The cornea

image of The cornea
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This chapter considers the cornea, its embryology, anatomy and physiology; investigation of disease and canine and feline conditions.

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12.1 Normal cornea in a young English Bulldog. The corneal outline is somewhat oval in shape, squared off nasally and temporally.
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12.2 Histology of the normal feline cornea. Note the relative thickness of the corneal structures. The wide and largely acellular corneal stroma contains scattered quiescent keratocytes (corneal fibrocytes) between its regularly arranged collagen lamellae. It is bordered superficially by the non-keratinizing, stratified squamous corneal surface epithelium (top layer) and at the deep or inner aspect by Descemet’s membrane and the corneal endothelium (bottom layers).
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12.3 Histology of the healthy feline corneal surface epithelium. Note the normal maturation process, from the larger generative basal epithelial cells at the deep margin of the epithelium, to the smaller ‘wing’ cells in the intermediate zone, and the superficial squamous cells near the surface. The basal epithelial cells are oriented perpendicular to the underlying basement membrane zone and the deeper, regularly arranged corneal stromal lamellae.
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12.4 Haab’s striae in a cat with glaucoma.
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12.6 Large, deep central ulcer with melting stroma.
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12.7 The small central descemetocele appears dark compared with the surrounding oedematous stroma.
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12.8 The transparent descemetocele in this cat’s eye, which is at risk of imminent perforation, allows visualization of the intraocular structures.
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12.9 Intracorneal haemorrhage visible in the superior cornea associated with vascularization.
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12.10 Phases of corneal wound healing. Corneal ulcer highlighted by fluorescein and surrounded by a diffuse stromal infiltrate of inflammatory cells, a ring of vascularization and secondary oedema. When viewed from the side, the cornea is obviously thickened and has an exaggerated curvature. As it heals, transparency and more normal corneal thickness are slowly restored, whilst some scarring with pigment deposition and residual vascularization remain.
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12.11 Subepithelial bullae secondary to marked stromal oedema caused by the ulcerative process. One large bulla is seen in the superior cornea and a cluster of smaller bullae are visible centrally.
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12.14 Persistent pupillary membrane in contact with the cornea, causing permanent, focal, corneal opacities.
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12.15 A small conjunctival dermoid. Dermoid affecting the bulbar conjunctiva, limbus and cornea.
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12.18 Conjunctival pedicle grafting. The pedicle must be long and wide enough to cover the corneal defect without having any tension in any direction. The edge of the corneal ulcer must be strong enough to hold the suture material. It is necessary to remove the corneal epithelium at the margins of the lesion, to promote adhesion of the graft to the corneal stroma, and to decide whether devitalized cornea needs to be removed, which invariably will make the defect larger and/or deeper. The surgeon should measure the distance from the base of the pedicle (the point at which the pedicle will pivot on to the corneal surface) to the distal edge of the ulcer, as well as the width of the ulcer. These measurements are used to prepare the pedicle from bulbar conjunctiva. The use of bulbar conjunctiva that arises from the inferionasal aspect is generally discouraged because the location of the third eyelid fold complicates graft preparation. Once prepared, the pedicle should be able to rest on top of the ulcer and cover it completely without retraction caused by excessive tension. The base of the pedicle is then sutured on to the limbus and at least four additional single interrupted sutures (cardinal sutures) are used to secure the pedicle to the edge of the ulcer. Once secured, further sutures are used around the edge of the ulcer to fill in the gaps. Excessive manipulation of the pedicle graft during surgery, failure of vascularization of the pedicle and/or progressive collagenolysis postoperatively may contribute to graft dehiscence. Drawn by S.J. Elmhurst BA Hons (www.livingart.org.uk) and reproduced with her permission.
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12.19 Corneoconjunctival transposition (CCT). The peripheral cornea and conjunctiva are transposed centrally to the ulcer site. The edge of the ulcer is squared off. Without removing the outlined ‘cut out’ around the edge of the ulcer, the rest the graft is outlined on the corneal surface. The path of the graft must be such that the graft is approximately 1 mm wider than the ulcer bed. The graft path is slightly diverging and crosses over the limbus on to the conjunctiva. A diamond knife, beaver blade or a restricted depth blade may be used. The corneal and conjunctival sections of the graft are then undermined using a lamellar dissector. The ideal thickness of the corneal portion of the graft is approximately 50% of the corneal depth. Extreme care must be taken not to damage the graft or enter the anterior chamber at any point. Once the graft is freed from the eye, except at its conjunctival base, it is transposed centrally over the ulcer bed. The outlined tissue at the site of the corneal ulcer is removed and the graft is sutured in place. As with conjunctival pedicle grafting, the surgeon must ensure that there is no graft tension and that the graft is not damaged during surgery. Oedematous corneal tissue is difficult to handle and easy to harm. Drawn by S.J. Elmhurst BA Hons (www.livingart.org.uk) and reproduced with her permission.
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12.20 Lower lateral eyelid entropion in an English Bulldog. The lid margin has been manually everted to expose the associated corneal ulcer, which has been stained with fluorescein.
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12.21 Paracentral, deep corneal ulcer associated with KCS in a 1-year-old English Bulldog. Distichiasis is also present affecting the lateral upper eyelid.
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12.22 Spontaneous chronic corneal epithelial defect (SCCED). Fluorescein stains the stroma past the apparent edge of the ulcer. Loose epithelium is seen folding on to itself around the ulcer edge.
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12.23 After flushing the ocular surface with a 1:50 saline:povidone–iodine solution, debridement may be performed with a dry cotton tipped applicator. This may be followed by superficial grid keratotomy with a 27 gauge needle and bandage contact lens fitting. A temporary lateral tarsorrhaphy in the shape of a horizontal mattress suture can be used to help secure the lens.
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12.24 Placement of a third eyelid flap for palliative management of corneal ulceration if more direct surgical support procedures are contraindicated (e.g. due to systemic health concerns). A needle with 2 metric (3/0 USP) non-absorbable suture is inserted through an appropriate stent (thick rubber band, Penrose drain or soft intravenous tubing) and then through the upper eyelid into the superior conjunctival fornix. The needle is then inserted though the anterior surface of the third eyelid, with the aim of encircling the vertical cartilage ‘stem’ of the third eyelid, taking care to avoid penetrating the conjunctiva on the posterior bulbar surface. Following suture placement, the third eyelid should be everted to confirm that the conjunctiva has not been penetrated on its posterior surface, as this could lead to corneal irritation and abrasion. The needle is then redirected through the superior conjunctival fornix, penetrating both the upper eyelid skin and stent. If possible, two further horizontal mattress sutures should be placed in a similar fashion, taking small bites through the horizontal cartilage on either side of the initial suture. The sutures are then tightened to pull the third eyelid across the globe, taking care to ensure that the third eyelid is anchored in the superior conjunctival fornix, without undue tension or distortion of the third eyelid. After tying, the suture ends should be left long, to facilitate their temporary untying at subsequent recheck visits (the suture knots should be carefully ‘unpicked’ and loosened with a hypodermic needle if necessary), releasing the third eyelid flap to permit observation of the eye. If necessary, the third eyelid flap sutures can be retied without the need for repeated surgical procedures. Drawn by S.J. Elmhurst BA Hons (www.livingart.org.uk) and reproduced with her permission.
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12.26 Corneal laceration resulting in a flap. Note the hair trapped within the corneal stroma, overlying the pupil (arrowed).
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12.27 Cat claw injury through the cornea, resulting in a penetrating horizontal corneal laceration and a clot of fibrin in the anterior chamber. Traumatic perforation of the cornea, resulting in a prominent fibrin plug.
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12.29 Superficial ulcer of the axial cornea within the interpalpebral fissure of a patient with neuroparalytic keratitis after total ear canal ablation surgery.
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12.30 Epithelial dystrophy in a Shetland Sheepdog. (Courtesy of S Crispin)
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12.31 Diffuse corneal oedema in a patient with endothelial dystrophy.
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12.32 Chronic superficial keratitis infiltrate in a German Shepherd Dog.
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12.33 The cornea of the dog in Figure 12.31 following treatment with topical dexamethasone phosphate drops at a tapering dose (4 x daily for 1 week, then 3 x daily for 1 week, then 2 x daily for 1 week, then 1 x daily for 1 week) and ciclosporin (0.2%) (2 x daily for 8 weeks, then 1 x daily long term). Note the superficial scarring and pigment deposition that remains.
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12.34 Pigmentary keratitis affecting the visual axis of a Pug.
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12.36 Subepithelial stromal lipid dystrophy with deposition of refractile lipid crystals in a characteristic ring in a Cavalier King Charles Spaniel. There is also an unrelated diabetic cataract in this eye.
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12.37 Diffuse superficial stromal lipid dystrophy in a Siberian Husky. The reflection of the overhead light on the cornea is arrowed.
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12.38 Calcium degeneration (calcium keratopathy) in a patient that presented with pain. The lesion retained fluorescein.
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12.39 Calcium degeneration (calcium keratopathy) in a reticulated pattern.
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12.40 Epithelial inclusion cyst of unknown origin near the limbus in a Chihuahua.
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12.41 Histology of an epithelial inclusion cyst (*) presumed to be secondary to surgical trauma. There is a linear, healed, surgical corneal incision to the right of the cyst (arrowed).
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12.42 Persistent pupillary membrane strands (arrowed) attaching to the posterior cornea in a cat, resulting in permanent, focal corneal opacity.
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12.43 A conjunctival dermoid hidden under the upper eyelid of a cat is producing long hairs that irritate the ocular surface.
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12.44 A single distichiasis lash in the lower eyelid, resulting in superficial keratitis in this cat.
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12.46 A superficial, thin, linear ulcer is seen stained with fluorescein in the cornea of this cat with active feline herpesvirus-1 infection (arrowed).
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12.47 Extensive symblepharon in a cat.
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12.48 Corneal sequestrum showing brown discoloration of the left cornea.
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12.49 A more longstanding, dense sequestrum in the right eye of the same cat as in Figure 12.48 . A dark plaque with no epithelial cover has developed.
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12.50 A dense black plaque is visible with this advanced, though small, corneal sequestrum, which has elicited a prominent vascular response.
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12.51 Lamellar keratectomy may be superficial or deep. Deep keratectomy requires tectonic support of the cornea and therefore grafting is recommended. Generally, keratectomy should be as superficial as possible, whilst at the same time attempting to remove the entirety of the lesion. The procedure should be performed with the aid of an operating microscope. A partial thickness incision is made around the sequestrum with a diamond knife, beaver blade or a restricted depth blade. The edge of the lesion is then grasped with corneal forceps and either a lamellar dissector, beaver blade or diamond knife is used in a sweeping motion to undermine it at the required depth. The surgeon must ensure that the corneal curvature is followed during the lamellar dissection so as not to progressively undermine the lesion at a much deeper level than intended and risk perforation. Once the lesion has been completely undermined, resection of the undermined flap can be completed with corneal scissors. Drawn by S.J. Elmhurst BA Hons (www.livingart.org.uk) and reproduced with her permission.
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12.52 A spontaneous superficial ulcer with loose epithelial edges in a cat.
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12.53 Eosinophilic keratitis in a cat.
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12.54 Eosinophilic lesions in the same cat as in Figure 12.50 showing uptake of fluorescein stain.
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12.55 Acute bullous keratopathy in a cat. (Courtesy of J Mould)
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