Animal model for uveitic glaucoma

Deconstructing aqueous humor outflow - The last 50 years

Herein partially summarizes one scientist-clinician's wanderings through the jungles of primate aqueous humor outflow over the past ~45 years. Totally removing the iris has no effect on outflow facility or its response to pilocarpine, whereas disinserting the ciliary muscle (CM) from the scleral spur/trabecular meshwork (TM) completely abolishes pilocarpine's effect. Epinephrine increases facility in CM disinserted eyes. Cytochalasins and latrunculins increase outflow facility, subthreshold doses of cytochalasins and epinephrine given together increase facility, and phalloidin, which has no effect on facility, partially blocks the effect of both cytochalasins and epinephrine. H-7, ML7, Y27632 and nitric oxide - donating compounds all increase facility, consistent with a mechanosensitive TM/SC. Adenosine A1 agonists increase and angiotensin II decrease facility. OCT and optical imaging techniques now permit visualization and digital recording of the distal outflow pathways in real time. Prostaglandin (PG) F2α analogues increase the synthesis and release of matrix metalloproteinases by the CM cells, causing remodeling and thinning of the interbundle extracellular matrix (ECM), thereby increasing uveoscleral outflow and reducing IOP. Combination molecules (one molecule, two or more effects) and fixed combination products (two molecules in one bottle) simplify drug regimens for patients. Gene and stem cell therapies to enhance aqueous outflow have been successful in laboratory models and may fill an unmet need in terms of patient compliance, taking the patient out of the delivery system. Functional transfer of genes inhibiting the rho cascade or decoupling actin from myosin increase facility, while genes preferentially expressed in the glaucomatous TM decrease facility. In live NHP, reporter genes are expressed for 2+ years in the TM after a single intracameral injection, with no adverse reaction. However, except for one recent report, injection of facility-effective genes in monkey organ cultured anterior segments (MOCAS) have no effect in live NHP. While intracameral injection of an FIV. BOVPGFS-myc.GFP PGF synthase vector construct reproducibly induces an ~2 mmHg reduction in IOP, the effect is much less than that of topical PGF2⍺ analogue eyedrops, and dissipates after 5 months. The turnoff mechanism has yet to be defeated, although proteasome inhibition enhances reporter gene expression in MOCAS. Intracanalicular injection might minimize off-target effects that activate turn-off mechanisms. An AD-P21 vector injected sub-tenon is effective in 'right-timing' wound healing after trabeculectomy in live laser-induced glaucomatous monkeys. In human (H)OCAS, depletion of TM cells by saponification eliminates the aqueous flow response to pressure elevation, which can be restored by either cultured TM cells or by IPSC-derived TM cells. There were many other steps along the way, but much was accomplished, biologically and therapeutically over the past half century of research and development focused on one very small but complex ocular apparatus. I am deeply grateful for this award, named for a giant in our field that none of us can live up to.

Changes in intraocular pressure and aqueous flare in eyes with multiple evanescent white dot syndrome

PURPOSE

To study the changes in intraocular pressure (IOP) and aqueous flare in eyes with multiple evanescent white dot syndrome (MEWDS) during the disease course.

STUDY DESIGN

Retrospective observational study.

METHODS

Twenty-one patients with unilateral MEWDS were retrospectively evaluated. IOP values were compared between the affected and fellow eyes 2 weeks, 1 month, and 3 months following disease onset in 17 patients, and within 7 days from disease onset in 11 patients. Aqueous flare values measured using a laser flare-cell meter in ten eyes between 1 weeks and 1 month from disease onset were compared between the affected and fellow eyes.

RESULTS

IOP values were significantly lower in the affected eyes than in the fellow eyes at both 2 weeks (P=0.002) and 1 month from disease onset (P=0.02). However, IOP values of affected eyes did not show significant differences from the fellow eyes within 7 days ((P=0.11) and 3 months of onset (P=0.30). Aqueous flare values were significantly increased in the affected eyes compared to those in the fellow eyes (P=0.010) and significantly correlated with IOP values (r=-0.67, P=0.035).

CONCLUSION

IOP values mildly decreased in association with aqueous flare values in the acute phase in eyes with MEWDS.

Outflow facility and extent of angle closure in a porcine model

PURPOSE

To establish the extent of anterior chamber angle circumference needed to maintain a physiological outflow facility (C). This could create a model to investigate focal outflow regulation.

METHODS

Twenty anterior segments of porcine eyes were assigned to five groups, each with a different degree of cyanoacrylate-mediated angle closure: 90° (n = 4), 180° (n = 4), 270° (n = 4), 360° (n = 4), and four unoccluded control eyes. The outflow facility was measured at baseline, 3, 12, 24, and 36 h after angle closure. Outflow patterns were evaluated with canalograms and the histomorphology was compared.

RESULTS

Baseline outflow facilities of the five groups were similar (F = 0.922, p = 0.477). Occlusion of 360° induced a significant decrease in facility from baseline at all time-points (p ≤ 0.023 at 3, 12, 24, and 36 h). However, no difference from baseline was found in any of the partially occluded (0-270°) groups (F ≥ 0.067, p ≥ 0.296 at 3, 12, 24, and 36 h). The canalograms confirmed the extent of occlusion with flow through the unblocked regions. Histology revealed no adverse effects of blockage on the TM or aqueous plexus in the unoccluded angle portions. The unoccluded TM appeared normal.

CONCLUSION

Cyanoacrylate-mediated angle occlusion created a reproducible angle closure model. Ninety degrees of unoccluded anterior chamber angle circumference was sufficient to maintain physiological outflow. This model may help understand how outflow can be regulated in healthy, nonglaucomatous TM.

Comparison of Short-term Postoperative Hypotony Rates of 23-gauge vs 25-gauge Needles in Formation of the Scleral Tract for Baerveldt Tube Insertion into the Anterior Chamber

Introduction

To compare the early postoperative hypotony rates and intraocular pressure (IOP) in two groups of eyes using either 23-gauge (23G) or 25-gauge (25G) needle in the creation of the anterior chamber entry tract for Baerveldt tube. The primary outcome measure was incidence of hypotony, and secondary outcome measures included comparison of mean IOP and other early complications.

Materials and methods

Ours was a retrospective case review of consecutive patients who underwent 350 mm² Baerveldt implantation in two units over a 2-year period. Data including IOP and complications were collected at 1 day, 1 week, and 1 month following surgery from patients’ notes. Statistical analysis between groups was determined using the unpaired 2-tailed f-test for continuous variables and chi-squared test for categorical variables. Statistical significance was defined at the 0.05 level.

Results

A total of 58 eyes of 58 patients were included in this study. Preoperative mean IOP in the 25G group was significantly higher (26.4 ± 6.8 mm Hg) when compared with the 23G group (21.6 ± 4.0 mm Hg) (p = 0.002). The mean postoperative IOP remained significantly higher in the 25G group at day 1 (p=0.004), week 1 (p = 0.008), but not at month 1 (p = 0.744). Four patients in the 23G group had hypotony within 1 month postsurgery compared with no cases in the 25G group (chi-squared test p = 0.038).

Conclusion

There was a significantly higher risk of early hypotony and lower IOP in the larger 23G group at days 1 and 7, although the IOP was similar in both groups by 1 month.

Clinical significance

After all glaucoma drainage device (GDD) tube implantation, regardless of which needle is used to create the tract, the entry site should always be checked with 2% fluorescein drop and 10.0 nylon suture is used with or without autologous Tenon’s tissue to close any leakage.

How to cite this article: Lim KS, Garg A, Cheng J, Muthusamy K, Beltran-Agullo L, Barton K. Comparison of Short-term Postoperative Hypotony Rates of 23-gauge vs 25-gauge Needles in Formation of the Scleral Tract for Baerveldt Tube Insertion into the Anterior Chamber. J Curr Glaucoma Pract 2018;12(1):36-39.

iDrugs and iDevices Discovery Research: Preclinical Assays, Techniques, and Animal Model Studies for Ocular Hypotensives and Neuroprotectants

Discovery ophthalmic research is centered around delineating the molecular and cellular basis of ocular diseases and finding and exploiting molecular and genetic pathways associated with them. From such studies it is possible to determine suitable intervention points to address the disease process and hopefully to discover therapeutics to treat them. An investigational new drug (IND) filing for a new small-molecule drug, peptide, antibody, genetic treatment, or a device with global health authorities requires a number of preclinical studies to provide necessary safety and efficacy data. Specific regulatory elements needed for such IND-enabling studies are beyond the scope of this article. However, to enhance the overall data packages for such entities and permit high-quality foundation-building publications for medical affairs, additional research and development studies are always desirable. This review aims to provide examples of some target localization/verification, ocular drug discovery processes, and mechanistic and portfolio-enhancing exploratory investigations for candidate drugs and devices for the treatment of ocular hypertension and glaucomatous optic neuropathy (neurodegeneration of retinal ganglion cells and their axons). Examples of compound screening assays, use of various technologies and techniques, deployment of animal models, and data obtained from such studies are also presented.

Pathogenesis of Uveitic Glaucoma

Uveitic glaucoma consists one of the most serious complications of intraocular inflammation and, despite its rarity, is considered as one of the leading causes of preventable loss of vision worldwide. It has been associated with a wide spectrum of inflammatory diseases, but its pathogenesis is still not fully comprehended. It appears that the type of inflammation, the steroid-response and the anatomical alterations of the anterior chamber play a pivotal role. To our knowledge, the mechanisms may be both acute and chronic. The main targets of the treatment are to control the inflammation and reduce the intraocular pressure (IOP). The management of glaucoma associated with uveitis remains an extremely challenging condition for ophthalmologists. The successful treatment of uveitic glaucoma is inextricably correlated with prompt and immediate therapeutic decisions. Very often a solid collaboration between clinicians from different specialties may be required for treating the underlying disease.

Further understanding of its pathogenesis can indicate therapeutic targets and may lead to the development of new and more efficient therapeutic approaches. New glaucoma surgical modalities may ameliorate the prognosis after surgical intervention, but this calls for further evaluation. This study aims to highlight the complexity of uveitic glaucoma analyzing the main pathogenetic mechanisms and the correlations with the inflammatory response.

The Pathogenesis of Raised Intraocular Pressure in Uveitis

AIM

To analyze current understanding of the factors that contribute to raised intraocular pressure (IOP) in patients with uveitis.

METHODS

A pubmed literature review was carried out using words including "uveitic glaucoma", "IOP AND uveitis", "ocular hypertension AND uveitis", "inflammation AND glaucoma", "aqueous dynamics" AND "glaucoma/uveitis".

RESULTS

Of the two studies looking at the aqueous dynamics in experimentally induced uveitis, both found aqueous flow decreased acutely, and one found that uveoscleral outflow increased. This is likely to reflect the types of uveitis that present acutely with hypotony. A study examining patients with Fuch's heterochromic cyclitis found no difference in aqueous flow or uveoscleral outflow. No studies have examined aqueous dynamics in types of uveitis that present with acutely raised IOP. Levels of prostaglandins rise in acute uveitis, which has been shown to increase uveoscleral and trabecular outflow, without affecting aqueous flow. Studies have demonstrated that raised levels of trabecular protein reduce trabecular outflow. Steroid treatment, inflammatory cells, free radicals and enzymes are also likely to contribute to the development of raised pressure. When considering the impact of the pathogenesis of raised pressure in uveitis on its treatment, prostaglandins may provide good intraocular pressure control, but there are concerns regarding their theoretical ability to worsen the inflammatory response in uveitis. Studies have not conclusively proven this to be the case. Surgical success rates vary, but trabeculectomy plus an antimetabolite, deep sclerectomy plus an antimetabolite, and Ahmed valve surgery have been used.

CONCLUSIONS

Uveitic glaucoma is caused by a number of different diseases, some of which present with acute hypotony, others with acutely raised IOP, and others which demonstrate an increase in IOP over time. Further studies should be carried out to examine the differing pathogenesis in these types of diseases, and to establish the best treatment options.

Morphological and hydrodynamic correlates in monkey eyes with laser induced glaucoma

This study investigated the relationship between decreased outflow facility (C) and changes in hydrodynamic aqueous humor outflow patterns and morphology in cynomolgus monkey eyes with unilateral chronically elevated intraocular pressure (IOP). Argon laser photocoagulation burns to the trabecular meshwork (TM) were made in one eye of each monkey (N = 3), leaving the contralateral eye as a normotensive control. IOPs were followed by pneumatonometry for 16-70 months. C was measured by fluorophotometry before sacrifice. To label the hydrodynamic patterns of outflow, the eyes were enucleated and perfused with fluorescent microspheres (0.5 microm; 0.002%) at the last pressure measured before death minus 7 mmHg. The eyes were perfusion-fixed at the same pressure. Confocal images were taken along the inner wall (IW) of the Schlemm's canal (SC). The total length (TL) and the filtration length (FL) of the IW decorated by tracers were measured in frontal sections. The average percent effective filtration length (PEFL = FL/TL) was calculated for each eye. Sections exhibiting SC were processed and examined under light and electron microscopy. The average IOP was significantly higher in laser-treated eyes (mean +/- SD = 61.33 +/- 4.16 mmHg) than controls (22.67 +/- 4.16 mmHg, P = 0.002). The average C was 13-fold lower in laser-treated eyes (0.03 +/- 0.02 microl/min/mmHg) than controls (0.39 +/- 0.17 microl/min/mmHg, P = 0.057). By confocal microscopy, in control eyes, SC was open and a segmental distribution of microspheres was found in the TM with a greater concentration near the collector channel (CC) ostia. Much less tracer labeling was seen along SC in laser-treated eyes than control eyes. The average PEFL in controls (47.47 +/- 10.79%) was 6-fold larger than in laser-treated eyes (8.40 +/- 4.81%, P = 0.048). The average distance between the inner and outer wall of SC was 5-fold greater in control eyes (18.99 +/- 6.03 microm) than in laser-treated eyes (3.47 +/- 0.33 microm, P = 0.048). By light microscopy, there was extensive pigmentation throughout the TM, denser extracellular matrix in the JCT region, and most of SC collapsed with focal herniations of the IW and JCT protruding into the CC ostia in laser-treated eyes. By electron microscopy, few or no microspheres were observed in laser-treated areas and the areas with SC collapse. More microspheres were observed near the CC ostia area in non-lasered areas. In conclusion, in the laser-induced glaucoma model, laser damage results in a reduction in the available area for outflow across the IW of SC which contributes to the decrease in C and thus elevation of the IOP. Constriction of SC, caused by the chronic elevation of IOP, further decreases the available area for outflow across the IW which decreases C even more in a vicious cycle. This study suggests that the available area for aqueous humor outflow across the IW of SC may play a role in regulating outflow resistance and maintaining IOP.

Rodent models of glaucoma

Glaucoma is a progressive, age-related optic neuropathy and a leading cause of irreversible blindness in the world. Animal models of glaucoma are essential to our continued efforts of elucidating the natural course of the disease and to developing therapeutic interventions to halt or reverse the progression of the condition. Over the past 10-15 years, rodents have become a popular model organism to study glaucoma, because of their high degree of availability, relatively low cost, short life-span, and amenability to experimental and genetic manipulation. In this review, we examine the numerous in vivo and in vitro rodent models of glaucoma, discuss the methods used to generate them, summarize some of the major findings obtained in these models, and identify individual strengths and weaknesses for the various systems.

Repeat sample intraocular pressure variance in induced and naturally ocular hypertensive monkeys

PURPOSE

To compare repeat-sample means variance of laser induced ocular hypertension (OH) in rhesus monkeys with the repeat-sample mean variance of natural OH in age-range matched monkeys of similar and dissimilar pedigrees.

MATERIALS & METHODS

Multiple monocular, retrospective, intraocular pressure (IOP) measures were recorded repeatedly during a short sampling interval (SSI, 1-5 months) and a long sampling interval (LSI, 6-36 months). There were 5-13 eyes in each SSI and LSI subgroup. Each interval contained subgroups from the Florida with natural hypertension (NHT), induced hypertension (IHT1) Florida monkeys, unrelated (Strasbourg, France) induced hypertensives (IHT2), and Florida age-range matched controls (C). Repeat-sample individual variance means and related IOPs were analyzed by a parametric analysis of variance (ANOV) and results compared to non-parametric Kruskal-Wallis ANOV.

RESULTS

As designed, all group intraocular pressure distributions were significantly different (P < or = 0.009) except for the two (Florida/Strasbourg) induced OH groups. A parametric 2 x 4 design ANOV for mean variance showed large significant effects due to treatment group and sampling interval. Similar results were produced by the nonparametric ANOV. Induced OH sample variance (LSI) was 43x the natural OH sample variance-mean. The same relationship for the SSI was 12x.

CONCLUSION

Laser induced ocular hypertension in rhesus monkeys produces large IOP repeat-sample variance mean results compared to controls and natural OH.

Elevated intraocular pressure and optic nerve injury models in the rat

Models of experimentally elevated intraocular pressure in rats provide valuable opportunities to discover and study mechanisms of pressure-induced optic nerve damage. The structure and vasculature of the rat optic nerve head have several anatomic similarities and differences from the primate that allow useful comparisons and insights into human glaucoma. Specifically, the ultrastructural relationship between astrocytes, retinal ganglion cell axons and the connective tissues of the optic nerve head appear quite similar to the primate, and have a high potential for revealing cellular mechanisms of axonal injury. Three widely used models of creating elevated IOP in rats exist. However, they are not all equivalent and appear to differ in the relationship they exhibit between the level of pressure and extent of optic nerve damage. This indicates that these models may differ in the mechanisms by which they produce elevated eye pressure. All of these models are amenable to a variety of methods for evaluating damage. These include objective and subjective histologic assessment of the optic nerve, counting cells in the retinal ganglion cell layer of the retina and the use of retinal whole mounts to count retinal ganglion cells that have been back-labeled with dye applied to the superior colliculus. In the decade since their introduction, these versatile models have provided important insights into mechanisms of pressure-induced optic nerve damage using sensitive molecular biology techniques. They have also allowed the evaluation of several potential strategies for neuroprotection in glaucoma, ranging from currently available drugs to gene transfer studies.

Prospects for relevant glaucoma models with retinal ganglion cell damage in the rodent eye

Retinal ganglion cell (RGC) death is the end result of practically all diseases of the optic nerve, including glaucomatous optic neuropathy. Understanding the factors determining susceptibility of the retina or the optic nerve to glaucomatous damage, and the means to prevent it, requires good animal models. Here we review the different, current models in rodents that have been used to study RGC damage, discuss their value, and their adequacy as models for human glaucoma.

A sensitive ocular perfusion apparatus measuring outflow facility

PURPOSE

To build an improved apparatus for measuring outflow facility by one- and two-level, constant pressure perfusion.

METHODS

A computer-controlled and -monitored tower adjusts the height of a fluid reservoir by a stepper motor every 5 min, to keep intraocular pressure (IOP) constant at one or two desired pressure levels (e.g. 2.5 and 12.0 mm Hg above the spontaneous IOP). The fluid flow rate is calculated by the equation Q = (Pres - IOP)/R, where Pres is the pressure of the reservoir as calculated by the term "pgh," h being the height of the reservoir, measured by the rotation of stepper motor, g, the gravity constant, and p, the fluid density. IOP is measured by the transducer, and R is the resistance of 180 microns tubing. Outflow facility is calculated by delta Q/ delta P. delta Q is the difference between flow rate at the two pressure levels, and delta P is the intraocular pressure difference. Perfusion was performed on live albino rat eyes by the two-level constant pressure technique, and on artificial steel eyes at a single constant pressure perfusion (20 mm Hg).

RESULTS

Outflow facility of albino rat eyes and artificial steel eyes perfused for 40 and 30 min, respectively was 0.0507 +/- 0.0039 microliter/min/mm Hg and 0.3546 +/- 0.0034 microliter/min/mm Hg (mean +/- SD), respectively.

CONCLUSIONS

Our perfusion apparatus is highly stable, sensitive, and capable of measuring subtle changes in outflow facility in vivo and in vitro.

Glaucoma associated with uveitis

Raised intraocular pressure is a common and frequently serious complication of anterior uveitis. The milieu of inflammatory cells, the mediators they release, and the corticosteroid therapy used to treat the uveitis can participate in the pathogenesis of uveitic glaucoma. These factors alter the normal anatomic structure of the anterior chamber and angle, influencing aqueous production and outflow. These changes act to disrupt the homeostatic mechanisms of intraocular pressure control. Structural changes in the angle can be acute, such as in secondary angle closure with pupillary block glaucoma, or chronic, such as combined steroid-induced and secondary open angle glaucoma. Management of uveitic glaucoma may be difficult because of the numerous mechanisms involved in its pathogenesis. Diagnostic and therapeutic decisions are guided by careful delineation of the pathophysiology of each individual case. The goal of treatment is to minimize permanent structural alteration of aqueous outflow and to prevent damage to the optic nerve head. This article reviews the pathogenesis of uveitic glaucoma, with specific attention to etiology. Medical and surgical therapies are also discussed, with emphasis on the more recent developments in each category.