Chapter 8 Surgery of the cornea and sclera
Corneal diseases occur frequently in the dog and cat. In the dog, corneal diseases may be primary or secondary to other ophthalmic diseases. Secondary corneal diseases occur frequently in the brachycephalic breeds and with keratoconjunctivitis sicca. In dogs, the common causes of corneal diseases are trauma, inflammations and ulcerations, degenerations, and dystrophies. Congenital abnormalities and neoplasia of the canine cornea are infrequent. In cats, corneal diseases are common, and are usually associated with inflammations, trauma, and sequestration (corneal black spot, corneal mummification). In both cats and dogs, trauma of the cornea occurs most frequently in animals under 1 or 2 years of age.
Corneal diseases cause varying degrees of opacification. Invasion of blood vessels, pigment cells, neoplastic cells, lipid material, and leukocytes from the limbus reduce corneal transparency. Edema results from local corneal inflammation or impaired corneal endothelia that can no longer remove fluids from the cornea. With reduction in corneal transparency, vision can be gradually impaired; with total corneal opacification, vision can be lost temporarily or permanently.
Treatment of corneal diseases in dogs and cats is quite successful using medical, surgical, and a combination of these therapies. Corneal diseases are often noticed early by the pet’s owner because of the onset of pain, blepharospasm, photophobia, conjunctival hyperemia and chemosis, tearing, and rubbing of the eyelids. As a result, the possibility of successful treatment is higher. Medical treatment of corneal diseases usually involves the direct instillation of drugs on the affected tissue. Topical drugs include solutions, suspensions, and/or ointments. When corneal diseases are progressing or are unusually severe, the topical route may be supplemented with drugs provided systemically and subconjunctivally. Corneal penetration by most antibiotics is limited by the lipophilic corneal epithelium. Chloramphenicol is still the antibiotic of choice when the epithelium is intact, and therapeutic levels of antibiotic are necessary in the cornea and anterior chamber. With corneal ulceration, the epithelial barrier is markedly reduced, and the administration of broad-spectrum topical antibiotics is recommended. The most frequently used topical antibiotics include gentamicin, chloramphenicol, tobramycin, the fluoroquinolones and the combination of neomycin, polymyxin B, and bacitracin. In corneal diseases with vascularization of the cornea and/or secondary iridocyclitis, systemic antibiotics are often indicated and are highly successful.
Surgical treatment of corneal diseases in the dog and cat is often the primary modality. The normal cornea, exposed suddenly to trauma or ulceration, often requires several days to initiate satisfactory inflammatory and healing responses. In the meantime, the infectious agents, proteases, and collagenases (from bacteria and damaged corneal cells) can cause rapid degradation of the cornea, and threaten the integrity of the cornea and maintenance of vision. Surgical treatment can jump-start the healing process and markedly reduce the length of the lag phase for healing, as well as provide vital structural corneal support. Corneal surgery includes partial keratectomy (or removal for treatment or biopsy), keratotomies (single or multiple incisions), transposition (movement from one site to another), primary closure for small corneal ulcers and lacerations, and transplantation (autogenous and homologous) or grafting of corneal tissues to replace cornea lost to disease.
Dog and cat corneas are relatively large compared to those of humans, probably to assist in night vision. Animals with large corneas are typically nocturnal, as large corneas allow greater amounts of light to enter the pupil during reduced illumination. Most animal corneas are roughly elliptical in shape with the vertical diameter slightly less than the horizontal diameter. The normal dog cornea measures 12–16 mm vertically and 13–17 mm horizontally, and is 0.45–0.55 mm thick centrally and 0.50–0.65 mm thick peripherally. The normal cat cornea measures 15–16 mm vertically and 16–17 mm horizontally, and is about 0.58 mm thick centrally and peripherally. Corneal measurements, both diameters and thicknesses, have not been established for different ages and the different breeds in either the dog or cat.
The cornea, along with the sclera, forms the fibrous tunic of the globe (Fig. 8.1). The zone where the cornea gradually becomes opaque and changes to sclera is the limbus. The limbus is sufficiently forward of the aqueous humor filtration angle or the iridociliary cleft (or ciliary cleft) to prevent direct visualization of the aqueous outflow pathways. Dog and cat corneas are divided into axial (central) and peripheral, with the central area most important for vision. Often the cornea is divided into quadrants. The central cornea is generally the thinnest and most often affected with ulcerations.
Fig. 8.1 The surgical and microanatomy of the dog and cat cornea. (a) The anatomic relationships of the cornea to the other tissues in the anterior segment of the eye. (b) The microscopic layers of the dog and cat cornea include: (A) epithelium; (B) stroma; (C) Descemet’s membrane; and (D) the endothelium. H & E, 25×.
Dog and cat corneas have four different microscopic regions. From external to internal, these subdivisions include: 1) epithelia with basal membrane; 2) thick stromal layer; 3) Descemet’s membrane; and 4) the posterior single layer endothelia. A modified anterior region of the corneal stroma, Bowman’s membrane, is absent in the dog and cat, but present in humans and most birds. The epithelial layer is normally about 5–7 cells thick and consists of: 1) outer two to three layers of non-keratinized squamous cells; 2) middle two to three layers of polyhedral or wing cells; and 3) a single layer of basal columnar cells that are positioned on a basement membrane (Fig. 8.2). The apparent turnover of corneal basal epithelia is about 7 days. The basement membrane, produced by the basal corneal epithelia, attaches the basal epithelial cells via hemidesmosomes to the anterior stroma. Defects in the canine basal corneal epithelia and basement membrane are thought to contribute directly to the development of recurrent corneal erosions. The corneal stroma, or substantia propria, accounts for about 90% of the corneal thickness, and consists of parallel bundles of collagen fibrils, few fibrocytes (also called keratocytes), and a matrix of glycosaminoglycans. The arrangement of these fibrils and the matrix of glycosaminoglycans become distorted with disease and is the basis of corneal opacification.
Corneal sensory nerves, distributed from the mid posterior stroma from the ophthalmic branch of the trigeminal nerve, eventually terminate in subepithelial plexuses to provide free nerve endings in the epithelial wing cell layer. As a result, superficial corneal ulcers are usually more painful in dogs than ulcerations involving the deep corneal stroma. Corneal sensitivity may be reduced in the brachycephalic breeds, possibly predisposing the cornea to damage.
Descemet’s membrane is the basement membrane produced by the posterior cells, the endothelia. Descemet’s membrane, a relatively thick basement membrane that increases in thickness with aging, is clear and somewhat elastic. Surgical repair is necessary to prevent imminent corneal rupture when exposed Descemet’s membrane or a descemetocele is clinically visible.
The single layer of endothelia forms the posterior layer of the cornea, and consists of hexagonal-shaped cells that interdigitate with each other laterally with different cell junctions, including zonulae occludentes, maculae adherentes, and nexi. These metabolically active cells are the principal site for removal of water from the cornea via an Na+K+ ATPase ‘pump’. Surgical and traumatic damage, as well as aging and decreased numbers of endothelia, alter this state of ‘deturgescence’, and edema of the cornea may result.
The oval adult equine cornea measures 26–28 mm vertically and 32–38 mm horizontally, with a radius of curvature of about 17 mm. The thickness also varies, with the center measuring as thin as 0.56 mm and the periphery 0.8 mm. Corneal thickness measurements can vary by the in-vivo or in-vitro method used (direct measurement in fixed tissue), ultrasonography or ultrasound pachymetry and specular microscopy. Ultrasonic pachymetry of the central equine cornea reveals 785 ± 2.98 μm to 858 μm (mean). Dorsal and lateral measurements are 932.5 μm (mean) and 879.5 μm (mean), respectively. Corneal endothelial count varies by age, and a count of 3216 cells/mm2 has been reported in the horse.
The oval cornea of the adult cow is roughly pear-shaped and measures 22–24 mm vertically and 27–32 mm horizontally. Its thickness also varies with the center thicker (range from center and periphery: 0.75 to 0.85 mm). The radius of curvature for the bovine cornea varies from 14.7 mm (vertically) and 16.8 mm (horizontally).
Diseases alter corneal transparency. Invasion of the cornea with blood vessels from the limbus and bulbar conjunctiva; accumulation of extracellular and intracellular fluids and edema; infiltration with the different types of leukocyte; migration of pigment cells from the limbus, conjunctiva, and anterior synechiae; and deposition of lipid, cholesterol, and calcium products all reduce the cornea’s ability to transmit images. Fortunately, dog and cat corneas have considerable capacity for repair and the re-establishment of transparency. Corneal nutrition is from three sources: precorneal/preocular film, limbal vasculature, and the aqueous humor posteriorly.
The corneal epithelia respond quickly to damage by undergoing mitosis and sliding of new wing cells into the corneal defect. The entire cornea can be re-epithelialized in 7–10 days, although firm adhesion of the new epithelia by hemidesmosomes requires several weeks. New corneal epithelium is usually semipermeable to topical fluorescein, staining a faint green. During the corneal ulcerative process, proteases, collagenases, and other enzymes are released from degenerating corneal cells, leukocytes, and certain bacteria. These enzymes degrade the collagen fibrils and glycosaminoglycans, potentiating the ulcer’s progression even in the absence of sepsis.
Superficial corneal ulcers are often more painful than deeper ulcerations. Both the corneal epithelium and anterior corneal stroma possess pain and pressure receptors that are part of the long ciliary nerves that arise from the ophthalmic branch of the trigeminal (fifth) nerve. Not only does pain occur from stimulation of these nerve endings, but also an axonal reflex that results in a secondary iridocyclitis (reflex miosis, conjunctival and anterior uveal hyperemia), and altered blood–aqueous barrier (aqueous flare). Hence, corneal ulceration commonly causes secondary iridocyclitis, which requires treatment concurrent with the corneal ulcer therapy.
As the corneal ulcer is being rapidly covered by healing epithelia, the corneal stroma has a slower and longer repair phase. Often the stroma is invaded by blood vessels from the limbus and bulbar conjunctiva, as well as leukocytes from tears, blood vessels, and limbus. Fibroblasts, converted from keratocytes and histiocytes, slowly produce new collagen fibrils and local glycosaminoglycans matrix. This process takes several weeks to a few months. Following deep keratectomy, complete recovery of corneal stroma to normal thickness may take months, and the stroma may not totally return to normal thickness. It has been suggested that the limit of three superficial keratectomies for the dog appears related to failure of total stromal regeneration.
After superficial keratectomy, the new collagen fibrils may not perfectly realign with adjacent normal corneal lamellae, resulting in variable amounts of scarring. Fortunately, scarring after superficial keratectomies in the dog and cat is limited. In fact, if one ranks the tendency for corneal scarring among animals, it appears that the cat and cow have the least tendency, the dog ranks in the middle, and the horse has the greatest tendency for corneal scarring after disease or surgery.
Chronic corneal irritation in animals usually results in invasion of the cornea with melanocytes from limbus and bulbar conjunctiva, epithelial cells, and histiocytes. This intracellular melanin pigment, observed clinically as brown-to-black areas in the cornea, appears histologically in the corneal basal and wing cell layers, and in anterior corneal stroma. Once the canine cornea becomes pigmented, the opacification from this pigmentation is difficult to eliminate medically or excise surgically, but can usually be controlled sufficiently to allow clinical vision.
Repair of defects in Descemet’s membrane depend on the formation of new basement membrane by corneal endothelium and requires several weeks. Posterior endothelial regeneration is influenced by animal species and age: in young animals the defect is covered by new corneal endothelia derived primarily by mitosis; in older animals these defects are covered primarily by endothelial cell enlargement by adjacent cells. In older animals, it appears that Descemet’s membrane progressively thickens. Once Descemet’s membrane is cut, the membrane curls and retracts. The exposed posterior stroma is rapidly covered with a fibrin clot. Adjacent endothelial cells, by either mitosis or enlargement, cover the fibrin clot, and over several weeks produce a new Descemet’s membrane. The rabbit corneal endothelia seem to rapidly undergo mitosis and slide to cover areas on Descemet’s membrane or posterior stroma within days. Regeneration of endothelia in the dog and cat is poorly understood. Like children, the puppy and kitten corneal endothelia demonstrate remarkable and rapid regeneration by mitosis. However, the density of corneal endothelia in the dog gradually declines with age, suggesting that regeneration is not occurring. The occurrence of persistent corneal edema and primary endothelial dystrophies in dogs indicate that regeneration of corneal endothelial cells in older animals does not occur.
The normal cornea also becomes slightly thicker in older dogs, presumably from less effective corneal dehydration associated with decreased numbers of corneal endothelia. The density of corneal endothelial cells in the dog, critical to maintain a cornea devoid of edema, ranges from 1200 to 1500 cells mm2, with 2500–3000 cells mm2 being normal. In corneal tissue banks for humans, at least 2000 cells mm2 is considerable essential for corneal donor tissue for corneal transplantation. Because of limited numbers of endothelial cells in older dogs, corneal edema is more apt to occur after cataract surgery. Cataract surgery in dogs generally results in the loss of 10–20% of the corneal endothelia. Hence, in very old dogs having cataract surgery, damage to the corneal endothelium during phacoemulsification must be considered seriously, and more postoperative corneal edema is expected.
The preoperative treatment of corneal diseases in animals depends on the condition. With limbal and corneal neoplasms, dermoids, focal corneal lipidosis, and corneal cysts, treatment of the cornea prior to surgery is not usually necessary. In contrast, corneal inflammations, foreign bodies, ulcerations, and partial and full-thickness lacerations may require adequate preoperative treatment to ensure the success of the surgical procedure, especially if entry into the anterior chamber is likely. With corneal defects, i.e., ulcerations, descemetoceles, corneal ulcerations with iris prolapse, corneal foreign bodies, and partial-to-full-thickness lacerations, topical and systemic antibiotics are indicated to prevent the infectious process from spreading intraocularly.
Topical antibiotics most frequently include chloramphenicol, gentamicin, tobramycin, and the combination of neomycin, polymyxin B, and bacitracin. Most often the bacteria recovered from corneal ulcers by culture are Staphylococcus and Streptococcus spp., which are sensitive to most antibiotics. Systemic antibiotics, such as amoxicillin or cephalexin, are administered when corneal disease is advanced and integrity of the globe threatened.
With inflammatory corneal diseases, secondary involvement of the anterior uvea is common. Miosis, flare in the anterior chamber, fibrin formation, lowered intraocular pressure, photophobia, and swelling of the iris and ciliary body usually accompany corneal ulcerations and lacerations. Instillations of iridocycloplegics, such as 1% atropine or 0.3% scopolamine with 10% phenylephrine, are indicated to reduce ocular pain, decrease the likelihood of posterior synechiae and cataract formation, and retract the iris from full-thickness axial corneal wounds. The physical objective of mydriatic treatment is to moderately dilate the pupil, but still permit some iris movement that discourages the formation of posterior synechiae. The strong iridocycloplegics, such as 1–3% atropine, are long acting and can markedly decrease aqueous tear production. An acutely dry cornea does not heal readily! As the pupil size can be ascertained daily through most corneas, the intensity and concentration of mydriatic(s) can be adjusted quickly to maximize therapeutic effects and minimize side effects.
Corneal ulcers in the dog occasionally progress even though sepsis is not demonstrable. This expansion of the edges of the corneal ulcer may result from the local release of proteases, collagenases, and other enzymes liberated by dying corneal and inflammatory cells. Specific treatment to combat this effect may be achieved by topical 5.0% acetylcysteine or preferably autogenous serum.
Non-steroidal anti-inflammatory drugs (NSAIDs) such as flunixin meglumine (0.5 mg/kg IV; Banamine®, Schering-Plough, Kenilworth, NJ) or carprofen (2 mg/kg PO; Rimadyl®, Pfizer Animal Health, Exton, PA) are used in the dog to reduce postoperative iridocyclitis, pain, and conjunctival and eyelid swelling. These drugs may mimic the effects of steroids on the eye but with few side effects; however, like corticosteroids, they appear to delay corneal vascularization.
The dog and cat eyes rotate during gas anesthesia ventromedially, and collapse once the anterior chamber has been opened. The use of non-depolarizing neuromuscular blocking agents such as atracurium (0.2 mg/kg IV; Glaxo-Wellcome Research, Triangle Park, NC) paralyzes the animal, including the extraocular muscles. As a result, the eye position remains normal and the lack of extraocular muscle tone reduces intraocular tissue displacement once the anterior chamber is entered. Because of the paralysis of all striated musculature, including the muscles associated with breathing, artificial ventilation in these anesthetized patients is essential until the drug-induced paralysis ceases or is reversed (see Chapter 3).
With general anesthesia, loss of tear production, and lack of the protective blink reflex, the cornea quickly dries. During surgery, the corneal surface should be intermittently irrigated with lactated Ringer’s solution or balanced saline solution.
Preoperative treatment is not uncommon in horses, and often directed at the primary corneal disease. Although the most common topical antibiotics for the horse include the triple antibiotics (neomycin, bacitracin, and polymyxin) and gentamicin or tobramycin, ciprofloxacin may have the greatest activity and least number of resistant organisms. Topical gentamicin, perhaps because of its high frequency of use as the first-line topical antibiotic for nearly four decades, has been reported to now encounter the highest number of resistant Pseudomonas and Streptococcus organisms. As a result, intensive or progressive corneal ulcers in the horse should be cultured, if possible, to help guide antibiotic therapy.
Mydriasis is often indicated for horses with corneal diseases to reduce the ocular pain, dilate the pupil, and stabilize the blood–aqueous barrier. Of the species requiring mydriatics, the horse appears the most sensitive to the systemic effects of atropinization. Hence, mydriatics are administered to effect (a maximally dilated pupil) for a day or two, and then reduced to maintain the dilated pupil to minimum levels. Close daily observation of the horse for intestinal motility and output of feces is important. Any decrease in intestinal motility or apparent abdominal distress should initiate immediate reduction or cessation of topical anticholinergic mydriatic therapy.
Antifungal medications are important in the horse because of the not-infrequent fungal involvement in corneal disease in many areas of the world. Natamycin, also known as pimaricin, (5%) is the only commercially available antifungal agent in the United States; its reported activity is for Fusarium and some Aspergillus species. Other antifungals must be individually prepared and include miconazole (1%), fluconazole (0.2%), voriconazole (1%), itraconazole (1%), and amphotericin (0.15%). These topical antifungals are often administered for several weeks. The search for effective systemic antifungals continues!
The horse eye is characterized by a profound inflammatory response which often must be medically controlled in order to treat the infectious agents, but prevent excessive inflammation which can cause anterior and posterior synechiae formation, secondary cataracts, and phthisis bulbi (destruction of the ciliary body with markedly reduced aqueous formation rates, intraocular pressure less than 5 mmHg, cataract formation, and retinal detachment). Topical and systemic corticosteroids must be administered carefully in the horse, as potential adverse effects are not infrequent. Topical and especially systemic non-steroidals, such as flunixin meglumine (1 mg/kg PO, IV, or IM q12h) are effective in controlling secondary uveitis, reducing uveal exudation, and relieving ocular discomfort.
For horses with severe corneal disease, the subpalpebral medication system to administer medications to the eye is most useful when the medications are delivered to the eye six to eight times daily for many weeks.
The investment in surgical instruments depends on the expertise of the veterinary surgeon and the anticipated patient load with corneal surgical diseases (see Table 1.4, p. 12). Magnification is essential for corneal surgeries. Head loupes can provide magnification at levels of 2.5 to 4×; the operating microscope provides at least 10× and is generally preferred. With considerable corneal surgery and keratoplasty, the operating microscope is recommended. Either standard or microsurgical instruments may be used, or a combination of both sizes. Minimal instruments include the following.
• For exposure of the cornea and a possible lateral canthotomy: tenotomy or Steven’s scissors, eyelid speculum, tissue forceps (Bishop–Harmon), and needle holder (standard; with lock: Castroviejo). For additional information on lateral canthotomy, see Chapter 2.
• For corneal tissues: Colibri and tying forceps (with 1 × 2 teeth), Beaver scalpel handle and blades (Nos 6400, 6500, and keratome), corneal scissors (right and left handed), iris scissors, disposable electrocautery, Martinez corneal dissector, associated cannulas, needle holder (microsurgical size and without lock), and a set of corneal trephines (at 0.5 mm increments). The diamond knife is very useful for corneal surgery and can provide exact control of the depth of the corneal incision in increments of 0.2, 0.3, 0.4, 0.5, 0.6 and 0.8 mm.
• For keratoplasty, additional instruments include corneal cutting block (often Teflon®) and punch. At least one or two different sizes of Flieringa rings are useful for corneal transplantation, and are temporarily sutured to the bulbar conjunctiva and episclera to prevent collapse of the globe (see Chapter 1).
Both absorbable and non-absorbable 6-0 to 10-0 sutures are used for corneal surgeries. The least reactive suture is the non-absorbable nylon, and is essential for keratoplasty, but removal of the sutures is usually necessary. Absorbable sutures, such as polyglactin 910 or polyglyconate, are the usual choices with either reverse cutting or spatula needles. Suture techniques include simple interrupted, simple continuous, double saw-tooth, and others. Ideally a corneal stitch should be 75–90% of the corneal thickness with ‘bites’ about 2 mm of tissue for maximum tissue holding (especially with edematous corneal tissue).
The ophthalmic surgical instrumentation for performing corneal or corneoscleral surgeries in large animals is identical to that for small animals. For these surgeries using limited magnification, standard size instruments are most useful; if the operating microscope is used as well as general anesthesia, microsurgery ophthalmic instruments are often employed.
Superficial corneal diseases are usually confined to corneal epithelia and anterior stroma, and may be treated surgically. For instance, corneal dermoids generally extend into the anterior stroma and may involve adjacent bulbar conjunctiva (Fig. 8.3). Treatment is superficial keratectomy. Corneal lipidosis often affects the anterior corneal stroma, and may be removed by superficial keratectomy. Recurrent corneal erosions in the dog appear related to corneal epithelial dystrophy and defects in the basement membrane which result in defective adhesion during healing of these superficial erosions and frequent recurrences (Fig. 8.4). Several surgical procedures, including superficial keratectomy, have been used to treat this condition. The superficial keratectomy procedure may be used for corneal sequestra in cats limited to the anterior corneal stroma.
Fig. 8.3 Corneal dermoid in a St Bernard puppy. Covered with long coarse hair, that is highly irritating, the congenital mass involves the lateral cornea, limbus, and bulbar conjunctiva. Recommended treatment is superficial keratectomy.
In this section, surgical techniques that involve corneal epithelium and anterior aspects of the corneal stroma are presented and include: 1) superficial keratectomy (partial and complete); 2) superficial punctate, grid or linear keratotomy; and 3) corneal biopsy.
Keratectomy may prove useful in the early stages of ulcerative keratitis when infection is confined to the corneal epithelium and anterior third or half of the cornea stroma, and in later stages of stromal keratitis when the epithelium has healed. Removing necrotic tissue by keratectomy speeds healing, minimizes scarring, and decreases the stimulus for keratitis and iridocyclitis.
In superficial keratectomy the corneal epithelia and variable amounts of anterior stroma are excised. The procedure may involve the entire cornea or only part of the cornea. The thickness of stroma removed depends on the corneal disease. When the superficial keratectomy is limited to the outer one-third of the cornea, the postoperative wound is treated medically as a corneal ulcer. However, when the partial superficial keratectomy extends for more than one-half to two-thirds of the corneal stroma, the defect is covered with a conjunctival graft. While regeneration of corneal epithelia appears complete after superficial keratectomy, total recovery of the corneal stromal thickness is questionable. A single cornea subjected to three superficial keratectomies appears to have a stroma of about one-half to two-thirds normal thickness thereafter.
Indications for superficial keratectomies include corneal dermoid, chronic superficial keratitis (pannus), pigmentary keratitis, chronic and recurrent corneal erosions, corneal and/or limbal neoplasia, ulcerative keratitis with sequestration in cats (Fig. 8.5), corneal superficial dystrophies (usually lipidosis and calcification), and superficial corneal foreign bodies. In some of these diseases, the cause(s) has not been determined, and although the involved corneal tissue(s) appears completely excised, recurrence may occur. For some of these conditions, such as pigmentary keratitis, the superficial keratectomy may re-establish a clear cornea, but if the predisposing factors, such as lagophthalmia, nasal fold trichiasis, or tear film disorder, are not resolved, the cornea will become pigmented again.
Fig. 8.5 Corneal sequestration in cats consists of a distinct central brown-to-black area of necrotic stroma. (a) Corneal vascularization and inflammation may surround the lesion. (b) Immediate postoperative appearance after superficial keratectomy. If the keratectomy is limited to less than one-third of the corneal stroma, it is treated as a corneal ulcer. A bulbar conjunctival graft may also be used to cover the surgical defect, and may decrease the possibility for recurrence of sequestrum.
There are several different modifications of the superficial keratectomy procedure. As a rule, only the diseased area within epithelial and anterior stromal layers is excised. While the normal dog and cat corneas are about 0.5–0.6 mm thick, the abnormal cornea may exceed 1.0 mm in thickness. When the entire cornea is diseased, superficial keratectomy may be performed using a limited-depth corneal trephine or diamond knife, or by dividing the cornea into four quadrants (much like cutting a pie or cake) and separating the opaque superficial layers of cornea from the deeper clear corneal stroma.
Instruments used to perform the superficial keratectomy include: eyelid speculum, smooth and 1 × 2 teeth tissue forceps (Bishop–Harmon or Colibri), Beaver scalpel handle and No. 6400 microsurgical blade or diamond knife, strabismus or tenotomy scissors, irrigator bulb, and small cannula. Additional instruments to perform the lateral canthotomy may be necessary when improved exposure of the corneal site is necessary. Other corneal instruments that can assist in the superficial keratectomy are the Martinez dissector, a corneal trephine whose depth can be preset to 0.2–0.3 mm, and the diamond knife with a micrometer (which limits the depth of the corneal incision). This procedure is generally performed under general anesthesia.
Superficial keratectomy is usually limited (partial) to the diseased cornea. The periphery of the diseased area is encircled with an incision using the Beaver scalpel handle and No. 6400 microsurgical blade, or the diamond knife with the micrometer set at 0.15 or 0.25 mm, or a preset corneal trephine (0.15–0.25 mm). The incision should be of sufficient depth to remove the base of the diseased cornea, as estimated preoperatively by slit-lamp biomicroscopy (Fig. 8.6a). Often, the cornea is vascularized, and limited hemorrhage occurs during the incision. To prevent hemorrhage from obscuring the incision, a continuous stream of 0.9% sterile saline is directed at the leading aspects of the corneal incision as it is performed. This hemorrhage will usually cease once sufficient time has elapsed to permit clotting. If a conjunctival graft is applied to the keratectomy site, the surgical defect may be constructed as a square lesion.
Fig. 8.6 In the superficial keratectomy procedure, a section of corneal epithelium and the superficial stroma are excised. (a) The periphery of the corneal dermoid is incised with the Beaver No. 6400 microsurgical blade to a depth of about 0.2–0.3 mm. (b) The edge of the lesion is grasped and lifted with a 1 × 2 teeth thumb forceps, and separation of the lesion from the underlying clear stroma is continued by scalpel dissection. (c) During the dissection of the stroma, the microsurgical blade is held tangential to avoid entry into the deeper stromal lamellae. (d) The Martinez corneal dissector or separator may also be used (instead of the microsurgical blade) during this part of the surgery for lamellar dissection. (e) Once the stromal dissection has been completed, the lesion is carefully removed, resulting in a corneal defect.
Once the corneal lesion has been outlined, the edge of the superficial keratectomy section is grasped carefully with 1 × 2 teeth tissue forceps to permit separation of the diseased cornea from the underlying stroma (Fig. 8.6b). The dissection plane within the stroma should remain in the same parallel lamellae throughout the superficial keratectomy. If the Beaver knife is used, the instrument must be held tangential to the corneal stroma to prevent progressive deeper dissection into the stroma (Fig. 8.6c). Alternately, the Martinez dissector facilitates this dissection to remain within the respective corneal lamellae (Fig. 8.6d).
Once the diseased cornea has been completely separated from the stroma, it is lifted from the surgical wound (Fig. 8.6e). If some tags of stroma remain, they are carefully cut with utility or tenotomy scissors. If additional areas of diseased cornea are still present, the procedure may be repeated in these areas.
Partial superficial keratectomies are generally limited to the outer one-half of the cornea unless a conjunctival graft, corneoconjunctival graft, or lamellar corneal graft is attached afterwards to the surgical wound. Postoperatively, the superficial keratectomy wound is not usually covered with a nictitating membrane flap or complete temporary tarsorrhaphy. Topical broad-spectrum antibiotics are instilled four to six times daily. Topical 1% atropine is instilled to maintain a moderate to completely dilated pupil (one drop daily or every other day). Topical atropine can reduce aqueous tear formation, as measured by the Schirmer tear test, by 50–75%. A marked decrease in tears can prolong corneal re-epithelialization by several days.
Every day or every 2 days the re-epithelialization of the superficial keratectomy wound is evaluated with and without topical fluorescein. Re-epithelialization usually starts within 48 h. The entire canine cornea can re-epithelialize within 7–10 days. New epithelia are somewhat translucent, stain faintly with topical fluorescein, and adhere incompletely. Each day the area of fluorescein retention (devoid of corneal epithelia) should become smaller, as re-epithelialization occurs 360°. If re-epithelialization is slow or ceases, topical aqueous 0.5% povidone–iodine solution is carefully applied to the wound edges to stimulate epithelial activity.
Corneal healing after superficial keratectomy for the treatment of feline corneal sequestration is often slower than normal and more scarring results. Once re-epithelialization of the superficial keratectomy site is complete, topical antibiotics are continued for few days. Final corneal clarity may require several weeks during which the corneal stroma becomes reorganized. Topical corticosteroids, such as 0.25–0.5% prednisolone or 2.5% hydrocortisone, may be administered two to four times daily, or cyclosporine administered once daily may be used to minimize corneal scarring, but are not usually necessary. Although some corneal scarring may result after the superficial keratectomy procedure, the opacity is not usually dense in the dog and cat.
Postoperative complications after superficial keratectomies are infrequent. Bacterial infection of the surgical wound is rare, provided appropriate topical antibiotics are administered. Postoperative reduction in tear production will delay corneal healing, and increase the possibility of corneal vascularization and more scarring. After superficial keratectomies in brachycephalic breeds, the cornea should be evaluated daily or every other day. Lagophthalmos, infrequent blinking, central corneal exposure, and marginal tear production are often associated with corneal diseases in these breeds, and can prolong corneal re-epithelialization of the superficial keratectomy site excessively. Postoperative corneal scarring may be more in dogs than in cats.
Use of only superficial keratectomies to treat pigmentary keratitis in dogs is not successful. The surgical areas often re-pigment within months. For the superficial keratectomy to have reasonable success for canine pigmentary keratitis, additional medical and/or surgical treatments are indicated, such as permanent medial or lateral canthoplasty, removal of nasal fold trichiasis, and medically increasing tear production using topical cyclosporine or oral pilocarpine. These treatment modalities address the basic problems that contributed to the development of the original corneal pigmentation.
Superficial keratectomy for the clinical management of canine chronic superficial keratitis (pannus) is not curative, but can remove the dense corneal pigmentation in advanced disease and immediately restore vision in these dogs. However, the healing periods after superficial keratectomies for the treatment of this disease are not predictable. Recurrence of pannus and corneal vascularization may occur before or concurrent with the corneal re-epithelialization, and necessitate topical corticosteroids or cyclosporine before re-epithelialization is complete. Beta radiation may be indicated to retard corneal vascularization during immediate postoperative healing in selected patients. Long-term control of pannus still depends on daily instillations of corticosteroids and/or cyclosporine, adjusted for the severity of the disease.
Overall results with superficial keratectomies are very good. Recurrence of the original corneal disease may again opacify the cornea, but the procedure provides a temporarily clear cornea. Removal of corneal scar tissue, corneal dermoids, embedded corneal foreign bodies, and other corneal opacities with superficial keratectomies is usually curative.
Keratectomy may be indicated in cases of melting ulcers in horses. Keratectomy is thought to speed healing by removing necrotic and infected tissue, and encouraging vascularization, minimizing scarring, and decreasing the stimulus for anterior uveitis. Necrotic tissues that are often present in case of melting ulcers should be removed and this can be achieved with a cellulose sponge or a fine-toothed forceps (e.g., 0.12 mm Colibri forceps). Additionally, careful debridement can be done with microsurgical corneal scissors, a microsurgical blade or a corneal dissector. A corneal incision to outline the lesion to be removed can be done with a corneal trephine, a diamond knife or a microsurgical blade (e.g., Beaver No. 6400 microsurgical blade). The depth of the incision in the stroma should be adequate to remove the lesion completely. Once the initial incision is made, the edge of the tissue to be removed is grasped by a forceps (e.g., 0.12 mm Colibri forceps), and a corneal dissector (e.g., Martinez corneal dissector) is introduced and held parallel to the cornea. The dissector is used tangentially to separate the corneal lamella without penetrating deeper than the original cutting plane. The cornea is then separated until the opposite incision line is reached. Depending on the stromal defect, a conjunctival graft may be placed subsequently.
Superficial keratectomy is also performed during any grafting procedure to prepare the bed for conjunctival, amniotic membrane or corneal graft. The complications of superficial keratectomy are minimal, but include slow healing, infection, granulation tissue formation, perforation, and excessive scar formation. Superficial keratectomy may also be used in the treatment of superficial corneal scarring, with postoperative treatment of the corneal healing response (Fig. 8.7).
Fig. 8.7 Treatment of corneal scarring by superficial keratectomy and amnion graft in a horse. (a) A granulomatous scar is present at the surgical site 2 years postoperatively for superficial keratectomy and permanent conjunctival graft for corneal squamous cell carcinoma. (b) A slight scar is present at the surgical site five months postoperatively.
Superficial punctate, grid, and linear keratotomies are relatively new surgical procedures used to treat chronic corneal erosions and refractory corneal ulcers in dogs and other species (Fig. 8.8). Investigations into corneal recurrent erosions (indolent ulcer or Boxer ulcer) in the dog indicate defective basal corneal epithelia and basement membrane. Defects in the basement membrane, including paucity of hemidesmosomes and multiple layers of basement membrane, appear to contribute directly to the onset of these highly painful but shallow corneal ulcers, and to their variable but often prolonged healing. Both surgical procedures attempt to improve adhesion of the defective epithelia and basement membrane to the anterior stroma by making multiple shallow grooves in the epithelia and anterior stroma that provide deeper attachment sites. As a result, basal corneal epithelia increase their surface contact and adhesion through these incomplete needle or linear tracks to the anterior stroma. As the entire cornea is involved, these procedures are used for most of the entire cornea including the actual erosion site. Punctate keratotomy leaves smaller scars than grid keratotomy. The keratotomy procedures are usually preceded by debridement using a cotton swab and topical anesthesia of the edges of these chronic erosions for 1 or 2 weeks. If epithelialization has not occurred in 12–14 days, one of these keratotomies is performed. Partial-to-complete superficial keratectomy has also been used for this condition. Other surgical procedures for recurrent and slow healing corneal erosions, such as transplantation of small lenticules of new and healthy epithelium (keratoepithelioplasty), microdiathermy of Bowman’s membrane, and neodymium:yttrium aluminum garnet (Nd:YAG) laser photo-induced adhesion of corneal epithelia, have not been reported in the dog.
Fig. 8.8 Corneal erosions, stained with topical fluorescein, are characterized by slow healing and recurrence. New surgical procedures, such as the superficial punctate and grid keratotomies, attempt to enhance healing and prevent recurrences by expanding the adhesion of the epithelia and basement membrane to the anterior stroma.
Superficial punctate keratotomy may be performed in quiet dogs with only topical anesthesia, or with some sedation in less cooperative dogs. The grid procedure may be performed under local anesthesia, but general anesthesia is recommended if most of the cornea is involved. Prior to both procedures, loose corneal epithelia surrounding the chronic erosion are debrided with thumb forceps or sterile cotton swabs.
In superficial punctate keratotomy, multiple anterior stromal punctures are performed with a 20–23 g disposable hypodermic needle or a diamond corneal knife with the micrometer set at 0.10–0.2 mm (Fig. 8.9a). The hypodermic needle is grasped directly or clamped with a small hemostat to expose 0.1–0.2 mm of the tip. The needle should enter the cornea at a 45–90° angle. The 23 g hypodermic needle will penetrate deeper into the corneal stroma than the 20 g hypodermic needle. Alternatively, the Nd:YAG laser may be used with multiple impulses set at 2 mJ.
Fig. 8.9 In the superficial punctate keratotomy procedure, multiple partial-thickness hypodermic needle punctures are made in the erosion and adjacent cornea. (a) After thumb forceps removal of the loose epithelia about the erosion, the epithelia and 0.1–0.2 mm of the anterior stroma are repeatedly partially penetrated with a 20 g hypodermic needle grasped with a hemostat to prevent deeper corneal penetration. (b) About 15–25 partial corneal punctures are positioned within the erosion and adjacent area.
About 15–25 punctures are usually made about 1.5 mm apart surrounding the erosion and extending into the adjacent normal cornea (Fig. 8.9b). The cornea is slightly indented when the 0.1–0.2 mm depth is achieved. If inadvertent complete puncture of the cornea results, rapid self-sealing occurs.
In the superficial grid keratotomy procedure, the corneal epithelia and anterior stroma are incised numerous times in a grid, cross-hatching or linear pattern. The majority of the grid incisions are adjacent to the corneal erosion, but this procedure may cover most of the corneal surface. The linear incisions for superficial grid keratotomy are made with a 20 g disposable hypodermic needle, Beaver No. 6400 microsurgical blade, or a diamond knife with the micrometer set at 0.2–0.3 mm deep (Fig. 8.10a). Incisions at 90° to the first series of incisions complete the grid keratotomy (Fig. 8.10b). Smaller gauge hypodermic needles are not recommended, as their incisions extend too deep. The grids are about 1–1.5 mm apart.
Fig. 8.10 In the superficial grid keratotomy procedure, the corneal epithelia and anterior stroma are incised in a grid or cross-hatching manner within the corneal erosion and adjacent area. (a) The initial corneal incisions, about 0.1–0.25 mm deep, may be performed with the Beaver No. 6400 microsurgical blade, diamond knife or a disposable 20 g hypodermic needle. (b) A second set of crosshatching incisions are placed at 90° to the initial incisions. The grids should be 1.0–1.5 mm apart.
The superficial grid keratotomy procedure has been partially replaced by a linear keratotomy which is easier to perform in the clinical patient with only topical corneal anesthesia. In this modification, the corneal epithelia and superficial stroma are incised in vertical linear incisions using a 20–22 g hypodermic needle. The incisions, about 1 mm apart, overlap the corneal erosion by a few millimeters in all directions. As with the superficial punctate and grid keratotomies, smaller gauge hypodermic needles (smaller than 22 g) are avoided because of their tendency to penetrate too deeply.
Postoperative management after these three procedures is topical broad-spectrum antibiotics instilled every 8 h for 5–7 days. Healing of the erosions should occur during this period. The success rate of these procedures for the treatment of recurrent corneal erosions in dogs is about 80–90% within 2 weeks.
Alternative treatments appear to yield lower success rates. Treatment of canine recurrent erosions with only aqueous iodine cautery of the erosion requires an average of 46 days for complete re-epithelialization. Contact lenses for corneal erosions yield 73% success, the major limitation being retention of the lens. If the lens is retained for 7–10 days, the success rate increases to 92%. Other forms of treatment for this disorder include nictitating membrane flaps, temporary tarsorrhaphy, and bulbar conjunctival grafts (see Chapters 5 and 6). The success rates of these treatment methods have focused on short-term management of the healing of the recurrent erosions. The real value of these procedures, yet to be established, is long-term prevention of recurrent corneal erosions.
Postoperative complications after superficial punctate and grid keratotomies are infrequent. Inadvertent puncture of the cornea is rare, after the technique has been mastered. Fortunately, these punctures will self-seal immediately, but a small corneal scar results. Both techniques may produce faint anterior stromal scars, appearing as individual spots or a grid. This scar formation is usually negligible when compared to the repeated effects of recurrent corneal erosions of the canine cornea, including influx of vascularization and pigmentation, and the occasional deposition of lipids, cholesterol, and calcium.
Recently the clinical use of these different keratotomies in the cat has been questioned because of the development of corneal sequestrum following their use. Perhaps the feline herpes virus (FHV-1) is present in some corneal sequestra, and, following keratotomy, the virus can penetrate deeper into the stroma. As a result, pending further clinical studies, keratotomies in cats must be most cautiously performed.
Biopsy of the cornea is usually performed to establish a diagnosis. Diagnosis of specific infectious agents (bacterial/fungal) and possible neoplasia is provided by corneal biopsy. Corneal biopsies may be limited to the anterior epithelium, anywhere in the stroma, or even full thickness. Under the rubric of corneal biopsies is corneal cytology (obtained by deep scraping), and superficial and deep keratectomies. Deep keratectomies are performed in the same manner as superficial keratectomies, but the surgical wound is usually covered with a bulbar conjunctival graft or corneal graft.
Excisional corneal biopsies can combine both diagnosis and initial treatment. When ulcerated or inflamed cornea is biopsied, all necrotic and involved tissues should be excised to enhance the possibility of diagnosis, demonstrate infectious agents, and facilitate attachment and retention of the conjunctival graft.
The corneal biopsy procedure is modified by the depth of the corneal disease: 1) superficial keratectomy for diseases of the epithelia and anterior one-half of corneal stroma; 2) deep keratectomy for corneal diseases involving the posterior one-half of the corneal stroma; and 3) full-thickness keratectomy with homologous corneal grafts when the entire depth of the cornea is involved. These surgical procedures are presented in the respective sections in this chapter.
Keratectomy to obtain tissue for culture or histology in melting ulcers is similar to the techniques used in small animals. Large amounts of necrotic cornea can be removed with tenotomy scissors to speed healing of melting ulcers in horses and cows.
Stromal biopsies to aid in the diagnosis of stromal abscesses or immune-mediated keratopathies in horses can be obtained in the standing horse. Utilizing sedation and topical anesthesia, a linear incision in the corneal epithelium is made with the Beaver No. 6400 or 6900 microsurgical blade (Fig. 8.11). A Martinez corneal dissector is then used to separate the epithelium from the stroma. A small biopsy punch is utilized to obtain a stromal sample, and the tissue placed on a sponge in a histologic cassette in fixative. The corneal epithelium is sutured with 8-0 sutures in a simple interrupted pattern. Topical antibiotics and NSAIDs can be used post-biopsy.
Fig. 8.11 Corneal stromal biopsy in a horse with suspected immune-mediated keratitis. (a) Incision of the corneal epithelium in a sedated standing horse. (b) The corneal dissector is used to expose the corneal stroma. (c) Corneal forceps and scissors are used to remove a section of stroma for histopathology. d) Biopsy site is apposed by sutures.
Corneal ulcerations are frequent in dogs but less common in cats. In the dog, corneal ulcers may be initiated by minor trauma. In certain breeds, particularly brachycephalic dogs, corneal ulceration may be associated with several predisposing factors. In brachycephalic breeds the eye is very prominent, suffers lagophthalmia, and the rate of blinking may be less than normal. Corneal microtrauma may occur from distichia and nasal fold trichiasis. The precorneal film may be thin and abnormal centrally, placing the central corneal epithelia in constant jeopardy. Retention of rose Bengal by the central corneal epithelia in these dogs suggests that these cells are degenerating at a faster rate than normal. These eyes are frequently also victims of marginal or low levels of aqueous tear production. The combinative effect is the development of a central-to-paracentral corneal ulcer that rapidly becomes deeper and larger (Fig. 8.12).
Clinical signs associated with pain are often minimal or absent. Initial medical treatment usually includes topical broad-spectrum antibiotics, topical autogenous serum, and mydriatics. If the corneal ulcer continues to progress to involve the deep corneal stroma, becomes a descemetocele, or perforates with iris prolapse, surgical treatment with conjunctival or corneal grafts is recommended. Maintenance of vision and the least corneal scarring are achieved when conjunctival grafts are positioned before development of corneal perforation and iris prolapse.
Administration of antibiotic treatment to all potential corneal ulcerations is recommended. Surgical correction of these ulcerations will usually be successful, provided the infectious agents are eliminated. Bacteria, usually Staphylococcus and Streptococcus spp., are frequently isolated from canine corneal ulcers. These organisms seem to originate from the conjunctival surfaces. Staphylococcus spp. are usually susceptible to chloramphenicol, bacitracin, and gentamicin; Streptococcus spp. are usually susceptible to chloramphenicol, erythromycin, carbenicillin, and cephalothin. Infrequently recovered in small animals, Pseudomonas spp. are susceptible to gentamicin, tobramycin, polymyxin B, and amikacin.
Small deep corneal ulcers may be closed by direct suturing. The maximum diameter of corneal ulcers that can be apposed by sutures is about 3 mm or less. Control and hopefully elimination of the potential pathogenic bacteria from the ulcer site contributes directly to the success or failure of this method. After general anesthesia and surgical preparation of the eyelids and conjunctiva, the eye is draped and an eyelid speculum positioned. The corneal ulcer is closely examined, and any necrotic or suspect tissue carefully removed by Beaver No. 6400 microsurgical blade (Fig. 8.13a). These tissues should be cultured and evaluated histologically. The edges of the corneal ulcer are carefully treated with aqueous 0.5% povidone–iodine solution applied with sterile cotton swabs to attempt sterilization of the ulcer. The same solution may be applied carefully to the ulcer base, but only if some deep corneal stroma remains.
Fig. 8.13 Primary closure of corneal ulcers may be attempted with deep corneal ulcers <3 mm diameter. (a) To ensure suture stability, any necrotic or potentially infected corneal epithelia and stroma are excised with the Beaver No. 6400 microsurgical blade. These tissues should be cultured for bacteria and examined microscopically. (b) Two or three 5-0 to 6-0 simple interrupted absorbable sutures are pre-placed. If the cornea appears quite friable, interrupted mattress sutures may be used. The corneal bites for these sutures should be about 2 mm and deep in the normal corneal stroma. (c) All sutures are tightened and tied to effect primary ulcer closure. Some temporary corneal distortion is usually evident for several days postoperatively.
Two or three 5-0 to 6-0 braided polyglactin 910 simple interrupted sutures, or a combination of a central interrupted horizontal mattress suture and two simple interrupted sutures, are used to appose the ulcer’s edges (Fig. 8.13b). Sutures are placed into the deep corneal stroma. Some distortion of the cornea develops as the sutures are tied and ulcer edges apposed (Fig. 8.13c). Fortunately, as corneal healing occurs during the next 7–10 days, the corneal curvature gradually returns to normal.
For success with this procedure, sutures must be placed in viable corneal stroma. Placement of the sutures in necrotic and friable stroma usually results in sutures tearing from the stroma in 24–48 h. Braided polyglactin 910 sutures are recommended as this material is not adversely affected by sepsis. After recovery from general anesthesia, medical treatment is continued with topical and occasionally systemic broad-spectrum antibiotics, topical autogenous serum, and mydriatics. Maintenance of the wound beyond 5–7 days usually results in a successful apposition. If premature dehiscence of the wound and sutures occurs, treatment of the ulcer with a bulbar conjunctival graft is recommended.
Conjunctival autografts are frequently used in small animal ophthalmology for the clinical management of deep and large corneal ulcers, descemetoceles, mycotic ulcers, stromal abscesses, after sequestrum removal in cats, keratomalacia, and for perforated corneal ulcers with and without iris prolapse. Conjunctival autografts were presented in Chapter 7. Conjunctival autografts consist of either bulbar or palpebral conjunctival mucosa with epithelium, and connective tissue. These autografts can be transposed and sutured directly to the edges of the corneal ulcer or defect to provide additional support and tissue for a cornea weakened by deep ulceration, descemetocele, or perforation with or without iris prolapse. The transplanted conjunctival autograft provides additional tissues and no risk of host rejection.
Conjunctival autografts provide sufficient tissue to strengthen most weakened corneas and prevent staphyloma formation, but are not as strong as corneal grafts. When harvested from the limbus, the transplanted limbal conjunctiva contains stem cells capable of additional generation and transition into corneal epithelium. Conjunctival autografts contain blood vessels and lymphatics to offer significant antibacterial, antifungal, antiviral, antiprotease, and anticollagenase effects. With conjunctival transplants, leukocytes, antibodies, serum, and α2-macroglobulin (thought to be the anticollagenase factor) are immediately incorporated into the corneal ulcer bed. Through the conjunctival blood vessels, systemic antibiotics can enter the ulcer site in higher levels. The fibrovascular or deeper layer of the conjunctival transplant offers immediate fibroblasts and collagen to begin rebuilding the corneal stroma. Conjunctival grafts usually result in corneal scars of various sizes and depths. Postoperative topical corticosteroids and/or cyclosporine can reduce this postoperative scar tissue formation to a minimum, but corneal scarring after conjunctival grafts should be anticipated.
Conjunctival autografts from either bulbar or palpebral conjunctiva should be thin, and not include Tenon’s capsule or the bulbar fascia. The inclusion of Tenon’s capsule may contribute to surgical failure by increasing the traction on the transplanted conjunctival graft. Transpalpebral conjunctival autografts contain limited portions of the fibrous tarsal layer which may be necessary to maintain the graft base from the deeper aspects of the eyelid to the corneal surface. Conjunctival autografts are more difficult to perform than nictitating membrane flaps, but are easier than corneoconjunctival and corneoscleral transpositions, and the different types of keratoplasty. The most frequent type of bulbar conjunctival graft is the pedicle type (Fig. 8.14).
The porcine small intestinal submucosa graft is a biomaterial consisting primarily of proteins and, to a lesser extent, carbohydrates and lipids. SIS grafts have been reported in dogs, cats, rabbits, and horses. The SIS graft, derived from the porcine jejunum, is composed of three distinct layers: 1) tunica muscularis mucosa; 2) tunica mucosa; and 3) the stratum compactum layer of the tunica mucosa. Following processing and mechanical debridement, a few remaining endothelial cells and fibrocytes are lyzed with a hypotonic wash, leaving a sheet of collagen with a smooth surface (stratum compactum) and a rough surface (tunica muscularis mucosa). The SIS graft is sterilized by ethylene oxide, and is supplied commercially as 7 × 10 cm sheets, and 10 or 15 mm diameter ophthalmic discs.
After debridement of the septic corneal ulcer, the SIS graft is carefully trimmed and ‘fitted’ to cover in excess of 1–1.5 mm of the entire corneal ulcerative bed. After securing the graft to the ulcer’s edges with several 7-0 to 9-0 simple interrupted absorbable sutures, the entire SIS graft is covered with a bulbar pedicle conjunctival graft.
The SIS graft provides a scaffold for corneal healing as well as additional strength to the overlying bulbar conjunctival graft. Rabbit corneal SIS graft studies suggest that the graft collagen sheet is actually incorporated into the healing process. SIS grafts have been reported to fill the limbocorneal defect after limbal melanocytoma excision and covered with bulbar conjunctiva, for full-thickness corneal ulcerative disease in dogs and covered with conjunctival grafts, after corneal ulcers and corneal sequestra in cats and not covered with conjunctival grafts, and after corneal ulceration and corneal stromal abscess formation in horses and covered with conjunctival grafts.
These grafts are convenient to use, are commercially available and ready for use, avoid potential virus transmission as is possible with feline-based grafts, and are easy to handle during surgery. If placed in an uncontrolled septic corneal ulcer, covering with a conjunctival graft is highly recommended.
Amniotic grafts have been reported to repair deep corneal ulcers in horses and for experimentally induced full-thickness corneal defects dogs. The amniotic grafts for both the horse and dog studies were harvested from normal equine placenta and are not available commercially.
In the dog study, after harvest, the amniotic grafts were preserved in 98% sterile glycerol (full-thickness scleral grafts are commonly preserved in glycerol). The equine amniotic graft is obtained aseptically as a 5 mm2 section of amnion after death or cesarean section. After harvest, the tissues are preserved in 98% sterile glycerol; immediately before use the tissue is rehydrated in sterile saline solution.
The equine pericardium has also been used in surgery to correct canine lateral canthal entropion, deep corneal ulcerations in dogs, fill the orbital cavity of dogs after enucleation, and as a scleral graft.
Nictitating membrane flaps provide more support to the diseased cornea than temporary complete tarsorrhaphy in small animals. Nictitating membrane flaps are used to cover and protect a weakened cornea, but are not usually a source of tissues for the cornea. Nictitating flaps are recommended for superficial corneal diseases, including corneal erosions, neuroparalytic and neurotropic keratitis, temporary exposure keratitis, superficial corneal ulcers, and acute keratoconjunctivitis sicca, and to reinforce a bulbar conjunctival graft. Surgical procedures for nictitating membrane flaps are presented in Chapter 7.
Corneoscleral transposition shifts the peripheral cornea into central corneal defects and moves adjacent sclera into the peripheral cornea. As this procedure is primarily used for the repair of corneal ulcerations it is included in this section; however, corneoscleral and corneoconjunctival transpositions may also be considered as distinct types of autologous sliding lamellar keratoplasty. As a result, axial (central) and visually important cornea remains clear, but the peripheral cornea with the transposed sclera becomes somewhat translucent. A modification of this surgery, the corneoconjunctival autograft, was presented in Chapter 7. Corneoscleral transpositions are used to treat deep corneal ulcerations, descemetoceles, and feline corneal sequestra involving the deeper stroma. The advancement of corneoscleral tissues into central corneal ulcers, suspected as septic or not treated with antibiotics, is not recommended as these grafts may also be destroyed with the progressive melting process. If the tip of the corneoscleral graft is already vascularized, the chance of slough from infectious agents is reduced. Hence, before corneoscleral transpositions are performed, the corneal ulcer’s edges should be scraped and examined microscopically for bacterial and fungal organisms. Alternatively, the corneal ulcer may be treated hourly with topical broad-spectrum antibiotics for several hours to attempt sterilization of the ulcer site.
After the onset of general anesthesia, and surgical preparation of the eyelids, conjunctiva, and cornea with aqueous 0.5% povidone–iodine solution, the eye is draped and an eyelid speculum positioned. The corneal ulcer is carefully debrided to remove all potentially necrotic and/or infected tissues (Fig. 8.15a). Once these tissues have been removed, the corneal defect may be 1–2 mm larger. The corneoscleral advancement graft is prepared. Two slightly diverging corneal incisions with the Beaver No. 6400 microsurgical blade are performed, extending from the corneal bed to the limbus (Fig. 8.15b). These incisions are approximately one-half of the stromal thickness. At the limbus, the bulbar conjunctiva and Tenon’s capsule are incised by tenotomy scissors for about 15–20 mm and reflected caudally to expose the sclera (Fig. 8.15c).
Fig. 8.15 In corneoscleral transposition, cornea and adjacent sclera are slid from the periphery into a central corneal defect. (a) With the Beaver No. 6400 microsurgical scalpel blade, all necrotic and suspicious corneal epithelia and stroma are carefully excised. (b) Two slightly diverging corneal incisions are excised from the edge of the corneal ulcer to the limbus with the Beaver No. 6400 microsurgical scalpel blade to prepare the corneoscleral graft. (c) At the limbus, the bulbar conjunctiva and epibulbar fascia (Tenon’s capsule) are incised by Steven’s tenotomy scissors to expose the underlying sclera. (d) The ends of both corneal incisions are extended into the superficial (about 0.2–0.3 mm deep) sclera with the Beaver No. 6400 microsurgical blade. Hemorrhage from these scleral incisions is controlled by point electrocautery. (e) The tip of the corneoscleral graft is grasped with 1 × 2 teeth thumb forceps, and the corneal stromal dissection is started with a corneal dissector or separator. (f) After the corneoscleral graft is completely separated from the deeper corneal and scleral tissues, its base is incised with Steven’s tenotomy scissors. (g) The corneoscleral graft is carefully trimmed to fit the corneal defect, and should be 0.5–1 mm larger than the defect to compensate for tissue shrinkage. It is attached by 7-0 to 9-0 simple interrupted absorbable sutures. (h) After the corneoscleral graft is secured by sutures, the bulbar conjunctiva is apposed to the limbus with a 7-0 to 9-0 simple continuous absorbable suture. (i) The 3-week postoperative appearance of a corneoconjunctival graft in a cat after removal of a corneal sequestrum.
The ends of the two corneal incisions are extended into the sclera at a distance equal to the height of the corneal ulcer bed (Fig. 8.15d). These incisions should be about 0.2–0.3 mm deep. Hemorrhage during these incisions is anticipated and judicious cautery with the disposable battery-powered electrocautery unit is necessary for hemostasis. The tip of the corneoscleral transposition is elevated with 1 × 2 teeth thumb forceps and a corneal dissector, and separated from the corneal bed to the end of the scleral incisions (Fig. 8.15e). The corneal separator, in contrast to sharp dissection with a scalpel blade, facilitates dissection within the corneal lamellae without the danger of shifting the plane of tissue separation.
Once separation from the underlying corneosclera is complete, the base of the graft is incised with tenotomy scissors (Fig. 8.15f). The length and width of the tip of the corneoscleral graft should be 1–2 mm larger than the corneal ulcer bed. The corneoscleral graft is positioned in the corneal ulcer bed, trimmed if necessary, and apposed with 7-0 to 9-0 simple interrupted braided polyglactin 910 sutures (Fig. 8.15g). The edge of the bulbar conjunctiva is apposed back to the limbus with a 7-0 to 9-0 simple continuous braided polyglactin 910 suture (Fig. 8.15h).
Postoperative treatment after corneoscleral transpositions includes topical and systemic broad-spectrum antibiotics, and topical mydriatics sufficient to maintain a moderately dilated and mobile pupil. After 7–10 days, topical corticosteroids are started to reduce corneal scarring. Clearing of the cornea usually starts 2–6 weeks postoperatively. Success rates with corneoscleral transpositions are 75– 80% (Fig. 8.15i).
Complications after this surgery include septic involvement of the tip of the transposition with the ulcer process, suture loss, and abscesses. Iridocyclitis is often intense in these eyes, and vigorous mydriatic therapy indicated. With inadequate control of the iridocyclitis, deposits of iridal tissue on the anterior lens capsule, posterior synechiae, and secondary cataract formation result.
Conjunctival grafts or flaps are frequently used in equine ophthalmology for clinical treatment of deep, melting, and large corneal ulcers, descemetoceles, and perforated corneal ulcers. Melting ulcers should be stabilized with medical therapy, if possible, before the surgical placement of the graft to prevent protease digestion of any absorbable suture that will hold the conjunctival graft in place. The preoperative application of topical antiproteolytic agents before a conjunctival graft slows or stops ulcer progression in many cases and provides a healthier cornea for suturing. All conjunctival flaps consist of thin conjunctival tissue transposed onto the cornea to cover a severe corneal lesion and provide sufficient tissue to strengthen most weakened melting corneas. They are not as strong as corneal grafts. Conjunctival autografts consist of either bulbar or palpebral conjunctival mucosa with epithelium, and connective tissue (fibroblasts, blood vessels, and lymphatics), thus offering a new and highly viable epithelium and significant antibacterial, antifungal, antiprotease, and anticollagenase effects. With conjunctival grafts, polymorphonuclear leukocytes, antibodies, serum, and α2-macroglobulin are immediately placed in the corneal ulcer bed. The fibrovascular, or deeper, layer of the conjunctival transplant offers immediate fibroblasts and collagen with which to begin rebuilding the corneal stroma.
Conjunctival autografts are more difficult to perform than nictitating membrane flaps but simpler than surgeries such as corneoconjunctival grafts, corneoscleral transpositions, and penetrating keratoplasty (PK). They are also easier to perform in the horse than in other species, because horses have a great deal of very mobile bulbar conjunctiva.
There are different types of conjunctival graft based on the source of the mucosa (bulbar or palpebral) and the type of graft (total or 360°, bridge or bipedicle, hood or 180°, pedicle or rotational, and island). Conjunctival grafts are usually harvested from adjacent bulbar conjunctiva; however, the palpebral conjunctiva can also be used. The disadvantage of the palpebral conjunctival flap is that the eyelid is mobile and some tension will be applied to the sutured conjunctival graft, possibly leading to a higher rate of graft dehiscence. However, as the bulbar conjunctival flap moves with the eye, no tension is applied to the flap itself. It is also not recommended to use the conjunctiva near the nictitating membrane if possible because its movements can put tension on the graft, resulting in premature graft release.
Conjunctival autografts from either bulbar or palpebral conjunctiva should be thin, and not include Tenon’s capsule or the bulbar fascia. It seems beneficial to administer a drop of 2.5% phenylephrine prior to dissection of the conjunctiva to limit hemorrhage. Tenon’s capsule should be stripped or cut from the graft so that the graft easily covers the corneal defect without any tension, prior to suture placement. The inclusion of Tenon’s capsule may contribute to surgical failure by increasing postoperative traction on the transplanted conjunctival graft. Conjunctival grafts should have tension-relieving sutures placed at the limbus to prevent the graft pulling away from the ulcer bed prematurely. Conjunctival pedicle grafts using the bulbar conjunctiva from the dorsal or temporal quadrants are preferred, because the conjunctiva in those areas is surgically available and the pedicle flap covers only the ulcer surface, allowing postoperative observation of the pupil and anterior chamber. The pedicle graft is a focal transplant and does not cover the entire cornea. With all the flaps, it is important that the corneal graft bed and ulcer be properly and carefully prepared. The recipient bed for the conjunctival graft is prepared by debridement and keratectomy of the corneal ulcer with a Beaver No. 6400 microsurgical blade, thereby removing loose epithelium and necrotic corneal tissues. Great care should be taken to prevent corneal perforation during this debridement.
Temporary tarsorrhaphy is performed concurrently with conjunctival grafts to minimize blinking movement, prevent excessive lid trauma to the graft and its sutures, and allow quick graft adherences to the stroma.
The most common complication from any type of conjunctival grafting procedure is dehiscence of the graft from the corneal lesion (retraction of the graft). This may occur because the corneal lesion is progressing (worsening) and damaging the cornea at the points where the sutures secure the graft. Cytokines released by the tissue may induce infarction of the flap vessels and cause premature release. Excessive tension on the graft, or allowing a significant portion of the fibrous Tenon’s capsule to remain attached to the graft, may result in premature dehiscence of the graft. Proper suture placement in healthy cornea using a thin conjunctival graft concurrent with appropriate medical therapy will greatly decrease the complications following conjunctival graft surgery. Conjunctival grafts usually result in corneal scars of various sizes and degrees. Scarring can be minimized, however, by removal of necrotic cornea with keratectomy before graft placement. Corticosteroids are not recommended but they can be used topically very carefully after surgery to reduce postoperative scar tissue formation to a minimum, but some degree of corneal scarring after conjunctival grafts should be anticipated.
This type of graft is indicated in eyes with a large melting ulcer that affects most of the cornea. After Castroviejo eyelid speculum placement to expose the eye and conjunctiva, and a drop of 2.5% phenylephrine, the dorsal bulbar conjunctiva is grasped with forceps (0.12 mm Colibri forceps or Bishop–Harmon forceps) and incised with tenotomy scissors (Westcott scissors or Steven’s tenotomy scissors) 2 mm from the limbus (limbus-based 360° conjunctival graft). The incision is then continued 360° around the limbus (i.e., peritomy). The bulbar conjunctiva is then separated from the underlying Tenon’s capsule by alternating blunt–sharp dissection that is continued up to 10 mm behind the limbus. The conjunctival graft should be thin to minimize traction, and the loose edges of the graft should rest on the central cornea without spontaneously retracting. The conjunctiva is then pulled over the cornea and sutured to itself in the center of the cornea in a linear pattern (horizontal mattress) using 7-0 or 8-0 absorbable sutures. A 360° conjunctival graft is easy to perform and effective for large corneal lesions; however, it covers the entire cornea which makes vision impossible, prevents monitoring of lesion progression, and leaves large corneal scars.