Central Corneal Thickness of Healthy Lowlanders at High Altitude: A Systematic Review and Meta-Analysis

Purpose: Central corneal thickness, a marker of corneal hydration and metabolism, was reported to increase at high elevations. This study aimed to assess the effect of chronic high-altitude exposure on the central corneal thickness of healthy lowlanders with unoperated corneas, and determine if a relationship exists between exposure time and corneal edema formation. Materials and Methods: The PubMed, Embase, Scopus, Cochrane Library, and Airiti Library databases were searched up to 2017 January 31 for prospective cohort studies performed above 2500 m in healthy lowlanders with measurements of the central corneal thickness. Subjects with prior eye surgery, contact lens, and non-hypobaric hypoxic exposure were excluded. Results: Seven studies of 207 adults were included. The pooled effect of high-altitude exposure on the central corneal thickness for < 12 hours, 3–5 days, 6–7 days, and > 10 days was a mean difference of 13.4 (95% confidence interval: 5.1–21.6) μm with moderate heterogeneity (p < 0.05, I² = 59%), 19.3 (95% confidence interval: 9.7–29) μm with low heterogeneity (p = 0.88, I² = 0%), 20.4 (95% confidence interval: 10.3–30.5) μm with low heterogeneity (p = 0.73, I² = 0%), and 30.8 (95% confidence interval: 20.4–41.2) μm with low heterogeneity (p = 0.69, I² = 0%), respectively. Baseline differences between pre-exposure and post-exposure were not statistically significant. Regression analysis revealed a significant linear relation between high-altitude exposure time and corneal edema formation that exceeded 5% after 10 days. Conclusions: High-altitude exposure induces central corneal thickening with significant linear progression over time, whereas it takes over 10 days to reach clinical significance in healthy lowlanders with unoperated corneas, and changes in central corneal thickness are reversible after descent to lower elevations.