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

What intrinsic factors affect the central corneal thickness?

The cornea is one of the tissues responsible for covering and protecting the inner structures of the eye. Central corneal thickness (CCT) is defined as the distance between the anterior epithelial surface and the posterior surface of the endothelial layer. This parameter plays a very important role regarding intraocular pressure (IOP) measurement, evaluation of corneal uniformity, selection of a suitable technique for corneal refractive surgery and the planning of surgical procedures to overcome corneal disease. This comprehensive review elucidates the multifaceted factors influencing the central corneal thickness. Recognising the impact of these factors not only enhances our understanding of corneal dynamics but also contributes significantly to the refinement of diagnostic and therapeutic strategies in ophthalmology.

Effects of ambient atmospheric pressure on intraocular pressure measured using a Goldman applanation tonometer in normal eyes under ordinary conditions

PURPOSE

Little is known about the effects of ambient atmospheric pressure (AP) on intraocular pressure (IOP) under ordinary conditions. This study aimed to investigate the effects of AP on Goldmann applanation tonometer-measured IOP (GAT-IOP) in normal eyes under everyday atmospheric conditions adjusting for effects of possible confounding factors including other climatic factors.

METHODS

Data obtained from 2,431 normal healthy eyes of 2,431 subjects (mean age: 56.9 years) who participated in this population-based glaucoma survey in Japan were analyzed via multivariable linear regression analysis, where the GAT-IOP was the response variable; explanatory variables were the ocular and systemic factors and calendar factors reportedly correlated with GAT-IOP as well as AP, relative humidity, temperature, wind speed, and weather. The Bonferroni correction was adopted to obtain P values.

RESULTS

Only AP of the outside air when each subject was examined was positively correlated with GAT-IOP (regression coefficient (RC) = 0.0460; P = 0.0051). Central corneal thickness, body mass index, and systolic blood pressure were positively correlated (RC = 0.0133, 0.0754, 0.0131; P < 0.0001, P < 0.0001, P = 0.0016, respectively), and corneal curvature radius and age were negatively correlated (RC=-0.7017, -0.0225; P = 0.0144, P = 0.0111, respectively) with GAT-IOP. The seasonal effect on GAT-IOP was also significant (P < 0.0001), and in reference to winter, summer was correlated with lower GAT-IOP and spring was correlated with higher GAT-IOP (RC=-0.5133, 0.4590; P = 0.0087, P = 0.0001, respectively).

CONCLUSIONS

AP was found to have a significantly positive correlation with GAT-IOP in normal healthy eyes under ordinary conditions, though the actual impact of AP on IOP in normal individuals under ordinary conditions would be minimal.

Influence of meteorological factors on intraocular pressure variability using a large-scale cohort

The effects of meteorological conditions on IOP using a large-scale health examination cohort were investigated. There were a total of 811,854 measurements from 126,630 eyes of 63,839 subjects in 9 years from a health checkup cohort followed up annually for age, sex, body height, body mass index (BMI), systolic blood pressure (SBP), diastolic blood pressure (DBP), and IOP. The effects of these variables and the meteorological data of daily average temperature (TP), daily average local atmospheric pressure (AP), daily average volumetric humidity (VH), and daily amount of rainfall (RF) on the day of IOP measurement on IOP were investigated. Several variables were significantly associated with IOP, including sex, age, body height, BMI, SBP, DBP, average TP, average AP, average VH, RF, white blood cell count, red blood cell count, hemoglobin, aspartate aminotransferase, alanine aminotransferase, guanosine triphosphate, calcium, and HbA1c. This study indicated a correlation between meteorological factors and IOP. Higher AP and RF were associated with elevated IOP, whereas higher TP and VH were associated with decreased IOP.

Glaucoma burden and its profile in a tertiary care centre of North-East India: A retrospective hospital-based study

Purpose

To estimate the proportion and pattern distribution of glaucoma disease in a teaching hospital of North-East India.

Materials and Methods

Retrospective hospital-based study of glaucoma cases from January 2014 to December 2022.

Results

Out of a total of 89725 new patients, 449 patients had glaucoma with a prevalence of 0.50% (95% CI: 0.46%-0.55%) of which 361 (80.4%) were ≥40 years and 88 (19.6%) were <40 years. The commonest type was POAG (32.1%) followed by PACG (17.6%) while JOAG was 7.8%. Males (67.2%) were predominant with a male: female ratio of 2.0:1. The mean age of patients was 53.4 (17.1) years. PACG was found to be significantly higher among females (p=0.001) whereas POAG was higher among males (p=0.003). There was no association of either age or systemic conditions with the prevalence of POAG and PACG. Hypertension and diabetes were present among 139 (31.0%) and 103 (23.0%) respectively while 13 (2.9%) had chronic obstructive pulmonary disease (COPD), 5 had coronary artery disease (CAD) and 5 hypothyroidism.

Conclusion

The prevalence of glaucoma though lower compared to other parts of the country, the number was significant, warranting screening activities owing to its silent nature. PACG was significantly higher among females while POAG was higher among males. Systemic factors like COPD, CAD and hypothyroidism require further studies for better understanding of the temporal association with glaucoma.

Comparison of central corneal thickness measured by automatic and manual analysis of optical coherence tomography

PURPOSE

To compare central corneal thickness (CCT) measured with optical coherence tomography (OCT) using an automatic algorithm (A-OCT) vs. manual measurements (M-OCT) with respect to the gold standard ultrasound pachymetry (USP).

METHODS

CCT measurements were performed on both eyes of 28 healthy subjects at four times of the day. A-OCT used an automatic software analysis of the corneal image, M-OCT was performed by two operators by setting a digital calliper on the corneal borders, and USP was performed after corneal anesthesia. Measurements were compared using a three-way repeated measures ANOVA. Bland-Altman plots were used to evaluate the agreement between OCT measurements and USP.

RESULTS

Both A-OCT and M-OCT significantly underestimated the USP measures, with the mean difference, i.e., the systematic error, being larger for A-OCT (- 19.0 µm) than for M-OCT (- 6.5 µm). Good reproducibility between the two operators was observed. Bland-Altman plots showed that both OCT methods suffered from proportional errors, which were not affected by time and eye.

CONCLUSIONS

Measuring CCT with OCT yielded lower values than USP. Therefore, clinicians should be aware that corneal thickness values may be influenced by the measurement method and that the various devices should not be used interchangeably in following up a given patient. Intriguingly, M-OCT had less systematic error than A-OCT, an important outcome that clinicians should consider when deciding to use an OCT device.

Influence of hypobaric hypoxic conditions on ocular structure and biological function at high attitudes: a narrative review

Background

With the development of science and technology, high-altitude environments, involving aviation, aerospace, and mountainous regions, have become the main areas for human exploration, while such complex environments can lead to rapid decreases in air and oxygen pressure. Although modern aircrafts have pressurized cabins and support equipment that allow passengers and crew to breathe normally, flight crew still face repeated exposure to hypobaric and hypoxic conditions. The eye is a sensory organ of the visual system that responds to light and oxygen plays a key role in the maintenance of normal visual function. Acute hypoxia changes ocular structure and function, such as the blood flow rate, and can cause retinal ischemia.

Methods

We reviewed researches, and summarized them briefly in a review.

Results

The acute hypobaric hypoxia affects corneal, anterior chamber angle and depth, pupils, crystal lens, vitreous body, and retina in structure; moreover, the acute hypoxia does obvious effect on visual function; for example, vision, intraocular pressure, oculometric features and dynamic visual performance, visual field, contrast sensitivity, and color perception.

Conclusion

We summarized the changes in the physiological structure and function of the eye in hypoxic conditions and to provide a biological basis for the response of the human eye at high-altitude.

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.

Effects of acute stress, general anesthetics, tonometry, and temperature on intraocular pressure in rats

Intraocular pressure (IOP) is important for eye health as abnormal levels can led to ocular tissue damage. IOP is typically estimated by tonometry, which only provides snapshots of pressure history. Tonometry also requires subject cooperation and corneal contact that may influence IOP readings. The aim of this research was to investigate IOP dynamics of conscious animals in response to stressors, common anesthetics, tonometry, and temperature manipulations. An eye of male Brown-Norway rats was implanted with a fluid-filled cannula connected to a wireless telemetry system that records IOP continuously. Stress effects were examined by restricting animal movements. Anesthetic effects were examined by varying isoflurane concentration or injecting a bolus of ketamine. Tonometry effects were examined using applanation and rebound tonometers. Temperature effects were examined by exposing anesthetized and conscious animals to warm or cool surfaces. Telemetry recordings revealed that IOP fluctuates spontaneously by several mmHg, even in idle and anesthetized animals. Environmental disturbances also caused transient IOP fluctuations that were synchronous in recorded animals and could last over a half hour. Animal immobilization produced a rapid sustained elevation of IOP that was blocked by anesthetics, whereas little-to-no IOP change was detected in isoflurane- or ketamine-anesthetized animals if body temperature (BT) was maintained. IOP and BT decreased precipitously when heat support was not provided and were highly correlated during surface temperature manipulations. Surface temperature had no impact on IOP of conscious animals. IOP increased slightly during applanation tonometry but not rebound tonometry. The results show that IOP is dynamically modulated by internal and external factors that can activate rapidly and last long beyond the initiating event. Wireless telemetry indicates that animal interaction induces startle and stress responses that raise IOP. Anesthesia blocks these responses, which allows for better tonometry estimates of resting IOP provided that BT is controlled.