Molecular mechanisms of posterior vitreous detachment

[Biomechanics of diabetic vitreopapillary traction syndrome]

Diabetic vitreopapillary traction syndrome (VPT) is a variant of diabetic retinopathy (DR) that can lead to vision loss in advanced stages. This review reports on the biomechanics of the vitreous in the pathogenesis of proliferative DR, in particular diabetic VPT. The article analyzes and summarizes literature data, presents the views of different authors on this problem, and provides the results of Russian and foreign scientific research on this pathology. It is concluded that further research in this area can lead to a significant improvement in the results of therapy, timely diagnosis, and preservation of vision in patients with DR.

Hyaluronic acid-lipid binding

Background

Phospholipid (PL)–hyaluronic acid (HA) interactions are relevant to aging-associated vitreous humor liquefaction, therapies for dry eye disease, skin-care products and synovial joint lubrication. Phosphatidyl choline–HA interactions have been well characterized. However, other major lipids found in tears, vitreous humor and synovial joints have not. The purpose of this study was to bridge this gap of knowledge.

Methods

HA (1600 kDa) at 5 mg/mL, was mixed with various lipids ranging in concentration from 0.1 to 10 mg/mL in D₂O. HA–PL binding was measured from the decrease in HA proton resonance intensity with binding using a nuclear magnetic resonance spectrometer.

Results

Cholesterol weakly bound to HA, followed by monoglyceride and palmitoyl palmitate < phosphatidyl choline, phosphatidic acid and sphingomyelin. The maximum amount of PL bound was 14 ± 1 µmoles inferring a 1 to 1 molar ratio of bound PL to HA dimer. Monoglyceride and palmitoyl palmitate required two to three times more lipid to achieve 100% bound HA compared to PL.

Conclusions

Physiological levels of HA, phosphatidyl choline and sphingomyelin would result in only 4% of the hydrophobic hydrogens of HA to be bound. HA–PL binding interactions could be important for therapeutic use of HA in eye drops in future studies to treat dry eye and to trap PL entering the VH to keep them from forming light scattering micelles. HA–lipid binding may also be relevant to the therapeutic effects of topical skin-care products. Both head group and hydrocarbon chain moieties influence HA–lipid interactions.

Risk Factors for Residual Vitreous Cortex at the Fovea after Posterior Vitreous Detachment during Vitrectomy in Ocular Trauma

Purpose

To investigate the frequency and patterns of the residual vitreous cortex (RVC) at the fovea after posterior vitreous detachment (PVD) during vitrectomy after ocular trauma and the risk factors associated with its occurrence.

Methods

A cross-sectional, multicenter, and observational study was conducted in 72 consecutive patients who underwent pars plana vitrectomy after ocular trauma. In patients with PVD after vitrectomy, RVC patterns were visualized using triamcinolone acetonide staining and were classified as diffuse or focal. A multivariate logistic regression analysis was performed to evaluate the association between RVC and various factors, such as the patient's age, preoperative visual acuity, days after injury, the existence of spontaneous PVD, and the type of injury prior to vitrectomy.

Results

Of the 72 eyes with trauma, 35 eyes (48.6%) showed RVC on the macula, of which 19 (54.3%) were of the diffuse type and 16 (45.7%) were of the focal type. A multivariate logistic regression analysis showed that age (OR = 0.933, P=0.025) and days after injury (OR = 0.862, P=0.013) had a significant impact on RVC.

Conclusions

RVC at the fovea is very common after spontaneous or surgical PVD during vitrectomy. Younger patients are more likely to show RVC if vitrectomy is performed soon after injury.

Pharmacologic vitreolysis with plasmin and hyaluronidase in diabetic rats

OBJECTIVE

To evaluate the feasibility of inducing posterior vitreous detachment (PVD) with pharmacologic vitreolysis in diabetic rats.

METHODS

Forty diabetic rats were randomly divided into four groups and treated with different drugs by intravitreous injection respectively as follows: Group A: hyaluronidase (5 U); Group B: plasmin (0.25 U); Group C: hyaluronidase (5 U) plus plasmin (0.25 U); and Group D: balanced salt solution (2 microL). Ten normal rats in Group E were used as controls and were treated with hyaluronidase (5 U) plus plasmin (0.25 U). Clinical observation, visual electrophysiology tests (electroretinogram), transmission electron microscopy, and scanning electron microscopy were performed on the rats 1 week after the injection.

RESULTS

Scanning electron microscopy results showed that PVD did not occur at all with hyaluronidase alone (Group A, 0 of 10) and balanced salt solution alone (Group D, 0 of 10). Partial PVD was found in all eyes treated with plasmin alone (Group B, 10 of 10), whereas complete PVD was present in most but not all eyes treated with both hyaluronidase and plasmin (Group C, 8 of 10). All nondiabetic eyes treated with hyaluronidase and plasmin had total PVD (Group E, 10 of 10). No significant difference in electroretinogram was observed in each group before and after the injection (P < 0.05).

CONCLUSION

Hyaluronidase (5 U) alone is ineffective, whereas plasmin (0.25 U) alone induces partial PVD, a very dangerous state for the diabetic eye. Combination of hyaluronidase and plasmin can induce complete PVD in 12-week old diabetic rats. However, it is more difficult to induce PVD in diabetic rats than in healthy rats. No obvious toxic reaction was observed in each group.

Pathogenesis and classification of proliferative diabetic vitreoretinopathy

PURPOSE

To review the current knowledge regarding the pathogenesis of proliferative diabetic vitreoretinopathy (PDVR) and to present recommendations for its clinical staging.

DESIGN

Focused literature review and authors' clinical experience.

RESULTS

Although several biochemical mediators may be responsible for the pathogenesis of PDVR, no common biochemical pathway exists. Of those mediators, vascular endothelial growth factor is the one most studied so far. However, since in proliferative diabetic retinopathy (PDR) the thickened posterior vitreous cortex is one of the main factors in the development of proliferations, a consequent shrinkage of the posterior vitreous cortex leads to hemorrhages and tractive retinal detachments. Therefore, PDR should be called PDVR. In consequence, the authors present a new morphological classification of PDVR.

CONCLUSIONS

There is no consensus about the biochemical pathway responsible for the progression of PDVR. Although several classifications are described in the literature, the classification suggested here is important in the judgment of, the communication about and the therapy of diabetic retinopathy. Furthermore, it is the only reliable classification for predicting the surgical outcome in diabetics.

Packages of vitreous collagen (type II) in the human retina: an indication of postnatal collagen turnover?

The purpose of this study was to evaluate the vitreoretinal border in the (pre-)equatorial area in nonpathologic human donor eyes, because the majority of retinal defects induced by posterior vitreous detachment (PVD) are located there. Nine eyes (24-80 years) were fixed and embedded in Technovit 8100. After evaluation by light microscope, areas of interest were selected for immunotransmission electron microscope. Anti-type II collagen antibody was used to stain vitreous fibrils and lamellae; anti-type IV collagen antibody was used to identify the internal limiting lamina (ILL); anti-vimentin and anti-CD-68 antibodies stained retinal Muller cells and macrophages, respectively. Observations included fusing of lamellae with the ILL, an intravitreal course of the ILL, and clear focal interruptions in the ILL. In addition, an obvious finding was the presence of intraretinal packages of type II collagen. Interestingly these collagen packages were closely related to Muller cells and, in several eyes, also to macrophages, cell debris and interruptions in the ILL. In our opinion, the collagen packages can reflect the net result of a process of interactive remodelling, in which both breakdown and synthesis of vitreous and ILL collagens take place. Connections between vitreous and intraretinal collagen networks can make the (pre-)equatorial area more vulnerable to tearing and retinal detachment in the case of liquefaction and PVD.

Ultrastructure of the vitreoretinal interface following plasmin assisted vitrectomy

AIMS

To investigate the ultrastructure of the vitreoretinal interface following plasmin induced posterior vitreous detachment.

METHODS

Plasmin (1 or 2 U/0.1 ml) was injected into the vitreous cavity of 24 eyes of freshly slaughtered pigs. The 24 fellow eyes received calcium-free and magnesium-free PBS and served as a control. After incubation at 37 degrees C for 30 and 60 minutes, the globes were placed in fixative and hemisected. Specimens for light, scanning, and transmission electron microscopy were obtained from the posterior pole, the equator, and the vitreous base using a corneal trephine.

RESULTS

All plasmin treated eyes showed posterior vitreous detachment. However, the inner limiting membrane (ILM) was covered by remnants of cortical vitreous at the posterior pole and at the equator. There was a direct correlation between the concentration and exposure times of plasmin and the degree of vitreoretinal separation. Eyes exposed to 1 U plasmin for 30 minutes had a dense network of residual collagen fibrils while those exposed to 1 U plasmin for 60 minutes had only sparse collagen fibrils covering the ILM. Eyes treated with 2 U plasmin for 60 minutes had a smooth retinal surface, consistent with a bare ILM. At the vitreous base there was no vitreoretinal separation. In all control eyes the vitreous cortex was completely attached to the retina. There was no evidence of retinal damage in any plasmin treated eye.

CONCLUSION

Plasmin induces a cleavage between the vitreous cortex and the ILM without morphological changes to the retina. In contrast with previous reports, plasmin produces a smooth retinal surface and additional surgery is not required in this experimental setting. The degree of vitreoretinal separation depends on the concentration and length of exposure to plasmin.

Induction of posterior vitreous detachment in rabbits by intravitreal injection of tissue plasminogen activator following cryopexy

The purpose of this study was to generate intravitreal plasmin after intravitreal injection of tissue plasminogen activator (TPA) and cryopexy, and to assess its proteolytic effect on the vitreoretinal border region.Twenty-four hr after a mild cryopexy, 25 microg recombinant tissue plasminogen activator (TPA) was injected into the vitreous cavity, the fellow eye received an intravitreal injection of the same volume of buffered salt solution. Light, scanning and transmission electron microscopy was performed in 24 eyes that underwent vitrectomy 1 week later. Plasmin was measured prior and 2 hr after intravitreal TPA injection (4 eyes). Hyaluronic acid (8 eyes) and vitronectin (4 eyes) were measured 1 week after TPA- or BSS-injection and compared to untreated controls. In all eyes treated with TPA, histopathologic examination by scanning and transmission electron microscopy demonstrated a complete detachment of the vitreous from the surface of the retina as well as from the posterior surface of the lens. After BSS-injection, vitreous cortex attachment to the retina was demonstrated in all eyes. Two hr after TPA-injection, plasmin increased to 9.75 mU ml(-1)(s.d.+/-2.3). Neither a decrease of hyaluronic acid nor an increase of transglutaminase, that might alter the vitreous structure leading to a collapse of the vitreous, were detected in treated eyes. There was no increase of vitronectin indicating proliferative activity.A temporary breakdown of the blood-retinal barrier by cryopexy combined with intravitreal injection of TPA is a sufficient technique to induce a posterior vitreous detachment enzymatically. The method may be useful prior to mechanical vitrectomy.

Variations of posterior vitreous detachment

AIMS

To identify variations in posterior vitreous detachment (PVD) and establish a clinical classification system for PVD.

METHODS

400 consecutive eyes were examined using biomicroscopy and vitreous photography and classified the PVD variations-complete PVD with collapse, complete PVD without collapse, partial PVD with thickened posterior vitreous cortex (TPVC), or partial PVD without TPVC.

RESULTS

In each PVD type, the most frequently seen ocular pathologies were as follows: in complete PVD with collapse (186 eyes), age related changes without vitreoretinal diseases (77 eyes, 41.4%) and high myopia (55 eyes, 29.6%); in complete PVD without collapse (39 eyes), uveitis (23 eyes, 59.0%) and central retinal vein occlusions (8 eyes, 20.5%); in partial PVD with TPVC (64 eyes), proliferative diabetic retinopathy (30 eyes, 46.9%); and inpartial PVD without TPVC (111 eyes), age related changes without vitreoretinal diseases (62 eyes, 55.9%). This PVD categorisation was significantly associated with the prevalence of each vitreoretinal disease (p < 0.0001, chi 2 test on contingency table).

CONCLUSIONS

PVD variations can be classified into four types, which is clinically useful because each type corresponds well to specific vitreoretinal changes.

Serum-induced collagen gel contraction

PURPOSE

To understand the molecular events underlying disease-related vitreous gel contraction, the effect of serum components on collagen was investigated.

METHODS

Bovine vitreous or dermal collagen was incubated with a mixture of transglutaminase (TG; factor XIIIa) and fibronectin (FN), and the biochemical changes of collagen were monitored by gel electrophoresis. In addition, serum-induced changes in the volume of the collagen gel were monitored.

RESULTS

Gel electrophoresis revealed a new high-molecular-weight band (M(r) 240,000) presumably due to intermolecular cross-links of collagen peptides and FN. The serum components also were shown to cause a significant decrease in the volume of the collagen gel. CONCLUSION. Collagen gel contraction could be attributed to the collagen-FN-collagen cross-links catalyzed by TG.