Intraocular pressure (IOP) is crucial for maintaining the eye's structure and visual function. Its accurate measurement is vital for evaluating drug efficacy and understanding glaucoma pathophysiology. Clinical IOP measurements are often indirect (e.g., applanation tonometry), potentially underestimating true pressure. The eye's internal spaces (anterior and posterior chambers) differ in composition (aqueous humor vs. vitreous body), and the presence of the lens might affect IOP distribution. Previous studies investigating IOP differences between chambers faced challenges due to the vitreous gel's viscosity, leading to incomplete evaluation of vitreous cavity pressure. This study aimed to use a novel waterproof device lacking a tubular structure to overcome these limitations, allowing for accurate and simultaneous measurement of anterior chamber and vitreous cavity IOPs, even in the presence of vitreous gel, thus providing insights into the impact of the lens and vitreous gel on IOP distribution.

Several studies have examined IOP differences between the anterior and vitreous chambers in porcine eyes. One study reported significant differences, but vitreous gel viscosity hampered complete evaluation. Another study, removing the vitreous gel, showed no significant difference at pressures below 50 mmHg, suggesting minimal lens influence under those conditions. However, the influence of the lens when the vitreous gel was present remained unclear. Existing methods for direct IOP measurement, while providing accurate anterior chamber IOP readings, have limitations when measuring vitreous cavity IOP due to the viscous nature of the vitreous gel and potential clogging of the pressure sensor.

Twenty-four freshly enucleated porcine eyes were used. A control experiment verified the accuracy of a disk-shaped pressure sensor embedded in artificial gel, demonstrating high accuracy (0.5-0.67 mmHg difference from true water pressure). IOP was measured simultaneously and continuously in both the anterior chamber (using a needle-pressure sensor) and vitreous cavity (using a disk-shaped sensor). For aphakic eyes, the lens was removed using phacoemulsification. Saline was injected into the anterior chamber to increase IOP to approximately 50 mmHg, then gradually decreased to 10 mmHg. At four anterior chamber IOP levels (40, 30, 20, and 10 mmHg), vitreous IOP was measured. The difference between vitreous and anterior IOP (ΔIOPv-a) was calculated. Linear regression analysis determined the correlation between anterior and vitreous IOP and between ΔIOPv-a and anterior IOP, in both phakic and aphakic groups. Repeated-measures ANOVA was used to analyze the difference in vitreous IOP between anterior IOP conditions and between the two groups.

The disk-shaped pressure sensor demonstrated high accuracy in measuring pressure within the artificial gel, validating its use in the vitreous cavity. Repeated-measures ANOVA revealed a significant interaction between vitreous IOP, the presence of the lens, and anterior chamber IOP levels (F[3,258] = 5.8564, p<0.001). A strong positive correlation existed between anterior and vitreous chamber IOP in both phakic (R = 0.96, p<0.001) and aphakic (R = 0.97, p<0.001) groups. However, no significant correlation was found between ΔIOPv-a and anterior chamber IOP in either group (phakia group: R = -0.18, p = 0.034; aphakia group: R = -0.029, p = 0.73). ΔIOPv-a values were consistently positive (around 4–5 mmHg in the phakia group and 3 mmHg in the aphakia group), suggesting that factors beyond gravity alone contribute to this difference. Table 1 shows the verification results of the pressure measurement in the gel, demonstrating the sensor's high accuracy. Table 2 presents the vitreous IOP in both phakic and aphakic groups across different anterior IOP levels. Figure 2 displays the strong correlation between anterior and vitreous IOP in both groups, and Figure 3 illustrates the lack of correlation between ΔIOPv-a and anterior chamber IOP.

The results indicate that the novel sensor accurately measures vitreous IOP in the presence of vitreous gel, showing a strong correlation with anterior chamber IOP. The consistent positive ΔIOPv-a, exceeding the expected gravity effect, suggests other factors influence IOP distribution. Possible explanations include the non-physiological condition created by sensor insertion, potentially increasing vitreous IOP more than anterior IOP. The lens's role as a partition between chambers might be a contributing factor, influencing ΔIOPv-a depending on the anterior IOP. The findings contrast with a previous study reporting high ΔIOPv-a, likely due to sensor clogging; our sensor's design eliminated this issue. The lack of correlation between ΔIOPv-a and anterior IOP implies that the difference isn't simply a function of anterior chamber pressure. Factors like vitreous gel deformation and the partition effect of the lens seem to play crucial roles. The negligible influence of fluid flow and height differences on the measurements is considered based on analysis of Bernoulli's theorem. Measuring ΔIOPv-a is vital, as clinically used IOP measurements (anterior chamber) may not accurately reflect the actual vitreous IOP affecting optic nerve damage. This has implications for glaucoma and potentially myopia management.

This study demonstrates the effectiveness of a novel sensor for accurate, simultaneous measurement of anterior chamber and vitreous cavity IOPs in porcine eyes. The results highlight the importance of considering the difference between these pressures, which is affected by both the lens and the vitreous body. Further research should investigate IOP distribution under different physiological conditions and explore the clinical implications of these findings, especially regarding glaucoma management and myopia progression.

The study's limitations include the assessment of ΔIOPv-a only in the supine position, and the absence of IOP measurement in the posterior chamber between the iris and lens. Future studies should address these limitations by assessing the impact of different eye positions and including posterior chamber IOP measurements. Additionally, the study is conducted on porcine eyes, and results might not entirely translate to human eyes.