Reduced oxygen concentration at high altitudes may induce pupil miosis accompanied by increased iris volume, study finds. This image from the study shows how hypobaric hypoxia-induced pupil miosis flattened the iris and widened the anterior chamber angle in individuals with convex irises (B, D), while those with concave irises (A, C) experienced increased concavity, leading to a wider angle. Photo: Xie Y, et al. J Glaucoma. September 24, 2024. Click image to enlarge.

The reduced oxygen concentration found at high altitudes, known as hypobaric hypoxia, can affect numerous ocular parameters, one being intraocular pressure (IOP). As the authors of a new study, published in Journal of Glaucoma, explained, “One common factor contributing to IOP alteration is an altered outflow resistance, which is influenced by the geometry of the aqueous humor outflow facilities in the anterior chamber angle.” Pupil constriction has also been observed at high altitudes. To further investigate these effects–specifically from short-term exposure–researchers measured anterior chamber biometric parameters in healthy individuals from low-altitude environments before and after brief exposure to hypobaric hypoxia using swept-source anterior-segment OCT (AS-OCT).

The prospective study involved 25 healthy individuals (50 eyes) residing at altitudes between 40m and 100m above sea level, where baseline swept-source AS-OCT and IOP assessments were performed. Participants were then passively exposed to simulated conditions equivalent to 4000m above sea level for three hours. At the two-hour mark, researchers measured participants’ acute mountain sickness symptoms such as headache, gastrointestinal distress and fatigue or weakness. Fifteen minutes after exiting the hypobaric chamber, swept-source AS-OCT measurements and IOP assessments were repeated. 

After hypobaric hypoxia exposure, participants showed significantly reduced IOP (16.4mm Hg vs. 14.9mm Hg) and pupil diameter (5.36mm vs. 4.78mm) compared to sea level. Significant changes were also noted before vs. after exposure for angle opening distance, trabecular-iris space area, angle recess area, iris curvature and iris volume, while central corneal thickness, anterior chamber depth and lens vault remained stable. 

After adjusting for age, differences in angle opening distance and trabecular-iris space area were associated with changes in iris curvature. Notably, no anterior segment parameter changes were associated with IOP or correlated with acute mountain sickness. 

“Several factors, such as exercise, may have effects on these anterior chamber biometric parameters,” the study authors noted in the discussion portion of their paper. “This study excluded these confounding factors and found hypobaric hypoxia, the major environmental factor at high altitudes, leads to pupil miosis and widening of the anterior chamber angle.

“The pupil serves as a gateway to the brain,” they continued. “As humans at high altitudes experience sleep disturbances and daytime sleepiness, a decreased pupil diameter may indicate decreased central nerve activation.” It is interesting, however, that pupil constriction had no association with acute mountain sickness symptoms in this study, including fatigue.

Other studies have observed alterations in several pupillary dynamics at high altitudes, highlighting the need for further research using anterior segment imaging to better understand the mechanisms behind this phenomenon.