Acute calcification in alkali-injured rabbit cornea treated with synthetic inhibitor of metalloproteinases (SIMP)

Assessment of Phosphate and Osmolarity Levels in Chronically Administered Eye Drops

Objectives

To assess phosphate and osmolarity levels of chronically administered eye drops commercially available in Turkey.

Materials and Methods

A total of 53 topical eye drops including 18 antiglaucoma drugs, 4 nonsteroidal anti-inflammatory drugs (NSAIDs), 10 corticosteroids, 7 antihistaminics, and 14 artificial tears identified using the Vademecum Modern Medications Guideline (2018) were included in the study. Phosphate levels were assessed using Roche Cobas C501 analyzer (Roche Diagnostics GmbH, Mannheim, Germany) and the respective kits. Osmolarity was assessed using Vescor Vapro 5600 vapor pressure osmometer (Sanova Medical Systems, Vienna, Austria). Mean phosphate and osmolarity levels were obtained after averaging three measurements. Eye drops were categorized as isoosmolar, hypoosmolar and hyperosmolar based on physiologic tear osmolarity range (296.5±9.8 mOsm/L).

Results

The highest phosphate concentration was found in the antiglaucoma group (20.3±35.4 mmol/L), followed by antihistaminics (17.3±17.9 mmol/L), corticosteroids (15.2±19.1 mmol/L), artificial tears (0.8±1.0), and NSAIDs (0.04±0.08). Percentage of medications in the hyperosmolar category was highest in the NSAI group (75%), followed by antihistaminics (43%), corticosteroids (20%), and antiglaucoma drugs (19%). Nearly all of the artificial tear formulations were in the hypoosmolar (71%) or isoosmolar (21%) categories.

Conclusion

Approximately 40% of glaucoma medications and approximately 60% of corticosteroid and antihistaminic medications had a phosphate concentration higher than the physiologic tear phosphate level (1.45 mmol/L).

Topical thrombin-related corneal calcification

PURPOSE

To report a highly unusual case of corneal calcification after brief intraoperative use of topical thrombin.

METHODS

A 44-year-old man underwent sclerouvectomy for ciliochoroidal leiomyoma, during which 35 UNIH/mL lyophilized bovine thrombin mixed with 9 mL of diluent containing 1500 mmol/mL calcium chloride was used. From the first postoperative day, corneal and anterior lenticular capsule calcifications developed, and corneal involvement slightly enlarged thereafter.

RESULTS

A year later, 2 corneal punch biopsies confirmed calcification mainly in the Bowman layer. Topical treatment with 1.5% ethylenediaminetetraacetic acid significantly restored corneal clarity. Six months later, a standard extracapsular cataract extraction with intraocular lens placement improved visual acuity to 20/60.

CONCLUSION

This case suggests that topical thrombin drops with elevated calcium concentrations may cause acute corneal calcification in Bowman layer and on the anterior lens capsule.

Corneal calcification following intensified treatment with sodium hyaluronate artificial tears

AIM

To report a potential adverse effect of intensified treatment with sodium hyaluronate artificial tears.

METHODS

Five cases of deep calcium deposition in the cornea associated with ocular surface disease and frequent use of hyaluronic acid artificial tears are described. All patients used one formulation of phosphate buffered hyaluronate eye drops when rapid calcification developed. All eyes required corneal graft surgery for visual rehabilitation. Specimens at keratoplasty were available for light microscopy and investigation by dispersive x ray analysis. The phosphate concentration in the medication used for topical treatment was measured and compared to alternative hyaluronate preparations.

RESULTS

Light microscopy showed dense mineralisation of the entire stroma. The crystalline deposits consisted of hydroxyapatite, Ca5(PO4)3OH. A 50-fold higher concentration of phosphate was measured in the sodium hyaluronate eye drops used for treatment (50.9 mmol/l) when compared with normal serum. The other hyaluronate formulations showed phosphate concentrations from <0.1 mmol/l to 10.9 mmol/l.

CONCLUSIONS

The hyaluronate artificial tear formulation "Hylo-Comod" favours the formation of insoluble crystalline calcium phosphate deposits in presence of epithelial keratopathy. This is because of its high phosphate concentration and typically frequent instillation. Manufacturers and prescribers should be aware that topical preparations may contain considerable amounts of phosphate which may lead to sight threatening corneal complications.

Phosphate concentration in artificial tears

BACKGROUND

Irrigating solutions and eye drops may contain phosphates as part of their buffer system. In the presence of epithelial keratopathy, a high concentration of phosphate favours corneal calcification. Knowledge of the phosphate concentration in artificial tear products helps to prevent this sight-threatening complication. This study gives an overview on the amount of phosphate contained in artificial tears.

METHODS

Fifty-nine samples of commercially available artificial tear preparations were tested. The quantification of phosphate was performed using the molybdate method on a Modular P autoanalyzer.

RESULTS

Twenty-six of 59 (44%) artificial tear products had a phosphate concentration above physiological levels (>1.45 mmol/l). A phosphate concentration above 25 mmol/l was found in nine products (15%), a concentration higher than 50 mmol/l in three (5%).

CONCLUSIONS

Many artificial tear formulations contain unphysiological levels of phosphate, but very high concentrations are found only in a few products. These preparations have the potential to favour the formation of insoluble crystalline calcium phosphate deposits when used on a damaged corneal surface, and should therefore be used cautiously.

Acute Corneal Calcification Following Chemical Injury

PURPOSE

To describe the clinical and ultrastructural features of 3 cases of acute corneal calcification following accidental chemical injury.

METHODS

Three men presented over an 18-month period with unilateral eye injuries sustained when applying an industrial fire retardant. This product is predominantly a gypsum aggregate (calcium sulfate dihydrate) plaster combined under pressure with a set-time accelerator (aluminum sulfate). In each case the tear pH was initially alkaline, and the eyes were irrigated with phosphate-buffered saline according to protocol. Within hours a dense corneal opacity had developed that showed only minor resolution over 3 years of follow-up. Two eyes required corneal graft surgery for visual rehabilitation. Light and electron microscopy and energy dispersive analysis of x-rays (EDAX) was performed on excised tissue.

RESULTS

Light and electron microscopy showed dense mineralization of the anterior stroma with discrete crystalline deposits in the deeper stroma. EDAX of the crystals showed high emission peaks for calcium and phosphorus.

CONCLUSIONS

The insolubility, elemental composition, and ultrastructural appearance suggest that the opacity was caused by calcium phosphate deposition. The absence of phosphorus from the listed components of the fire retardant suggests that the use of phosphate-buffered irrigation fluid or the subsequent use of phosphate-buffered drops may have contributed to the deposition of this insoluble crystalline deposit.

Effect of different irrigating solutions on aqueous humour pH changes, intraocular pressure and histological findings after induced alkali burns

PURPOSE

To evaluate the effects of hypo-osmolar tap water and isotonic saline solution on the intracameral pH, intraocular pressure and histological changes in alkali burned rabbit eyes.

METHODS

Four groups of four rabbit corneas each were burned with 2 N sodium hydroxide, and then rinsed with 0.5 l or 1.5 l of saline solution or tap water, respectively. Changes in pH were monitored with an intracameral microelectrode. Intraocular pressure (IOP) was monitored by a transducer placed in the vitreous cavity. After enucleation, histology was performed.

RESULTS

The pH increased after 1.5 min following alkali application. Irrigation with different solutions affected the maximum pH levels reached. Following the tap water rinse, the maximum rise was significantly lower than after the saline solution rinse. The maximum rise following rinsing with 1.5 l of tap water showed a significant delay. The increase in IOP was 23 +/- 10 mmHg without differences between the groups; the original pressure was recovered after 18 +/- 9 min. Histology of the eyes revealed a significant oedema in all corneas. Other ocular structures appeared unchanged.

CONCLUSIONS

The hypo-osmolarity of tap water led to remarkable corneal oedema. Enlargement of the diffusion barrier and intracorneal dilution inhibit elevated intracameral pH levels. The difference in maximum pH levels reached may influence the degree of subsequent intraocular structure injury. Therefore, the use of iso-osmolar saline solution proves to be less efficacious than tap water as an irrigation agent for ocular burns.

The role of platelet-activating factor in the corneal response to injury

Platelet-activating factor (PAF) is a potent bioactive lipid that is generated in the cornea after injury and whose actions are mediated through specific receptors. Studies from our laboratory have shown that PAF interactions with its receptor activate several transmembrane signals involved in inflammation, wound healing, and apoptosis. The wide variety of responses to PAF implicate this lipid as a central player in many responses of the cornea after a pathologic stimulus. An exciting facet of PAF is that it induces the expression of specific genes involved in the remodeling of components of the extracellular matrix, such as some metalloproteinases, urokinase plasminogen activator, and selective inhibitors of metalloproteinases. These enzymes, when overexpressed, could lead to corneal ulceration. Continuous exposure to PAF during prolonged inflammation produces increase keratocyte apoptosis and inhibition of epithelial adhesion to the basement membrane. As a consequence, there is a marked delay in wound healing, which is not countered by the actions of growth factors. In this review, we present data mainly from our laboratory showing actions of PAF in corneal epithelium in vivo and in vitro in corneal models of injury as well as in cells in culture. We also discuss the signal-transduction mechanisms involved in the different actions of PAF. A therapeutic role for PAF antagonists in blocking the effects of PAF is guaranteed in the future.

Platelet-activating factor induces the gene expression of TIMP-1, -2, and PAI-1: imbalance between the gene expression of MMP-9 and TIMP-1 and -2

Previous studies in the laboratory have shown that platelet-activating factor (PAF), a potent inflammatory mediator that accumulates rapidly in the cornea after an injury, stimulates the expression of urokinase (uPA) and matrix metalloproteinase-1 (MMP-1) and -9 (MMP-9). Tissue inhibitors of MMPs (TIMPs) and plasminogen activator inhibitor (PAI-1) are produced in conjunction with these enzymes and are important regulators of their activity. Here, the authors investigated how PAF affects the expression of PAI-1, TIMP-1 and -2 relative to that of uPA, MMP-1, and -9 in rabbit corneal epithelial cells. Rabbit corneas were incubated in MEM medium containing 100 nM cPAF. To block the effects of PAF in some studies, corneas were preincubated for 1 hr in the presence of the PAF antagonist BN50730 (10 microM). At several time intervals, mRNA was extracted from epithelial cells and the levels of gene expression for the enzymes and their inhibitors were determined by real-time PCR. All quantitations were normalized to the 18s rRNA values (endogenous control) and changes in gene expression were reported as fold increase relative to untreated controls. PAF produced a 20-fold increase in the gene expression of PAI-1 at 8 hr, while similar fold increases in uPA mRNA expression occurred at 2 hr. PAF treatment also stimulated the expression of TIMP-1 and -2 genes, with a six-fold increase in TIMP-1 expression occurring at 36 hr and a four-fold increase in TIMP-2 expression at 24 hr. Maximal induction of MMP-1 and -9 mRNA, on the other hand, occurred at 4 and 8 hr, respectively. Induction of MMP-1 gene expression was similar to that of its inhibitors TIMP-1 and -2, while MMP-9 mRNA induction exceeded that of these inhibitors by 100-fold. The PAF-induced expression of PAI-1, TIMP-1 and -2 mRNAs was abolished by pre-treatment with BN50730. These data indicate that PAF activates the gene expression of TIMP-1, -2, and PAI-1 in corneal epithelium by a receptor-mediated mechanism. Furthermore, PAF induced overexpression of MMP-9 mRNA relative to that of TIMP-1 and -2, suggesting an imbalance between the expression of this proteolytic enzyme and its inhibitors, which may contribute to changes in the wound-healing process and ultimately lead to corneal ulcer development.

Anaylsis of birefringence during wound healing and remodeling following alkali burns in rabbit cornea

The use of synthetic inhibitors of metalloproteinases (SIMP) or medroxyprogesterone (MP) can prevent or significantly delay the ulceration of alkali-injured corneas by influencing collagen degradation. We have examined the remodeling of rabbit corneal stroma following alkali injury and have assessed the effect of SIMP and MP treatment. Following a defined alkali injury to the rabbit cornea, animals were divided into three subgroups, one group treated with topical beta-mercaptomethyl tripeptide (SIMP), one treated by subconjunctival injection of MP and one treated with a control solution. The corneal tissue was taken at 3 days, 1, 2, 3, 4, 9 and 26 weeks after alkali injury and prepared for light microscopy and transmission electron microscopy (TEM). A quantitative measurement of birefringence, in terms of the optical path difference (OPD), was made using a modified polarized microscopy technique based on the analysis of interference colours. The results showed that SIMP effectively prevented deep corneal ulceration. MP could delay the ulceration and the corneas treated with MP appeared to have better transparency than the other groups. There was a significant difference of the OPD between the anterior (5.8 +/-0.3 nm) and posterior (7.8 +/-0.4 nm) stroma of the normal cornea (P<0.001). The OPD values from the central corneas from alkali-injured eyes were generally lower than normal during the first 4 weeks and then gradually recovered to the normal level or above, except for the posterior stroma of the MP-treated eyes. We found that the OPD changes were very dependent on the presence of corneal lesions. The stroma near corneal ulceration, scar tissue, calcified stroma and the retro-corneal collagen layer showed a significant reduction of birefringence (lower OPD values). These OPD values remained much lower than normal up to the end of the experiment. TEM showed disrupted corneal stroma in all three groups, with thinner scar tissue in the MP group. The fibril diameters did not change significantly 3 days and 1 week after the alkali burns (27.1+/-2.3 nm in the control group, 27.3+/-2.2 nm in the SIMP group and 27.7+/-2.1 nm in the MP group) and there were no differences compared with 29.7+/-1.7 nm of the normal cornea (P>0.05). After 2 weeks of tissue remodeling, the fibril diameters in alkali-injured corneas showed a large variation (the range was between 11.5 and 80 nm) with a bimodal distribution, especially in the control group. The technique presented here for birefringence evaluation can provide an alternative way to monitor wound healing and tissue remodeling, both visually and quantitatively.

The use of X-ray scattering techniques to determine corneal ultrastructure

The manner in which X-rays are scattered or diffracted by the cornea provides us with valuable insights into the fine structure of the corneal stroma. This is because when X-rays pass through a cornea a diffraction pattern is formed due to scattering from regularly arranged collagen molecules and fibrils that comprise the bulk of the stromal matrix. Collagen provides the cornea with most of its strength, and its proper organisation is believed to be important for tissue transparency. Ever since 1978, when the first X-ray diffraction patterns were obtained from the cornea using radiation from a powerful synchrotron source, biophysicists have recorded and analysed a huge number of X-ray diffraction patterns from many different corneas. This article aims to explain the ideas that underpin our use of X-ray diffraction to investigate corneal ultrastructure, and show how the knowledge gained to date has far-reaching implications for tissue biomechanics, disease changes and transparency.