Excerpt from Central Sterile Corneal Ulceration

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Background

A corneal ulcer is defined as a disruption of the epithelial layer with involvement of the corneal stroma. This condition is associated with inflammation, either sterile or infectious.

The primary purpose of this article is to highlight the pathogenesis of noninfectious stromal ulceration. The infective causes and mechanisms of autoimmune ulcerative keratitis, particularly peripheral, are not included within this article.

See related CME at Cornea and External Disease.

Pathophysiology

An understanding of the pathophysiology of sterile corneal ulceration requires a review of the processes involved in epithelial and stromal wound healing, as well as an examination of the role of precorneal tear film, corneal nerves, proteolytic enzymes, and cytokines.

Epithelial wound healing

Corneal ulceration always begins with an epithelial defect. A persistent epithelial defect allows the corneal stroma to be exposed to the external environment and permits the process of stromal degradation.

Within minutes after a small corneal epithelial injury, cells at the edge of the abrasion begin to migrate centripetally to cover the defect rapidly at a rate of 60-80 µm/h. A longer delay of 4-5 hours is seen in larger defects. This delay is required for preparatory cellular changes prior to rapid cell movement.

Epithelial cells adjacent to the area of the defect flatten, lose their hemidesmosome attachments, and migrate on transient focal contact zones that are formed between cytoplasmic actin filaments and extracellular matrix proteins. Vinculin, a plasma protein, links fibers to talin, which is a cell membrane protein. It, in turn, is linked to integrin. Contraction of actin fibers pulls the cell body forward. Vinculin, integrin, fibronectin, fibrinogen, and fibrin are formed continuously and cleaved to allow for cell migration. Plasmin is the protease responsible for cleaving fibrinogen and fibrin at these focal contact zones.

The basement membrane is also important for epithelial migration, and abnormalities in basement membrane structure, whether due to trauma (eg, recurrent erosion syndrome) or dystrophy (eg, basement membrane dystrophy), can lead to persistence of corneal epithelial defects and stromal ulceration.

After 24-30 hours, mitosis begins to restore epithelial cell population. Basal and limbal stem cells contribute to mitosis. A sufficient supply of progenitor stem cells to facilitate epithelial cell proliferation is important for the cornea. A deficiency of limbal stem cells, from either disease (eg, aniridia) or trauma (eg, chemical burn), can preclude adequate epithelial wound healing.

Stromal wound healing

Stromal wound healing occurs via stromal keratocyte migration, proliferation, and deposition of extracellular matrix molecules, including collagen (specifically type III), adhesion proteins (eg, fibronectin, laminin), and glycosaminoglycans. These processes are facilitated by a phenotypic change among quiescent keratocytes to become active myofibroblasts, a task mediated by transforming growth factor-beta (of presumptive epithelial origin).

Stromal necrosis and degradation

The corneal wound repair process is intricately linked to a complex inflammatory response that must be precisely regulated to ensure proper healing.

Invasion of monocytes/macrophages is critical in wound healing; however, in the corneal stroma, excessive infiltration of monocytes/macrophages is considered to be unfavorable because they secrete matrix metalloproteinases (MMPs) and other proteins undesirable for tissue healing. Numerous cytokines and growth factors that are up-regulated in corneal cells further contribute to tissue inflammation.

Matrix metalloproteinases (MMPs) are a group of structurally related endopeptidases that require a metal cofactor. To date, more than 25 have been identified and are categorized into 6 groups according to their substrate specificity. The main function of metalloproteinases is to degrade extracellular matrix and basement membrane components. MMP-2 and MMP-9 are known as gelatinases and are involved in cleaving collagen types IV, V, VII, and X, as well as fibronectin, laminin, elastin, and gelatins. MMP-1 (neutrophil collagenase) and MMP-8 (fibroblast or keratocyte collagenase) are involved in cleaving collagen types I, II, and III.

Barely detected in an unwounded cornea, MMPs are strongly induced during wound healing. Metalloproteinases are secreted as proenzymes by neutrophils infiltrating the wound, injured epithelial cells, and keratocytes. They are activated by proteolytic cleavage of the N-terminal region in the extracellular compartment. In vivo, tissue inhibitors of metalloproteinases (TIMPs) inhibit collagenase activity. TIMPs represent a multigene family that includes at least 4 members. They exert their action by blocking the activation of MMPs and inhibiting proteinase activity.

A relatively higher degree of collagenolysis relative to synthesis is thought to result in degradation, progressive corneal thinning, and, hence, ulceration of the corneal stroma. MMPs are induced at the transcriptional level by various cytokines and growth factors, such as interleukin 1 (IL-1), interleukin 6 (IL-6), tumor necrosis factor-alpha (TNF-alpha), epidermal growth factor (EGF), platelet derived growth factor (PDGF), fibroblast growth factor (FGF), and transforming growth factor-beta (TGF-beta).

Synthetic inhibitors of mammalian metalloproteinase (SIMP) have been studied to determine their effect on the cornea after an alkali burn. It has been shown that SIMP effectively inhibits corneal ulceration when started earlier in treatment as well as in established ulcers.¹ 

Studies have shown a role for the extracellular matrix metalloproteinase inducer (EMMPRIN), a cell membrane glycoprotein enriched on epithelial cells during corneal wound healing. It has been shown that it is up-regulated on epithelial cells by EGF and TGF-beta. This, in turn, induces fibroblasts, by direct interaction, to increase their own level of EMMPRIN, leading to induction of MMP. Inhibition of EMMPRIN may represent a promising future therapeutic strategy in situations of excess extracellular matrix degradation associated with chronic wound healing.²

Since all metalloproteinase enzymes require metal cofactors Ca2+ and Zn2+, such chelating agents as ethylenediaminetetraacetic acid (EDTA), acetylcysteine, and penicillamine inhibit collagenase activity; however, these agents have been found to have limited efficacy in vivo. Tetracyclines also possess anticollagenolytic activity.

As a result of collagen breakdown, tripeptide products of collagen are released. These are chemotactic for neutrophils, which migrate into the injured tissue where they release additional MMPs as well as superoxide radicals. These agents potentiate further collagenolytic action and corneal degradation. Superoxide dismutase (SOD) enzymatically reduces the superoxide radical to hydrogen peroxide, thus effectively eliminating highly reactive oxygen metabolites before any further damage. Isozymes of SOD are widely found in the corneas of mammals. Therefore, the use of topical SOD is helpful in preventing corneal damage. Studies have shown a beneficial effect of lecithinated SOD, which is retained on the ocular surface longer than native SOD when applied as an eye drop solution.³

In cells along the leading edge of the wound, there is a specific activation of the Ser/Thr kinase, Cdk5. Cdk5 activity limits the accumulation of active Src. Active Src promotes epithelial cell migration. However, excessive Src activity can also cause degradation of E-cadherin and a complete loss of cell-cell adhesion, so its activity and localization must be stringently controlled. Inhibiting Cdk5 activity in organ culture after debridement wounding significantly increases the rate of migration but also causes some separation of cells along the leading edge.

The topical application of a Cdk5 inhibitor, olomoucine, increases the rate of debridement wound closure without causing appreciable dissociation or detachment of epithelial cells.⁴

The role of corneal nerves

The cornea is densely innervated by fibers of the ophthalmic division of the trigeminal nerve and sympathetic nerve fibers from the superior cervical ganglion. Corneal nerves provide important protective and trophic functions, and interruption of corneal innervation may result in altered epithelial morphology and function, poor tear film, and delayed wound healing. Decreased corneal sensation from denervation can result in stromal ulceration and perforation. These ulcers result from decreased metabolic and mitotic rates in the corneal epithelium and reduced acetylcholine, choline acetyltransferase, and substance P concentrations.

In 1954, the classic experiment by Sigelman et al demonstrated that ocular surface changes associated with neurotropic keratitis in denervated animals persist despite tarsorrhaphy, suggesting a trophic effect of the corneal nerves.⁵ Evidence suggests that sensory neuron loss leads to a severe depletion of acetylcholine in an otherwise acetylcholine rich tissue, resulting in a relative decrease in epithelial cell growth.

Other studies attributed the depletion of substance P associated with sensory denervation as the cause of the changes associated with neurotrophic keratitis. It has been reported that substance P administered with insulinlike growth factor 1 (IGF-1) or EGF synergistically facilitates corneal epithelial migration and adhesion. Nakamura and coworkers (1999) determined that only the four-amino-acid sequence (FGLM) from the C terminal of substance P is necessary.⁶ This finding has implications for the clinical use of topically applied neuropeptides, since full-length peptides are more readily degraded and inactivated by peptidases in the tear film and corneal epithelium.

Clinical trials of nerve growth factor (NGF) by Bonini et al (2000) demonstrated a beneficial effect in promoting corneal epithelial wound healing and, possibly, in improving sensitivity in patients with neurotrophic keratitis.⁷ The mechanism of action of NGF on the ocular surface is not well defined. It may involve a direct mechanism of sensory innervation and the proliferation and differentiation of epithelial cells. An indirect mechanism, such as increasing the neuropeptides that promote epithelial healing or invoking immune cells through the release of cytokines, could also be involved.

The role of the precorneal tear film in ulceration

The exposure of the bare corneal stroma to its environment secondary to deficient or impaired epithelial wound healing is thought to contribute to stromal degradation through environmental factors, cytokines, lytic enzymes, and neutrophils in the tear film. Direct neutrophil adhesion to the corneal stroma theoretically allows hydrolytic and collagenolytic enzymes, including MMP-8 (neutrophil collagenase), to contribute to the degradation of the corneal stromal extracellular matrix.

Dohlman et al (1969) and subsequently Kenyon et al (1979) demonstrated that a glued on methylacrylate lens applied to a rabbit alkali burn model of corneal ulceration protected the stroma from collagenolysis by neutrophils and injured epithelial cells.^{8, 9} Keratocyte fibroblasts also may contribute to this milieu. The prevention of neutrophil infiltration and the promotion of epithelialization are thought to be at least some of the mechanisms responsible for the beneficial effect of amniotic membrane graft use in preventing stromal ulceration.

In addition, cytokines, such as hepatocyte growth factor (HGF), keratocyte growth factor (KGF), and EGF, are produced by the lacrimal gland and, thus, are present in tears. HGF is up-regulated in response to corneal injury in parallel with increased aqueous tear production. In the wounded cornea, these cytokines may play an important role in regulating epithelial healing. Inflammatory cytokines, including IL-1alpha, are detectable in normal human tears and may be important in causing further degradation of the corneal stroma, either directly by inducing keratocyte apoptosis or by recruiting inflammatory cells via their chemotactic properties. In addition, an irregular tear film and a decreased tear film breakup time over the area of the bare stroma can cause a delle effect that may contribute to an unfavorable cellular environment for the viability and proliferation of stromal keratocytes.

The role of cytokines
 
The complex autocrine and paracrine functions of the cytokines involved in the interactions between the corneal epithelium and stromal keratocytes are important in achieving the appropriate responses to corneal wound healing. While their precise triggers and interactions are still being elucidated, cytokines can induce and mediate many of the fundamental steps involved in wound healing.

Epithelial cell migration, proliferation, and differentiation are influenced by the stromal keratocyte cytokines, KGF and HGF. The cornea is not unique with respect to the stromal-epithelial interactions of these 2 cytokines, which are mediators of similar interactions in the breast, skin, and lung. Although the expression profiles of these cytokines lend themselves toward a linear interpretation of their stromal-epithelial interactions, these cytokines clearly are modulated further in vivo by the effects of other cytokines and truncated receptors of these molecules.

In what is likely to be merely the tip of the iceberg with respect to the understanding of cytokine-cytokine interactions, both KGF and HGF mRNA production are altered by the fibroblast cytokines, EGF, TGF-alpha, PDGF, and IL-1. In addition, EGF, PDGF, IL-1alpha, IL-6, and TNF at low concentrations appear to enhance fibronectin (FN)-induced epithelial cell migration.

Not to be eclipsed by stromal influences, epithelial cells modulate important keratocyte responses to epithelial cell injury. Keratocyte wound healing processes, including MMP production and regulation, HGF and KGF production, and keratocyte apoptosis, are mediated via various cytokines, including stimulators like IL-1 and soluble Fas ligand and major inhibitor TGF-beta2. Anterior stromal keratocyte cell death is an important feature of corneal wounding and stromal degradation.

Plasminogen is synthesized in the cornea and can be activated to plasmin by a plasminogen activator. This synthesis is stimulated by IL-1alpha and IL-1beta. In turn, plasmin is able to activate latent collagenase. This system could lead to the collagen degradation of corneal ulceration. Studies have demonstrated that uPA (urokinase plasminogen activator), but not tPA (tissue plasminogen activator), is induced in the migrating epithelial cells during corneal epithelial wound healing. Amiloride, a specific uPA inhibitor, effectively decreases uPA activity in the cornea as well as in the tear fluid and favorably affects corneal healing.

Frequency

United States

The incidence rate depends on the etiology of the corneal ulcer.

Mortality/Morbidity

Corneal scarring, decreased vision, neovascularization, perforation, and blindness are associated with this condition.

Sex

Because of an increased incidence of injuries, this condition may be seen more frequently in males than females.

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