To investigate the effects of short-term acute hypobaric hypoxia on tear film stability in healthy subjects.

A total of 22 healthy volunteers (44 eyes) recruited from Beihang University between October 2023 and October 2024 were enrolled as study subjects. Among them, there were 12 males (24 eyes) and 10 females (20 eyes) with a mean age of (22.3±1.5) years (ranging from 20 to 24 years). Subjects underwent tear film stability measurements in a hypobaric chamber at ground level (50 m), 3500 m, 4000 m, 4500 m, and after returning to ground level. Tear breakup time (TBUT), tear film lipid layer thickness (LLT), tear meniscus height (TRH), and complete blink counts were measured using an ocular surface interferometer. The Shapiro-Wilk test was used to assess normality for all data. TRH and TBUT were normally distributed and expressed as ±s; LLT and complete blink counts were non-normally distributed and expressed as median (interquartile range [IQR]). Differences in parameters across different altitudes were analyzed using the Generalized Linear Mixed Model (GLMM). Post-hoc pairwise comparisons for normally distributed data were performed using Bonferroni correction, and the Wilcoxon signed-rank test was used for non-normally distributed data.

All subjects completed the entire experimental procedure without supplemental oxygen. The values of TBUT, TRH, LLT, and complete blink counts at sea level (approximately 50 m), 3500 m, 4000 m, 4500 m and after returning to ground level were (9.89±2.60) s, (0.19±0.09) mm, 56.16 (41.00, 66.00) nm, 2 (2, 2) times, (9.28±2.47) s, (0.22±0.11) mm, 55.71 (41.00, 66.00) nm, 2 (2, 2) times, (7.57±2.86) s, (0.18±0.10) mm, 55.43 (41.00, 66.00) nm, 2 (2, 3) times, (7.94±3.07) s, (0.22±0.13) mm, 55.53 (41.00, 66.00) nm, 3 (2, 3) times, (9.50±2.74) s, (0.17±0.08) mm, 50.61 (41.00, 66.00) nm, and 2 (2, 2) times, respectively.With increasing altitude, TBUT showed an overall decreasing trend, reaching the shortest value at 4500 m under severe hypoxic conditions. When the altitude decreased and subjects returned to ground level, TBUT increased again, gradually recovering to near baseline levels. LLT exhibited a trend of initial thickening followed by reduction as altitude gradually increased. When subjects returned to ground level from high altitude, LLT did not recover but showed the decrease instead. TRH showed certain fluctuations with changes in altitude. After Bonferroni correction, TBUT at baseline was longer than that at 3500 m, 4000 m, and 4500 m with significant difference (P<0.05). TBUT at 3500 m was longer than that at 4000 m and 4500 m with significant difference (P<0.05). At 4000 m and 4500 m, TBUT was shorter than that after returning to ground level with significant difference (P<0.05). With increasing altitude, compared to ground level, LLT showed a decreasing trend at 3500 m and 4000 m, with a slight increase at 4500 m. There were no statistically significant differences in LLT values at 3500 m, 4000 m, and 4500 m compared to ground level (P>0.05). Additionally, binocular LLT after returning to ground level was thinner than baseline values and values at 3500 m, 4000 m and 4500 m with significant difference (P<0.05). After Bonferroni correction, TRH distribution was relatively uniform at ground level, 4000 m, and after returning to ground level, but showed greater fluctuation at 3500 m. Furthermore, GLMM analysis comparing TRH under different conditions showed that TRH at 3500 m was higher than baseline with significant difference (P<0.05); compared to 3500 m, TRH at 4000 m was significantly decreased (P<0.05), while TRH at 4500 m was increased with significant difference (P<0.05); and TRH was decreased again after returning to ground level compared to 4500 m with significant difference (P<0.05). The number of complete blinks within 20 s was 2 at ground level, increasing to 3 at 4000 m and 4500 m as altitude increased. At 4500 m, the number of complete blinks was statistically significantly different compared to ground level, 4000 m, and after returning to ground level (Z=-1.999, -2.238, -2.586; P<0.05). Additionally, the number of complete blinks at 4000 m was statistically significantly different compared to after returning to ground level (Z=-2.015, P<0.05).

Short-term acute hypobaric hypoxia induces early functional changes in tear film-related parameters, characterized mainly by reduced tear film stability and altered blinking behavior, while tear volume remains relatively unaffected. Tear film-related indicators are sensitive to acute hypoxic stimuli and may provide objective evidence for early assessment of dry eye risk in high-altitude and aviation-related populations.