Trabecular repopulation by anterior trabecular meshwork cells after laser trabeculoplasty

Aberrant glycosylation in the human trabecular meshwork

PURPOSE

To determine the difference in protein glycosylation and glycosylation enzyme levels between glaucomatous and control trabecular meshwork (TM).

EXPERIMENTAL DESIGN

Glaucomatous and normal donor (n = 12 each) TM tissues, lectin fluorescence, fluorophore-assisted carbohydrate analyses, and quantitative MS were used to determine the glycosylation levels. Primary TM cells and glycosylation inhibitors were used to determine their effect on cell shape and motility.

RESULTS

In contrast to elevated levels of glycoproteins determined by lectin fluorescence, simultaneous hyper- and hypo-glycosylation in glaucomatous TM was revealed by fluorophore-assisted carbohydrate analyses. Analyses of enzymes showed elevation of beta-glycosidase 1 and decrease in galactosyltransferase family 6 domain containing protein 1 in the glaucomatous TM. Quantitative MS identified select protein level changes between glaucomatous and normal TM. Primary TM cells were treated with inhibitors to elicit hypo-glycosylation, which affected cell shape, motility, and fluorescent tracer transport across a layer of TM cells.

CONCLUSIONS AND CLINICAL RELEVANCE

Global protein glycosylation is aberrant in glaucomatous TM compared to controls. The results presented here suggest that the alteration in global TM protein glycosylation encompassing cellular and extracellular matrix proteins contributes to glaucoma pathology likely mediated through changes in properties of TM cells.

Morphologic changes in trabecular meshwork after patterned and argon laser trabeculoplasty in cats

PURPOSE

Patterned laser trabeculoplasty (PLT) was introduced as it claimed to cause less thermal damage and provide more uniform coverage of the trabecular meshwork than argon laser trabeculoplasty (ALT). The objective of this study was to investigate morphologic changes in the trabecular meshwork after PLT or ALT in cats.

MATERIALS AND METHODS

We performed ALT in the right eyes and PLT in the left eyes of 6 domestic cats. A seventh cat was assigned as a control. Two ranges of laser energy were used for PLT: supra-threshold energy of 400-450 mW in 3 cats and sub-threshold energy of 250 to 350 mW in 3 cats. Specimens were obtained at 1, 4 and 9 weeks after treatment. Structural changes in the trabecular meshwork were evaluated by light microscopy and scanning electron microscopy.

RESULTS

The trabecular meshwork after supra-threshold PLT revealed coagulative damage such as a crater-like lesion with disruption of trabecular beams in early period and extensive membranous obliteration in the late period (at 4 and 9 weeks after treatment), which were comparable to tissue changes after ALT. Sub-threshold PLT resulted in thinning of the uveal meshwork and denudation of trabecular endothelial cells whereas it did not disrupt trabecular beams. Nevertheless, following sub-threshold PLT, partial membranous coverages were observed in the trabecular meshwork in the late period.

CONCLUSIONS

When used at sub-threshold power, PLT caused less thermal damage to the trabecular meshwork than ALT. However, it did not effectively prevent late scarring of the trabecular meshwork in cats.

Mechanisms of selective laser trabeculoplasty: a review

Selective laser trabeculoplasty is a safe and effective treatment for glaucoma, with greater cost effectiveness than its pharmacological and surgical alternatives. Nevertheless, although the basic science literature on selective laser trabeculoplasty continues to grow, there remains uncertainty over the mechanism by which laser trabeculoplasty reduces intraocular pressure. To address this uncertainty, the evidence behind several potential biological and mechanical mechanisms of selective laser trabeculoplasty were reviewed. In particular, cytokine secretion, matrix metalloproteinase induction, increased cell division, repopulation of burn sites and macrophage recruitment were discussed. Refining our understanding of these mechanisms is essential both to understanding the pathophysiology of ocular hypertension and developing improved therapies to treat the condition.

Human trabecular meshwork cells exhibit several characteristics of, but are distinct from, adipose-derived mesenchymal stem cells

PURPOSE

To support the growing promise of regenerative medicine in glaucoma, we characterized the similarities and differences between human trabecular meshwork (HTM) cells and human mesenchymal stem cells (hMSCs).

METHODS

HTM cells and hMSCs were phenotypically characterized by flow cytometry. Using quantitative polymerase chain reaction, the expression of myoc, angptl7, sox2, pou5f1, and notch1 was determined in both cell types with and without dexamethasone (Dex). Immunosuppressive behavior of HTM cells and hMSCs was determined using T cells activated with phytohemagglutinin. T-cell proliferation was determined using BrdU incorporation and flow cytometry. Multipotency of HTM cells and hMSCs was determined using adipogenic and osteogenic differentiation media as well as aqueous humor (AH). Alpha-smooth muscle actin (αSMA) expression was determined in HTM cells, hMSCs, and HTM tissue.

RESULTS

Phenotypically, HTM and hMSCs expressed CD73, CD90, CD105, and CD146 but not CD31, CD34, and CD45 and similar sox2, pou5f1, and notch1 expression. Both cell types suppressed T-cell proliferation. However, HTM cells, but not hMSCs, upregulated myoc and angptl7 in response to Dex. Additionally, HTM cells did not differentiate into adipocytes or osteocytes. Culture of hMSCs in 20%, but not 100%, AH potently induced alkaline phosphatase activity. HTM cells in culture possessed uniformly strong expression of αSMA, which contrasted with the limited expression in hMSCs and spatially discrete expression in HTM tissue.

CONCLUSIONS

HTM cells possess a number of important similarities with hMSCs but lack multipotency, one of the defining characteristics of stem cells. Further work is needed to explore the molecular mechanisms and functional implications underlying the phenotypic similarities.

Identification and characterization of mesenchymal stem cells derived from the trabecular meshwork of the human eye

Mesenchymal stem cells (MSC) have been isolated from several adult human tissues. Their propensity to differentiate into cell types of connective tissue, such as osteocytes, chondrocytes, and adipocytes, suggests that MSC may function as a reserve of progenitor cells that repair and maintain healthy adult tissues. Dysfunction of the trabecular meshwork (TM), a connective tissue at the anterior region of the human eye that regulates intraocular pressure, plays a major role in the pathogenesis of glaucoma. The mechanobiology and pharmacological aspects of the TM tissue have been relatively well studied in disease states. Less well understood is if there are progenitor cells within the TM that contribute to maintenance of this tissue. In this study, we have identified and characterized an expandable population of cells that have stem cell-like properties. In particular, these cells express the markers CD73, CD90, and CD105, which are typically associated with MSC. Thus, we have named these cells TM-MSC. As further evidence that these cells are MSC, they were differentiated in vitro into adipocytes, osteocytes, and chondrocytes. Through genomic characterization, we show that TM-MSC have gene expression patterns most similar to MSC derived from other tissues. TM-MSC express genes found on adult TM tissue, suggesting that TM-MSC are progenitor cells that serve to maintain a healthy TM.

Progenitors for the corneal endothelium and trabecular meshwork: a potential source for personalized stem cell therapy in corneal endothelial diseases and glaucoma

Several adult stem cell types have been found in different parts of the eye, including the corneal epithelium, conjunctiva, and retina. In addition to these, there have been accumulating evidence that some stem-like cells reside in the transition area between the peripheral corneal endothelium (CE) and the anterior nonfiltering portion of the trabecular meshwork (TM), which is known as the Schwalbe's Ring region. These stem/progenitor cells may supply new cells for the CE and TM. In fact, the CE and TM share certain similarities in terms of their embryonic origin and proliferative capacity in vivo. In this paper, we discuss the putative stem cell source which has the potential for replacement of lost and nonfunctional cells in CE diseases and glaucoma. The future development of personalized stem cell therapies for the CE and TM may reduce the requirement of corneal grafts and surgical treatments in glaucoma.

Stemming vision loss with stem cells

Dramatic advances in the field of stem cell research have raised the possibility of using these cells to treat a variety of diseases. The eye is an excellent target organ for such cell-based therapeutics due to its ready accessibility, the prevalence of vasculo- and neurodegenerative diseases affecting vision, and the availability of animal models to demonstrate proof of concept. In fact, stem cell therapies have already been applied to the treatment of disease affecting the ocular surface, leading to preservation of vision. Diseases in the back of the eye, such as macular degeneration, diabetic retinopathy, and inherited retinal degenerations, present greater challenges, but rapidly emerging stem cell technologies hold the promise of autologous grafts to stabilize vision loss through cellular replacement or paracrine rescue effects.

Rapid and delayed death of cultured trabecular meshwork cells after selective laser trabeculoplasty

BACKGROUND AND OBJECTIVE

Selective laser trabeculoplasty (SLT) is becoming increasingly employed to reduce elevated intraocular pressure in glaucoma patients. SLT is known to target the ocular trabecular meshwork (TM), but the exact response mechanisms to this treatment have not been clearly delineated. The aim of the present study, therefore, was to investigate the modes of death of cultured bovine TM cells subjected to SLT in vitro.

MATERIALS AND METHODS

Bovine TM cell cultures were established, pigmented with exogenous melanin and irradiated with a Q-switched, frequency doubled, Nd:YAG laser, at different energy settings (0.05-1.0 mJ). Influences on cells were determined for up to 10 days post-treatment by trypan blue exclusion, terminal deoxynucleotidyl transferase dUTP nick-end labeling (TUNEL) and by morphological assessment. Furthermore, homogeneous mixtures of pigmented and non-pigmented TM cells were irradiated to ascertain selectivity of laser effects.

RESULTS

At higher energy levels (1.0, 0.75 mJ), immediate loss of cells was detected at the irradiated site. Trypan blue exclusion analysis showed that necrotic cell death subsequently occurred up to 8 hours following irradiation, peaking at 60 minutes. This was followed by delayed cell death peripheral to the irradiated area which was characteristic of apoptosis and which peaked at 2-3 days post-treatment. When mixed cultures were tested, laser treatment selectively killed pigmented cells at an energy level equivalent to the lower cell killing threshold in the initial studies (0.2 mJ) but at the higher laser energy of 0.35 mJ, all cells were non-selectively killed.

CONCLUSIONS

SLT treatment killed pigmented TM cells in culture by a variety of processes (instant vaporization, rapid necrosis, delayed apoptosis), depending on the magnitude of the energy used and the distance from the center of the irradiated zone. These data may assist in the elucidation of the mechanism of action of the SLT procedure on TM cells in situ.

Impact of laser trabeculoplasty on Trabectome® outcomes

BACKGROUND AND OBJECTIVE

To report the impact of prior laser trabeculoplasty on clinical outcomes of Trabectome (NeoMedix Corporation, Tustin, CA) surgery.

PATIENTS AND METHODS

This is a retrospective, non-comparative study of consecutive cases of Trabectome surgery with 36 months of follow-up. Main outcome measures were intraocular pressure (IOP), number of glaucoma medications, and the occurrence of secondary procedures.

RESULTS

In the trabeculoplasty group, mean IOP measured 16.5 +/- 4.0 mm Hg with an average decrease of 24% from preoperative IOP at 12 months. In eyes without previous trabeculoplasty, IOP measured 15.7 +/- 3.0 mm Hg with an average decrease of 30% at 12 months. Adjunctive medications decreased to 2.1 and 1.5 glaucoma medications, respectively. Secondary procedures were performed in 12% and 10% of eyes in each group.

CONCLUSION

Previous laser trabeculoplasty does not appear to significantly impact IOP, but may increase the need for glaucoma medication in patients undergoing Trabectome surgery.

[Available laser systems and mechanism of action in laser trabeculoplasty]

Intraocular pressure-lowering laser procedures such as argon laser trabeculoplasty and selective laser trabeculoplasty are nowadays an essential part of glaucoma therapy. This article presents the currently most popular laser systems. These various laser procedures are based on the physical structure of the different lasers and on variable generation of laser light. They vary in their effect on the trabecular meshwork on a histopathological level, as well as in the active principles of intraocular pressure reduction. The mechanical, biological, and repopulation theories are currently used to explain the intraocular pressure-lowering effect of the different laser systems; these are discussed in detail.

Laser trabeculoplasty induces changes in the trabecular meshwork glycoproteome: a pilot study

Laser trabeculoplasty (LT) is a commonly used modality of treatment for glaucoma. The mechanism by which LT lowers the intraocular pressure (IOP) is unknown. With the use of cat eyes, selective laser trabeculoplasty (SLT) with a Q-switched frequency doubled Nd:YAG laser was used to treat the trabecular meshwork (TM). Laser treated TM was then subjected to proteomic analysis for detection of molecular changes and histological analysis for the detection of structural and protein expression patterns. In addition, the protein glycosylation patterns of laser treated and nontreated TM was assessed and differentially glycosylated proteins were proteomically identified. SLT laser treatment to the TM resulted in elevated glycosylation levels compared to nonlasered TM. TM laser treatment also resulted in protein expression levels changes of several proteins. Elevated levels of biglycan, keratocan and prolargin were detected in laser treated TM compared to nonlasered controls. Further investigation is anticipated to provide insight into how glycosylation changes affect TM proteins and TM regulation of aqueous outflow in response to laser trabeculoplasty.

Argon laser trabeculoplasty versus selective laser trabeculoplasty

The clinical role of laser trabeculoplasty remains a controversial topic with many different opinions. In the following three viewpoints the authors have taken contrasting positions on whether argon laser trabeculoplasty remains the "gold standard" or has been supplanted by selective laser trabeculoplasty. Questions are raised regarding the position of laser trabeculoplasty within the stepping of the treatment paradigm for open-angle glaucoma. The appropriate use of laser trabeculoplasty at different stages of disease severity is analyzed. A broader perspective with new insights on laser trabeculoplasty from these articles will hopefully lead to a better understanding of its clinical role in practice.

Stem cells in the trabecular meshwork: present and future promises

Primary open-angle glaucoma is recognized as a disease of aging, and studies show a relationship between aging and trabecular meshwork (TM) cell density. Human TM cell division occurs primarily in the anterior, non-filtering region. A commonly used glaucoma treatment, laser trabeculoplasty (LTP), triggers and increases cell division, as well as cell migration of these anterior TM cells. These freshly-divided migrating cells repopulate the burned laser sites, suggesting that they are stem cells. Several studies concerning this putative TM stem cell will be discussed.

Specialized podosome- or invadopodia-like structures (PILS) for focal trabecular meshwork extracellular matrix turnover

PURPOSE

There are distinctive areas of colocalization of matrix metalloproteinase (MMP)-2 and -14 on trabecular meshwork (TM) cells that resemble podosomes or invadopodia. Studies were conducted to determine whether TM cells exhibit podosome- or invadopodia-like structures (PILS) and whether they produce focal extracellular matrix (ECM) turnover.

METHODS

Porcine and human TM cells and perfused anterior segment organ cultures were studied. Localization of PILS components on TM cells and in sections from anterior segments was determined by immunohistochemistry and confocal microscopy. Cells were grown on type I collagen labeled with fluorescein isothiocyanate (FITC) for degradation analysis. Confocal time lapse images were taken of labeled TM cells on FITC-collagen.

RESULTS

Immunostaining for MMP-2, MMP-14, and the typical PILS components cortactin, caldesmon, alpha-actinin, N-WASP, Arp-3, and cdc42 colocalized on these distinctive structures. Integrin-alphaV and -beta1, fibronectin, and versican colocalized with PILS components. TM cells on FITC-conjugated collagen developed focal regions of degradation. Time-lapse imaging showed dramatic and controlled movement of TM cell processes during this ECM degradation and fragment internalization. MMP-2, MMP-14, and cortactin colocalized at regions that appear to be PILS on cells within the outflow pathway in sections of human anterior segments.

CONCLUSIONS

TM cells exhibit areas where PILS components colocalize with MMP-2 and -14. Similar structures are found in sections, suggesting that PILS occur in situ in the outflow pathway. The collagen degradation suggests that PILS may serve as focal sites for targeted ECM turnover, an event linked to modifications of aqueous outflow resistance and intraocular pressure homeostasis.

Mechanisms of action and efficacy of argon laser trabeculoplasty and selective laser trabeculoplasty

PURPOSE OF REVIEW

Since the 1980s, laser trabeculoplasty has served as an effective way to lower intraocular pressure in patients with primary or secondary open angle glaucomas, both as an initial therapy or in conjunction with hypotensive medications. This manuscript will describe the proposed mechanisms of action of argon laser trabeculoplasty and selective laser trabeculoplasty, as well as review current studies of the therapeutic effect of these interventions.

RECENT FINDINGS

The exact mechanisms by which argon laser and selective laser trabeculoplasty lower intraocular pressure are not known. There are several theories, however, and we discuss the three most common ones: the mechanical theory, the cellular (biologic) theory, and the cell division theory. Since both lasers are applied to the same tissue and produce similar results, they most likely produce their effects in comparable ways. We also describe the results of several studies comparing these devices. Most show them to be equally effective at lowering intraocular pressure; however, there are a few circumstances when selective laser trabeculoplasty may be a better option than argon laser trabeculoplasty.

SUMMARY

Argon laser and selective laser trabeculoplasty are safe and effective procedures for lowering intraocular pressure. The results of ongoing clinical trials will help further define their role in the management of patients with open angle glaucoma.

Effect of Laser Trabeculoplasty on Nocturnal Intraocular Pressure in Medically Treated Glaucoma Patients

PURPOSE

To evaluate the effects of laser trabeculoplasty on 24-hour intraocular pressure (IOP) in a group of medically treated open-angle glaucoma patients.

DESIGN

Prospective experimental study.

PARTICIPANTS

Eighteen open-angle glaucoma patients.

METHODS

Laser trabeculoplasty (180 degrees ) was performed on 28 eyes of 18 glaucoma patients. Twenty-four-hour IOP data were collected in a sleep laboratory before and 45 to 80 days after the procedure. Measurements of sitting and supine IOP were taken during the 16-hour diurnal/wake period, and measurements of supine IOP were taken during the 8-hour nocturnal/sleep period in 2-hour intervals.

MAIN OUTCOME MEASURES

Changes in the mean, peak, and range of IOP during the office-hour, diurnal, nocturnal, and 24-hour periods.

RESULTS

Compared with the baselines, changes in the mean, peak, and range of IOP were not significant during the office-hour period and during the diurnal period in either the sitting or the supine position. The mean, peak, and range of IOP were reduced significantly during the nocturnal period in the supine position. Mean and peak 24-hour IOP were reduced significantly in the habitual body positions (sitting during the diurnal period and supine during the nocturnal period). The reduction of mean 24-hour IOP in the supine position also was significant.

CONCLUSIONS

In this group of medically treated open-angle glaucoma patients, laser trabeculoplasty reduced IOP more consistently during the nocturnal period than during the diurnal period.

Characterization of free-floating spheres from human trabecular meshwork (HTM) cell culture in vitro

It has been observed in several tissues that direct isolation of cells in serum-free media and on nonadhesive substrates results in the formation of spherical clusters of cells known as free-floating spheres. Such free-floating spheres have been hypothesized to contain undifferentiated multipotent progenitor cells. Our goal was to isolate and characterize such free-floating spheres from HTM cell primary cultures. For this purpose, HTM cells were incubated in serum-free media and on a nonadhesive substrate. Individual free-floating spheres generated in these conditions were isolated in 96-well plates, and their proliferative capacity was evaluated by monitoring their size increase over time. The expression of the TM markers, MGP and CHI3L1, was examined using recombinant adenoviruses containing the respective promoters. Morphology of the free-floating spheres was analysed in semithin sections, and the gene expression profile was obtained using Human Genome U133 Plus 2.0 Affymetrix microarrays. HTM cells incubated in serum-free media and on nonadhesive substrate generated free-floating spheres that could be grown for more than 3 months. Addition of serum to the culture media promoted the attachment of the spheres to the substrate, migration of cells from the spheres, and differentiation into cells phenotypically similar to normal TM cells. Gene profiling analysis demonstrated strong similarities between the gene expression profiles of the spheres and HTM cell monolayers. Both infection with the recombinant adenoviruses and gene array analysis demonstrated the expression of CHI3L1 and MGP, indicating that free-floating spheres likely originate from HTM cells. Gene array analysis also showed expression of the marker for neural precursor cells nestin, as well as leukemia inhibitory factor, a gene involved in the maintenance of the undifferentiated state of progenitor cells. Analysis of semithin sections indicated that these TM free-floating spheres were highly dynamic structures demonstrating a distinct radial gradient of cell proliferation, survival, and apoptosis. Extensive up- and down-regulation of gene expression was associated with the processes of sphere attachment and cell migration after the addition of serum. These results suggest that HTM primary cultures might contain relatively undifferentiated or progenitor cells. The availability of TM progenitor cell cultures could constitute a useful tool to investigate cell therapy approaches targeting the TM in glaucoma.

Primary open-angle glaucoma

Primary open-angle glaucoma is a progressive optic neuropathy and, perhaps, the most common form of glaucoma. Because the disease is treatable, and because the visual impairment caused by glaucoma is irreversible, early detection is essential. Early diagnosis depends on examination of the optic disc, retinal nerve fibre layer, and visual field. New imaging and psychophysical tests can improve both detection and monitoring of the progression of the disease. Recently completed long-term clinical trials provide convincing evidence that lowering intraocular pressure prevents progression at both the early and late stages of the disease. The degree of protection is related to the degree to which intraocular pressure is lowered. Improvements in therapy consist of more effective and better-tolerated drugs to lower intraocular pressure, and more effective surgical procedures. New treatments to directly treat and protect the retinal ganglion cells that are damaged in glaucoma are also in development.

Genetic modification of human trabecular meshwork with lentiviral vectors

Glaucoma, a group of optic neuropathies, is the leading cause of irreversible blindness. Neuronal apoptosis in glaucoma is primarily associated with high intraocular pressure caused by chronically impaired outflow of aqueous humor through the trabecular meshwork, a reticulum of mitotically inactive endothelial-like cells located in the angle of the anterior chamber. Anatomic, genetic, and expression profiling data suggest the possibility of using gene transfer to treat glaucomatous intraocular pressure dysregulation, but this approach will require stable genetic modification of the differentiated aqueous outflow tract. We injected transducing unit-normalized preparations of either of two lentiviral vectors or an oncoretroviral vector as a single bolus into the aqueous circulation of cultured human donor eyes, under perfusion conditions that mimicked natural anterior chamber flow and maintained viability ex vivo. Reporter gene expression was assessed in trabecular meshwork from 3 to 16 days after infusion of 1.0 x 10(8) transducing units of each vector. The oncoretroviral vector failed to transduce the trabecular meshwork. In contrast, feline immunodeficiency virus and human immunodeficiency virus vectors produced efficient, localized transduction of the trabecular meshwork in situ. The results demonstrate that lentiviral vectors permit efficient genetic modification of the human trabecular meshwork when delivered via the afferent aqueous circulation, a clinically accessible route. In addition, controlled comparisons in this study establish that feline and human immunodeficiency virus vectors are equivalently efficacious in delivering genes to this terminally differentiated human tissue.

Comparison of the morphologic changes after selective laser trabeculoplasty and argon laser trabeculoplasty in human eye bank eyes

OBJECTIVE

To compare the histopathologic changes in the human trabecular meshwork (TM) after argon laser trabeculoplasty (ALT) and selective laser trabeculoplasty (SLT) with a Q-switched, frequency-doubled, neodymium:yttrium-aluminum-garnet laser.

DESIGN

Human "in vitro" experimental study. TISSUE AND CONTROLS: Eight human autopsy eyes were obtained within 18 hours of death from persons aged 71 to 78 years.

METHODS

The anterior segment of autopsy eyes was isolated, and one half of each trabecular meshwork underwent SLT and the other half ALT. Specimens were evaluated with scanning and transmission electron microscopy.

MAIN OUTCOME MEASURES

Structural changes in the TM were detected by scanning electron microscopy, and cellular or intracellular changes were seen with transmission electron microscopy.

RESULTS

Evaluation of the TM after ALT revealed crater formation in the uveal meshwork at the junction of the pigmented and nonpigmented TM. Coagulative damage was evident at the base and along the edge of craters, with disruption of the collagen beams, fibrinous exudate, lysis of endothelial cells, and nuclear and cytoplasmic debris. Evaluation of the TM after SLT revealed no evidence of coagulative damage or disruption of the corneoscleral or uveal trabecular beam structure. Minimal evidence of mechanical damage was present after SLT, and the only ultrastructural evidence of laser tissue interaction was cracking of intracytoplasmic pigment granules and disruption of trabecular endothelial cells.

CONCLUSIONS

SLT applied "in vitro" to the TM of human eye bank eyes seemed to cause no coagulative damage and less structural damage to the human TM when compared with ALT and, therefore, may be a safer and more repeatable procedure.

How does nonpenetrating glaucoma surgery work? Aqueous outflow resistance and glaucoma surgery

Histologic, experimental, and theoretical studies of the aqueous outflow pathways point toward the juxtacanalicular region and inner wall of Schlemm's canal as the likely site of aqueous outflow resistance in the normal eye. At least 50% of the aqueous outflow resistance in the normal eye and the bulk of the pathologically increased resistance in the glaucomatous eye resides in the trabecular meshwork and the inner wall of Schlemm's canal. The uveoscleral, or uveovortex, pathway, which accounts for perhaps 10% of the aqueous drainage in the healthy aged human eye, can become a major accessory route for aqueous drainage after pharmacologic treatment. Surgeries designed to incise or remove the abnormal trabecular meshwork of glaucoma address the pathologic problem of the disease. Surgeries that unroof Schlemm's canal or expand the canal, such as viscocanalostomy, probably cause inadvertent ruptures of the inner wall and juxtacanalicular tissue, thus relieving the abnormal outflow resistance of glaucoma. This review is a summary of current thought on the pathophysiology of aqueous outflow resistance in glaucoma and, in light of this, provides an interpretation of the mechanism of pressure reduction created by these new surgeries.

Repair in the rabbit outflow system

An in vivo study was conducted to study repair processes in the injured rabbit outflow system. A uniform injury was produced by raising intraocular pressure (IOP) manometrically to 70 mmHg for 1 h. The recovery process, which was followed clinically for 8 weeks and morphologically for 6 weeks, led to the re-establishment of normal meshwork architecture within this period. The morphological studies included light microscopy, autoradiography and electron microscopy. The initial lesion consisted of large deficits in the meshwork with breakdown of cell-to-cell connections, loss of extracellular materials and disruption of the vessels of the aqueous plexus. There was a significant lowering of IOP in the first week of recovery, which thereafter climbed back to normal. Also in the first week the meshwork became infiltrated with inflammatory cells which cleared by 4 weeks. There was some meshwork cell death by either necrosis or apoptosis. The majority of meshwork cells became activated within the first few days and remained activated for at least the first 2 weeks. Tritiated proline incorporation was maximal between 1 and 2 weeks. Tritiated thymidine labelling was seen throughout, but only after the inflammation subsided was it clear that meshwork cells in all regions of the meshwork were proliferating. Our study provided no evidence that normal meshwork cells have a basal proliferative turnover level. Our injury model involved complete repair of the outflow tissues and that required meshwork cells to become activated, mobilise, undertake synthetic activity and proliferate. This is the first example, other than argon laser trabeculoplasty, where meshwork cells in vivo have been induced to divide. Possible therapeutic implications for glaucoma are discussed.

Effect of topical anti-inflammatory treatment on the outcome of laser trabeculoplasty. The Fluorometholone-Laser Trabeculoplasty Study Group

PURPOSE

We investigated the effect of anti-inflammatory treatment on the outcome of argon laser trabeculoplasty.

METHODS

In this multicenter, double-masked, randomized, placebo-controlled, parallel comparison study, 140 chronic open-angle glaucoma patients received either 0.25% fluorometholone or vehicle four times a day unilaterally, beginning 24 hours before and continuing one week after argon laser trabeculoplasty. The laser surgeon placed 50 to 60 burns over the inferior 180 degrees of the trabecular meshwork. The patients were followed up frequently for five weeks after the procedure.

RESULTS

Following argon laser trabeculoplasty, signs of anterior chamber inflammation were significantly lower in the fluorometholone group. However, there was no significant difference between the fluorometholone and the vehicle groups in the incidence of increased intraocular pressure in the immediate post-argon laser trabeculoplasty period. Intraocular pressure decreased significantly in both groups from day 1 throughout the follow-up period. A significant between-group difference in intraocular pressure decrease was found only at week 5 (7.83 +/- 6.27 [S.D.] mm Hg for the fluorometholone group vs 6.63 +/- 5.79 mm Hg for the vehicle group, P = .046). No drug-related clinically significant adverse events were observed.

CONCLUSIONS

Use of fluorometholone is effective in attenuating inflammation and has no clinically significant impact on the outcome of argon laser trabeculoplasty or on the incidence of intraocular pressure spikes during the immediate post-argon laser trabeculoplasty period.

Early changes in matrix metalloproteinases and inhibitors after in vitro laser treatment to the trabecular meshwork

Extracellular matrix turnover in the trabecular meshwork may play a role in regulating aqueous humor outflow. Laser trabeculoplasty is a common treatment for open-angle glaucoma. The mechanism of this treatment is not understood. We investigated changes in the levels and expression of the matrix metalloproteinases and their tissue inhibitors (TIMPs) in this tissue using cultured human anterior segment explants and standard clinical-parameter laser treatment. Medium gelatinase A activity levels are relatively high for sham-treated controls and are not changed dramatically following laser treatment. Medium gelatinase B and stromelysin activity levels are low in sham-treated explants and increase significantly by 24 h after treatment. TIMP1 levels, as assessed by immunoblots of Western transfers, are initially low. However, by 24 h TIMP1 levels have increased significantly. Using semi-quantitative reverse transcription and the polymerase chain reaction, mRNA levels of stromelysin, gelatinase B and TIMP1 are shown to increase after laser treatment, while gelatinase A and TIMP2 remain relatively constant. The increases in trabecular stromelysin and gelatinase B in response to laser trabeculoplasty may have important implications for the mechanism of action of this treatment for open-angle glaucoma.

Argon laser trabeculoplasty in pigmentary glaucoma

PURPOSE

To evaluate the long-term effect of argon laser trabeculoplasty (ALT) in pigmentary glaucoma.

METHODS

The authors retrospectively analyzed results of ALT in 32 eyes of 32 patients with medically uncontrolled pigmentary glaucoma. Data were longitudinally adjusted to normalize the disparity in time of follow-up and evaluated by life-table analysis.

RESULTS

Mean age (+/- standard deviation) was 45.1 +/- 13.1 years (range, 23-72 years) (males, 46.3 +/- 13.7 years; females, 42.9 +/- 12.2 years). Mean baseline intraocular pressure (IOP) was 27.8 +/- 5.3 mmHg. Mean follow-up time was 33.0 +/- 5.0 months (range, 1 week [immediate failures] to 96 months). Three eyes were lost to follow-up at 3 months. Eleven eyes required trabeculectomy between 1 week (2 eyes) and 37 months after laser treatment. Life-table analysis indicated a cumulative success for all eyes of 80% at 1 year, 62% at 2 years, and 45% at 6 years. Mean IOP was significantly reduced (P < 0.001; paired Student's t test) for male and female eyes at all intervals calculated. Age was a significant factor in determining time to failure.

CONCLUSION

Argon laser trabeculoplasty is effective in pigmentary glaucoma. Younger patients had a greater chance of success than older patients at all intervals. This became highly significant (P < 0.001) after 3 years.

The effect of topical corticosteroids on laser-induced peripheral anterior synechiae

PURPOSE

To assess the influence of different topical steroid agents and a non-corticosteroid medication after Argon laser trabeculoplasty (ALT) on the development of peripheral anterior synechiae (PAS) and the reduction of intraocular pressure (IOP).

METHOD

In two separate prospective, randomised, group-controlled studies, topical fluorometholone 0.1% (FML Liquifilm) was compared with dexamethasone 0.1% (Maxidex) (Study A), or naphazoline hydrochloride 0.1% (Albalon) (Study B) after ALT for chronic open-angle glaucoma, with particular reference to the formation of PAS and the IOP response.

RESULTS

In Study A (N = 109) eyes treated with Maxidex had a significantly higher incidence of PAS than those treated with FML - 45% compared with 22%, P < 0.05 (normal deviate test). In Study B (N = 75) the incidence of PAS was equal in eyes treated with FML or Albalon (23%). In the two studies combined (N = 184), the development of PAS was associated with a significantly lower mean response of IOP to ALT - 1.47 mmHg compared with 3.22 mmHg, 0.01 < P < 0.05 (Student t-test).

CONCLUSION

The incidence of PAS after ALT is significantly higher with the post-laser use of Maxidex than with FML, and is the same for Albalon as for FML. The therapeutic benefit of ALT is significantly reduced if PAS develop.

Factors influencing in vivo trabecular cell replication as determined by 3H-thymidine labelling; an autoradiographic study in cats

Because trabecular cell loss is thought to play a role in the pathogenesis of some forms of glaucoma, identification of factors which influence trabecular cell division may provide some insight into the mechanisms and potential treatment of this disease. We studied several commonly encountered clinical situations which may affect the trabecular meshwork: phagocytosis of debris, hyphema, and argon laser trabeculoplasty. The effect of these factors on trabecular cell division was determined using 3H-thymidine autoradiography. Laser trabeculoplasty induced a 307% increase in 3H-thymidine labelling of trabecular cells while phagocytosis and hyphema had no effect on cell labelling. In addition, subgroups of eyes with a loss of trabecular cells or with inflammation did not have increased labelling. Thus laser trabeculoplasty, known to increase outflow facility, was associated with increased trabecular cell labelling while factors often associated with decreased outflow facility and glaucoma were not.

Fibronectin induces migration of bovine trabecular meshwork cells in vitro

The migration of cultured bovine trabecular meshwork cells in response to fibronectin was studied using a 48-well migration chamber. Fibronectin was shown to be a potent chemoattractant for these cells, with a dose optimum of 30 micrograms ml-1. At this dose, the number of migrated cells was approximately 12 times that of the control, which was serum-free medium. Analysis of the migratory response revealed it to be mainly a chemokinesis, but chemotaxis was a substantial component. Electron microscopy demonstrated that adhesion and spreading of cells on the surface of the pore-containing membrane are necessary prerequisites to their migration through a pore. Microspikes are abundant during the movement of cellular processes into and through the pores. The migrated cells are extremely thin with a large surface area, but despite this phenotypic alteration the ultrastructure of migrated cells is unchanged.

Morphological effects of argon laser trabeculoplasty upon the glaucomatous human meshwork

Thirty four trabeculectomy specimens from open angle glaucoma patients who had under argon laser trabeculoplasty as part of their treatment, were studied by scanning electron microscopy. Three of these were also examined by transmission electron microscopy. The intervals between laser therapy and surgery ranged from one month to five years. A sub-group consisted of six patients who had received laser treatment on more than one occasion prior to surgery. Electron microscopy revealed distortion of trabecular beams, loss of trabecular endothelial cells and the development of a cellular sheet extending from the region of Schwalbe's line and covering the anterior surfaces of the anterior portion of the uveal meshwork. The sheet occluded the trabecular spaces of the region; thus, when extensive, contributing to laser trabeculoplasty failure. It is concluded that argon laser trabeculoplasty induces a repair process, in the form of repopulating a cellular deficient meshwork, but which can become detrimental as a consequence of its success.