Investigation of intraocular pressure of the anterior chamber and vitreous cavity of porcine eyes via a novel method

Intraocular pressure (IOP) sustains the eyeball's structure and is necessary for visual function. Clinically, IOP is commonly measured indirectly at the corneal surface by tonometry, which can underestimate true IOP but correlates with direct anterior chamber measurements in animal models. The intraocular space is partitioned by the lens into anterior and posterior segments; differing fluids and tissues (aqueous humor anteriorly, vitreous gel posteriorly) and the presence or absence of the lens may create IOP gradients between the anterior chamber and vitreous cavity. Determining any difference is important because indirect IOP measures guide glaucoma therapy. Prior studies in porcine eyes reported differences between anterior and vitreous IOPs, but accurate vitreous measurements were limited by vitreous gel viscosity and sensor clogging; another study found no difference when the vitreous gel was removed under pressures <50 mmHg, leaving the effect of the lens with intact gel unclear. The authors introduce a waterproof, disk-shaped, non-tubular pressure sensor designed to avoid clogging and allow direct measurement within the vitreous gel. Their objective was to measure anterior chamber and vitreous cavity IOPs simultaneously without removing the vitreous gel and to assess device usefulness by examining correlations between these pressures.

The paper references: (1) concordance and potential underestimation of IOP by applanation tonometry relative to direct measurements in rabbit and porcine eyes; (2) earlier porcine-eye studies that attempted simultaneous anterior and vitreous IOP measurements but faced errors due to vitreous gel clogging; and (3) a study showing no anterior–vitreous IOP difference when the vitreous gel was removed below ~50 mmHg, suggesting gel presence and lens status may influence pressure distribution. These works frame the need for a sensor capable of accurate vitreous gel pressure measurement to resolve inconsistencies in reported anterior–posterior IOP differences.

Study design and specimens: Twenty-four freshly enucleated porcine eyes (measured within 12 hours post-enucleation at room temperature) were obtained from a meat supplier; ethics approval was not required. Sensor validation in gel: A disk-shaped pressure sensor (PDA-PB; Tokyo Measuring Instruments Lab) was embedded within an artificial gel (98% moisture; gelatin, salt, water) shaped as a 2 cm cube and sealed within a thin polyethylene bag. The bagged gel was submerged to water depths of 10, 15, and 20 cm (true hydrostatic pressures: 7.35, 11.0, 14.7 mmHg). After stabilization, pressures recorded inside the gel were compared to true water pressures; performed in triplicate to evaluate accuracy and repeatability. Anterior chamber IOP measurement: A custom device with a 27-gauge needle attached to a wireless pressure gauge (KDM30; Krone) measured water pressure at the needle tip. The needle was secured in the anterior chamber, and continuous measurements were recorded. Vitreous cavity IOP measurement: A 10-mm scleral incision along the equator allowed insertion of the disk-shaped pressure sensor into the vitreous gel. The incision was sutured to prevent gel leakage, with only the sensor cable externalized and connected to a transducer (DC-004P; Tokyo Measuring Instruments Lab) that converted resistance changes to pressure. Continuous measurements were recorded. Gauges and sensors were zeroed at atmospheric pressure and verified to detect 2 cm H2O (~1.5 mmHg); measurements reflect absolute pressure changes relative to zeroing. Any atmospheric drift affected both channels equally; thus, differences between anterior and vitreous IOP were unaffected by atmospheric changes. Authors note intra-chamber pressures may vary spatially due to gravity, flow, and gel deformation; measured values reflect local pressures at sensor positions. Simultaneous measurement protocol: Eyes were positioned upright. After placing the vitreous sensor, two 27-gauge needles were inserted into the anterior chamber: one connected to a syringe for saline injection to raise IOP toward ~50 mmHg and one to the pressure gauge. Syringe and gauge were fixed. After saline injection, anterior and vitreous IOPs were recorded simultaneously and continuously as anterior IOP decayed. Phakia versus aphakia: For aphakic eyes, phacoemulsification and aspiration (Whitestar Signature Pro) removed the lens while preserving the posterior capsule; incisions were sutured. A total of 24 eyes were studied: 12 phakic (lens present) and 12 aphakic (lens removed). For each eye, saline was injected to raise anterior IOP to ~50 mmHg, then allowed to decline to 10 mmHg. When anterior IOP reached target levels (40, 30, 20, 10 mmHg), the vitreous IOP was recorded and ΔIOPv-a (vitreous minus anterior IOP) computed. Each experiment was repeated twice with re-injection after a brief interval. Statistics: Mean ± SD reported. Two-factor repeated-measures ANOVA with vitreous IOP as the dependent variable; main factors: group (phakia, aphakia) and anterior IOP condition (40, 30, 20, 10 mmHg). Linear regressions assessed correlations between anterior and vitreous IOP and the relationship between ΔIOPv-a and anterior IOP in each group. Bonferroni-corrected significance threshold p<0.025.

Sensor validation: In gel, measured pressures were 6.68, 10.5, and 14.2 mmHg at true hydrostatic pressures of 7.35, 11.0, and 14.7 mmHg, respectively; absolute differences 0.5–0.67 mmHg with negligible variation across repeats, indicating high accuracy and stability. Vitreous IOP by anterior IOP condition (mean ± SD): - At anterior 40 mmHg: phakia 44.2±3.61 mmHg; aphakia 42.7±3.17 mmHg. - At anterior 30 mmHg: phakia 34.9±2.97; aphakia 33.0±2.85. - At anterior 20 mmHg: phakia 25.1±2.69; aphakia 23.0±2.61. - At anterior 10 mmHg: phakia 15.7±2.11; aphakia 12.9±2.22. Repeated-measures ANOVA: - Significant effect of anterior IOP condition on vitreous IOP: F[3,258]=13610.70, p<0.001. - Significant group-by-condition interaction: F[3,258]=5.8564, p<0.001. - No significant main effect of group (phakia vs aphakia): F[1,22]=3.3645, p=0.080. Correlations: - Vitreous vs anterior IOP: strong positive correlations in both groups: phakia R=0.96, p<0.001; aphakia R=0.97, p<0.001. - ΔIOPv-a vs anterior IOP: no significant correlation in either group within the examined range: phakia R=−0.18, p=0.034 (not significant at Bonferroni p<0.025 threshold); aphakia R=−0.029, p=0.73. Magnitude of ΔIOPv-a: - Predominantly positive and relatively stable across conditions: approximately 4–5 mmHg in phakia and ~3 mmHg in aphakia, exceeding what can be explained solely by gravity (~1.8 mmHg for an estimated 24 mm height difference).

The study addressed whether a novel non-tubular, disk-shaped sensor can accurately measure vitreous cavity IOP in the presence of intact vitreous gel and how vitreous IOP relates to anterior chamber IOP under physiological ranges. Validation in an artificial gel demonstrated sub-mmHg accuracy and high repeatability, supporting the device's suitability for gel environments. Simultaneous measurements showed vitreous and anterior IOPs are strongly correlated from 10 to 40 mmHg, indicating the device provides physiologically consistent readings. Despite strong correlations, ΔIOPv-a was consistently positive and larger than could be attributed to gravity alone, suggesting additional factors. The authors propose that sensor insertion may slightly increase vitreous cavity volume/pressure locally and that the lens acts as a partition influencing pressure transmission; aphakic eyes showed lower ΔIOPv-a, consistent with reduced anterior partitioning. The significant group-by-condition interaction supports a pressure-transmission difference across lens status that varies with anterior IOP, even though overall mean differences between groups were not significant. Findings contrast with prior reports of large anterior–posterior differences likely caused by vitreous gel clogging tubular sensors; the non-tubular design here avoided such artifacts. Results align with studies showing minimal anterior–posterior differences when the vitreous gel is absent under <50 mmHg, suggesting that gel presence and measurement method do not produce large disparities within physiological ranges. Analyses using Bernoulli’s theorem indicated negligible contributions from flow kinetics and hydrostatic height under experimental conditions. Clinically, this implies that corneal tonometry may not fully reflect vitreous IOP, which could be relevant for understanding optic nerve head biomechanics in glaucoma and potentially myopia progression. Accurate vitreous IOP assessment may refine interpretations of IOP-related risk when lens or vitreous conditions are altered.

A novel, non-tubular, disk-shaped pressure sensor accurately measured pressure within a gel and enabled direct, simultaneous measurement of vitreous cavity IOP alongside anterior chamber IOP in porcine eyes with intact vitreous gel. Vitreous IOP strongly correlated with anterior IOP over 10–40 mmHg, while the vitreous–anterior difference (ΔIOPv-a) was positive and relatively stable, modestly exceeding gravity effects and differing slightly by lens status, with a significant group-by-condition interaction. These results suggest the device overcomes prior limitations due to gel clogging and that anterior IOP may not fully represent vitreous IOP. Future research should validate the method in vivo, examine posture and eye-movement effects on ΔIOPv-a, measure posterior chamber pressure, and assess impacts of pathological or surgical alterations of the lens and vitreous on posterior segment pressure.

- Orientation constraint: Measurements reflected the supine orientation (eyeballs facing upward); effects of other positions (e.g., sitting, eye movements) on ΔIOPv-a remain unknown. - Unmeasured posterior chamber: IOP in the posterior chamber (between iris and lens) was not measured, limiting insight into pressure gradients across all intraocular compartments.