Intraocular pressure changes during high-altitude acclimatization

Physiology-Enhanced Data Analytics to Evaluate the Effect of Altitude on Intraocular Pressure and Ocular Hemodynamics

Altitude affects intraocular pressure (IOP); however, the underlying mechanisms involved and its relationship with ocular hemodynamics remain unknown. Herein, a validated mathematical modeling approach was used for a physiology-enhanced (pe-) analysis of the Mont Blanc study (MBS), estimating the effects of altitude on IOP, blood pressure (BP), and retinal hemodynamics. In the MBS, IOP and BP were measured in 33 healthy volunteers at 77 and 3466 m above sea level. Pe-retinal hemodynamics analysis predicted a statistically significant increase (p < 0.001) in the model predicted blood flow and pressure within the retinal vasculature following increases in systemic BP with altitude measured in the MBS. Decreased IOP with altitude led to a non-monotonic behavior of the model predicted retinal vascular resistances, with significant decreases in the resistance of the central retinal artery (p < 0.001) and retinal venules (p = 0.003) and a non-significant increase in the resistance in the central retinal vein (p = 0.253). Pe-aqueous humor analysis showed that a decrease in osmotic pressure difference (OPD) may underlie the difference in IOP measured at different altitudes in the MBS. Our analysis suggests that venules bear the significant portion of the IOP pressure load within the ocular vasculature, and that OPD plays an important role in regulating IOP with changes in altitude.

The Mont Blanc Study: The effect of altitude on intra ocular pressure and central corneal thickness

The aim of the Mont Blanc Study was to investigate the relationship between intraocular pressure (IOP), central corneal thickness (CCT), and altitude in healthy subjects. Thirty-three eyes of 33 healthy volunteers (mean age: 24.8 years, 17 females) had their IOP measured with Perkins and I-Care tonometers and their CCT using ultrasound pachymetry at three locations in Italy with different altitudes: Pavia, (PV), 77 meters above sea level (a.s.l); Courmayeur (CM), 1300 meters a.s.l; Pointe Helbronner (PH), 3466 meters a.s.l.). The measurements were performed at 9 am, 11 am, 1 pm and 3 pm (±30’) in indoor settings (mean temperature of 19°C) in PV and PH. At 9 am, CCT and IOP were measured outdoor (mean temperature of -1.4°C) at PH. The mean values of the IOP curve decreased from PV to PH with the Perkins (p = 0.02) and I-Care tonometers (p = 0.001). Instead, CCT increased upon ascension from PV to PH (p = 0.01), and from CM to PH (p = 0.002). When exposed to sub-zero temperature, the IOP increased (p<0.001), while the CCT did not change (p = 0.30). The results suggest that IOP significantly decreased and CCT significantly increased upon ascension from the sea level to higher altitudes.

Intraocular pressure response affected by changing of sitting and supine positions

PURPOSE

To assess the intraocular pressure (IOP) time response to change in body position from sitting to supine and from supine to sitting immediately and during rest in each position.

METHODS

Forty-four visually healthy volunteers were recruited for the study. The experiment consisted of the initial sitting position (baseline state), the subsequent lying period and the final sitting period. Both periods were 30 min long. The IOP was measured in the baseline state, immediately after each position change and then in minutes 5, 15, 25 and 30 during each period. The Icare Pro^® rebound tonometer was used.

RESULTS

The mean IOP increased after each position change (2.6 ± 2.4 mmHg after lying down and 2.1 ± 3.1 mmHg after sitting up) and then gradually decreased with time. The mean IOP was 1.41 ± 2.4 mmHg higher in the lying period than in the sitting period; the mean difference was smaller for the lower baseline (0.9 ± 2.2 mmHg) than the higher baseline (1.9 ± 2.5 mmHg). The mean IOP in the final sitting was significantly lower (2.5 ± 1.9 mmHg) than in the initial sitting position. The effect of sex was insignificant.

CONCLUSIONS

There was an immediate increase in IOP as a response to both changes in the body position and the subsequent gradual decrease with time. The IOP difference between lying and sitting position was depended on baseline.

Intraocular pressure and axial length changes during altitude acclimatization from Beijing to Lhasa

Purpose

To investigate changes in intraocular pressure (IOP) and axial length (AL) on the ascent to high altitude from Beijing to Lhasa.

Patients and methods

Twenty volunteers (17 men, 3 women) who had been sent to work in Lhasa, Tibet for more than 6 months were enrolled. One of their journeys from Beijing to Lhasa was chosen as the time for the examination. IOP, AL, corneal curvature (K), and blood pressure (BP) were measured in Beijing (altitude 43 m) and Lhasa (altitude 3658 m). Their first examination was conducted at least 1 day before arriving in Lhasa and the second examination after they had stayed in Lhasa for 7 days. The data from the highland and lowland examinations were analyzed with a paired-sample T test and Pearson’s correlation coefficient was calculated for the association between IOP and average BP.

Results

The mean IOP was 12.65±2.34 mmHg in Beijing and 14.85±3.1 mmHg in Lhasa. The mean AL was 24.61±1.50mm in Beijing, and 24.98±1.45 mm in Lhasa. The IOP and AL showed a significant elevation in highland compared with lowland (P<0.05). The mean K was 43.58±2.25 D in Beijing and 43.56±2.21 D in Lhasa and no significant difference was found in this study (P>0.05). A positive correlation between variance of IOP and ACD was found (r = 0.475, P<0.05) and no correlation between IOP and average BP was noted.

Conclusions

High altitude may lead to a small but significant change in IOP and axial length. However, the shape of the corneal surface was not influenced by the hypobaric and hypoxic conditions.

Association Between Arterial Blood Gas Variation and Intraocular Pressure in Healthy Subjects Exposed to Acute Short-Term Hypobaric Hypoxia

Purpose

To investigate the association between changes in arterial blood gases and intraocular pressure (IOP) after acute, short-term exposure to simulated elevation of 4000 m above sea level.

Methods

Twenty-five healthy young lowlanders participated in this prospective study. IOP was measured in both eyes with an Accupen tonometer. Arterial blood gas parameters (partial oxygen pressure [PaO₂], partial carbon dioxide pressure [PaCO₂], pH, and bicarbonate ion [HCO₃ ^-]) were checked using a blood gas analyzer. Measurements were taken at sea level (T1), at 15-minute (T2) and at 2-hour (T3) exposure times to simulated 4000 m above sea level in a hypobaric chamber, and upon return to sea level (T4). Associations between arterial blood gas parameters and IOP were evaluated using multivariate linear regression.

Results

PaO₂ significantly decreased at T2 and T3, resolving at T4 (P < 0.001). pH significantly increased at T2 and returned to baseline at T3 (P = 0.004). Actual and standard bicarbonate ion both dropped with IOP at T3 and T4. IOP significantly decreased from 16.4 ± 3.4 mm Hg at T1 to 15.1 ± 2.1 mm Hg (P = 0.041) at T3 and remained lower (14.9 ± 2.4 mm Hg; P = 0.029) at T4. IOP was not correlated with pH. Multivariate linear regression showed that lower IOP was associated with lower standard bicarbonate ion (beta = -1.061; 95% confidence interval, -0.049 to -2.074; P = 0.04) when adjusted for actual bicarbonate and diastolic blood pressure.

Conclusions

Hypobaric hypoxia triggers plasma bicarbonate ion reduction which, rather than pH, may decrease aqueous humor formation and subsequently cause IOP reduction. These findings may shed light on the mechanism of IOP regulation at high altitude.

Translational Relevance

Hypoxia-triggered reduction in plasma bicarbonate ion may decrease aqueous humor production, leading to IOP reduction at high altitude. These findings may provide new insight into a potential mechanism of IOP regulation. Hypobaric hypoxia at high altitude is an environmental factor that can reduce IOP and, therefore, deserves further study.

Intraocular Pressure Changes of Healthy Lowlanders at Different Altitude Levels: A Systematic Review and Meta-Analysis

Background: High altitude, characterized by hypobaric hypoxia, low temperature, and intensive ultraviolet radiation, is identifiably one of the examples of scientific enquiry into aviation and space analogs. However, little is known about the ocular physiological response, especially intraocular pressure (IOP) changes at high altitude. Objectives: This study aimed to systematically review of high altitude exposure on IOP for healthy lowlanders with unoperated eyes. Methods: A comprehensive systematic literature search was conducted in the electronic databases until September 1st, 2019. A meta-analysis was performed following the preferred reporting items for systematic review and meta-analysis statement (PRISMA). We systematically searched the studies conducted over 2,000 m above sea level (a.s.l) in healthy lowlanders with measurements of IOP. Meta-analyses (random effect model and heterogeneity tests), subgroup analyses (altitude, duration, type, and pattern of exposure), sensitivity analysis, funnel plot, Begger's and Egger's test for publication bias were performed. Quality assessment was conducted using the Newcastle-Ottawa scale. The meta-analysis was registered in the PROSPERO database (CRD42019136865). Results: Of 9595 publications searched, 20 publications (n = 745) qualified for inclusion, with non-significant decrease in overall IOP [standardized mean difference (SMD): 0.14, 95% CI: -0.12-0.40; p = 0.30] with high heterogeneity (p < 0.001, I 2 = 82%). However, subgroup analyses revealed significant decrease of IOP at high altitude of 3,000-5,500 m a.s.l (SMD: 0.57, 95% CI: 0.07-1.06; p = 0.03) whereas increase of IOP at extreme altitude of over 5,500 m a.s.l (SMD: -0.34, 95% CI: -0.61-0.06; p = 0.02). And the duration of exposure more than 72 hours (h) was likely to induce a decrease of IOP bordering on statistical significance at the 5% level (SMD: 1.29, 95% CI: 0.02-2.56; p = 0.05). Simultaneously, we also observed significant decrease of IOP for active exposure (e.g., physical activity and hiking, SMD: 0.81, 95% CI: 0.05-1.57; p = 0.04). Conclusion: Our analysis shows exposure to the altitude over 3,500 m a.s.l, duration of exposure more than 72 h and active exposure pattern may have modest, but significant effects on IOP. The higher altitude, the duration of exposure as well as physical activity seem to play crucial roles in the effects of high-altitude exposure on IOP.

Longitudinal observation of intraocular pressure variations with acute altitude changes

BACKGROUND

Higher intraocular pressure (IOP) is a major risk factor for developing glaucoma, and the leading cause of irreversible blindness worldwide. High altitude (HA) may be involved in IOP, but the reported results were conflicting. Ascent to HA directly by plane from low altitude regions is an acute, effortless exposure. However, the effects of such exposure to different altitudes on IOP have rarely been reported.

AIM

To investigate changes in IOP after rapid effortless exposure to HA in stages and compare it with systemic parameters.

METHODS

Fifty-eight healthy subjects (116 eyes) were divided into three groups: 17 low-altitude (LA) residents [44 m above sea level (ASL)], 22 HA residents (2261 m ASL) and 19 very HA (VHA) residents (3750 m ASL). The LA group flew to HA first. Three days later, they flew with the HA group to VHA where both groups stayed for 2 d. Then, the LA group flew back to HA and stayed for 1 d before flying back to 44 m. IOP, oxygen saturation (SpO₂) and pulse rate were measured. The linear mixed model was used to compare repeated measurements.

RESULTS

IOP in the LA group significantly decreased from 18.41 ± 2.40 mmHg at 44 m to 13.60 ± 3.68 mmHg at 2261 m ASL (P < 0.001), and then to 11.85 ± 2.48 mmHg at 3750 m ASL (P = 0.036 compared to IOP at 2261 m ASL) and partially recovered to 13.47 ± 2.57 mmHg upon return to 44 m. IOP in the LA group at HA and VHA was comparable to that in the local residents (12.2 ± 2.4 mmHg for HA,11.5 ± 1.8 mmHg for VHA). IOP was positively associated with SpO₂ while negatively associated with pulse rate.

CONCLUSION

IOP in the LA group gradually reduced as altitude elevated in stages and became comparable to IOP in local residents. Hypoxia may be associated with IOP, which deserves further study.

Intraocular Pressure Response to Short-Term Extreme Normobaric Hypoxia Exposure

Purpose: The purpose of the study was to determine the intraocular pressure response to normobaric hypoxia and the consequent recovery under additional well-controlled ambient conditions. Second, the study attempted to determine if the intraocular pressure changes were dependent on its baseline, initial heart rate, sex and arterial oxygen saturation. Methods: Thirty-eight visually healthy volunteers (23 women and 15 men) of an average age 25.2 ± 3.8 years from 49 recruited participants met the inclusion criteria and performed the complete test. Initial intraocular pressure (baseline), heart rate, and arterial oxygen saturation were measured after 7 min of rest under normal ambient conditions at an altitude 250 m above sea level. Each subject then underwent a 10 min normobaric hypoxic exposure and a subsequent 7 min recovery under normoxic conditions. Within hypoxic period, subjects were challenged to breathe hypoxic gas mixture with fraction of inspired oxygen of 9.6% (~6.200 m above sea level). Intraocular pressure and arterial oxygen saturation were re-measured at 4 and 10 min during the hypoxia and at 7 min after hypoxia termination. Results: Intraocular pressure increased in 1.2 mmHg ± 1.9 mmHg and 0.9 mmHg ± 2.3 mmHg at 4 and 10 min during the hypoxic period and returned approximately to the baseline at 7 min of recovery. The influence of sex was not statistically significant. The arterial oxygen saturation decreased in 14.9 ± 4.2% at min 4 and 18.4 ± 5.8% at min 10 during hypoxia and returned to the resting value at 7 min of recovery. The decrease was slightly higher in the case of women if compared with men. The hypoxia induced changes in intraocular pressure were significantly correlated with the arterial oxygen saturation changes, whereas the relationship with intraocular pressure baseline and initial heart rate were insignificant. Conclusion: There was a significant increase in intraocular pressure as a response to short-term normobaric hypoxia, which returned to the baseline in 7 min after hypoxia. The increase was dependent on the induced oxygen desaturation.

High Altitude-associated Changes in Intraocular Pressure Abrogated by Trabeculectomy

PURPOSE

To highlight the effect of ascent to high altitude on intraocular pressure (IOP) in a patient with primary open-angle glaucoma, who had previously undergone trabeculectomy in 1 eye.

METHODS

Case report.

RESULTS

A 66-year-old mountaineer with primary open-angle glaucoma and previous right trabeculectomy performed self-tonometry using a rebound tonometer (Icare HOME) before and during an expedition in the Himalaya. In the nonoperated eye, there was a statistically significant increase in IOP as the patient ascended to 5000 m over 8 days (R=0.790, P=0.001), consistent with recent literature. IOP increased by 1.73 mm Hg with each 1000 m increase in altitude. In the trabeculectomized eye there was no significant increase in IOP (R=0.219, P=0.172).

CONCLUSIONS

Filtration surgery may be protective against IOP fluctuations associated with ascent to high altitude. Self-tonometry complements standard glaucoma care by providing opportunities for IOP monitoring outside office hours and in remote locations.

Effect of High Altitude Exposure on Intraocular Pressure Using Goldmann Applanation Tonometry

Willmann, Gabriel, Kai Schommer, Maximilian Schultheiss, M. Dominik Fischer, Karl-Ulrich Bartz-Schmidt, Florian Gekeler, and Andreas Schatz. Effect of high altitude exposure on intraocular pressure using Goldmann applanation tonometry. High Alt Med Biol. 18:114-120, 2017.

AIMS

The aim of the study was to quantify changes of intraocular pressure (IOP) during exposure to 4559 m using the state-of-the-art method of Goldmann applanation tonometry for IOP measurement and to detect correlations between IOP and acute mountain sickness (AMS) in a prospective manner.

METHODS

IOP was measured using a Goldmann applanation tonometer AT 900^® (Haag-Streit, Switzerland) and central corneal thickness (CCT) with the anterior segment module of a Spectralis™ HRA+OCT^® device (Heidelberg Engineering, Germany) at baseline and high altitude. Assessment of AMS was performed using the Lake Louise and AMS-C questionnaires, and Pearson's correlation coefficient was calculated for association between IOP and AMS.

RESULTS

Raw IOP values at high altitude were not significantly changed compared to baseline. IOP adjusted to the increase in CCT at high altitude, which is known to alter IOP levels, showed a significant reduction for corrected IOP values on day 3 of exposure (morning -2.1 ± 1.2 mmHg; evening -2.3 ± 1.1 mmHg; p < 0.05). No correlation of IOP with AMS or clinical parameters (heart rate and SpO₂) at high altitude was noted.

CONCLUSIONS

IOP showed a significant reduction of IOP levels when corrected for increased CCT values at high altitude. Furthermore, the prospective measurement of IOP is not useful in diagnosing AMS or for the prediction of more severe high altitude related illnesses as the decrease in IOP and symptoms of AMS do not correlate during altitude exposure.

The effect of hypoxia on intra-ocular, mean arterial, retinal venous and ocular perfusion pressures

INTRODUCTION

High altitude hypoxia is linked to decreased blood oxygen saturation with a related increase of Endothelin-1 (ET-1) blood plasma levels. As a consequence of such elevated ET-1 levels, alterations of retinal venous and ocular perfusion pressures are suspected.

PURPOSE

To measure the effect of hypoxia on intra-ocular pressure, mean arterial pressure, retinal venous pressure and to calculate ocular perfusion pressure.

METHOD

An experimental, prospective cohort study with 33 healthy subjects was conducted in which the subjects were confronted with long-term (days) environmental hypoxia at high altitudes. Mean arterial pressure, arterial blood oxygen saturation, intra-ocular pressure, retinal venous and ocular perfusion pressure were measured at 300 m/1'000 ft (baseline), 4200 m/13'800 ft and 6000 m/19'700 ft above sea level.

RESULTS

Arterial oxygen saturation (-13.06% ± 4.69, p = < 0.001; -23.46% ± 5.7,p = < 0.001), retinal venous pressure (+7.16 m Hg±8.2, p = < 0.001;+9.9 mm Hg±8.5, p = < 0.001) and ocular perfusion pressure (-8.49 mm Hg±10.6, p = < 0.001; -6.02 mm hg±11.2, p = 0.006) changed significantly from baseline at both high altitude of 4200 and 6000 m. Intra-ocular pressure did not change significantly at all altitudes (+1.16 mm Hg±4.5, p = 0.227; +0.84 mm Hg±4.8, p = 0.286) and mean arterial pressure changed significantly only at an altitude of 6000 m (+3,8 mm Hg±21.1, p = 0.005) from baseline.

CONCLUSION

As hypoxia increases with higher altitude, arterial oxygen saturation and ocular perfusion pressure decreased, retinal venous pressure increased, intra-ocular pressure remains stable and mean arterial pressure was elevated only at 6000 m.

Hypobaric hypoxia: effects on intraocular pressure and corneal thickness

Objective. The purpose of this study focused on understanding the mechanisms underlying ocular hydrodynamics and the changes which occur in the eyes of subjects exposed to hypobaric hypoxia (HH) to permit the achievement of more detailed knowledge in glaucomatous disease. Methods. Twenty male subjects, aged 32 ± 5 years, attending the Italian Air Force, were enrolled for this study. The research derived from hypobaric chamber, using helmet and mask supplied to jet pilotes connected to oxygen cylinder and equipped with a preset automatic mixer. Results. The baseline values of intraocular pressure (IOP), recorded at T1, showed a mean of 16 ± 2.23 mmHg, while climbing up to 18,000 feet the mean value was 13.7 ± 4.17 mmHg, recorded at T2. The last assessment was performed returning to sea level (T4) where the mean IOP value was 12.8 ± 2.57 mmHg, with a significant change (P < 0.05) compared to T1. Pachymetry values related to corneal thickness in conditions of hypobarism revealed a statistically significant increase (P < 0.05). Conclusions. The data collected in this research seem to confirm the increasing outflow of aqueous humor (AH) in the trabecular meshwork (TM) under conditions of HH.

Intraocular pressure after exposure to moderate altitude

BACKGROUND

To study intraocular pressure (IOP) alteration in healthy individuals following a rapid effortless increase in altitude from 1900 m above sea level (ASL) to 3740 m ASL.

METHODS

Intraocular pressure, blood pressure, pulse rate, and arterial oxygen tension were determined in both eyes of healthy volunteers at the lower altitude. Participants were taken to a higher altitude of 3740 m ASL (1840-m altitude gain) via gondola lift, which took 30 minutes. All measurements were repeated at the higher altitude. Pearson and Spearman correlation analysis was conducted to assess the correlations among the variables. A paired t-test and linear regression were also used to compare IOP before and after ascending. The accepted level of significance for all tests was p <0.05.

RESULTS

Fifty-four healthy volunteers participated in the study. Four eyes of three subjects with IOP higher than 21 mmHg were excluded. Intraocular pressure ± SD (range) decreased from 14.9 ± 2.6 mmHg (9-21 mmHg) to 14.3 ± 2.4 mmHg (11-20 mmHg) (p = 0.02) after the ascent. Arterial oxygen saturation decreased from 95.4 % to 91.5 % (p < 0.001). Neither of the participants complained of any ocular or systemic symptoms during or after ascending to the higher altitude. Mean IOP, before and after ascending, was positively correlated with systolic blood pressure before and after the increase in altitude (Pearson correlation coefficient, 0.41, p = 0.002 and Pearson correlation coefficient, 0.37, p = 0.006, respectively). Intraocular pressure changes did not correlate with age, pulse rate, or arterial oxygen saturation.

CONCLUSION

A rapid, effortless increase in altitude (over a moderate range in altitude) decreases IOP in healthy individuals. The observed decrease may not be clinically significant; however, it shows the versatility of IOP control mechanisms in response to alteration in altitude and temperature.

The relation between intraocular pressure change and plasma natriuretic peptide under simulated hypobaric conditions

Purpose:

To ascertain whether the changes in intraocular pressure (IOP) that occur during hypobaric hypoxic exposure are related to plasma N-terminal pro-brain natriuretic peptide (BNP) levels.

Materials and Methods:

The study group comprised 26 healthy participants (all male, mean age 23.1 years). IOP was measured at local ground level, (792 m above sea level), then while in a chamber providing hypobaric hypoxic conditions (the subjects were exposed to a pressure equivalent to 9144 m for 1-3 min), and again after exit from the chamber. In each condition, the mean of three consecutive measurements of IOP was calculated for each eye. For BNP measurements, blood samples were drawn before the participants entered the chamber and just after they left the chamber.

Results:

IOP during hypobaric hypoxic exposure (18.00 ± 3.70 mmHg) was significantly greater than that before (15.66 ± 2.10 mmHg, P < 0.001) or after (16.10 ± 2.63 mmHg, P = 0.001) the exposure. IOP levels before and after the exposure were not significantly different (P = 0.136). Plasma BNP levels measured before and after exposure to hypobaric hypoxic conditions were not significantly different (P = 0.462).

Conclusion:

Plasma BNP levels did not change after short-term hypobaric hypoxic exposure, while the IOP increased. This increase may have been caused by some other systemic factors. As the hypobaric hypoxic conditions were reversed, IOP decreased to normal levels.

The effect of short-term hypobaric hypoxic exposure on intraocular pressure

PURPOSE

To evaluate the effect of short-term hypobaric hypoxic conditions (STHC) on intraocular pressure (IOP) in healthy subjects.

METHODS

The study group was comprised of 30 healthy participants (60 eyes) with a mean age of 24.8 years. IOP was measured with a Tono-Pen XL tonometer at ground level (792 m above sea level, prehypoxic) during hypobaric hypoxic exposure (equivalent to 9144 m) and at ground level again after this exposure (post-hypoxic). For each condition, the mean of 3 consecutive measurements of IOP was calculated. In each condition, central corneal thickness was also measured.

RESULTS

IOP in the hypobaric hypoxic condition (18.23 +/- 2.84 mmHg) was significantly greater than IOP in both the prehypoxic condition (16.52 +/- 2.84 mmHg) (p < 0.001) and the post-hypoxic condition (17.02 +/- 2.52 mmHg) (p < 0.01). When corrected for hypoxia-related changes in CCT, IOP under hypobaric hypoxic conditions was still greater than IOP in both the prehypoxic and post-hypoxic conditions. There was no significant difference between IOP levels in the prehypoxic and post-hypoxic conditions (p > 0.05).

CONCLUSION

STHC caused a significant increase in IOP in healthy participants. But this significant increase in IOP cannot be solely explained by a CCT-related overestimation error due to the increase in CCT. Individuals with IOP-related disorders such as glaucoma should be cautious when facing potential exposure to hypobaric hypoxic conditions.