ANNUAL SPECIAL ISSUE "Journal of Biomechanical Engineering-Year(s) in Review"

We review some highlights of the past several years at ASME J Biomechanical Engineering.

A Potential Role of Acute Choroidal Expansion in Nonarteritic Anterior Ischemic Optic Neuropathy

Purpose

Nonarteritic anterior ischemic optic neuropathy (NAION) has been associated with a thickened choroid at the optic nerve head (ONH). Here, we use computational modeling to better understand how choroidal expansion and choroidal geometry influence tissue deformation within the ONH relative to intraocular pressure (IOP) and intracranial pressure (ICP) effects.

Methods

Using a model of the posterior eye that included the sclera, peripapillary sclera, annular ring, pia mater, dura mater, neural tissues, Bruch's membrane, choroid, and lamina cribrosa, we examined how varying material properties of ocular tissues influenced ONH deformations under physiological and supra-physiological, or “pathological,” conditions. We considered choroidal expansion (c. 35 µL of expansion), elevated IOP (30 mm Hg), and elevated ICP (20 mm Hg), and calculated peak strains in the ONH relative to a baseline condition representing an individual in the upright position.

Results

Supra-physiological choroidal expansion had the largest impact on strains in the prelaminar neural tissue. In addition, compared to a tapered choroid, a “blunt” choroid insertion at the ONH resulted in higher strains. Elevated IOP and ICP caused the highest strains within the lamina cribrosa and retrolaminar neural tissue, respectively.

Conclusions

Acute choroidal expansion caused large deformations of the ONH and these deformations were impacted by choroid geometry. These results are consistent with the concept that compartment syndrome due to the choroid geometry and/or expansion at the ONH contributes to NAION. Prolonged deformations due to supra-physiological loading may induce a mechanobiological response or ischemia, highlighting the potential impact of choroidal expansion on biomechanical strains in the ONH.

Glaucoma and biomechanics

PURPOSE OF REVIEW

Biomechanics is an important aspect of the complex family of diseases known as the glaucomas. Here, we review recent studies of biomechanics in glaucoma.

RECENT FINDINGS

Several tissues have direct and/or indirect biomechanical roles in various forms of glaucoma, including the trabecular meshwork, cornea, peripapillary sclera, optic nerve head/sheath, and iris. Multiple mechanosensory mechanisms and signaling pathways continue to be identified in both the trabecular meshwork and optic nerve head. Further, the recent literature describes a variety of approaches for investigating the role of tissue biomechanics as a risk factor for glaucoma, including pathological stiffening of the trabecular meshwork, peripapillary scleral structural changes, and remodeling of the optic nerve head. Finally, there have been advances in incorporating biomechanical information in glaucoma prognoses, including corneal biomechanical parameters and iridial mechanical properties in angle-closure glaucoma.

SUMMARY

Biomechanics remains an active aspect of glaucoma research, with activity in both basic science and clinical translation. However, the role of biomechanics in glaucoma remains incompletely understood. Therefore, further studies are indicated to identify novel therapeutic approaches that leverage biomechanics. Importantly, clinical translation of appropriate assays of tissue biomechanical properties in glaucoma is also needed.

Consensus Recommendation for Mouse Models of Ocular Hypertension to Study Aqueous Humor Outflow and Its Mechanisms

Due to their similarities in anatomy, physiology, and pharmacology to humans, mice are a valuable model system to study the generation and mechanisms modulating conventional outflow resistance and thus intraocular pressure. In addition, mouse models are critical for understanding the complex nature of conventional outflow homeostasis and dysfunction that results in ocular hypertension. In this review, we describe a set of minimum acceptable standards for developing, characterizing, and utilizing mouse models of open-angle ocular hypertension. We expect that this set of standard practices will increase scientific rigor when using mouse models and will better enable researchers to replicate and build upon previous findings.

Anterior Segment Anatomy and Conventional Outflow Physiology of the Tree Shrew (Tupaia belangeri)

Purpose

Rodent and primate models are commonly used in glaucoma research; however, both have their limitations. The tree shrew (Tupaia belangeri) is an emerging animal model for glaucoma research owing in part to having a human-like optic nerve head anatomy, specifically a collagenous load-bearing lamina. However, the anterior segment anatomy and function have not been extensively studied in the tree shrew. Thus, the purpose of this study was to provide the first detailed examination of the anterior segment anatomy and aqueous outflow facility in the tree shrew.

Methods

Aqueous outflow dynamics were measured in five ostensibly normal eyes from three tree shrews using the iPerfusion system over a range of pressures. Gross histological assessment and immunohistochemistry were performed to characterize anterior segment anatomy and to localize several key molecules related to aqueous outflow.

Results

Anterior segment anatomy in tree shrews is similar to humans, demonstrating a scleral spur, a multilayered trabecular meshwork and a circular Schlemm's canal with a single lumen. Average outflow facility was 0.193 µL/min/mm Hg (95% confidence interval, 0.153-0.244), and was stable over time. Outflow facility was more similar between contralateral eyes (approximately 5% average difference) than between eyes of different animals. No significant dependence of outflow facility on time or pressure was detected (pressure-flow nonlinearity parameter of 0.01 (95% % confidence interval, -0.29 to 0.31 CI µL/min/mm Hg).

Conclusions

These studies lend support to the usefulness of the tree shrew as a novel animal model in anterior segment glaucoma and pharmacology research. The tree shrew's cost, load-bearing collagenous lamina cribrosa, and lack of washout or anterior chamber deepening provides a distinct experimental and anatomic advantage over the current rodent and nonhuman primate models used for translational research.

Assessment of the viscoelastic mechanical properties of the porcine optic nerve head using micromechanical testing and finite element modeling

Optic nerve head (ONH) biomechanics is centrally involved in the pathogenesis of glaucoma, a blinding ocular condition often characterized by elevation and fluctuation of the intraocular pressure and resulting loads on the ONH. Further, tissue viscoelasticity is expected to strongly influence the mechanical response of the ONH to mechanical loading, yet the viscoelastic mechanical properties of the ONH remain unknown. To determine these properties, we conducted micromechanical testing on porcine ONH tissue samples, coupled with finite element modeling based on a mixture model consisting of a biphasic material with a viscoelastic solid matrix. Our results provide a detailed description of the viscoelastic properties of the porcine ONH at each of its four anatomical quadrants (i.e., nasal, superior, temporal, and inferior). We showed that the ONH's viscoelastic mechanical response can be explained by a dual mechanism of fluid flow and solid matrix viscoelasticity, as is common in other soft tissues. We obtained porcine ONH properties as follows: matrix Young's modulus E=1.895[1.056,2.391] kPa (median [min., max.]), Poisson's ratio ν=0.142[0.060,0.312], kinetic time-constant τ=214[89,921] sec, and hydraulic permeability k=3.854×10^-1[3.457×10^-2,9.994×10^-1] mm⁴/(N.sec). These values can be used to design and fabricate physiologically appropriate ex vivo test environments (e.g., 3D cell culture) to further understand glaucoma pathophysiology. STATEMENT OF SIGNIFICANCE: Optic nerve head (ONH) biomechanics is an important aspect of the pathogenesis of glaucoma, the leading cause of irreversible blindness. The ONH experiences time-varying loads, yet the viscoelastic behavior of this tissue has not been characterized. Here, we measure the time-dependent response of the ONH in porcine eyes and use mechanical modeling to provide time-dependent mechanical properties of the ONH. This information allows us to identify time-varying stimuli in vivo which have timescales matching the characteristic response times of the ONH, and can also be used to design and fabricate ex vivo 3D cultures to study glaucoma pathophysiology in a physiologically relevant environment, enabling the discovery of new generations of glaucoma medications focusing on neuroprotection.

MRI-based quantification of ophthalmic changes in healthy volunteers during acute 15° head-down tilt as an analogue to microgravity

Spaceflight is known to cause ophthalmic changes in a condition known as spaceflight-associated neuro-ocular syndrome (SANS). It is hypothesized that SANS is caused by cephalad fluid shifts and potentially mild elevation of intracranial pressure (ICP) in microgravity. Head-down tilt (HDT) studies are a ground-based spaceflight analogue to create cephalad fluid shifts. Here, we developed non-invasive magnetic resonance imaging (MRI)-based techniques to quantify ophthalmic structural changes under acute 15° HDT. We specifically quantified: (i) change in optic nerve sheath (ONS) and optic nerve (ON) cross-sectional area, (ii) change in ON deviation, an indicator of ON tortuosity, (iii) change in vitreous chamber depth, and (iv) an estimated ONS Young's modulus. Under acute HDT, ONS cross-sectional area increased by 4.04 mm² (95% CI 2.88-5.21 mm², p < 0. 000), while ON cross-sectional area remained nearly unchanged (95% CI -0.12 to 0.43 mm², p = 0.271). ON deviation increased under HDT by 0.20 mm (95% CI 0.08-0.33 mm, p = 0.002). Vitreous chamber depth decreased under HDT by -0.11 mm (95% CI -0.21 to -0.03 mm, p = 0.009). ONS Young's modulus was estimated to be 85.0 kPa. We observed a significant effect of sex and BMI on ONS parameters, of interest since they are known risk factors for idiopathic intracranial hypertension. The tools developed herein will be useful for future analyses of ON changes in various conditions.

Publisher Correction: Human stem cells home to and repair laser-damaged trabecular meshwork in a mouse model

A Correction to this paper has been published: https://doi.org/10.1038/s42003-021-01958-w

Individual-Specific Modeling of Rat Optic Nerve Head Biomechanics in Glaucoma

Glaucoma is the second leading cause of blindness worldwide and is characterized by the death of retinal ganglion cells (RGCs), the cells that send vision information to the brain. Their axons exit the eye at the optic nerve head (ONH), the main site of damage in glaucoma. The importance of biomechanics in glaucoma is indicated by the fact that elevated intraocular pressure (IOP) is a causative risk factor for the disease. However, exactly how biomechanical insult leads to RGC death is not understood. Although rat models are widely used to study glaucoma, their ONH biomechanics have not been characterized in depth. Therefore, we aimed to do so through finite element (FE) modeling. Utilizing our previously described method, we constructed and analyzed ONH models with individual-specific geometry in which the sclera was modeled as a matrix reinforced with collagen fibers. We developed eight sets of scleral material parameters based on results from our previous inverse FE study and used them to simulate the effects of elevated IOP in eight model variants of each of seven rat ONHs. Within the optic nerve, highest strains were seen inferiorly, a pattern that was consistent across model geometries and model variants. In addition, changing the collagen fiber direction to be circumferential within the peripapillary sclera resulted in more pronounced decreases in strain than changing scleral stiffness. The results from this study can be used to interpret data from rat glaucoma studies to learn more about how biomechanics affects RGC pathogenesis in glaucoma.

Transpupillary collagen photocrosslinking for targeted modulation of ocular biomechanics

The central vision-threatening event in glaucoma is dysfunction and loss of retinal ganglion cells (RGCs), thought to be promoted by local tissue deformations. Here, we sought to reduce tissue deformation near the optic nerve head by selectively stiffening the peripapillary sclera, i.e. the scleral region immediately adjacent to the optic nerve head. Previous scleral stiffening studies to treat glaucoma or myopia have used either pan-scleral stiffening (not regionally selective) or regionally selective stiffening with limited access to the posterior globe. We present a method for selectively stiffening the peripapillary sclera using a transpupillary annular light beam to activate methylene blue administered by retrobulbar injection. Unlike prior approaches to photocrosslinking in the eye, this approach avoids the damaging effects of ultraviolet light by employing red light. This targeted photocrosslinking approach successfully stiffened the peripapillary sclera at 6 weeks post-treatment, as measured by whole globe inflation testing. Specifically, strain was reduced by 47% when comparing treated vs. untreated sclera within the same eye (n = 7, p=0.0064) and by 54% when comparing the peripapillary sclera of treated vs. untreated eyes (n = 7, p<0.0001). Post-treatment characterization of RGCs (optic nerve axon counts/density, and grading), retinal function (electroretinography), and retinal histology revealed that photocrosslinking was associated with some ocular toxicity. We conclude that a transpupillary photocrosslinking approach enables selective scleral stiffening targeted to the peripapillary region that may be useful in future treatments of glaucoma.

Anti-fibrotic activity of a rho-kinase inhibitor restores outflow function and intraocular pressure homeostasis

Glucocorticoids are widely used as an ophthalmic medication. A common, sight-threatening adverse event of glucocorticoid usage is ocular hypertension, caused by dysfunction of the conventional outflow pathway. We report that netarsudil, a rho-kinase inhibitor, decreased glucocorticoid-induced ocular hypertension in patients whose intraocular pressures were poorly controlled by standard medications. Mechanistic studies in our established mouse model of glucocorticoid-induced ocular hypertension show that netarsudil both prevented and reduced intraocular pressure elevation. Further, netarsudil attenuated characteristic steroid-induced pathologies as assessed by quantification of outflow function and tissue stiffness, and morphological and immunohistochemical indicators of tissue fibrosis. Thus, rho-kinase inhibitors act directly on conventional outflow cells to prevent or attenuate fibrotic disease processes in glucocorticoid-induced ocular hypertension in an immune-privileged environment. Moreover, these data motivate the need for a randomized prospective clinical study to determine whether netarsudil is indeed superior to first-line anti-glaucoma drugs in lowering steroid-induced ocular hypertension.

Identifiability of tissue material parameters from uniaxial tests using multi-start optimization

Determining tissue biomechanical material properties from mechanical test data is frequently required in a variety of applications. However, the validity of the resulting constitutive model parameters is the subject of debate in the field. Parameter optimization in tissue mechanics often comes down to the "identifiability" or "uniqueness" of constitutive model parameters; however, despite advances in formulating complex constitutive relations and many classic and creative curve-fitting approaches, there is currently no accessible framework to study the identifiability of tissue material parameters. Our objective was to assess the identifiability of material parameters for established constitutive models of fiber-reinforced soft tissues, biomaterials, and tissue-engineered constructs and establish a generalizable procedure for other applications. To do so, we generated synthetic experimental data by simulating uniaxial tension and compression tests, commonly used in biomechanics. We then fit this data using a multi-start optimization technique based on the nonlinear least-squares method with multiple initial parameter guesses. We considered tendon and sclera as example tissues, using constitutive models that describe these fiber-reinforced tissues. We demonstrated that not all the model parameters of these constitutive models were identifiable from uniaxial mechanical tests, despite achieving virtually identical fits to the stress-stretch response. We further show that when the lateral strain was considered as an additional fitting criterion, more parameters are identifiable, but some remain unidentified. This work provides a practical approach for addressing parameter identifiability in tissue mechanics.

A Porcine Organ-Culture Glaucoma Model Mimicking Trabecular Meshwork Damage Using Oxidative Stress

Purpose

Re-cellularization of the trabecular meshwork (TM) using stem cells is a potential novel treatment for ocular hypertension associated with glaucoma. To assess the therapeutic efficacy of this approach, improved in vivo and ex vivo models of TM pathophysiology are needed. Here, we investigate whether oxidative stress, induced by hydrogen peroxide (H2O2), can model glaucomatous ocular hypertension in the readily available porcine anterior segment organ culture model.

Methods

The impact of H2O2 on TM cell viability and function was first evaluated in vitro using primary porcine TM cells. Oxidative stress was then induced by H2O2 infusion into perfused porcine anterior segments. Trabecular meshwork function was assessed by tracking matrix metalloproteinase (MMP) activity and the ability of the preparation to maintain intraocular pressure (IOP) homeostasis after a flow challenge (doubled fluid infusion rate). Finally, the TM was evaluated histologically.

Results

H2O2 treatment resulted in a titratable reduction in cellularity across multiple primary TM cell donor strains. In organ culture preparations, H2O2-treated eyes showed impaired IOP homeostasis (i.e., IOPs stabilized at higher levels after a flow challenge vs. control eyes). This result was consistent with reduced MMP activity and TM cellularity; however, damage to the TM microstructure was not histologically evident in anterior segments receiving H2O2.

Conclusions

Titrated H2O2 infusion resulted in TM cellular dysfunction without destruction of TM structure. Thus, this porcine organ culture model offers a useful platform for assessing trabecular meshwork therapies to treat ocular hypertension associated with glaucoma.

Quantification of arterial, venous, and cerebrospinal fluid flow dynamics by magnetic resonance imaging under simulated micro-gravity conditions: a prospective cohort study

BACKGROUND

Astronauts undergoing long-duration spaceflight are exposed to numerous health risks, including Spaceflight-Associated Neuro-Ocular Syndrome (SANS), a spectrum of ophthalmic changes that can result in permanent loss of visual acuity. The etiology of SANS is not well understood but is thought to involve changes in cerebrovascular flow dynamics in response to microgravity. There is a paucity of knowledge in this area; in particular, cerebrospinal fluid (CSF) flow dynamics have not been well characterized under microgravity conditions. Our study was designed to determine the effect of simulated microgravity (head-down tilt [HDT]) on cerebrovascular flow dynamics. We hypothesized that microgravity conditions simulated by acute HDT would result in increases in CSF pulsatile flow.

METHODS

In a prospective cohort study, we measured flow in major cerebral arteries, veins, and CSF spaces in fifteen healthy volunteers using phase contrast magnetic resonance (PCMR) before and during 15° HDT.

RESULTS

We found a decrease in all CSF flow variables [systolic peak flow (p = 0.009), and peak-to-peak pulse amplitude (p = 0.001)]. Cerebral arterial average flow (p = 0.04), systolic peak flow (p = 0.04), and peak-to-peak pulse amplitude (p = 0.02) all also significantly decreased. We additionally found a decrease in average cerebral arterial flow (p = 0.040). Finally, a significant increase in cerebral venous cross-sectional area under HDT (p = 0.005) was also observed.

CONCLUSIONS

These results collectively demonstrate that acute application of -15° HDT caused a reduction in CSF flow variables (systolic peak flow and peak-to-peak pulse amplitude) which, when coupled with a decrease in average cerebral arterial flow, systolic peak flow, and peak-to-peak pulse amplitude, is consistent with a decrease in cardiac-related pulsatile CSF flow. These results suggest that decreases in cerebral arterial inflow were the principal drivers of decreases in CSF pulsatile flow.

Smarce1 and Tensin 4 Are Putative Modulators of Corneoscleral Stiffness

The biomechanical properties of the cornea and sclera are important in the onset and progression of multiple ocular pathologies and vary substantially between individuals, yet the source of this variation remains unknown. Here we identify genes putatively regulating corneoscleral biomechanical tissue properties by conducting high-fidelity ocular compliance measurements across the BXD recombinant inbred mouse set and performing quantitative trait analysis. We find seven cis-eQTLs and non-synonymous SNPs associating with ocular compliance, and show by RT-qPCR and immunolabeling that only two of the candidate genes, Smarce1 and Tns4, showed significant expression in corneal and scleral tissues. Both have mechanistic potential to influence the development and/or regulation of tissue material properties. This work motivates further study of Smarce1 and Tns4 for their role(s) in ocular pathology involving the corneoscleral envelope as well as the development of novel mouse models of ocular pathophysiology, such as myopia and glaucoma.

Automated MRI-based quantification of posterior ocular globe flattening and recovery after long-duration spaceflight

BACKGROUND/OBJECTIVES

Spaceflight associated neuro-ocular syndrome (SANS), a health risk related to long-duration spaceflight, is hypothesized to result from a headward fluid shift that occurs with the loss of hydrostatic pressure gradients in weightlessness. Shifts in the vascular and cerebrospinal fluid compartments alter the mechanical forces at the posterior eye and lead to flattening of the posterior ocular globe. The goal of the present study was to develop a method to quantify globe flattening observed by magnetic resonance imaging after spaceflight.

SUBJECTS/METHODS

Volumetric displacement of the posterior globe was quantified in 10 astronauts at 5 time points after spaceflight missions of ~6 months.

RESULTS

Mean globe volumetric displacement was 9.88 mm³ (95% CI 4.56-15.19 mm³, p < 0.001) on the first day of assessment after the mission (R[return]+ 1 day); 9.00 mm³ (95% CI 3.73-14.27 mm³, p = 0.001) at R + 30 days; 6.53 mm³ (95% CI 1.24-11.83 mm³, p < 0.05) at R + 90 days; 4.45 mm³ (95% CI -0.96 to 9.86 mm³, p = 0.12) at R + 180 days; and 7.21 mm³ (95% CI 1.82-12.60 mm³, p < 0.01) at R + 360 days.

CONCLUSIONS

There was a consistent inward displacement of the globe at the optic nerve, which had only partially resolved 1 year after landing. More pronounced globe flattening has been observed in previous studies of astronauts; however, those observations lacked quantitative measures and were subjective in nature. The novel automated method described here allows for detailed quantification of structural changes in the posterior globe that may lead to an improved understanding of SANS.

The role of Piezo1 in conventional aqueous humor outflow dynamics

Controlling intraocular pressure (IOP) remains the mainstay of glaucoma therapy. The trabecular meshwork (TM), the key tissue responsible for aqueous humor (AH) outflow and IOP maintenance, is very sensitive to mechanical forces. However, it is not understood whether Piezo channels, very sensitive mechanosensors, functionally influence AH outflow. Here, we characterize the role of Piezo1 in conventional AH outflow. Immunostaining and western blot analysis showed that Piezo1 is widely expressed by TM. Patch-clamp recordings in TM cells confirmed the activation of Piezo1-derived mechanosensitive currents. Importantly, the antagonist GsMTx4 for mechanosensitive channels significantly decreased steady-state facility, yet activation of Piezo1 by the specific agonist Yoda1 did not lead to a facility change. Furthermore, GsMTx4, but not Yoda1, caused a significant increase in ocular compliance, a measure of the eye's transient response to IOP perturbation. Our findings demonstrate a potential role for Piezo1 in conventional outflow, likely under pathological and rapid transient conditions.

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Piezo1 is functionally expressed in the TM, the most important tissue controlling IOP

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Suppression of mechanosensitive channel leads to a significant decrease in facility

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Our data suggest a role for Piezo in pathological situations and rapid IOP transients

A biphasic approach for characterizing tensile, compressive and hydraulic properties of the sclera

Measuring the biomechanical properties of the mouse sclera is of great interest: altered scleral properties are features of many common ocular pathologies, and the mouse is a powerful tool for studying genetic factors in disease, yet the small size of the mouse eye and its thin sclera make experimental measurements in the mouse difficult. Here, a poroelastic material model is used to analyse data from unconfined compression testing of both pig and mouse sclera, and the tensile modulus, compressive modulus and permeability of the sclera are obtained at three levels of compressive strain. Values for all three properties were comparable to previously reported values measured by tests specific for each property. The repeatability of the approach was evaluated using a test-retest experimental paradigm on pig sclera, and tensile stiffness and permeability measurements were found to be reasonably repeatable. The intrinsic material properties of the mouse sclera were measured for the first time. Tensile stiffness and permeability of the sclera in both species were seen to be dependent on the state of compressive strain. We conclude that unconfined compression testing of sclera, when analysed with poroelastic theory, is a powerful tool to phenotype mouse scleral changes in future genotype-phenotype association studies.

In vivo estimation of murine iris stiffness using finite element modeling

The iris plays an important role in certain types of glaucoma, including primary angle-closure glaucoma and pigmentary glaucoma. Iris mechanics are also important in influencing trabecular meshwork deformation in response to intraocular pressure changes in some animal species. Although mice are widely used to study ocular disease, including glaucoma, the in vivo biomechanical properties of the murine iris are unknown. Thus, the primary objective of this study was to estimate murine iris biomechanical stiffness. We used optical coherence tomography (OCT) images of the anterior segment of living mice (n = 13, age = 7.3 ± 3.2 [mean ± SD] months) at sequentially increasing IOP levels, observing IOP-dependent iris deformations. We then used an inverse finite element model to predict iris deformations under the same conditions, estimating iris stiffness by maximizing agreement between OCT data and numerical simulations. Our results show an in vivo murine iris stiffness of 96.1 ± 54.7 kPa (mean ± SD), which did not correlate with age but was dependent on gender. Our results further showed strong evidence of reverse pupillary block, with mean posterior chamber pressure remaining at approximately 12 mmHg even as anterior chamber pressure was set to much higher levels. Our approach to monitoring iris stiffness in vivo is applicable to study potential changes of iris stiffness in various pathophysiological conditions and thus has significant potential for clinical care of ocular disease involving iris biomechanics.

Drug-Free, Nonsurgical Reduction of Intraocular Pressure for Four Months after Suprachoroidal Injection of Hyaluronic Acid Hydrogel

Glaucoma is the leading cause of irreversible blindness. Current treatments use drugs or surgery to reduce intraocular pressure (IOP). In this study, a drug-free, nonsurgical method is developed that lowers IOP for 4 months without requiring daily patient adherence. The approach involves expanding the suprachoroidal space (SCS) of the eye with an in situ-forming hydrogel injected using a microneedle. This study tests the hypothesis that SCS expansion increases the drainage of aqueous humor from the eye via the unconventional pathway, which thereby lowers IOP. SCS injection of a commercial hyaluronic acid (HA) hydrogel reduces the IOP of normotensive rabbits for more than 1 month and an optimized HA hydrogel formulation enables IOP reduction for 4 months. Safety assessment by clinical ophthalmic examinations indicate the treatment is well tolerated. Histopathology shows minor hemorrhage and fibrosis at the site of injection. Further analysis by ultrasound biomicroscopy demonstrates a strong correlation of IOP reduction with SCS expansion. Outflow facility measurements show no difference in pressure-dependent outflow by the conventional pathway between treated and untreated eyes, supporting the hypothesis. In conclusion, SCS expansion with an in situ-forming hydrogel can enable extended IOP reduction for treating ocular hypertension and glaucoma without drugs or surgery.

Using retinal function to define ischemic exclusion criteria for animal models of glaucoma

Most animal models of glaucoma rely on induction of ocular hypertension (OHT), yet such models can suffer from high IOPs leading to undesirable retinal ischemia. Thus, animals with IOPs exceeding a threshold (e.g. > 60 mmHg) are often excluded from studies. However, due to the intermittent nature of IOP measurements, this approach may fail to detect ischemia. Conversely, it may also inappropriately eliminate animals with IOP spikes that do not induce ischemic damage. It is known that acute ischemia selectively impairs inner retinal function, which results in a reduced b-wave amplitude. Here, we explore the potential of using electroretinography (ERG) to detect ischemic damage in OHT eyes. 74 Brown Norway rats received a unilateral injection of magnetic microbeads to induce OHT, while contralateral eyes served as controls. IOP was measured every 2-3 days for 14 days after microbead injection. Retinal function was evaluated using dark-adapted bright flash ERG (2.1 log cd•s/m²) prior to, and at 7 and 14 days after, injection. We investigated two criteria for excluding animals: (IOP Criterion) a single IOP measurement > 60 mmHg; or (ERG Criterion) a b-wave amplitude below the 99.5% confidence interval for naïve eyes. 49 of 74 rats passed both criteria, 7 of 74 failed both, and 18 passed one criterion but not the other. We suggest that ERG testing can detect unwelcome ischemic damage in animal models of OHT. Since brief IOP spikes do not necessarily lead to ischemic retinal damage, and because extended periods of elevated IOP can be missed, such ERG-based criteria may provide more objective and robust exclusion criteria in future glaucoma studies.

Effects of General Anesthesia on Intraocular Pressure in Rabbits

Measurement of intraocular pressure (IOP) is a standard procedure in ophthalmic research in animals, specifically in glaucoma research, and the control of IOP is essential during certain veterinary ophthalmic surgeries. We evaluated the effect of isoflurane on IOP in the clinically healthy laboratory rabbits and tested a way to minimize the alteration of IOP during isoflurane anesthesia. After measurement of the baseline IOP in each eye of 9 awake New Zealand white rabbits, animals were anesthetized by using either: (1) isoflurane without premedication, (2) a combination of ketamine and xylazine, or (3) isoflurane inhalation after an injection of ketamine-xylazine premedication. Isoflurane led to a sustained increase in IOP of approximately 12 mm Hg. In contrast, ketamine and xylazine decreased IOP by nearly 5 mm Hg (all values compared with baseline measurements in awake, unrestrained animals). The observed decrease in IOP after ketamine-xylazine anesthesia is consistent with anesthetic effects generally seen during anesthesia in other studies. The increased IOP after isoflurane anesthesia in rabbits in this study was an unexpected result that appears to be specific to this combination of anesthetic and animal species. Premedication with ketamine-xylazine diminished the effect of isoflurane inhalation on IOP. These results should be considered in the design of ophthalmic research studies using rabbits and in intraocular surgery where IOP stability is desired.