The effect of simulated altitude on vision in military personnel following small-incision lenticule extraction

Abstract: INTRODUCTION: The United States (U.S.) Military has been performing laser refractive surgery to enhance combat readiness, operational capability, and the quality of life for service members since fiscal year 2000. The latest laser refractive surgery adopted by the U.S. Military is small-incision lenticule extraction (SMILE) (Carl Zeiss Meditec AG, Jena, Germany), a type of keratorefractive lenticule extraction (KLEx). However, a limited number of studies have been conducted to determine whether varying levels of altitude have any effect on vision following the procedure. The purpose of this study was to evaluate the effect of decreased barometric pressure and atmospheric oxygen tension (simulated increased altitude up to 22,500 ft) on refractive stability in patients who have undergone bilateral SMILE. MATERIAL AND METHODS: Study subjects included active duty military personnel who had undergone bilateral SMILE using the VisuMax femtosecond laser (Carl Zeiss Meditec AG) for the correction of myopia. After being cleared to participate in the study by a military flight medicine physician, each participant underwent altitude chamber training. Uncorrected visual acuity (UCVA), 5% low contrast (LC) UCVA, refractive error, corneal thickness, and anterior chamber volume and depth were each measured at ground level, 10,000 ft, 15,000 ft, 18,000 ft, and 22,500 ft simulated altitude in a hypobaric chamber. Mixed-effects linear regression models were used to evaluate the statistical significance of changes in individual outcome measures across different altitudes. RESULTS: Data from 34 eyes (17 participants) was collected during the study. A statistically significant increase was found in UCVA (-0.04±0.01 LogMAR; P<.001) and LC UCVA (-0.05±0.01 LogMAR; P<.001) as simulated altitude increased from ground level to 22,500 ft. A myopic shift (-0.14±0.05 D; P=.050) was observed at 22,500 ft, as well as decreases in corneal thickness at central (-3.94±0.83 µm; P<.001) and thinnest point (-3.65±0.90 µm; P<.001). There were no statistically significant changes in mean keratometry, anterior chamber volume, or anterior chamber depth. CONCLUSION: The current study suggests that while high altitude may influence certain aspects of visual performance and corneal properties, overall visual acuity and anterior segment parameters remain stable in military personnel who underwent SMILE.