Ординатура / Офтальмология / Английские материалы / Ocular Disease Mechanisms and Management_Levin, Albert_2010

collagens with interrupted terminals collagens) occur dif­ fusely throughout the entire corneal stroma. Furthermore, this lamellar interweaving also explains why the anterior third of the cornea mildly swells, whereas the remaining corneal stroma can swell to up three times its normal thick­ ness.10 This anisotropic elasticity characteristic of the human cornea in swelling is important since the anterior corneal surface accounts for two-thirds of the refractive power of the eye. Because fibrotic corneal scars have random directionally oriented interweaving collagen fibrils, they have also been found to resist swelling under edematous conditions.16

Therefore, although it is commonly stated that corneal thickness and interfibrillar spacing increase in a linear fashion to the hydration level of the corneal stroma,1,8 one needs to be aware that this relationship mainly applies to the mid and posterior stromal regions.

Etiology

Corneal edema is usually caused by one of two pathogenic mechanisms: endothelial cell dysfunction or high intraocu­ lar pressure (IOP). Common causes of endothelial dysfunc­ tion include Fuchs’ endothelial dystrophy, pseudophakic bullous keratopathy (i.e., from cataract surgery), trauma, other ophthalmic surgery (corneal transplantation, trabeculectomy/tube shunt glaucoma surgery), infections, and toxic anterior-segment surgery (Box 9.4).17–20 Common causes of high IOP include uncontrolled glaucoma (acute angle closure glaucoma, neovascular glaucoma, pseudoexfo­ liation glaucoma), postoperative pressure spikes from retained viscoelastics, and medications (topical steroids).

Pathophysiology

Embryology to birth

During embryogenesis, the corneal endothelium forms during the 5th week of gestation as the first wave of neural crest-derived mesodermal cells form a two-cell layered primi­ tive endothelium. By 8 weeks of gestation, a monolayer of cells is formed. The epithelium and endothelium remain

Box 9.4  Etiology

Endothelial cell dysfunction

•Fuchs’ endothelial cell dystrophy

•Pseudophakic bullous keratopathy (cataract surgery)

•Trauma

•Other intraocular surgeries (glaucoma shunts, corneal transplantation)

•Infections (corneal ulcers or endophthalmitis)

•Toxic anterior-segment syndrome (toxic substances in anterior chamber)

High intraocular pressure

•Uncontrolled glaucoma (acute angle closure glaucoma, neovascular glaucoma, pseudoexfoliation glaucoma)

•Postoperative pressure spikes (retained viscoelastics)

•Medications (topical steroids)

Pathophysiology

closely opposed until 7 weeks of gestation (49 days) when a second wave of neural crest-derived mesodermal cells begins to grow centrally from the limbus between the epi­ thelium and endothelium, producing the corneal stroma. By the 3rd month of gestation, Descemet’s membrane can be clearly recognized on histologic sections. Studies have inferred that during the 5th month of gestation the tight junctions completely form and the endothelial barrier is established; similarly, by 5–7 months’ gestation, the density of Na+/K+-ATPase pump sites eventually reaches adult levels so that the cornea becomes dehydrated and transparent.21,22 By the 7th month of gestation, the cornea resembles that of the adult in most structural characteristics other than size. At birth in the full-term infant, the horizontal diameter of the cornea is only around 9.8 mm (surface area 102 mm2), or approximately 75–80% the size of an adult human cornea (note at birth, that the posterior segment is <50% the size of an adult human cornea).

Infancy to adulthood

The endothelium of a newborn infant cornea is composed of a single layer of approximately 500 000 neural crestderived endothelial cells, each measuring around 5 m in thickness by 20 m in diameter, and covering a surface area of 250 m2.5,20,23–25 The cells lie on the posterior surface of the cornea and form an irregular polygonal mosaic. The tangential appearance of each corneal endothelial cell is uniquely irregular, usually uniform in size to one another, and typically six-sided hexagons (which is the most energyefficient and optimal shape to cover a surface area without leaving gaps).1 They abut one another in an interdigitating fashion with a 20 nm wide intercellular space between each other (Figure 9.3A and B). The intercellular space is known to contain discontinuous apical tight junctions (Figure 9.3C), or macula occludens tight junctions, and lateral gap junctions (Figure 9.3D). Thus, intercellular space of the corneal endothelium represents an incomplete diffusion barrier to small molecules. As corneal endothelial cells have numerous cytoplasmic organelles, particularly mitochon­ dria, they have been studied and are presumed to have the second highest aerobic metabolic rate of all cells in the eye next to retinal photoreceptors.7 At birth, the central endothe­ lial cell density of the human cornea is around 5000 cells/ mm2.5 Because the corneal endothelium has very limited in vivo regenerative capacity (endothelial cells are currently hypothesized to proliferate at too low a rate in vivo to replace dying cells) and because aging results in progressive cellular senescence, particularly in the central regions of the cornea in part through the activity of the cyclindependent kinase inhibitor p21, there is a well-documented decline in central endothelial cell density with age that typi­ cally involves two phases: a rapid and slow component (Figure 9.4).2–4,5,20,23–25 During infancy, the cornea continues to grow over the first 2 years of life, reaching adult size at 2 years of age with an average horizontal diameter of 11.7 mm (surface area 138 mm2). Thereafter, it changes very little in size, shape, and optical properties. However, the only sig­ nificant structure in the cornea that continues to grow after age 2 is the Descemet’s membrane as it gradually increases an additional 6–11 m in thickness from birth to death. Due to corneal growth and age-related or developmentally selec­

1000

0

Age (years)

Figure 9.4  Scatterplot with best-fit curve showing the average central corneal endothelial cell density for normal, healthy eyes of different ages. (Redrawn with permission from Williams KK, Noe RL, Grossniklaus HE, et al. Correlation of histologic corneal endothelial cells counts with specular microscopic cell density. Arch Ophthalmol 1992;110:1146–1149).

tive cell death, during the fast component of cell loss, the central endothelial cell density decreases exponentially to about 3500 cells/mm2 by age 5 and 3000 cells/mm2 by age 14–20.5,20,23–25 Thereafter, a slow component of cell loss occurs where central endothelial cell density decreases to a linear steady rate between 0.3 and 0.6% per year, resulting in cell density measurements around 2500 cell/mm2 in late adulthood.5,20,23–25 Because the corneal endothelium essen­ tially maintains its continuity by migration and expansion of surviving cells, it is not surprising that the percentage of hexagonal cells decreases (pleomorphism) and the coeffi­ cient of variation of cell area increases (polymegathism) with age.23

When reviewing this information it is important to realize that these are average central corneal endothelial cell counts from predominantly Caucasian US populations. Several studies reveal that important racial and geographic differ­ ences exist as Japanese, Filipino, and Chinese corneas have been found to have higher central cell density measurements than Caucasians, while Indian corneas have lower central

cell densities (Table 9.1).26-29 It is hypothesized that this range of central cell densities may be predominantly due to racial differences in corneal diameter and endothelial surface area between these groups (e.g., Japanese, Caucasian, and Indian horizontal corneal diameters averaged 11.2, 11.7, and 12.0 mm, respectively), but genetic and environmental factors are also possible. Additionally, these data only apply to central corneal endothelial counts since recent work has shown that higher endothelial cell densities can typically be found in more peripheral aspects of the cornea, where a potential “stem-like” endothelial cell population or storage zone may reside (Figure 9.5).30–32 Therefore, overall it appears that corneal endothelial cell numbers decrease on average about 50% from birth to death in normal subjects. As corneal decompensation or overt corneal edema typically does not occur until the central endothelial cell density approaches values around 500 cells/mm2 (90% decreased from infant values), there appears to be plenty of cellular reserve remain­ ing after an average human life span of 75–80 years.5,20,23–25 In fact, estimates suggest that normal human corneal endothelium should maintain corneal clarity up to a minimum of 224–277 years of life, if humans lived that long.25

The primary function of the corneal endothelium is to maintain the deturgescence (i.e., it keeps the cornea near 78% water) and clarity of the cornea through both barrier and a pump leak mechanism first described by David Maurice.33 Secondarily, it is also known to secrete an anteri­ orly located basement membrane called Descemet’s mem­ brane and a posteriorly located glycocalyx layer.1

Barrier function

The barrier function of the endothelium is dependent upon having a sufficient number of corneal endothelial cells to cover the posterior surface of the cornea and having integrity of endothelial cellular tight junctions, which are present in the intercellular spaces between endothelial cells (Figure 9.6). Macula occludens tight junctions are characterized by

Figure 9.5  Diagram (A) and graph (B) illustrating the central, paracentral, and peripheral corneal endothelial cell densities in healthy, normal subjects. ((A) Modified from Edelhauser HF. The resiliency of the corneal endothelium to refractive and intraocular surgery. Cornea 2000;19:263–273; (B) redrawn with permission from Edelhauser HF. The Proctor lecture: the balance between corneal transparency and edema. Invest Ophthalmol Vis Sci 2006;47:1755–1767.)

partial total obliteration of the 20 nm wide intercellular space and partial sub-total retention so that 10 nm intercel­ lular spaces remain. Clinically, the barrier function of the cornea can be assessed by the use of the specular microscope or the confocal microscope (endothelial cell density), and fluorophotometry (permeability). In healthy human corneas, this barrier prevents the bulk flow of fluid from the aqueous humor to the corneal stroma, but does allow moderate dif­ fusion of some nutrients, water, and other metabolites to cross into the stroma through the 20 nm wide intercellular

Pump leak mechanism

The classic temperature reversal studies provided the first evidence that the maintenance of corneal hydration and transparency was metabolically dependent.35 Corneal thick­ ness and corneal cloudiness were found to increase when intact eyes were refrigerated. This effect was observed to reverse (i.e., the tissue thinned and regained transparency) when the tissue was rewarmed (temperature reversal). Sub­ sequent in vitro corneal perfusion studies demonstrated that temperature reversal still occurred in the absence of the corneal epithelium, implicating active metabolically depend­ ent processes on the corneal endothelium as mediating corneal deturgescence.1 These studies also demonstrated that transporters, located primarily on a corneal endothelial cell’s lateral cell membrane, affected the transport of ions – prin­ cipally sodium (Na+) and bicarbonate (HCO3−) – out of the stroma and into the aqueous humor. An osmotic gradient is created and water is osmotically drawn from the stroma into the aqueous humor.36 It is important to note that this osmotic gradient only occurs if the endothelial barrier is maintained. The transport protein essential for endothelial “pump function” was later identified as Na+/K+-ATPase (Figure 9.7A).37,38 Subsequently, the number and density of Na+/K+-ATPase sites have also been quantified using [3H]- ouabain.39 These studies have shown that approximately 2.1 million Na+/K+-ATPase sites are present on the lateral mem­ brane of a single human corneal endothelial cell. This cor­ responds to an average pump site density of 4.4 trillion ATPase sites/mm2 along the lateral plasma membrane wall of an intact corneal endothelial cell.39 Clinically, the metabolic pump of the corneal endothelium can be assessed in vivo by measuring how quickly the corneal thickness (pachymetry) recovers after being purposefully swollen by wearing oxygen-impermeable contact lens or by measuring the diurnal change in corneal thickness (normal = 6 ± 3%; eyelid closure during sleep induces hypoxia and decreased evaporation loss).

A number of factors are known to alter endothelial pump function. Fortunately, physiologic compensatory mecha­ nisms prevent corneal edema from occurring to a certain degree when central endothelial cell densities are between 2000 and 750 cells/mm2. This occurs by either increasing the activity of pump sites already present, which requires more ATP production by the cell, and/or by increasing the density of pump sites on the lateral membranes of endothelial cells (Figure 9.7B and C).39 A similar phenomenon occurs in the proximal tubule cells of the human kidney to adjust for an increased salt load. For example, in Fuchs’ endothelial dys­ trophy, the cornea has been found to remain clear and of normal thickness despite having very low endothelial cell densities and increased endothelial monolayer permeability to fluorescein (5.30 × 10-4 cm/min).39 Apparently, this occurs because the metabolic activity and density of the Na+/K+ pump sites increase to compensate for the increased permeability.40 The point at which compensatory mecha­ nisms appear to fail is when the central endothelial cell density reaches around 500 cells/mm2 (range of 750−250 cells/mm2) (Figure 9.7B and C).5,41 At this low cell count, the permeability has greatly increased to such a point that the endothelial cells – which are spread so thin – do not have enough room on their lateral cell membranes for more meta­

Pathophysiology

When the corneal endothelial barrier and metabolic pump are functioning normally, the corneal stroma has a total Na+ concentration of 179 mEq/L (134.4 mEq/L free and 44.6 mEq/L bound to stromal proteoglycans), while the aqueous humor has a total Na+ concentration of 142.9 mEq/L (all free).43 Therefore, after accounting for chloride activity and stromal imbibition pressure, an osmotic gradient of +30.4 mmHg exists, causing water to diffuse from the stroma to the aqueous humor (Figure 9.8A). When the corneal endothelium is damaged, there is a loss of both the corneal barrier and pump function, followed by a loss of the ionic gradients, ultimately resulting in corneal edema and stromal swelling (Figure 9.8B).

Pathophysiology of corneal edema

The Donnan effect states that the swelling pressure in a charged gel (e.g., the corneal stroma) results from ionic imbalances. The fixed negative or anionic charges on corneal stromal proteoglycan glycosaminoglycan (GAG) side-chains

– one carboxylic acid and one sulfate ester side chain per disaccharide repeat on a dermatan sulfate GAG polymer, and one or two sulfate ester side-chains per disaccharide repeat on a keratan sulfate GAG polymer – have a central role in this effect. The antiparallel GAG duplexes (tertiary structure) produce long-range electrostatic repulsive forces that induce an expansive force termed swelling pressure. Because the corneal stroma also has cohesive and tensile strengths that resist expansion, the normal swelling pressure of the non­ edematous corneal stroma is around 55 mmHg.44,45 If the stroma is further compressed (e.g., increasing IOP or mechanical applanation) or expanded (e.g., corneal edema), the swelling pressure will correspondingly increase or decrease. Conversely, the negatively charged GAG sidechains also form a double-folded helix in aqueous solution (secondary structure) that attracts and binds Na+ cations, which results in an osmotic effect, leading to the diffusion and subsequent absorption of water. Thus, the central corneal thickness is maintained with an average value of

Figure 9.7  Diagram (A) illustrates the corneal endothelial cell pump, which is due to many Na+/K+-ATPase pump sites on the lateral membrane of each corneal endothelial cell. (Modified from Dawson DG, Watsky MA, Geroski DH, et al. Physiology of the eye and visual system: cornea and sclera. In: Tasman W, Jaeger EA (eds) Duane’s Foundation of Clinical Ophthalmology on CD-ROM. Philadelphia: Lippincott Williams & Wilkins, 2006:v. 2 c. 4:1–76.) Diagram (B) and graph (C) illustrate the relationship between central endothelial cell density, barrier function, pump sites, and pachymetry. Note that the number pump sites are not all maximally used in the normal state (5000–2000 cells/mm2). With increased leaking (2000–750 cells/mm2), there is an adaptive phase in which the endothelial cells can maximally use all their pump sites and/or can form more pump sites to offset the leak up to a point. When the surface area of the lateral membranes of endothelial cells progressively becomes too small (750–0 cells/mm2), these adaptations reach a maximum and eventually decline. The point where endothelial cell pump site adaptations cross permeability (500 cells/mm2) is typically when corneal decompensation occurs. ((B) Modified from Chandler JW, Sugar J, Edelhauser HF. External diseases: cornea, conjunctiva, sclera, eyelids, lacrimal system, vol. 8. London: Mosby, 1994; (C) redrawn with permission from Dawson DG, Watsky MA, Geroski DH, et al. Physiology of the eye and visual system: cornea and sclera. In: Tasman W, Jaeger EA (eds) Duane’s Foundation of Clinical Ophthalmology on CD-ROM. Philadelphia: Lippincott Williams & Wilkins, 2006:v. 2 c. 4:1–76.)

Stroma

Increase of free [Na+]

Decrease of bound [Na+]

Figure 9.8  Diagram (A) illustrates the total transendothelial osmotic force due to Na+ activity, Cl− activity, and imbibition pressure. Although the Na+ activity within the aqueous humor is greater than that within the stroma (142.9 versus 134.4 mEq/L; P < 0.05.), using a reflection coefficient of 0.6, the calculated osmotic force due to Na+ is 98.5 mmHg. Similar calculations for Cl− and imbibition pressure result in osmotic forces of –8.1 and –60 mmHg, respectively. The sum of these forces results in a total osmotic force of +30.4 mmHg, which ultimately results in deturgescence of the cornea. Diagram (B) illustrates what happens to the ionic gradients and osmotic forces when the corneal endothelium is damaged and corneal edema and swelling set in. (Modified from Stiemke MM, Roman RJ, Palmer M, et al. Na+ activity in the aqueous humor and corneal stroma of the rabbit. J Exp Eye Res 1992:55:425–433.)

520 m (based on optical pachymetry) or 540 m (based on ultrasound pachymetry) because the fixed negatively charged proteoglycans induce a constant swelling pressure through anionic repulsive forces, and because the hydration level of corneal stroma is constantly maintained at around 78% water because corneal stromal proteoglycans imbibe water through cationic attractive forces.46 Interestingly, there is a difference in the water-absorbing characteristics between the anterior and posterior cornea from differences in the distribution of proteoglycans made in these two regions (anterior: more dermatan sulfate, low free water-binding capacity, high retentive water-binding capacity; posterior: more keratin sulfate, high free water-binding capacity, low retentive water-binding capacity).14 Under normal circum­ stances, the negative pressure drawing fluid into the cornea, called the imbibition pressure of the corneal stroma, is approximately –40 mmHg.47 This implies that the negative

72

Figure 9.9  Diagram demonstrating the delicate balance between stromal swelling pressure, endothelial pump function, and intraocular pressure (IOP). Usually if endothelial cell pump function fails and IOP remains normal, both stromal and epithelial edema occurs (B). Only when IOP increases above the swelling pressure of the stroma and the endothelium functions normally do we see epithelial cell edema alone (C) and only when IOP is near zero and the endothelium functions abnormally do we see stromal edema alone (D). (Modified from Hatton MP, Perez VL, Dohlman CH. Corneal oedema in ocular hypotony. Exp Eye Res 2004;78:549–552.).

charges on corneal proteoglycans are only about one-quarter (~27%) saturated, or bound, with Na+ and water, and that the remaining unbound proportion is still available to bind more Na+ and absorb more water if given either a compro­ mised endothelium or epithelium, or both. Normally, the highly impermeable epithelium and mildly impermeable endothelium keep the diffusion of electrolytes and fluid flow in the stroma to such a low level (resistance to diffusion of electrolytes and fluid flow = epithelium (2000) >> endothe­ lium (10) > stroma (1)) that the aqueous humor ionic gradi­ ent created by the endothelial cell metabolic pump can maintain stromal hydration in the normal range of 78%.

Although imbibitions pressure (IP) = swelling pressure (SP) when corneas are in the ex vivo state, IP is actually lower than SP in the in vivo state because of the hydrostatic pressure induced by IOP, which must now be accounted for. This is best represented by the equation IP = IOP – SP1 and explains why the hydration level of a patient’s cornea is not only dependent on having normal barrier functions, but also on having a normal IOP. Therefore, a loss of corneal barrier function, an IOP ≥ 55 mmHg, or a combination of the two results in corneal edema.48

Finally, while both epithelial and stromal edema com­ monly coexist, there are two notable exceptions (Figure 9.9). As the epithelium lacks fixed negatively charged proteogly­ cans and has much weaker cohesive and tensile strength values than the corneal stroma, its state of hydration is mainly dictated by IOP levels.49 Conversely, because colla­ gen fibrils in the corneal stroma are anchored at the limbus for 360°, they exert increasing or decreasing cohesive strength on the corneal stroma as the IOP elevates above or decreases below normal, respectively. This results in the transmission of stromal edema to the epithelial surface in cases of high IOP or to the stroma in cases of low IOP. Therefore, if IOP is ≥ 55 mmHg with normal endothelial barrier and pump function, epithelial edema usually occurs

by itself, or if endothelial cell dysfunction and hypotony (IOP ~0 mmHg) occur together, then stromal edema occurs alone.

Summary

In summary, although born with a substantial reserve of extra corneal endothelial cells for maintenance of normal corneal hydration and function, normal wear and tear on the human cornea from growth, development, and aging to an average human life span of 75–80 years of age reduce an individual’s central endothelial cell density on average by 50% (5000 cells/mm2 to 2500 cells/mm2). Compounding this normal decline are other potential exogenous stressors

Key references

(trauma, infections, corneal transplant procedures, and intraocular surgery at a very young age) that could potentially damage the endothelial monolayer further so that it reaches a 90% reduction in cell density (~500 cells/ mm2) from intent values. It is typically only around a central endothelial cell density of 500 cells/mm2 that corneal edema manifests clinically and corneal function drops precipitously.

Acknowledgment

This work was supported in part by NEI grants EY-00933, P30-EY06360, and an unrestricted departmental grant from Research to Prevent Blindness.

1.Dawson DG, Watsky MA, Geroski DH, et al. Physiology of the eye and visual system: cornea and sclera. In: Tasman W, Jaeger EA (eds) Duane’s Foundation of Clinical Ophthalmology on CD-ROM. Philadelphia: Lippincott Williams & Wilkins, 2006:v. 2 c. 4:1–76.

5.Edelhauser HF. Castroviejo lecture: the resiliency of the corneal endothelium to refractive and intraocular surgery. Cornea 2000;19:263–273.

6.Edelhauser HF. The balance between corneal transparency and edema. The Proctor lecture. Invest Ophthalmol Vis Sci 2006;47:755–767.

7.Hogan MJ, Alavarado JA, Weddell E. Histology of the Human Eye. Philadelphia: WB Saunders, 1971:55– 180.

8.Dawson DG, Holley GP, Schmack I, et al. Hydropic degenerative keratocytes contribute to corneal haze in edematous

human corneas. IOVS 2006;47:ARVO e-abstract B93.

20.Bourne WM. Biology of the corneal endothelium in health and disease. Eye 2003;17:912–918.

25.Armitage WJ, Dick AD, Bourne WM. Predicting endothelial cell loss and long-term corneal graft survival. Invest Ophthalmol Vis Sci 2003;44:326–331.

31.Amann J, Holley GP, Lee S, et al. Increased endothelial cell density in the paracentral and peripheral regions of the human cornea. Am J Ophthalmol 2003; 135:584–590.

32.Whikehart DR, Parikh CH, Vaugh AV,

et al. Evidence suggesting the existence of stem cells for the human corneal endothelium. Mol Vis 2005;11:816– 824.

33.Maurice DM. The cornea and sclera. In: Davson H (ed.) The Eye, vol. IB, 3rd edn. Orlando: Academic Press, 1984:1–158.

35.Harris JE. Symposium on the cornea. Introduction: factors influencing corneal hydration. Invest Ophthalmol 1962;1: 151–157.

38.Lim JJ, Ussing HH. Analysis of presteadystate Na+ fluxes across the rabbit corneal endothelium. J Membrane Biol 1982;65: 197–204.

39.Geroski DH, Matsuda M, Yee RW, et al. Pump function of the human corneal endothelium. Effects of age and corneal guttata. Ophthalmology 1985;92:759– 763.

42.Bonanno JA. Identity and regulation of ion transport mechanisms in the corneal endothelium. Prog Retin Eye Res 2003; 22:69–94.

48.Ytteborg J, Dohlman CH. Corneal edema and intraocular pressure. II. Clinical results. Arch Ophthalmol 1965;74:477– 484.

Overview

Corneal neovascularization (NV) and lymphangiogenesis are sight-threatening conditions that introduce vascular conditions into the normally avascular cornea (Box 10.1). Corneal NV is induced by various stimuli and is mainly associated with inflammation, trauma, transplantation, and infection of the ocular surface1,2; lymphangiogenesis is usually concurrent with hemangiogenesis in the human cornea.3 Both corneal NV and lymphangiogenesis are promoted or inhibited by a balance of factors, including the dynamics between vascular endothelial growth factor (VEGF), VEGF-C, VEGF-D, sFlt, VEGFR3, endostatin, and thrombospondin-1 and -2 contained in the cornea (Box 10.2). Recently, evidence has shown that soluble VEGF receptor (VEGFR-1) and ectopic VEGFR-3 are expressed in corneal epithelial cells, and act as decoy receptors for VEGF-A and VEGF-C/-D, respectively.4–7 These decoy receptors function to maintain corneal clarity and prevent corneal NV and lymphangiogenesis. In corneas that are diseased by inflammation, infection, degeneration, transplantation, or trauma, the normal balance of proand antiangiogenic factors is shifted toward proangiogenic status, leading to corneal NV and/or lymphangiogenesis. The pathogenesis of corneal NV and lymphangiogenesis may be influenced by growth factors, cytokines, matrix components, and matrix metalloproteinases (MMPs). New medical and surgical treatments that have been effective in corneal NV/lymphangiogenesis in animals and humans include immunosuppressant agents, angiostatic steroids, nonsteroidal anti-inflammatory drugs (NSAIDs), argon laser photocoagulation, and both photodynamic and antiangiogenic therapies.

The main purpose of this chapter is to describe the pathophysiology of corneal NV and lymphangiogenesis. The current treatments and potential antiangiogenic and/or antilymphangiogenic therapies will also be addressed at the end of this chapter.

Clinical background

Corneal NV and lymphangiogenesis together represent major public health burdens in the USA, affecting an estimated 1.4 million patients in any given year.1 These conditions are caused by a wide range of inflammatory, infectious, degenerative, toxic, and traumatic disorders; major ocular complica-

tions include corneal scarring, edema, lipid deposition, and inflammation (Box 10.3). Corneal NV and lymphangiogenesis not only significantly alter visual acuity, but also worsen the prognosis of subsequent penetrating keratoplasty.

Corneal NV originates from the perilimbal plexus of conjunctival venules and capillaries and may invade the cornea at any level. Two types of corneal NV can be clinically discerned: pannus and stromal NV. In pannus NV, the proliferation of blood vessels spreads between the epithelium and Bowman’s layer and is usually associated with ocular surface disorders such as infection, trauma, or metabolic dysfunction. In stromal NV, the vessels are usually in a straight line, following the anatomical divisions of the corneal lamellae and branching in a brushlike manner. This is most common in inflammatory status of the cornea, like stromal keratitis.

Clinically, patients with corneal NV may complain of a decrease in visual acuity. The diagnostic workup should include slit-lamp examination, which will identify the origin of the NV, as well as the depth of its invasion in the cornea. If corneal edema is presented at the same time, employment of pachymetry, specular microscopy, or confocal microscopy can aid in confirmation of the diagnosis.

Treatment of corneal NV has been widely investigated both medically and surgically. Current treatments for corneal NV in humans include steroids, NSAIDs, ciclosporin A, anti- VEGF-A antibody, argon laser, electrocoagulation, limbal transplantation, amniotic membrane transplantation, and conjunctival transplantation.

Pathology

In corneal pannus, a fibrous tissue with a significant vascular component is seen between the epithelium and Bowman’s layer; this is called “subepithelial fibrovascular pannus.” There are two types of corneal pannus: inflammatory pannus and degenerative pannus. Inflammatory pannus is associated with prominent leukocytic infiltration and includes polymorphonuclear leukocytes in the active stages. However, by the time the pathologist detects the inflammatory pannus, it is commonly constituted by overwhelming numbers of lymphocytes and plasma cells. Frequently, the Bowman’s layer is disrupted, and the vessels wander haphazardly through the anterior stroma. In degenerative pannus, there are fewer inflammatory cells. The vascular component has never been prominent and is liable to regress, leaving a

Box 10.1  Corneal neovascularization (NV)

and lymphangiogenesis

•Corneal NV and lymphangiogenesis are sight-threatening conditions that introduce vascular conditions into the normally avascular cornea

•Corneal NV and lymphangiogenesis are derived from:

Inflammatory disorders

Infection

Degenerative congenital disorders

Trauma and other causes

Box 10.2  Balance of angiogenic/antiangiogenic and lymphangiogenic/antilymphangiogenic factors

dictates corneal neovascularization and lymphangiogenesis

Both corneal neovascularization and lymphangiogenesis are promoted or inhibited by a balance of proangiogenic, antiangiogenic, prolymphangiogenic, and antilymphangiogenic factors:

•Basic fibroblast growth factor, vascular endothelial growth factor (VEGF), VEGF-C, VEGF-D

•sFlt, VEGFR3, endostatin, thrombospondin-1, -2, and others

Etiology

hyalinized, relatively acellular layer of fibrous tissue. This type of pannus is especially common in conditions that give rise to chronic epithelial edema, such as glaucoma.

Stromal NV differs from its superficial counterparts and is located beneath the Bowman’s layer. Although it can occur anywhere in the stroma, it is usually identified at the upper and middle third of this layer.

Etiology

The clinical situations precluding corneal NV are replete; however, they can be grouped into four categories (Table 10.1 and Figures 10.1–10.3):

Box 10.3  Corneal neovascularization and

lymphangiogenesis may cause ocular complications

Corneal neovascularization and lymphangiogenesis together represent major public health burdens and can cause many ocular complications, including:

•Corneal scarring

•Edema

•Lipid deposition

•Inflammation

•Transplantation rejection