentration, producing a new refractive error over a period of 24 hours or more. Additional study is required to identify with certainty the specific origin of the hyperopic shift at high altitude.

The etiology of refractive changes at high altitude after radial keratotomy: Hypoxia versus hypobaria

OBJECTIVE Refractive changes at high altitude that occur after radial keratotomy (RK) may be caused by hypoxia or hypobaria. DESIGN A prospective study was performed to evaluate the effects of hypoxia on RK and non-RK corneas. PARTICIPANTS There were 20 RK and 20 control eyes. INTERVENTION These eyes were subjected to ocular surface hypoxia using an air-tight goggle system at sea level for 2 hours. MAIN OUTCOME MEASURES Keratometry, cycloplegic refraction, and pachymetry were evaluated using repeated measures analysis of variance. RESULTS A significant hyperopic shift (P < 0.0001) and corneal flattening (P < 0.0013) occurred in all subjects with RK compared with those of control subjects. Corneal thickening occurred symmetrically in both groups. CONCLUSIONS These results suggest that refractive changes in subjects with RK occur at high altitude as a direct result of corneal hypoxia.

Effects of simulated high altitude on patients who have had radial keratotomy.

BACKGROUND Previous studies documented diurnal myopic shifts in patients who have had radial keratotomy (RK). Recently, hyperopic shifts in these patients exposed to high altitude have been reported. A direct mechanical effect of reduced barometric pressure on surgically altered corneas has been theorized to cause this hyperopic shift. Another hypothesis implicates the effect of hypobaric hypoxia on the RK incisions. The authors examined the effect of a 6-hour exposure to decreased barometric pressure on 14 normal and 18 RK corneas. METHODS Cycloplegic refraction, keratometry, corneal pachymetry, and tonometry were performed on seven control subjects and nine patients who have had RK. Measurements were obtained over 8 hours at sea level on day 1 of the study. Measurements were repeated on day 2 which included a 6-hour exposure to 12,000 feet simulated altitude in a hypobaric chamber. Results were compared between subjects and control subjects to determine the effect of a 6-hour exposure to decreased barometric pressure. RESULTS There was no statistically significant difference in refraction or keratometry readings between control subjects and subjects who have had RK. Central corneal thickness decreased in the afternoon in RK eyes compared with control eyes. There was no clinically significant difference in intraocular pressure between subjects who have had RK and control subjects. CONCLUSIONS A measurable hyperopic shift in RK corneas exposed to high altitude requires more than 6 hours to develop. A direct effect on corneal shape due to barometric pressure alone should produce a sudden change in refractive error. This study supports the hypothesis that a slow metabolic process is responsible for the previously documented hyperopic shifts induced by altitude. However, a barometric pressure effect requiring more than 6 hours to occur cannot be ruled out with the methodology used in this study.

Refractive changes caused by hypoxia after laser in situ keratomileusis surgery

OBJECTIVE To determine whether hypoxia induces refractive changes in subjects who have had laser in situ keratomileusis (LASIK) refractive surgery. DESIGN Prospective paired eye clinical trial. PARTICIPANTS There were 20 LASIK subjects (40 eyes) and 20 myopic non-LASIK controls (40 eyes). INTERVENTION Each subject had one eye exposed to ocular surface hypoxia (humidified nitrogen) by use of an airtight goggle system at sea level for 2 hours. The other eye was simultaneously exposed to humidified, compressed air (21% oxygen) with the same airtight goggle system. MAIN OUTCOME MEASURES Keratometry, cycloplegic refraction, and pachymetry were compared before and after exposure by use of repeated measures analysis of variance. RESULTS A significant myopic shift (P: < 0.01) occurred in LASIK corneas exposed to hypoxia compared with myopic control subjects. A significant increase in corneal thickening occurred symmetrically in both LASIK and control subjects exposed to hypoxia. There was a trend toward corneal steepening (keratometry) in LASIK subjects, but this was not statistically significant. CONCLUSIONS These results suggest that ocular surface hypoxia induces a myopic shift in LASIK subjects.

The Ascent of Mount Everest Following Radial Keratotomy

Several articles have described visual changes in radial keratotomy (RK) patients exposed to high altitude.1–4 Great notoriety was associated with the deleterious visual changes suffered by Beck Weathers, MD, during his disastrous attempt to climb Mt Everest following bilateral RK surgery.5 While ascending Mt Everest in 1996, some years after successful RKs, Dr Weathers suffered visual changes that worsened with increasing altitude. Poor vision at approximately 27 000 feet rendered him unable to safely ascend or descend. Left alone, he was forced to remain overnight at this altitude until he was finally able to blindly stumble into a high base camp. Although he survived the incident, he lost both hands to frostbite. In this brief report, we will describe the visual changes experienced by an RK climber who was able to successfully ascend Mt Everest in 1999, and we will briefly review the literature on this subject. In 1985, a 33-year-old male mountain climber underwent 20 and 16 incision RK in the right and left eyes, respectively, for the correction of 5.25 sphere and 3.75 to 1.00 75 diopters of myopia. In the initial years following RK, although he climbed to altitudes near 20 000 feet, he noted no visual difficulties. In his early 40s, he found it necessary to use 1.25 diopter reading glasses for near vision after staying overnight above 15 000 feet. These were no longer needed following descent below 5000 feet. A few years later, while in his mid 40s, he noticed a decrement in distant vision as well as near vision following exposure to altitudes above approximately 20 000 feet. He described this as a fuzziness to distant objects, which cleared slightly when straining his eyes. Again, both distant and near vision returned to normal following descent to below 5000 feet. Because of his visual changes at high altitudes, he sought consultation with

Analysis of anterior and posterior corneal curvature changes using Orbscan technology in radial keratotomy eyes exposed to hypoxia

Purpose: To study the changes in anterior and posterior corneal curvatures in radial keratotomy (RK) and normal eyes exposed to hypoxia using Orbscan (Bausch & Lomb) technology. Setting: Department of Ophthalmology, Madigan Army Medical Center, Tacoma, Washington, USA. Methods: At sea level, 11 RK subjects and 10 control subjects were exposed to ocular surface hypoxia in 1 eye by filtering humidified, compressed 100% nitrogen (0% oxygen) through an airtight goggle system for 2 hours. The contralateral eye was exposed to humidified, compressed air (21% oxygen) simultaneously through the airtight goggle system. Orbscan analysis was performed in each subject before and immediately after gas exposure. This analysis included measurement of the anterior axial and posterior axial corneal curvatures as well as corneal pachymetry. Results: In the RK eyes exposed to 100% nitrogen gas, there was a statistically significant flattening in the anterior axial corneal curvature (P = .0012) and in the posterior axial corneal curvature (P = .0067). Radial keratotomy and control eyes exposed to air and control eyes exposed to 100% nitrogen gas demonstrated no statistically significant change in the anterior axial or posterior axial corneal curvature. Conclusions: Corneal hypoxia induced a statistically significant flattening in the anterior axial and posterior axial corneal curvatures in eyes that had had RK. There was no significant change in these curvatures in RK eyes exposed to air and in control eyes exposed to air or 100% nitrogen gas.

Refractive Surgery Safety at Altitude

Over the years, many climbers and other visitors to high altitude have been attracted to refractive surgery in hopes of lessening their dependence on glasses. Although there has been a gradual refinement of these procedures over time, they continue to have the potential for visual changes with altitude exposure. The goal of this review is to provide guidance to clinicians as to how to better advise their patients on the pros and cons of these procedures.