CORNEA AND ULCERS

PHYSIOLOGY

The cornea is the most powerful refractive component of the eye, responsible for up to 65% of refraction. It has a smooth outer surface which continuously replaces epithelium. Although it is a tough impermeable barrier, it needs a healthy pre-ocular tear film to keep is healthy. The cornea receives most of its metabolic requirements through the aqueous humour with some contribution from the pre corneal tearfilm. The cornea is only 0.5-0.8mm thick and even thinner in birds and exotics.

ANATOMY

The cornea has four corneal layers: the outermost epithelium, the stroma, descemet’s membrane and the inner endothelium.

The anterior epithelium is of 8 - 10 layers thick and includes three cell types: non-keratonized, non-secretory squamous epithelium, wing cells and basal cells. Basal cells are the ‘youngest’ cells from the stem cells at the limbus and are linked with hemidesmosomes to the basement membrane. The wing cells are the transition cells microvilli on the superficial cells, helps to adhere the mucin from the precorneal tear film to the cornea. The superficial cells do degenerate and slough off.

The basal membrane is a very thin membrane anchoring the epithelial cells to the stroma. The stroma makes up 90 % of the cornea. The stroma consists of 75 – 80 % water. The remaining 20 – 25 % is made up of collagen, glycoproteins and glycosaminoglycans. Keratocytes are modified fibrocytes.

The trigeminal nerve [CN V] innervates the cornea. Superficial nerve fibres are situated directly beneath the basement membrane of the epithelial cells and are sensitive to touch. The deep layer is above Descemet’s membrane and is sensitive to increases in intra ocular pressure. Brachycephalic dogs have a lower density of corneal trigeminal nerve leading to decreased sensitivity of the cornea. Diabetic dogs have also a reduced corneal sensitivity.

The descemet’s membrane is an exaggerated basement membrane produced by the posterior endothelial cells. The endothelial cell layer is a single layer of cuboidal mesothelial cells that line the posterior surface of the cornea. The main function of the layer is the active transport of ions into the aqueous humour. This allows water to follow and keep the corneal ‘dehydrated’ and clear. It has a very limited ability to replicate, this regenerative ability is better in young animals. With age the quantity decrease and the cells enlarge. Loss of endothelial cells beyond the ability of the adjacent cells to compensate will lead to permanent corneal oedema. Endothelial cells may be damaged by mechanical irritation for example anterior synechia, increased intraocular pressure or surgical trauma.

TRANSPARENCY

The cornea is transparent due to the following features: Non-keratinized epithelium The collagen fibres have a uniform small diameter (25nm) and a lamellar

arrangement. Lack of blood vessels. Lack of pigment. Relative state of dehydration (80 % hydrated). Water enters the cornea under

the influence of intraocular pressure and the hydrophilic character of the collagen and mucopolysaccharides. The endothelial and epithelial cells act as barriers that limit the uptake of water. The endothelium also functions as a Na – K ATPase pump removing water from the corneal stroma.

In contrast the collagen bundles in the sclera vary in width and thickness. They branch and intertwine with each other. They are not parallel and regular spaced

CORNEAL WOUND HEALING

The corneal epithelium is a barrier to invasion and colonization of potentially pathogenic bacteria / fungi which is normally present on the surface of the cornea and conjunctiva. A defect in the corneal epithelium allows bacteria or fungi to adhere to the cornea and initiate infection.

EpitheliumWithin one hour after injury the basal cells start to flatten and proliferation of the wing cells start. Six hours after injury the cells at the wound edge start migrating to cover the defect. Polymorphonuclear cells enter the cornea through the precorneal tearfilm and scavenge cellular debris. Mitosis and centripetal migration of the limbal stem cells start 1 to 2 days after injury. The defect gets closed with epithelial sliding and mitoses. Corneal epithelium can be completely replaced in 2 weeks.

StromaThe wound is filled with a fibrin plug. Polymorphonuclear cells arriving through the tear film remove the cellular debris. If infection occurs a mucopurulent discharge will accumulate in the wound. If infection did not occur collagenase secreted by the leukocytes will establish a demarcation between healthy and necrotic tissue. Necrotic tissue will slough and epithelial cells will fill the crater. After 24 hours the adjacent keratocytes migrate to the wound edge, transforms

to fibroblasts and begin collagen and extracellular matrix secretion. This will stimulate cell adhesion, cell migration and protein synthesis. New collagen fibres and lamellae will be produced but in a disorganized arrangement. This will cause a scar and corneal opacity. Vascularization begins at the limbus 3 to 6 days after injury and progress toward the lesion at a rate of 1 mm per day. If the injury epithelise before the blood vessels reach it they will regress. If the lesion is not epithelised by the time the blood vessels reach it, the defect will be filled with granulation tissue. After healing the blood vessels regress spontaneously. The scar tissue will decrease over weeks and months.

Endothelial healingEndothelial cells have a minimal mitotic capacity. Replacement of endothelial cells occurs through enlargement and migration of adjacent cells which will reduce endothelial density.

PROTEASESProteinases play an important role in the normal turnover of epithelial cells and remodelling of corneal stroma. It will detect and remove damaged cells or collagen aro wear and tear of the cornea. Proteolytic enzyme activity is normally balanced by protease inhibitors, which prevents degradation of healthy tissue. Increased amounts of proteases can lead to rapid degradation of the corneal stroma.

Proteinase enzymes are:

- Matric metalloproteinases (MMP) and

- Serine proteineases neutrophil elastase (NE).

Natural proteinase inhibitors

-X1 proteinases inhibitor

-x2-macroglobulin

-TIMP’s (tissue inhibitors of metalloproteinases)

Epithelial cells, inflammatory cells and fibroblasts secrete proteases. Pseudomonas and Aspergillus also secrete MMP’s.

CORNEAL PATHOLOGICAL REACTIONS

Blue cornea - Oedema

Cornea oedema is the result of dysfunction of one of the two corneal layers responsible to keep the cornea dehydrated namely the corneal epithelial and endothelial cells.The excess fluid in the cornea will distort the lamellar arrangement of the collagen leading to opacity. An oedematous cornea appears blue or grey. Oedema may be localized to a small area or may involve the whole cornea.

Corneal oedema

Fluorescein negative Fluorescein positive

Non painfulUninflamed Normal IOPFlare absent

PainfulInflamedAbnormal IOPFlare Present

Senile endothelial

degenerationOR

Inherited endothelial dystrophy

GlaucomaUveitis

Lens luxationScleritis

Corneal ulcer

Monitor for bullae, ulcers

Complete ocular examination including

slitlamp and tonometry

Thorough ophthalmic examination and ulcer

treatment

CAUSES OF CORNEAL ULCERS

Trichiasis

Distichiasis

Ectopic cilia

Epithelial dystrophy / degeneration

Superficial chronic corneal epithelial dystrophy (SCCED)SCCED in dogs are chronic corneal erosions that are persistent and fail to resolve through normal wound healing processes Breeds most commonly affected are the Boxer and Corgi, but this condition is seen in all breeds. Left improperly treated SCCED can persist for months. The exact etiology is unknown. Most of these erosions occur spontaneously, but may be to other conditions, for example: distichiasis, ectopic cilia, trichiasis and mechanical trauma. One study showed a decreased density of hemidesmosomes adhering the epithelial cells to the basement membrane. Another study revealed either no basement membrane or adhesion complexes or only small discontinuous segments of basement membrane on the surface of the exposed stroma.

Signs:

May be unilateral or bilateral. Blepharospasm and epiphora. The erosion is a shallow slightly opaque lesion most often in the central cornea. An overhanging “lip” of epithelial cells is often present. The epithelium is loosened even over normal appearing portions of the cornea

and large areas of the epithelium can be removed with minimal debridement. Positive fluorescein stain that reveal epithelial undermining. (The positive area

gets bigger within minutes).

Treatment:

Medical treatment:

Aggressive debridement of the epithelium under topical anaesthetic using a cotton bud or a diamond burr. Topical antibiotic and morphine for 7 days. If no healing occurs or the erosion recurs surgical treatment is indicated.

Surgical treatment (Grid keratotomy):

The epithelial cells are debrided using a scalpel blade. Care must be taken to ensure that all the abnormal cells are removed. A striate keratotomy is performed using a 23 gauge needle with a bent tip. Multiple superficial scratches are made in the superficial cornea 0.5 to 1 mm apart in a crosshatching pattern. If the striations are too superficial they do not accomplish anything and if they are too deep they will lead to permanent

corneal scarring. To ensure the depth of the striations, good magnification preferably an operating microscope should be used. A soft disposable contact lens is placed on the cornea for 10 days. Post operative treatment includes topical antibiotic and morphine.

MORE CAUSES OF CORNEAL ULCERS:

Feline Herpes Virus 1 (FHV-1)

Etiology:

Feline Herpes virus (FHV-1) is an epitheliotrophic virus affecting the cornea and conjunctiva.

Signs

unilateral Blepharospasm Serous ocular discharge Conjunctivitis Discreet punctate corneal opacities en the early stages, progressing to

the pathognomonic dendritic ulcers. These lesions are the direct result from the cytopathic effect of the virus. Some cases may develop large, but very superficial corneal erosion very similar to SCCED in dogs.

Symblepharon Stromal keratitis may develop in chronic cases Sequestrum

Treatment for FHV-1

Topical antibiotics to prevent secondary bacterial infection. Idoxuridine [q4h] or Cidofovir [q12h] Oral Famcyclovir [90mg/kg bid] in severe cases. Grid keratotomies are contra indicated in cats with corneal erosions, this

may introduce the virus into the deeper stroma leading to a corneal sequestrum.

Systemic or topical cortisones can activate the latent FHV-1

MORE CAUSES OF CORNEAL ULCERS:

Mechanical abrasions - rubbing of the head, mechanical clippers, trauma for example cat scratches, thorns and other foreign bodies, eyelid tumours.

Keratoconjunctivitis sicca (KCS)

Entropion.

Lagophthalmos (inability to close the eyelids normally).

Exposure of the cornea due to prolapse of the eye or general anaesthesia.

Chemical burns. o Alkali burns are more severe than acid burns because alkaline substances

penetrate the cornea rapidly. Saponification of fatty acids causes cell disruption and cell death. It will hydrolyze intracellular glucosaminoglycans and denatures corneal collagen. This damaged tissue will stimulate an inflammatory response, release proteolytic enzymes which cause further damage like melting et. Treatment will include irrigation, anticolleganase, antibiotics, mydriatics and systemic NSAIS.

o Acidic products cause protein coagulation in the epithelium, but penetration and damage are limited and the lesion will more likely stay superficial further

Venom OphthalmiaMost snake venom is caustic (basic). The toxic agents forms soaps with the lipids of the corneal cells membranes and disrupt the glycosaminoglycan ground substance. Softening of tissues and devitalisation of corneocytes ensues. Secondary anterior uveitis is common.

Stromal ulcers

Stromal ulcers develop aro of trauma and/or bacterial infection when the epithelial healing fails. Common bacterial infections are Pseudomonas., Staph. and Strep. spp. In these cases is a bacterial culture and antibiogram with cytology very important. Bacteria adhere to the damaged corneal surface and invade the epithelium and stroma. Bacterial multiplication happen very fast and bacterial exotoxins, endotoxins and proteases are released. An influx of leukocytes and inflammatory mediators can cause further damage with a rapid progression.

Melting ulcers

Bacterial and fungal pathogens with corneal epithelial cells, corneal stromal fibroblasts and leukocytes, upregulate cytokines that induce MMP and NE production. This elicit an inflammatory and degradative process. The excessive levels of proteinases will cause rapid degeneration of the collagen of the stroma and case keratomalacia or corneal melting.

TREATMENT:

Determine and treat the initiating cause if possible. The treatment of corneal ulcers requires one or more of these therapeutic influences:

Antimicrobial Analgesic Anticollegenase Protection Support

Superficial ulcers only require analgesic and antimicrobial treatment, but deep ulcers also require support, protection and anticollagenase.

Antibiotic treatment:

Topical antibiotics are indicated to prevent bacterial infection in superficial ulcers or to eradicate already established infections.

Fucithalmic (Fusidic acid)

Fusidic acid is a narrow-spectrum antibiotic. Fusidic acid has potent antibacterial activity toward Gram-positive bacteria and Neisseria species. Fusidic acid is most notable for its activity against Staphylococci, whether coagulase-positive or negative, and regardless of resistance to methicillin and related penicillins. Fusidic acid is active against Haemophilus sp. but has almost no antibacterial activity against other Gram-negative organisms such as E. Coli, Proteus, Klebsiella and Salmonella. Fungi are also insensitive to fusidic acid.

Isee (Chloramphenicol)

It is a broad spectrum antibiotic but is bacteriostatic. Chloramphenicol spectrum includes anaerobes and Gram negative- and positive organisms like: Haemophilus influenzae, Strept. pneumoniae, Staph. aureus, Streptococcus viridians. It can also be used for Moraxella species and Enterobacteriaceae.

Exocin/ Octin (Ofloxacin)

Ofloxacin is a third generation Fluoroquinolone which has a broad spectrum against aerobes, Gram negative and Gram positive bacteria and Chlamydia.

Gram-negative bacteria: Acinetobacter calcoaceticus var. anitratum, and A. calcoaceticus var. iwoffi; Enterobacter Sp. including E. cloacae; Haemophilis Sp, including H. influenza and H. aegyptius; Klebsiella Sp., including K. Pneumoniae; Moraxella Sp., Morganella morganii; Proteus Sp., including P.

Mirabilis; Pseudomonas Sp.; including P. Aeruginosa, P. cepacia, and P. fluoroscens; and Serratia Sp., including S. marcescens.

Gram-positive bacteria: Bacillus Sp.; Corynebacterium Sp.; Micrococcus Sp.; Staphylococcus Sp., including S. aureus and S. epidermidis; Streptococcus Sp., including S. Pneumoniae (see above), S. viridans and Beta-haemolytic.

Vigamox (Ofloxacin)

Vigamox is a 4th generation Fluoroquinolone. Its spectrum includes Gram positive and Gram negative ocular pathogens (as above), as well as atypical microorganisms, anaerobes and Chlamydia.

Tobrex (Tobramycin)

Tobamycin is an aminoglycoside with a Gram positive and Gram negative spectrum with atypical microorganisms. Microorganims susceptible to Tobrex include: Pseudomonas aeruginosa , Escherichia coli, Klebsiella pneumoniae, Enterobacter aerogenes, Proteus mirabilis, Streptococci, Morganella morganii, most Proteus vulgaris strains, Haemophilus influenzae and H. aegyptius, Moraxella lacunata,  Acinetobacter calcoaceticus and some Neisseria species. Bacterial susceptibility studies demonstrate that in some cases, microorganisms resistant to gentamicin retain susceptibility to tobramycin.

Ciloxan (Ciprofloxacin) is a Fluoroquinolone with a Gram positive and Gram negative spectrum. The Ph of Ciloxam is 4.5 and can cause discomfort.

Type Sensitivity

Advantages

Disadvantages

Fucithalmi Gram + Narrow

c SpectrumIsee Gram +

(+) - Moraxella Bacteriostatic

Exocin/ Octin

Gram +(+) -

Chlamydia

Vigamox Gram + (+) -

Chlamydia

Tobrex Gram + (+) -

Ciloxan Gram + (+) -

Stings

Analgesic therapy:

Corneal ulcers are accompanied by pain, as a result of direct sensory nerve stimulation or through uveal stimulation.

Atropine, a parasympatholytic agent, stops the ciliary muscle spasm by blocking the parasympathetic receptors of the iris and ciliary body which leads to analgesia. Atropine also causes mydriasis and helps to stabalise the blood aqueous barrier. Atropine may cause profuse salivation and vomiting and can also cause reduction in tear production.

Morphine (10mg/ml 4-6x daily) is ideal to manage analgesia. (Topical use of 1% MSS in dogs with corneal ulcers provided analgesia and did not interfere with normal wound healing. (Am J Vet Res 2003;64:813–818).)

Never use topical anaesthetics as an analgesic because long term use of these drugs lead to corneal ulceration.

Anticollagenolytic drugs/ Protease inhibitors

Serum contains growth factors, fibronectin and vitamins which are important to support proliferation, migration, and differentiation of the corneal and conjunctival epithelium.

 Corneal epithelial cell morphology and function are better maintained by serum than by pharmaceutical tear substitutes. (Geerling, G. and Hartwig, D., 2006. Autologous serum eyedrops for ocular surface disorders. In Cornea and external eye disease (pp. 1-20). Springer, Berlin, Heidelberg.)

Oral Doxycycline is secreted in tears and inhibits MMP’s and other mediators of inflammation. It chelates Zinc which is a necessary co-factor for many enzymes and prevents the breakdown of collagen. Tetracycline is an oxygen scavenger and anti-inflammatory agent and can also be used for Chlamydial and Ehrlichia infections.

Topical Lubricant

Artelac Advanced

Systane Ultra

Optive Plus

Hy-care

SURGICAL TREATMENT

Amnion and conjuntival grafts are used for deep stromal ulcers, melting ulcers, descemetocele, corneal perforation or after keratectomies

Amnion

Amnion is the avascular tissue that forms the innermost layer of fetal placental membrane. The advantages of using amnion are: It is avascular, anti-fibrotic, anti-protease, has anti-inflammatory effects, contain growth factors, and are anti-immunogenic. It also supports epithelial migration, provides tectonic support, is permeable to topical medication, optimize the visual outcome and minimize the scar lesion.

Conjunctival flaps

Conjunctival flap is a fibrovascular tissue to fill and support a corneal defect. It is a source of direct blood supply with anticollagenolitic and antiprotease properties and supports healing.

Corneal Crosslinking [CXL]

Background:

Corneal collagen cross linking was developed for the treatment of primary and secondary corneal ectatic disease in humans including keratoconus. Between 2003 and 2007 additional indications for CXL were introduced: bullous keratopathy, early Fuch’s dystrophy and the treatment of infectious and non-infectious corneal melting ulcers.

CXL is a technique that creates intrafibrillar covalent bonds in the collagen fibres of the corneal stroma via the photoactivation of riboflavin by ultraviolet A [UV–A] light.

CXL is induced by introducing riboflavin to the cornea. The riboflavin acts as a photosensitiser when exposed to UV–A light.6 Riboflavin absorbs the UV–A producing reactive oxygen species [ROS]. These free radicals introduce new crosslinks between collagen fibres increasing the biomechanical stability of the cornea. The free radicals also directly damage and destroy micro-organisms and lead to apoptosis of cells in the irradiated area.

There are however certain risks to the eye from the exposure to UV–A, namely damage to corneal cells including endothelial cells as well as intraocular structures. Riboflavin limits the risks of direct damage, as it limits radiant transmission to deeper ocular structures by absorbing UV–A light. In human and porcine corneas damage occurs to a depth of 300ųm with surface irradiation of 3mW/cm2. It is therefore recommended that CXL is only done in corneas thicker than 400ųm as this will prevent damage to the corneal endothelial cells and intraocular structures.

Method

In dogs and cats the procedure is done under general anaesthesia. The ulcer is cleaned and the surrounding 2–3 mm of epithelial cells are removed with a scalpel or diamond polisher. 0.1 % Riboflavin solution is applied, 1 drop every 2 minutes for 30 minutes. Penetration of the Riboflavin is then confirmed by visualising the fluorescence of Riboflavin in the anterior chamber with slitlamp biomicroscopy using a cobalt blue light. The cornea is then rinsed and the cornea is irradiated. Various different protocols have been described varying from 3 mW/cm2 for 30 minutes to 30 mW/cm2 for 3 minutes. The Peschke unit used at the Johannesburg and Cape Animal Eye Hospitals allows irradiation at 45 mW/cm2 for 2 minutes.

Contraindications

CXL can lead to Herpesvirus exacerbation in humans.CXL should not be used in feline patients with active FHV keratitis. CXL is also contra-indicated in corneas thinner than 400ųm as this may lead to endothelial damage or damage to intraocular structures