Age-related changes in vitreous mobility as measured by video B scan ultrasound

Effective Prediction of Drug Transport in a Partially Liquefied Vitreous Humor: Physics-informed Neural Network Modeling for Irregular Liquefaction Geometry

As the medium for intravitreal drug delivery, the vitreous body can significantly influence drug delivery because of various possible liquefaction geometries. This work innovatively proposes a varying-porosity approach that is capable of solving the pressure and velocity fields in the heterogeneous vitreous with randomly-shaped liquefaction geometry, validated with a finite difference model. Doing so enables patient-specific treatment for intravitreal drug delivery and can significantly improve treatment efficacy. A physics-informed neural network (PINN) model is also established for the simulation, and three cases are used for validation. Despite limited information, the PINN model, together with the varying-porosity approach, captured fluid and drug transport in the partially liquefied vitreous. This opens the possibility for optimizing intravitreal drug delivery based on ultrasonography in clinical practice.

Vision degrading myodesopsia from vitreous floaters in the young: An important aspect of myopia

Vitreous floaters are visual phenomena resulting from floating opacities inside the eye that disturb vision. The opacities consist of microscopic collagen fibers that aggregate in myopia and during aging. These collagen fibers are then seen as irregular, sometimes worm-like shadows or structures of a translucent to black color. Because of the floating aspect, they tend to follow the movements of the eye, causing distress and having a negative impact on a patient's quality of life due to degradation in contrast sensitivity function. This is referred to as vision degrading myodesopsia (VDM). The overall importance of floaters and their effect on quality of life gained attention in recent years. While the existence of floaters alone is labeled as harmless, there is an increasing group of young people suffering from VDM. This coincides with the growing prevalence of myopia. Indeed, myopia and myopic vitreopathy are the major causes of VDM in the young. This aspect of myopia, however, is often overlooked. We review the current research status in floater formation, quality of life impact, symptom assessment, localization and therapeutic options for vitreous floaters from the perspective of a myopic, potentially younger patient group.

Finite Element Analysis of Changes in Deformation of Intraocular Segments by Airbag Impact in Eyes of Various Axial Lengths

Background

We studied the kinetic phenomenon of an airbag impact on eyes with different axial lengths using finite element analysis (FEA) to sequentially determine the physical and mechanical responses of intraocular segments at various airbag deployment velocities.

Methods

The human eye model we created was used in simulations with the FEA program PAM-GENERISTM. The airbag was set to impact eyes with axial lengths of 21.85 mm (hyperopia), 23.85 mm (emmetropia) and 25.85 mm (myopia), at initial velocities of 20, 30, 40, 50 and 60 m/s. The deformation rate was calculated as the ratio of the length of three segments, anterior chamber, lens and vitreous, to that at the baseline from 0.2 ms to 2.0 ms after the airbag impact.

Results

Deformation rate of the anterior chamber was greater than that of other segments, especially in the early phase, 0.2-0.4 ms after the impact (P < 0.001), and it reached its peak, 80%, at 0.8 ms. A higher deformation rate in the anterior chamber was found in hyperopia compared with other axial length eyes in the first half period, 0.2-0.8 ms, followed by the rate in emmetropia (P < 0.001). The lens deformation rate was low, its peak ranging from 40% to 75%, and exceeded that of the anterior chamber at 1.4 ms and 1.6 ms after the impact (P < 0.01). The vitreous deformation rate was lower throughout the simulation period than that of the other segments and ranged from a negative value (elongation) in the later phase.

Conclusion

Airbag impact on the eyeball causes evident deformation, especially in the anterior chamber. The results obtained in this study, such as the time lag of the peak deformation between the anterior chamber and lens, suggest a clue to the pathophysiological mechanism of airbag ocular injury.

Age-related changes on physicochemical properties of the artificial vitreous humor: A practical tool for enhancing ex vivo studies

The vitreous humor (VH) is a hydrophilic, jelly-like ocular fluid, which is located in the posterior chamber of the eye. The rheological, structural, and chemical properties of VH change significantly during aging, which further causes eye-associated diseases and could be a potential indicator for various diseases. In this study, artificial VH (A-VH) samples were created by taking into account different age groups to observe age-related changes in the physicochemical properties of these samples. This study aimed to measure the physicochemical properties of age-dependently prepared A-VH samples to determine the changes with aging in the physicochemical properties of A-VH samples. Phosphate-buffered saline (PBS)-based A-VH samples were prepared in three types representing adult, middle-aged, and elder individuals. Age-related changes in physicochemical properties (surface tension, osmolality, pH, relative viscosity, density, and refractive index) were analyzed by related equipment. The A-VH samples, prepared using PBS, showed strong similarity to authentic VH in terms of physicochemical properties. While the age-related changes studies have revealed some discrepancies between age-dependently prepared A-VH samples in terms of surface tension, osmolality, relative viscosity, and pH with high correlation coefficients (r2 > 0,94), density and refractive index values did not show any significant differences and correlation between types of A-VH representing 3 age groups. In conclusion, age-dependent A-VH samples were created successfully to use ex vivo method development studies, and the influence of aging on the physicochemical properties of VH was demonstrated as well.

The Role of Oxidative Stress in the Aging Eye

Given the expanding elderly population in the United States and the world, it is important to understand the processes underlying both natural and pathological age-related changes in the eye. Both the anterior and posterior segment of the eye undergo changes in biological, chemical, and physical properties driven by oxidative stress. With advancing age, changes in the anterior segment include dermatochalasis, blepharoptosis, thickening of the sclera, loss of corneal endothelial cells, and stiffening of the lens. Changes in the posterior segment include lowered viscoelasticity of the vitreous body, photoreceptor cell loss, and drusen deposition at the macula and fovea. Age-related ocular pathologies including glaucoma, cataracts, and age-related macular degeneration are largely mediated by oxidative stress. The prevalence of these diseases is expected to increase in the coming years, highlighting the need to develop new therapies that address oxidative stress and slow the progression of age-related pathologies.

Visualization and Grading of Vitreous Floaters Using Dynamic Ultra-Widefield Infrared Confocal Scanning Laser Ophthalmoscopy: A Pilot Study

Purpose: To describe the appearance of vitreous opacities using dynamic ultra-widefield infrared confocal scanning laser ophthalmoscopy (IRcSLO). Design: Retrospective case series. Methods: Eyes of patients complaining of myodesopsia were analyzed using dynamic ultra-widefield IRcSLO imaging (Nidek Mirante, Nidek Co., Ltd., Gamagori, Japan), and classified according to a vitreous opacity severity scale. Results: Thirty eyes of 21 patients were included in this study. The average age was 56 years. Symptom duration ranged from 1 to more than 365 days. The most common cause of vitreous floaters was posterior vitreous detachment (63.3%), followed by vitreous syneresis (23.3%), asteroid hyalosis (10%) and vitreous hemorrhage (3.3%). Opacities were classified as Grade 1 in three eyes (10%), Grade 2 in 10 eyes (33.3%), Grade 3 in 11 eyes (36.6%), Grade 4 in two eyes (6.6%) and Grade 5 in four eyes (13.3%). Patients with Grade 1 opacities were younger than patients with opacities Grade 2 or greater. A visible Weiss ring could be identified in 0% of eyes with Grade 1 opacities, 40% of eyes with Grade 2 opacities, 100% of eyes with Grade 3 opacities, and 100% of eyes with Grade 4 opacities. In patients with Grade 5 opacities, a Weiss ring could not be identified. Conclusion: Dynamic ultra-widefield IRcSLO imaging is a useful tool to evaluate patients with vitreous floaters. It allows for accurate visualization of the number, density, and behavior of the shadows that vitreous opacities project over a very wide area of the retina, which has a positive correlation with patient perception of floaters.

Determining vitreous viscosity using fluorescence recovery after photobleaching

PURPOSE

Vitreous humor is a complex biofluid whose composition determines its structure and function. Vitreous viscosity will affect the delivery, distribution, and half-life of intraocular drugs, and key physiological molecules. The central pig vitreous is thought to closely match human vitreous viscosity. Diffusion is inversely related to viscosity, and diffusion is of fundamental importance for all biochemical reactions. Fluorescence Recovery After Photobleaching (FRAP) may provide a novel means of measuring intravitreal diffusion that could be applied to drugs and physiological macromolecules. It would also provide information about vitreous viscosity, which is relevant to drug elimination, and delivery.

METHODS

Vitreous viscosity and intravitreal macromolecular diffusion of fluorescently labelled macromolecules were investigated in porcine eyes using fluorescence recovery after photobleaching (FRAP). Fluorescein isothiocyanate conjugated (FITC) dextrans and ficolls of varying molecular weights (MWs), and FITC-bovine serum albumin (BSA) were employed using FRAP bleach areas of different diameters.

RESULTS

The mean (±standard deviation) viscosity of porcine vitreous using dextran, ficoll and BSA were 3.54 ± 1.40, 2.86 ± 1.13 and 4.54 ± 0.13 cP respectively, with an average of 3.65 ± 0.60 cP.

CONCLUSIONS

FRAP is a feasible and practical optical method to quantify the diffusion of macromolecules through vitreous.

Distribution of Ocular Anterior and Posterior Segment Lengths Among a Cataract Surgical Population in Shanghai

Purpose: To investigate the distributions of the ocular anterior and posterior segment lengths among a cataract surgical population in Shanghai. Design: Cross-sectional study. Methods: Ocular biometric parameters of 23,462 eyes of 23,462 cataract surgery candidates were reviewed. Axial length (AL), anterior chamber depth (ACD), and lens thickness (LT) were obtained using IOL Master. Anterior segment length (ASL = ACD + LT), posterior segment length (PSL = AL - ASL) and the ratio of ASL to PSL (ASL/PSL) were calculated. Results: The mean ASL was 7.58 ± 0.39 mm, the mean PSL was 17.12 ± 2.64 mm. As the age grew, the ASL increased, and PSL increased firstly then decreased. Male subjects tended to have significantly longer ASL and shorter PSL than female subjects. With the increasing AL, the ASL was firstly decreased to trough at 20-22 mm AL group, then increased gradually, while the PSL increased rapidly. The ASL correlated positively with AL in normal, moderate and highly myopic eyes, negatively in short eyes. The PSL correlated positively with AL across the entire study population. The ASL/PSL was not constant in the eyes with different AL but had a relatively steep downward trend with the increasing AL in the short eyes, then decreased smoothly in normal, moderate and highly myopic eyes. Conclusions: In Chinese cataractous eyes, longer ASL and shorter PSL were associated with elder age and male gender. The change of ASL over AL was not linear, and the ASL was smallest in the eyes with AL of 20-22 mm. The elongation of the eyeball was mainly due to the extension of the posterior segment.

A simple indentation technique for identifying localized liquefaction of the vitreous body

Identification of vitreous liquefaction (VL) is important for investigating some eye pathologies related to changes in the mechanical properties of the vitreous, such as posterior vitreous detachment or retinal detachment. However, because of the extremely high-water content, characterization of the structural and mechanical properties of the vitreous remains a challenge. In the current study, a simple indentation test was performed on the fresh vitreous, partially liquefied vitreous and water, using a mechanical testing machine with a high-precision load cell. Pressure loads on the indenter changing with time for different samples were obtained to characterize their mechanical properties. VL identification was achieved based on a comparison of relative differences in some typical mechanical parameters (e.g. the pressure at the moment of the insertion (P₀), the pressure at the steady mechanical state (P_∞) and the time when the pressure on the indenter declined to one half the initial value (t_1/2)) between 1) partially liquefied vitreous and fresh vitreous samples and 2) partially liquefied vitreous and water samples. The results indicated that P₀ of the partially liquefied vitreous was almost half that of fresh vitreous (p < 0.05) and approximately 1.5 times that of water (p < 0.05). t_1/2 of the partially liquefied vitreous was approximately 1.4 times higher than that of fresh vitreous (p < 0.05) and approximately 80% of that of water (p < 0.05). It is concluded that P₀ and t_1/2 can be used as indicators to identify localized liquefaction of the vitreous.

Volumetric Optical Imaging and Quantitative Analysis of Age-Related Changes in Anterior Human Vitreous

Purpose

The purpose of this study was to characterize age-related changes in anterior human vitreous with 3-D swept source optical coherence tomography (SS-OCT) and evaluate associations with axial length (AL) and contrast sensitivity function (CSF).

Methods

There were 49 phakic eyes in 49 patients (40.0 ± 19.3 years) had 3-D volumetric scanning of the lens and retrolental vitreous with SS-OCT at 1050 nm. OCT-derived indices of vitreous optical density (VOD), vitreous opacification ratio (VOR), and lens optical density (LOD) were correlated with AL and double-pass assessment of retinal point spread function (Objective Scatter Index [OSI]). CSF was measured using an adaptive-optics visual simulator (area under log-log contrast sensitivity function [AULCSF]).

Results

Vitreous SS-OCT detected gel vitreous, liquefied lacunae, Berger's space, retrolental laminae, and fibrous opacifications. VOD, VOR, and LOD showed high reproducibility (intraclass correlation coefficients 0.968, 0.975, and 0.998, respectively). VOD was highly correlated with VOR (Pearson's R = 0.96, P < 0.000001). VOD, VOR, and LOD correlated with age (R = 0.48, 0.58, and 0.85, P < 0.001 for each). VOR and LOD correlated with OSI (R = 0.36, P = 0.0094, and R = 0.36, P = 0.0096, respectively). VOR correlated negatively with AULCSF (R = −0.53, P < 0.00009), which was related to OSI. Myopic eyes had higher OSI than nonmyopic eyes (P = 0.0121), consistent with correlation between OSI and AL (R = 0.37, P = 0.0091). Multivariable regression confirmed these findings.

Conclusions

SS-OCT visualized microstructural features of anterior human vitreous, where opacification is associated with increased light scattering and CSF degradation. SS-OCT enables high-resolution optical evaluation of vitreous opacities.

Macro- and Microscale Properties of the Vitreous Humor to Inform Substitute Design and Intravitreal Biotransport

Research on the vitreous humor and development of hydrogel vitreous substitutes have gained a rapid increase in interest within the past two decades. However, the properties of the vitreous humor and vitreous substitutes have yet to be consolidated. In this paper, the mechanical properties of the vitreous humor and hydrogel vitreous substitutes were systematically reviewed. The number of publications on the vitreous humor and vitreous substitutes over the years, as well as their respective testing conditions and testing techniques were analyzed. The mechanical properties of the human vitreous were found to be most similar to the vitreous of pigs and rabbits. The storage and loss moduli of the hydrogel vitreous substitutes developed were found to be orders of magnitude higher in comparison to the native human vitreous. However, the reported modulus for human vitreous, which was most commonly tested in vitro, has been hypothesized to be different in vivo. Future studies should focus on testing the mechanical properties of the vitreous in situ or in vivo. In addition to its mechanical properties, the vitreous humor has other biotransport mechanisms and biochemical functions that establish a redox balance and maintain an oxygen gradient inside the vitreous chamber to protect intraocular tissues from oxidative damage. Biomimetic hydrogel vitreous substitutes have the potential to provide ophthalmologists with additional avenues for treating and controlling vitreoretinal diseases while preventing complications after vitrectomy. Due to the proximity and interconnectedness of the vitreous humor to other ocular tissues, particularly the lens and the retina, more interest has been placed on understanding the properties of the vitreous humor in recent years. A better understanding of the properties of the vitreous humor will aid in improving the design of biomimetic vitreous substitutes and enhancing intravitreal biotransport.

A drug delivery analysis of large molecules in ocular vitreous chamber: Dependency on saccadic movements after intravitreal injection

The purpose of this study is to investigate the effect of vitreous sloshing induced by saccades on the intravitreal delivery of large molecule drugs. The vitreous body was considered in its age-related liquefaction condition. Fluid dynamics and large molecule distribution were described by the coupling of mass conservation's and Fick's laws with continuity and momentum equations for a Newtonian incompressible fluid in a 3D unsteady analysis. Two injection sites were analyzed, in both the mixing effect of a 50° periodic saccade leads to uniform drug distribution in 30 s of simulation, the initial bolus site being left after 3 s of simulation. In absence of saccadic movements, the dominant transport contribution is the diffusive one and large molecules hardly reach their uniform distribution inside the vitreous cavity. A model describing the intravitreal distribution of large molecules in presence of saccades was developed, improving the understanding of drug transport mechanism after an intravitreal injection and highlighting how advection contribution enhances its distribution in the vitreous chamber.

Vitreous and Vision Degrading Myodesopsia

Macromolecules comprise only 2% of vitreous, yet are responsible for its gel state, transparency, and physiologic function(s) within the eye. Myopia and aging alter collagen and hyaluronan association causing concurrent gel liquefaction and fibrous degeneration. The resulting vitreous opacities and collapse of the vitreous body during posterior vitreous detachment are the most common causes for the visual phenomenon of vitreous floaters. Previously considered innocuous, the vitreous opacities that cause floaters sometimes impact vision by profoundly degrading contrast sensitivity function and impairing quality-of-life. While many people adapt to vitreous floaters, clinically significant cases can be diagnosed with Vision Degrading Myodesopsia based upon echographic assessment of vitreous structure and by measuring contrast sensitivity function. Perhaps due to the ubiquity of floaters, the medical profession has to date largely ignored the plight of those with Vision Degrading Myodesopsia. Improved diagnostics will enable better disease staging and more accurate identification of severe cases that merit therapy. YAG laser treatments may occasionally be slightly effective, but vitrectomy is currently the definitive cure. Future developments will usher in more informative diagnostic approaches as well as safer and more effective therapeutic strategies. Improved laser treatments, new pharmacotherapies, and possibly non-invasive optical corrections are exciting new approaches to pursue. Ultimately, enhanced understanding of the underlying pathogenesis of Vision Degrading Myodesopsia should result in prevention, the ultimate goal of modern Medicine.

Effects of Collagenase Type II on Vitreous Humor—An In Situ Rheological Study

The purpose of this study is to quantify the impact of enzyme activity on the vitreous humor structure over time to understand the mechanical characteristics of the vitreous humor gel. Changes in the mechanical behavior of the vitreous occur due to many reasons including aging, which may lead to many vitreoretinal diseases. The degeneration process of the vitreous has been studied; however, in situ experimental procedures to validate the existing hypotheses are limited. We examined thirty-eight porcine eyes using in situ rheological creep tests to measure the mechanical properties of the vitreous humor of the eyes prior to, 1 h and 24 h after the intravitreal injection. Eyes in one group were injected with collagenase type II solution and eyes in the control group were injected with phosphate buffered saline solution (PBS) with calcium and magnesium chloride. Prior to the injection, viscosity and creep compliance intercept values between both groups were not statistically different. At 1 h and 24 h after the injection, vitreous properties in the eyes from the first group showed a statistically significant increase in the J intercept values (representing the inverse of elasticity) compared to the control group. In addition, 1 h and 24 h after the injection, vitreous viscosity was lower in the eyes from the first group than in the eyes from the control group. These findings are a foundation for future studies on the effectiveness of intravitreal drugs that modify the mechanical properties of the vitreous humor.

Rheological Properties and Age-Related Changes of the Human Vitreous Humor

The vitreous humor is a fragile, transparent hydrogel situated between the lens and the retina, occupying 80% of the eye's volume. Due to its viscoelastic behavior, the vitreous serves as a mechanical damper for the eye, absorbing impacts, and protecting the lens and retina. The vitreous liquefies with age, which compromises its function as a shock absorber and causes complications including retinal detachment, macular holes, and vitreous hemorrhage. Studies on the viscoelastic properties of the vitreous have been limited. Rheological testing of the vitreous has commonly been done on non-primate mammalian species. Human vitreous rheological properties have been previously reported; however, various measurement techniques were used, resulting in data that differed by orders of magnitude. Shear rheometry is commonly used to characterize soft tissues and hydrogels such as the vitreous humor. However, no human vitreous rheological data have been reported using this technique, preventing direct comparison to other published work. Additionally, no age-related changes in the mechanical properties of the human vitreous humor have been reported. Human vitreous samples (n = 39, aged 62 ± 15 years) were tested using a shear rheometer. Small amplitude oscillatory shear and creep experiments were performed. The linear viscoelastic region of the human vitreous was found to be below 1% strain. The solid phase of the old human vitreous was found to be stiffer than the young human vitreous and the porcine vitreous. The stiffness of the human vitreous gel also appeared to be positively correlated with age. Vitreous dehydration due to a decrease in hyaluronic acid concentration with age was proposed to cause the stiffening of the solid phase of the vitreous gel. Vitreous liquefaction, therefore, might be characterized as a simultaneous increase in liquid volume and localized stiffening of the vitreous gel. The phase separation of the vitreous humor with age has been hypothesized as the cause of many vitreous-related complications. This study provides viscoelastic properties and age-related changes of the human vitreous humor, which will aid in the design of biomimetic vitreous substitutes, enhancement in analyzing intravitreal transport of therapeutics, and understanding the pathological conditions of the vitreous humor.

Combined in-depth, 3D, en face imaging of the optic disc, optic disc pits and optic disc pit maculopathy using swept-source megahertz OCT at 1050 nm

Purpose

To demonstrate papillary imaging of eyes with optic disc pits (ODP) or optic disc pit associated maculopathy (ODP-M) with ultrahigh-speed swept-source optical coherence tomography (SS-OCT) at 1.68 million A-scans/s. To generate 3D-renderings of the papillary area with 3D volume-reconstructions of the ODP and highly resolved en face images from a single densely-sampled megahertz-OCT (MHz-OCT) dataset for investigation of ODP-characteristics.

Methods

A 1.68 MHz-prototype SS-MHz-OCT system at 1050 nm based on a Fourier-domain mode-locked laser was employed to acquire high-definition, 3D datasets with a dense sampling of 1600 × 1600 A-scans over a 45° field of view. Six eyes with ODPs, and two further eyes with glaucomatous alteration or without ocular pathology are presented. 3D-rendering of the deep papillary structures, virtual 3D-reconstructions of the ODPs and depth resolved isotropic en face images were generated using semiautomatic segmentation.

Results

3D-rendering and en face imaging of the optic disc, ODPs and ODP associated pathologies showed a broad spectrum regarding ODP characteristics. Between individuals the shape of the ODP and the appending pathologies varied considerably. MHz-OCT en face imaging generates distinct top-view images of ODPs and ODP-M. MHz-OCT generates high resolution images of retinal pathologies associated with ODP-M and allows visualizing ODPs with depths of up to 2.7 mm.

Conclusions

Different patterns of ODPs can be visualized in patients for the first time using 3D-reconstructions and co-registered high-definition en face images extracted from a single densely sampled 1050 nm megahertz-OCT (MHz-OCT) dataset. As the immediate vicinity to the SAS and the site of intrapapillary proliferation is located at the bottom of the ODP it is crucial to image the complete structure and the whole depth of ODPs. Especially in very deep pits, where non-swept-source OCT fails to reach the bottom, conventional swept-source devices and the MHz-OCT alike are feasible and beneficial methods to examine deep details of optic disc pathologies, while the MHz-OCT bears the advantage of an essentially swifter imaging process.

Electronic supplementary material

The online version of this article (10.1007/s00417-017-3857-9) contains supplementary material, which is available to authorized users.

Estimation of the mechanical properties of the eye through the study of its vibrational modes

Measuring the eye's mechanical properties in vivo and with minimally invasive techniques can be the key for individualized solutions to a number of eye pathologies. The development of such techniques largely relies on a computational modelling of the eyeball and, it optimally requires the synergic interplay between experimentation and numerical simulation. In Astrophysics and Geophysics the remote measurement of structural properties of the systems of their realm is performed on the basis of (helio-)seismic techniques. As a biomechanical system, the eyeball possesses normal vibrational modes encompassing rich information about its structure and mechanical properties. However, the integral analysis of the eyeball vibrational modes has not been performed yet. Here we develop a new finite difference method to compute both the spheroidal and, specially, the toroidal eigenfrequencies of the human eye. Using this numerical model, we show that the vibrational eigenfrequencies of the human eye fall in the interval 100 Hz-10 MHz. We find that compressible vibrational modes may release a trace on high frequency changes of the intraocular pressure, while incompressible normal modes could be registered analyzing the scattering pattern that the motions of the vitreous humour leave on the retina. Existing contact lenses with embebed devices operating at high sampling frequency could be used to register the microfluctuations of the eyeball shape we obtain. We advance that an inverse problem to obtain the mechanical properties of a given eye (e.g., Young's modulus, Poisson ratio) measuring its normal frequencies is doable. These measurements can be done using non-invasive techniques, opening very interesting perspectives to estimate the mechanical properties of eyes in vivo. Future research might relate various ocular pathologies with anomalies in measured vibrational frequencies of the eye.

Apparent Diffusion Coefficient (ADC) of the vitreous humor and Susceptibility Weighted Imaging (SWI) of the retina in abused children with retinal hemorrhages

This study speculated that the apparent diffusion coefficient (ADC) of the vitreous humor might be altered in the setting of abusive head trauma (AHT) with retinal hemorrhages (RH). Fourty-four subjects were analyzed (n=20 AHT cases; n=24 controls). There was no statistically significant difference in normalized ADC values between the cases and controls (-0.14 and -0.08 respectively, p=0.46), but analysis of RH by susceptibility weighted imaging (SWI) compared to dilated funduscopic exam demonstrated statistically significant correlation (p=0.003 and 0.012). Our results suggest that SWI serves as a more sensitive diagnostic tool for detection of ocular injury in AHT than ADC.

An experimental model of vitreous motion induced by eye rotations

BACKGROUND

During eye rotations the vitreous humour moves with respect to the eye globe. This relative motion has been suggested to possibly have an important role in inducing degradation of the gel structure, which might lead to vitreous liquefaction and/or posterior vitreous detachment. Aim of the present work is to study the characteristics of vitreous motion induced by eye rotations.

METHODS

We use an experimental setup, consisting of a Perspex model of the vitreous chamber that, for simplicity, is taken to have a spherical shape. The model is filled with an artificial vitreous humour, prepared as a solution of agar powder and hyaluronic acid sodium salt in deionised water, which has viscoelastic mechanical properties similar to those of the real vitreous. The model rotates about an axis passing through the centre of the sphere and velocity measurements are taken on the equatorial plane orthogonal to the axis of rotation, using an optical technique.

RESULTS

The results show that fluid viscoelasticity has a strong influence on flow characteristics. In particular, at certain frequencies of oscillation of the eye model, fluid motion can be resonantly excited. This means that fluid velocity within the domain can be significantly larger than that of the wall.

CONCLUSIONS

The frequencies for which resonant excitation occurs are within the range of possible eye rotations frequencies. Therefore, the present results suggest that resonant excitation of vitreous motion is likely to occur in practice. This, in turn, implies that eye rotations produce large stresses on the retina and within the vitreous that may contribute to the disruption of the vitreous gel structure. The present results also have implications for the choice of the ideal properties for vitreous substitute fluids.

Ultrasound-based quantification of vitreous floaters correlates with contrast sensitivity and quality of life

PURPOSE

Clinical evaluation of floaters lacks quantitative assessment of vitreous structure. This study used quantitative ultrasound (QUS) to measure vitreous opacities. Since floaters reduce contrast sensitivity (CS) and quality of life (Visual Function Questionnaire [VFQ]), it is hypothesized that QUS will correlate with CS and VFQ in patients with floaters.

METHODS

Twenty-two eyes (22 subjects; age = 57 ± 19 years) with floaters were evaluated with Freiburg acuity contrast testing (FrACT; %Weber) and VFQ. Ultrasonography used a customized probe (15-MHz center frequency, 20-mm focal length, 7-mm aperture) with longitudinal and transverse scans taken in primary gaze and a horizontal longitudinal scan through premacular vitreous in temporal gaze. Each scan set had 100 frames of log-compressed envelope data. Within each frame, two regions of interest (ROIs) were analyzed (whole-central and posterior vitreous) to yield three parameters (energy, E; mean amplitude, M; and percentage of vitreous filled by echodensities, P50) averaged over the entire 100-frame dataset. Statistical analyses evaluated E, M, and P50 correlations with CS and VFQ.

RESULTS

Contrast sensitivity ranged from 1.19%W (normal) to 5.59%W. All QUS parameters in two scan positions within the whole-central ROI correlated with CS (R > 0.67, P < 0.001). P50 in the nasal longitudinal position had R = 0.867 (P < 0.001). Correlations with VFQ ranged from R = 0.52 (P < 0.013) to R = 0.65 (P < 0.001).

CONCLUSIONS

Quantitative ultrasound provides quantitative measures of vitreous echodensity that correlate with CS and VFQ, providing objective assessment of vitreous structure underlying the functional disturbances induced by floaters, useful to quantify vitreous disease severity and the response to therapy.

Quantitative imaging of enzymatic vitreolysis-induced fiber remodeling

PURPOSE

Collagen fiber remodeling in the vitreous body has been implicated in cases of vitreomacular traction, macular hole, and retinal detachment, and also may occur during pharmacologic vitreolysis. The purpose of this study was to evaluate quantitative polarized light imaging (QPLI) as a tool for studying fiber organization in the vitreous and near the vitreoretinal interface in control and enzymatically perturbed conditions.

METHODS

Fiber alignment was measured in anterior-posterior sections of bovine and porcine vitreous. Additional tests were performed on bovine lenses and nasal-temporal vitreous sections. Effects of proteoglycan degradation on collagen fiber alignment using trypsin and plasmin were assessed at the microstructural level using electron microscopy and at the global level using QPLI.

RESULTS

Control vitreous showed fiber organization patterns consistent with the literature across multiple-length scales, including the global anterior-posterior coursing of vitreous fibers, as well as local fibers parallel to the equatorial vitreoretinal interface and transverse to the posterior interface. Proteoglycan digestion with trypsin or plasmin significantly increased fiber alignment throughout the vitreous (P < 0.01). The largest changes (3×) occurred in the posterior vitreous where fibers are aligned transverse to the posterior vitreoretinal interface (P < 0.01).

CONCLUSIONS

Proteoglycan loss due to enzymatic vitreolysis differentially increases fiber alignment at locations where tractions are most common. We hypothesize that a similar mechanism leads to retinal complications during age-related vitreous degeneration. Structural changes to the entire vitreous body (as opposed to the vitreoretinal interface alone) should be evaluated during preclinical testing of pharmacological vitreolysis candidates.

Materials characterization and mechanobiology of the eye

The eye responds to a great deal of internal and external stimuli throughout its normal function. Due to this, a mechanical or chemical analysis alone is insufficient. A systematic materials characterization is needed. A mechanobiological approach is required for a full understanding of the unique properties and function of the eye. This review compiles the mechanical properties of select eye components, summarizes mechanical and chemical testing platforms, and overviews modeling approaches. Analysis is done across studies, experimental methods, and between species in order to summarize what is known about the mechanobiology of the eye. Several opportunities for future research are identified.

Vitreous deformation during eye movement

Retinal detachment results in visual loss and requires surgical treatment. The risk of retinal detachment depends, among other factors, on the vitreous rheology, which varies with age. To date, the viscoelasticity of the vitreous body has only been measured in cadaver eyes. However, the ex vivo and in vivo viscoelasticity may differ as a result of the effect of intravitreal membranes. Therefore, an MRI method and appropriate postprocessing tools were developed to determine the vitreous deformation and viscoelastic properties in the eyes of living humans. Nineteen subjects (eight women and 11 men; mean age, 33 years; age range, 14-62 years) gazed at a horizontal sinusoidal moving target during the segmented acquisition of complementary spatial modulation of magnetization images. The center of the lens and the scleral insertion of the optic nerve defined the imaging plane. The vitreous deformation was tracked with a dedicated algorithm and fitted with the commonly used viscoelastic model to determine the model parameters: the modified Womersley number a and the phase angle b. The vitreous deformation was successfully quantified in all 17 volunteers having a monophasic vitreous. The mean and standard deviation of the model parameters were determined to be 5.5 ± 1.3 for a and -2.3 ± 0.2 for b. The correlation coefficient (-0.76) between a and b was significant. At the eye movement frequency used, the mean storage and loss moduli of the vitreous were around 3 ± 1 hPa. For two subjects, the vitreous deformation was clearly polyphasic: some compartments of the vitreous were gel-like and others were liquefied. The borders of these compartments corresponded to reported intravitreal membrane patterns. Thus, the deformation of the vitreous can now be determined in situ, leaving the structure of the intravitreal membranes intact. Their effect on vitreous dynamics challenges actual vitreous viscoelastic models. The determination of the vitreous deformation will aid in the quantification of local vitreous stresses and their correlation with retinal detachment.

Pulse-encoded ultrasound imaging of the vitreous with an annular array

The vitreous body is nearly transparent both optically and ultrasonically. Conventional 10- to 12-MHz diagnostic ultrasound can detect vitreous inhomogeneities at high gain settings, but has limited resolution and sensitivity, especially outside the fixed focal zone near the retina. To improve visualization of faint intravitreal fluid/gel interfaces, the authors fabricated a spherically curved 20-MHz five-element annular array ultrasound transducer, implemented a synthetic-focusing algorithm to extend the depth-of-field, and used a pulse-encoding strategy to increase sensitivity. The authors evaluated a human subject with a recent posterior vitreous detachment and compared the annular array with conventional 10-MHz ultrasound and spectral-domain optical coherence tomography. With synthetic focusing and chirp pulse-encoding, the array allowed visualization of the formed and fluid components of the vitreous with improved sensitivity and resolution compared with the conventional B-scan. Although optical coherence tomography allowed assessment of the posterior vitreoretinal interface, the ultrasound array allowed evaluation of the entire vitreous body.

Contribution of saccadic motion to intravitreal drug transport: theoretical analysis

PURPOSE

The vitreous humor liquefies with age and readily sloshes during eye motion. The objective was to develop a computational model to determine the effect of sloshing on intravitreal drug transport for transscleral and intra-vitreal drug sources at various locations

METHODS

A finite element model based on a telescopic implicit envelope tracking scheme was developed to model drug dispersion. Flow velocities due to saccadic oscillations were solved for and were used to simulate drug dispersion.

RESULTS

Saccades induced a three-dimensional flow field that indicates intense drug dispersion in the vitreous. Model results showed that the time scale for transport decreased for the sloshing vitreous when compared to static vitreous. Macular concentrations for the sloshing vitreous were found be much higher than that for the static vitreous. For low viscosities the position of the intravitreal source did not have a big impact on drug distribution.

CONCLUSION

Model results show that care should be taken when extrapolating animal data, which are mostly done on intact vitreous, to old patients whose vitreous might be a liquid. The decrease in drug transport time scales and changes in localized concentrations should be considered when deciding on treatment modalities and dosing strategies.

Oxidative damage and the prevention of age-related cataracts

PURPOSE

Cataracts are often considered to be an unavoidable consequence of aging. Oxidative damage is a major cause or consequence of cortical and nuclear cataracts, the most common types of age-related cataracts.

METHODS

In this review, we consider the different risk factors, natural history and etiology of each of the 3 major types of age-related cataract, as well as the potential sources of oxidative injury to the lens and the mechanisms that protect against these insults. The evidence linking different oxidative stresses to the different types of cataracts is critically evaluated.

RESULTS

We conclude from this analysis that the evidence for a causal role of oxidation is strong for nuclear, but substantially lower for cortical and posterior subcapsular cataracts. The preponderance of evidence suggests that exposure to increased levels of molecular oxygen accelerates the age-related opacification of the lens nucleus, leading to nuclear cataract. Factors in the eye that maintain low oxygen partial pressure around the lens are, therefore, important in protecting the lens from nuclear cataract.

CONCLUSIONS

Maintaining or restoring the low oxygen partial pressure around that lens should decrease or prevent nuclear cataracts.

Current treatment approaches in diabetic macular edema

PURPOSE

To review current treatment approaches in diabetic macular edema (DME).

METHODS

The underlying pathopathology, classifications and diagnostic examination techniques including fluorescein angiography, optical coherence tomography and stereoscopic biomicroscopy. Treatment modalities with focal or grid argon laser photocoagulation, pars plana vitrectomy with and without peeling of the inner limiting membrane (ILM), as well as intravitreal injections using triamcinolone acetonide or novel vascular endothelial growth factor (VEGF) inhibitors are described.

RESULTS

DME results from a series of biochemical and cellular changes, causing progressive leakage and exudation. Focal and grid photocoagulation remains the standard care for diabetic maculopathy. However, the availability of new agents raises the possibility of improvements if significant benefits can be validated in randomized clinical trials. Posterior vitreous attachments play a critical role through several mechanical or physiological mechanisms. Vitrectomy without ILM removal seems to be effective in reducing the retinal thickness and improving visual acuity.

CONCLUSION

A proper evaluation of the vitreous and retina is fundamental to select the most appropriate treatment approach in DME. While small microaneurysms with focal DME may be treated by conventional focal photocoagulation, diffuse DME which do not respond to grid photocoagulation may benefit from intravitreal injections using triamcinolone acetonide or novel VEGF inhibitors. Eyes with DME and additional vitreous traction may benefit from pars plana vitrectomy without ILM peeling. A combination of laser, pharmacological and surgical treatment modalities may be necessary to maintain central vision in eyes with DME.

High incidence of vitreomacular traction in recurrent choroidal neovascularisation after repeated photodynamic therapy

BACKGROUND

The causes of recurrent choroidal neovascularisation (CNV) after photodynamic therapy (PDT) remain controversial. Subretinal surgery was carried out after unsuccessful PDT.

AIMS

To determine intraoperatively the status of the posterior vitreous interface.

DESIGN

Interventional case series.

METHODS

Conventional three-port vitrectomy was carried out in 10 eyes with CNV that had undergone 1-4 PDT sessions. The vitreous cutter was held close to the edge of the optic nerve to evaluate the status of the posterior vitreous.

RESULTS

Lesion size showed an increase from 1.5 (standard deviation (SD) 0.53) to 2.3 (SD 0.83) macular photocoagulation study disc diameters, between the first and the last PDT. Intraoperative findings during vitrectomy showed little liquefaction of the vitreous gel and an incomplete posterior-vitreous detachment, with remarkably firm attachments at the macula in all cases (10/10).

CONCLUSION

We determined an abnormally high incidence of vitreous attachments in eyes with recurrent CNV. Vitreomacular attachments may trigger the progression or recurrence of CNV.

Putative aqueous humor misdirection syndrome as a cause of glaucoma in cats: 32 cases (1997-2003)

OBJECTIVE

To characterize the clinical and morphologic aspects of aqueous humor misdirection syndrome (AHMS) in cats and provide a hypothesis regarding its pathogenesis on the basis of detailed analysis of affected cats.

DESIGN

Retrospective study.

ANIMALS

32 cats (40 eyes).

PROCEDURE

Medical records of cats in which AHMS was diagnosed from July 1997 to August 2003 were reviewed. In certain cats, results of additional diagnostic testing were also obtained, including A-scan, B-scan, and high-resolution ultrasonography; streak retinoscopy; video keratometry; and infrared neutralizing videoretinoscopy as well as results of analysis of flash-frozen sections and histologic examination of enucleated globes.

RESULTS

Cats had a uniformly shallow anterior chamber, intact lens zonules, and a narrowed approach to an open iridocorneal angle. Mean age of affected cats was 11.7 years (range, 4 to 16 years), and female cats were significantly more often affected than male cats. Clinical signs included mydriasis, decreased pupillary light reflex, decreased menace response, and blindness. Glaucomatous changes to the optic nerve, incipient cataracts, and eventual blindness were seen. Intraocular pressure was > or = 20 mm Hg (range, 12 to 58 mm Hg) in 32 of 40 eyes. Ultrasonography and histologic examination revealed a thickened anterior vitreal face interposed between the lens and ciliary body, partial ciliary cleft collapse, and cavitated vitreal regions. Various treatment modalities were used.

CONCLUSIONS AND CLINICAL RELEVANCE

AHMS affects older cats, especially females, and may result in glaucoma, vision loss, and signs of ocular pain. Topical administration of carbonic anhydrase inhibitors decreased intraocular pressure.