The Stokes-Einstein equation and the physiological effects of vitreous surgery

Microstructure-Driven Self-Transport and Convection of Water on Membrane Surface for Ultra-Fast, Highly Sensitive, Low-Cost Lateral-Flow Assays

Lateral-flow assay (LFA) is one of the most commonly used detection technologies, in which the chromatographic membranes are currently used as the lateral-flow membrane (e.g., nitrocellulose membrane, NC Mem). However, several disadvantages of existing chromatographic membranes limit the performance of LFA, including relatively low flow velocity of sample solution and relatively more residuals of sample on membrane, which increase detection time and detection noise. Herein, a surface structure membrane (SS Mem) is proposed, which enables fast self-transport of water with a convection manner and realizes low residuals of sample on membrane surface after the flow. On SS Mem, the flow velocity of water is 7.1-fold higher, and the residuals of sample are decreased by 60-67%, comparing those in NC Mem. SS Mem is used as lateral-flow membrane to prepare lateral-flow strips of nanogold LFA and fluorescence LFA for rapid detection of SARS CoV-2 nucleocapsid protein. These LFAs require 210 s per detection, with limits of detection of 3.98 pg mL-1 and 53.3 fg mL-1, sensitivity of 96.5%, and specificity of 90%. The results suggest that SS Mem enables ultrafast, highly sensitive lateral-flow immunoassays and shows great potential as a new type of lateral-flow membrane to broaden the application of LFA.

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.

Protective effect of vitrectomy on the course of diabetic retinopathy: A case report

PURPOSE

To hypothesize the protective role of vitrectomy on the natural course of diabetic retinopathy.

METHODS

Case report.

RESULTS

A 66-year-old lady with a history of type 1 diabetes mellitus since the age of 10 was referred for evaluation. She received laser treatment in her left eye for proliferative diabetic retinopathy 6 years earlier, while her right eye underwent vitrectomy for retinal detachment 17 years before. Diabetic retinopathy was found to be substantially asymmetrical, as the vitrectomized right eye had only mild macular edema without significant vascular abnormalities, whereas the left eye had a complete confluent panretinal photocoagulation with advanced atrophic macular changes. Other reasons for this asymmetry, such as axial length disparity, carotid circulation impairment, or history of branch retinal vein occlusion, were not identified.

CONCLUSIONS

Vitreous removal, performed for a reason other than diabetic retinopathy, may inhibit the retinal complications often associated with longstanding diabetes. Further clinical studies should be considered to ascertain such evidence.

A New Slip Length Model for Enhanced Water Flow Coupling Molecular Interaction, Pore Dimension, Wall Roughness, and Temperature

In this paper, a slip length model is proposed to analyze the enhanced flow based on the Hagen–Poiseuille equation. The model considers the multimechanisms including wall-water molecular interactions, pore dimensions, fractal roughness, and temperature. The increasing wall-water interactions result in the greater slip length and flow enhancement factor. The increased temperature enhances the kinetic energy of water molecules that leads to great surface diffusion coefficient and small work of adhesion. The wall roughness can decrease the slip length and flow enhancement factor in hydrophilic nanopores. This work studies the effects of multimechanisms on slip length and flow enhancement factor theoretically, which can accurately describe the liquid flow in nanopores.

ASSOCIATION BETWEEN THE VITREOMACULAR INTERFACE AND OPTICAL COHERENCE TOMOGRAPHY CHARACTERISTICS IN WET AGE-RELATED MACULAR DEGENERATION

PURPOSE

To study the effect of the vitreomacular interface on various wet age-related macular degeneration (AMD) characteristics including the size and type of choroidal neovascularization (CNV), choroidal thickness, and activity of the CNV.

METHODS

This was a retrospective observational cross-sectional study. The study included 43 patients (51 eyes) with treatment-naive age-related macular degeneration. Twenty-six patients with wet AMD in one eye and dry AMD in the other eye were included in a paired-eye analysis. Patients underwent optical coherence tomography examination using Heidelberg Spectralis (spectral domain optical coherence tomography) at presentation to determine the type of CNV and the vitreomacular status. In addition, various parameters were measured including the choroidal thickness and horizontal width and vertical height measurements of the CNV.

RESULTS

There was no correlation between the height, width, activity or type of the CNV, and the presence or absence of vitreomacular adhesion. The mean choroidal thickness (using enhanced depth imaging) in cases with vitreomacular adhesion was 272.57 μm compared with 197.32 μm in cases with no vitreomacular adhesion, a statistically significant difference (P = 0.003). In the paired-eye study (21 patients), there was no significant difference between the eyes with wet AMD and dry AMD with regard to vitreomacular status or the choroidal thickness. In a subgroup analysis, patients with Type 1 CNV had a significantly higher percentage of vitreomacular adhesion compared with the other eye with dry AMD (P = 0.034).

CONCLUSION

In conclusion, the vitreomacular interface does seem to be associated with an increased choroidal thickness in cases of wet AMD. Furthermore, the association between the vitreomacular interface and wet AMD is more significant for Type 1 CNV.

The diffusion dynamics of PEGylated liposomes in the intact vitreous of the ex vivo porcine eye: A fluorescence correlation spectroscopy and biodistribution study

The diffusion dynamics of nanocarriers in the vitreous and the influence of nanocarrier physicochemical properties on these dynamics is an important aspect of the efficacy of intravitreal administered nanomedicines for the treatment of posterior segment eye diseases. Here we use fluorescence correlation spectroscopy (FCS) to determine liposome diffusion coefficients in the intact vitreous (D_Vit) of ex vivo porcine eyes using a modified Miyake-Apple technique to minimize the disruption of the vitreous fine structure. We chose to investigate whether the zeta potential of polyethylene glycol functionalized (i.e. PEGylated) liposomes altered liposome in situ diffusion dynamics in the vitreous. Non-PEGylated cationic nanocarriers have previously shown little to no diffusion in the vitreous, whilst neutral and anionic have shown diffusion. The liposomes investigated had diameters below 150nm and zeta potentials ranging from -20 to +12mV. We observed that PEGylated cationic liposomes had significantly lower D_Vit values (1.14μm²s^-1) than PEGylated neutral and anionic liposomes (2.78 and 2.87μm²s^-1). However, PEGylated cationic liposomes had a similar biodistribution profile across the vitreous to the other systems. These results show that PEGylated cationic liposomes with limited cationic charge can diffuse across the vitreous and indicate that the vitreous as a barrier to nanocarriers (Ø<500nm) is more complicated than simply an electrostatic barrier as previously suggested.

Retinal oxygen: from animals to humans

This article discusses retinal oxygenation and retinal metabolism by focusing on measurements made with two of the principal methods used to study O₂ in the retina: measurements of PO₂ with oxygen-sensitive microelectrodes in vivo in animals with a retinal circulation similar to that of humans, and oximetry, which can be used non-invasively in both animals and humans to measure O₂ concentration in retinal vessels. Microelectrodes uniquely have high spatial resolution, allowing the mapping of PO₂ in detail, and when combined with mathematical models of diffusion and consumption, they provide information about retinal metabolism. Mathematical models, grounded in experiments, can also be used to simulate situations that are not amenable to experimental study. New methods of oximetry, particularly photoacoustic ophthalmoscopy and visible light optical coherence tomography, provide depth-resolved methods that can separate signals from blood vessels and surrounding tissues, and can be combined with blood flow measures to determine metabolic rate. We discuss the effects on retinal oxygenation of illumination, hypoxia and hyperoxia, and describe retinal oxygenation in diabetes, retinal detachment, arterial occlusion, and macular degeneration. We explain how the metabolic measurements obtained from microelectrodes and imaging are different, and how they need to be brought together in the future. Finally, we argue for revisiting the clinical use of hyperoxia in ophthalmology, particularly in retinal arterial occlusions and retinal detachment, based on animal research and diffusion theory.

Spatial Variations in Vitreous Oxygen Consumption

We investigated the spatial variation of vitreous oxygen consumption in enucleated porcine eyes. A custom made oxygen source was fabricated that could be localized to either the mid or posterior vitreous cavity and steady state vitreous oxygen tension was measured as a function of distance from the source using a commercially available probe. The reaction rate constant of ascorbate oxidation was estimated ex vivo by measuring the change in oxygen tension over time using vitreous harvested from porcine eyes. Vitreous ascorbate from mid and posterior vitreous was measured spectrophotometrically. When the oxygen source was placed in either the mid-vitreous (N = 6) or the posterior vitreous (N = 6), we measured a statistically significant decrease in vitreous oxygen tension as a function of distance from the oxygen source when compared to control experiments without an oxygen source; (p<0.005 for mid-vitreous and p<0.018 for posterior vitreous at all distances). The mid-vitreous oxygen tension change was significantly different from the posterior vitreous oxygen tension change at 2 and 3mm distances from the respective oxygen source (p<0.001). We also found a statistically significant lower concentration of ascorbate in the mid-vitreous as compared to posterior vitreous (p = 0.02). We determined the reaction rate constant, k = 1.61 M^-1s^-1 ± 0.708 M^-1s^-1 (SE), of the oxidation of ascorbate which was modeled following a second order rate equation. Our data demonstrates that vitreous oxygen consumption is higher in the posterior vitreous compared to the mid-vitreous. We also show spatial variations in vitreous ascorbate concentration.

The impact of posterior vitreous adhesion on ischaemia in eyes with retinal vein occlusion

PURPOSE

To evaluate whether posterior vitreous adhesion (PVA) contributes to retinal ischaemia in eyes suffering from central (CRVO) or branch retinal vein occlusion (BRVO).

METHODS

Retrospective patient chart analysis of eyes with CRVO/BRVO receiving pars-plana vitrectomy (ppV). Prior to surgery fluorescence angiography was conducted to classify RVO as ischaemic or not.

RESULTS

Sixty eyes were included, thereof 36 (60%)/24 (40%) with CRVO/BRVO. In the CRVO group, 17 (47%)/19 (53%) eyes were classified as ischaemic/non-ischaemic. Respective results for BRVO-affected eyes were 16 (67%)/8 (33%). PVA/posterior vitreous detachment (PVD) was found in 33 (92%)/3 (8%) eyes with CRVO and in 23 (96%)/1 (4%) patients suffering from BRVO. Value differences of PVA/PVD between ischaemic- and non-ischaemic-typed RVO failed statistical significance for both, CRVO (p = 0.095) and BRVO (p = 1.0).

CONCLUSIONS

Posterior vitreous adhesion had no impact on retinal ischaemia in this investigation. As an attached posterior vitreous cortex acts as a scaffold and thus significantly increases neovascularization (NV) development in ischaemic-typed RVO, a prospective study evaluating the effect of enzymatic vitreolysis is indicated.

Effects of core vitrectomy in the treatment of age-related macular degeneration

PURPOSE

To investigate the clinical efficacy and safety of an additional core vitrectomy to the standard therapy in patients with exudative age-related macular degeneration (ARMD).

METHODS

In this prospective, controlled, single-centre study, 50 eyes of 50 patients (mean age: 74.1 ± 7.1; median 74 (69/78)) with ARMD were enrolled and randomized 1:1 to group 1 - core vitrectomy additional to three times injections of ranibizumab (3x Rbz) and Group 2 - 3x Rbz (control). 1 16 of 25 eyes in Group 1(64%) and 12 of 25 (48%) in Group 2 had a posterior vitreous detachment (PVD) prior to start of the study. Changes in best-corrected visual acuity (BCVA) using ETDRS charts, central macular thickness and macular volume (OCT) as well as the rate of reinjection with an OCT-based pro renata (PRN) protocol were monitored prospectively over 48 weeks. Forty-seven eyes completed follow-up at week 48.

RESULTS

In Group 1, 4 of 24 lost 1 line of BCVA (16.7%) and 3 of 24 lost 2 lines (12.5%), whereas 17 of 24 gained more than 1 line (70.8%) and improved in average by 9.8 letters. In Group 2, 3 of 23 remained stable and 20 of 23 gained more than or exactly 1 line (78.3%), resulting in 14.3 letters, with no loss of lines. Central macular thickness decreased by 85.58 μm (28.8%) in Group 1 and by 121.43 μm (32.68%) in Group 2 compared with baseline. In Group 1, four patients received three additional and two patients, two additional Rbz injections. In Group 2, three patients received three additional, three patients two and 12 patients one additional Rbz injections. This yielded in an average injection rate of 3.66 in Group 1 and 4.17 in Group 2 over 48 weeks. Posterior vitreous detachment (PVD) was identified in Group 1 in 16 of 24 (66.7%) and in Group 2 in 12 of 23 (52.2%) patients at baseline. At week 48, 6 of 8 (75%) of the patients in Group 1 with initial attached vitreous showed a vitreal detachment, whereas only 1 of 11 (9%) in Group 2 had a new occurred detachment of the vitreous. No systemic or ocular adverse events were noticed.

CONCLUSION

An initial core vitrectomy combined with a conventional ranibizumab injection regimen for exudative AMD patients was safe and lead to similar functional results with less intravitreal ranibizumab injections over 48 weeks.

Proteomics of vitreous humor of patients with exudative age-related macular degeneration

BACKGROUND

There is absence of specific biomarkers and an incomplete understanding of the pathophysiology of exudative age-related macular degeneration (AMD).

METHODS AND FINDINGS

Eighty-eight vitreous samples (73 from patients with treatment naïve AMD and 15 control samples from patients with idiopathic floaters) were analyzed with capillary electrophoresis coupled to mass spectrometry in this retrospective case series to define potential candidate protein markers of AMD. Nineteen proteins were found to be upregulated in vitreous of AMD patients. Most of the proteins were plasma derived and involved in biological (ion) transport, acute phase inflammatory reaction, and blood coagulation. A number of proteins have not been previously associated to AMD including alpha-1-antitrypsin, fibrinogen alpha chain and prostaglandin H2-D isomerase. Alpha-1-antitrypsin was validated in vitreous of an independent set of AMD patients using Western blot analysis. Further systems biology analysis of the data indicated that the observed proteomic changes may reflect upregulation of immune response and complement activity.

CONCLUSIONS

Proteome analysis of vitreous samples from patients with AMD, which underwent an intravitreal combination therapy including a core vitrectomy, steroids and bevacizumab, revealed apparent AMD-specific proteomic changes. The identified AMD-associated proteins provide some insight into the pathophysiological changes associated with AMD.

Vitreomacular adhesion and neovascular age-related macular degeneration

We explore the hypothesis that vitreomacular adhesion (VMA) and vitreomacular traction (VMT) play a role in the pathogenesis and clinical course of neovascular ("wet") age-related macular degeneration (AMD). Several biological theories are offered to explain this possible association, including direct tractional force, altered vitreous oxygenation, altered diffusion coefficients of intravitreal molecules, and alterations in the pharmacokinetics of intravitreal drugs. Release of VMT may improve the clinical course of neovascular AMD, and a few case series suggest that vitrectomy can lead to both a functional and anatomic improvement. A large, randomized, controlled clinical trial is underway, investigating pharmacologic release of VMA in eyes with neovascular AMD.

Comparison of cytokine levels from undiluted vitreous of untreated patients with retinal vein occlusion

PURPOSE

To compare cytokines in undiluted vitreous of treatment-naïve patients with macular oedema without vitreomacular traction secondary to branch (BRVO), central (CRVO) and hemi-central (H-CRVO) retinal vein occlusion.

METHODS

Ninety-four patients (median age 72 years, 42 men) underwent an intravitreal combination therapy, including a single-site 23-gauge core vitrectomy and the application of bevacizumab and dexamethasone due to vision-decreasing macular oedema. Among these were 43 patients with BRVO, 35 with CRVO and 16 patients with hemi-CRVO, which were distributed in a fresh or old retinal vein occlusion type (seven or more months after onset). Undiluted vitreous samples were analysed for interleukin 6 (IL-6), monocyte chemoattractant protein-1 (MCP-1) and vascular endothelial growth factor (VEGF-A) with cytometric BEAD assay. Vitreous samples from patients with idiopathic epiretinal membrane served as controls (n = 14).

RESULTS

The mean cytokine values were highest in the CRVO group with IL-6 = 64.7 pg/ml (SD ± 115.8), MCP-1 = 1015.8 pg/ml (±970.1) and VEGF-A = 278.4 pg/ml (±512.8), followed by the H-CRVO group with IL-6 = 59.9 pg/ml (SD ± 97.5), MCP-1 = 938.8 pg/ml (±561.1) and VEGF-A = 211.5 pg/ml (±232.4). The BRVO group had IL-6 = 23.2 pg/ml (SD ± 48.8), MCP-1 = 602.6 g/ml (±490.3) and VEGF-A = 161.8 pg/ml (±314.4). The values of MCP-1 and VEGF-A were significantly different for CRVO or H-CRVO versus BRVO. All values were significantly higher than in the control samples, which had 6.2 ± 3.4 pg/ml (IL-6), 253 ± 74 pg/ml (MCP-1) and 7 ± 4.9 pg/ml (VEGF-A). Within the old RVO type, only MCP-1 was significantly different for CRVO or H-CRVO versus BRVO.

CONCLUSIONS

Both inflammatory markers and VEGF-A were higher in CRVO and H-CRVO than in BRVO undiluted vitreous samples. It seems that monocyte recruitment to the vessel wall, which might underlie the importance of eosinophils in tissue remodelling after RVO, is of special interest owing to the significant difference in MCP-1 in the older RVO types.

The vitreomacular interface in retinal vein occlusion

PURPOSE

To evaluate the posterior vitreous adhesion status in patients with a history of central or branch retinal vein occlusion and to compare the results with the natural time-course of posterior vitreous detachment in healthy age-related controls.

METHODS

A retrospective chart review in terms of the posterior vitreous adhesion status was performed in 132 patients (133 eyes) with a history of a central (CRVO) or branch (BRVO) retinal vein occlusion. All patients underwent vitrectomy. Based on the operation reports, the vitreous adhesion status was classified as attached, partially detached or completely detached. The results were compared to the natural time-course of posterior vitreous detachment development in healthy age-related controls.

RESULTS

Eighty-one eyes met the inclusion and exclusion criteria. Fifty-two eyes (64%) had a history of CRVO and 29 eyes (36%) a history of BRVO, respectively. In the CRVO group, the posterior vitreous was attached in 47 eyes (90%) and completely detached in five eyes (10%). In the BRVO group, the posterior vitreous was attached in 27 eyes (93%), partially detached in 1 eye (3%) and completely detached in another eye (3%). A subdivision into age classes and a comparison with healthy age-related controls [data by Weber-Krause & Eckardt (1997) Ophthalmologe, 94, 619-623] showed in patients between 65 and 69 years of age an attached posterior vitreous cortex in 72% in healthy eyes, in 100% in CRVO (p = 0.109) and in 89% in BRVO (p = 0.440), in patients between 70 and 79 years of age an attached posterior vitreous cortex in 56% in healthy eyes, in 86% in CRVO (p = 0.010) and in 100% in BRVO (p = 0.038) and in patients between 80 and 89 years of age an attached posterior vitreous cortex in 43% in healthy eyes, in 100% in CRVO (p = 0.191) and in 67% in BRVO (p = 0.582) (Fisher's exact t-test).

CONCLUSION

In patients with a history of CRVO or BRVO, the posterior vitreous cortex stays attached more frequently in all age groups in comparison with the healthy age-related controls.

Vitrectomy may prevent the occurrence of diabetic macular oedema

PURPOSE

This study aimed to demonstrate that vitrectomy may prevent the occurrence of diabetic macular oedema (DMO).

METHODS

Three patients with diabetes type 1 underwent vitrectomy in one eye to treat complications of proliferative diabetic retinopathy.

RESULTS

During follow-up, all patients suffered unilateral macular oedema in the non-vitrectomized eye as a result of general metabolic changes. In two of these patients, the DMO resolved with management of the underlying medical condition.

CONCLUSIONS

These case reports suggest the vitreous may play a role in the occurrence of DMO associated with general risk factors. Further studies are needed to increase understanding of the mechanisms involved in the development and progression of DMO.

Vitreoretinal disease in the coming decade

PURPOSE OF REVIEW

The field of vitreoretinal disease and surgery has seen tremendous growth and innovation in recent years. In view of the latest advances, the present article highlights some of the current trends in vitreoretinal research that promise to be revolutionary in the coming decade.

RECENT FINDINGS

Pharmacologic vitreolysis may greatly impact the current approach to treatment of various vitreoretinopathies. New methods of drug delivery to the posterior segment either via nanoparticles or sustained-release intravitreal implants may supplant repetitive intravitreal injections. Gene therapy may become more common as the genetic basis of inherited retinal diseases is further elucidated. Stem-cell transplantation and the implantation of artificial retinal prostheses offer promise for long-term sight restoration.

SUMMARY

Based on recent advances in ongoing vitreoretinal research, the spectrum of treatable retinal disease is likely to expand significantly in the coming decade, with enormous public health impact, as well as significant changes in the practice patterns of retina specialists worldwide.

Physiology of vitreous surgery

Vitreous surgery has various physiological and clinical consequences, both beneficial and harmful. Vitrectomy reduces the risk of retinal neovascularization, while increasing the risk of iris neovascularization, reduces macular edema and stimulates cataract formation. These clinical consequences may be understood with the help of classical laws of physics and physiology. The laws of Fick, Stokes-Einstein and Hagen-Poiseuille state that molecular transport by diffusion or convection is inversely related to the viscosity of the medium. When the vitreous gel is replaced with less viscous saline, the transport of all molecules, including oxygen and cytokines, is facilitated. Oxygen transport to ischemic retinal areas is improved, as is clearance of VEGF and other cytokines from these areas, thus reducing edema and neovascularization. At the same time, oxygen is transported faster down a concentration gradient from the anterior to the posterior segment, while VEGF moves in the opposite direction, making the anterior segment less oxygenated and with more VEGF, stimulating iris neovascularization. Silicone oil is the exception that proves the rule: it is more viscous than vitreous humour, re-establishes the transport barrier to oxygen and VEGF, and reduces the risk for iris neovascularization in the vitrectomized-lentectomized eye. Modern vitreous surgery involves a variety of treatment options in addition to vitrectomy itself, such as photocoagulation, anti-VEGF drugs, intravitreal steroids and release of vitreoretinal traction. A full understanding of these treatment modalities allows sensible combination of treatment options. Retinal photocoagulation has repeatedly been shown to improve retinal oxygenation, as does vitrectomy. Oxygen naturally reduces VEGF production and improves retinal hemodynamics. The VEGF-lowering effect of photocoagulation and vitrectomy can be augmented with anti-VEGF drugs and the permeability effect of VEGF reduced with corticosteroids. Starling's law explains vasogenic edema, which is controlled by osmotic and hydrostatic gradients between vessel and tissue. It explains the effect of VEGF-induced vascular permeability changes on plasma protein leakage and the osmotic gradient between vessel and tissue. At the same time, it takes into account hemodynamic changes that affect the hydrostatic gradient. This includes the influence of arterial blood pressure, and the effect oxygen (laser treatment) has in constricting retinal arterioles, increasing their resistance, and thus reducing the hydrostatic pressure in the microcirculation. Reduced capillary hydrostatic pressure and increased osmotic gradient reduce water fluxes from vessel to tissue and reduce edema. Finally, Newton's third law explains that vitreoretinal traction decreases hydrostatic tissue pressure in the retina, increases the pressure gradient between vessel and tissue, and stimulates water fluxes from vessel into tissue, leading to edema.

Enzyme-induced posterior vitreous detachment in the rat produces increased lens nuclear pO2 levels

It has been proposed that disruption of normal vitreous humor may permit O(2) to travel more easily from the retina to the center of the lens where it may cause nuclear cataract (Barbazetto, I.A., Liang, J., Chang, S., Zheng, L., Spector, A., Dillon, J.P., 2004. Oxygen tension in the rabbit lens and vitreous before and after vitrectomy. Exp. Eye Res. 78, 917-924; Harocopos, G.J., Shui, Y.B., McKinnon, M., Holekamp, N.M., Gordon, M.O., Beebe, D.C., 2004. Importance of vitreous liquefaction in age-related cataract. Invest. Ophthalmol. Vis. Sci. 45, 77-85). In the present study, we injected enzymes intravitreally into guinea pigs (which possess an avascular retina) and rats (which possess a vascular retina) to produce either vitreous humor liquefaction plus a posterior vitreous detachment (PVD) (with use of microplasmin) or vitreous humor liquefaction only (with use of hyaluronidase), and 1-2 weeks later measured lens nuclear pO(2) levels in vivo using a platinum-based fluorophore O(2) sensor (Oxford-Optronix, Ltd.). Experiments were also conducted in which the animals were allowed to breathe 100% O(2) following intravitreal injection with either microplasmin or hyaluronidase in order to investigate possible effects on O(2) exchange within the eye. Injection of guinea pigs with either of the two enzymes produced no significant differences in lens pO(2) levels 1-2 weeks later, compared to controls. However, for the rat, injection of microplasmin produced a 68% increase in O(2) level in the center of the lens, compared to the controls (5.6mm Hg increasing to 9.4mm Hg, p<0.05), with no corresponding effect observed following similar use of hyaluronidase. Treatment of guinea pigs with microplasmin dramatically accelerated movement of O(2) across the vitreal space when the animals were later allowed to breathe 100% O(2) (for example, O(2) traveled to a location directly behind the lens 5x faster than control; p<0.01); however, the effect following treatment with hyaluronidase was significantly less. When microplasmin-injected rats breathed 100% O(2), the time required for O(2) to reach the center of the lens was 3x faster than control (0.4 min compared to 1.4 min, p<0.01). The results have implication with regard to the occurrence of age-related PVD in the human, and a possible acceleration of maturity-onset nuclear cataract. In addition, enzymatic creation of a PVD to increase the rate of O(2) exchange within the vitreal space may have potential application for treatment of retinal ischemic disease.

Vitreomacular traction and exudative age-related macular degeneration

Vitreomacular traction resulting from lacking, incomplete or anomalous posterior vitreous detachment is suspected to play a crucial role in the pathogenesis of different forms of age-related macular degeneration (AMD) along with the known mechanisms. It is probable that the fundamental pathomechanisms of AMD formation have already begun by the time tractional forces lead to a change for the worse. Vitreomacular traction alone is perhaps not able to induce AMD. It would seem sensible to consider vitreous changes when diagnosing and treating AMD patients because of the high coincidence of vitreomacular traction and choroidal neovascularization (CNV) and the often successful treatment of other diseases of the vitreoretinal interface by vitrectomy. The concept of the pathogenesis of AMD should therefore be extended to include the influence of the vitreous, especially where therapeutic concepts such as pharmacological vitreolysis and vitreous separation have been established as causative treatment of late forms of AMD.