Corneal Ulcers--A Plethora Of Solutions
Duane Flemming, D.V.M., J.D., D.A.C.V.O.
An ulcer is a defect or excavation in the surface of the cornea. Corneal ulcers occur in many forms, ranging from superficial epithelial erosions through Descemetoceles to complete perforations with prolapse of the intraocular contents. Corneal ulcers have many causes including, but not limited to, foreign bodies, exposure, dry eye, local trauma or infection. The great variety of ulcers necessitates a plethora of solutions. This discussion is not intended to be a complete instruction manual on the diagnosis and treatment of corneal ulcers. Rather, it is intended simply to clarify what the more common treatments are and to give some guidance as to their application.
For the purposes of this discussion only, medical therapy refers to the use of liquid or ointment medicants applied directly to the eye. Whether to use liquids or ointments is one of those ethereal questions without any really consistent answer. Certainly, there are specific circumstances where one or the other might be better. Factors to be considered include the integrity of the eye, the availability of the medicant, as well as the frequency and ease of use. The standard medicants are antibiotics, atropine and occasionally hyperosmotic agents. Other medicants are available but their use is beyond this discussion.
I believe it is fair to say that, as a general rule, all corneal ulcers should be treated with antibiotics. An exposed corneal stroma is at increased risk of infection and should be protected. The choice of antibiotic is determined by culture, appearance of the ulcer, or by the experience of the practitioner. There are many treatises available that provide specific guidance as to the selection of appropriate antibiotics for use in specific infections.
Atropine has two uses in the treatment of corneal ulcers. The first is to relieve pain. Axonal reflexes cause pain by inducing spasm of the ciliary muscles. Atropine relieves the pain by blocking the spasm. The second, and distinctly separate, purpose of atropine is to dilate the pupil and thereby prevent or reduce uveitis-induced adhesions of the iris to the lens. Unless the eye is in discomfort or uveitis is either present or suspected atropine is not necessary for adequate ulcer treatment.
Accumulation of extracellular fluid in the cornea can interfere with healing by preventing attachment of the epithelial basement membrane to the underlying stroma. Dehydration of the ulcer bed by hyperosmotic agents such as 5% NaCl may promote the attachment of basement membrane and speed epithelialization of the defect.
Removal of any loose or devitalized tissue is an essential element in the treatment of most corneal ulcers and may be accomplished, using topical anesthesia, while the animal is awake or, if necessary, under general anesthesia. With the eye stabilized and anesthetized , a dry cotton tipped swab is gently moved back and forth over the exposed edges of the ulcer. Generally, only the grossly loose and abnormal tissue will be easily removed but, occasionally, some apparently normal corneal epithelium may come off as well. Great care should be taken to not damage the underlying corneal stroma.
Chemical cautery may enhance healing of superficial ulcers by altering the exposed corneal surface and promoting adherence of the corneal epithelium to the underlying stroma. Because chemical cautery is a destructive process it must be done carefully and with a minimum of agent. Excess agent can get out of control and cause excess damage to the cornea and conjunctiva.
Over the years agents such as Tincture of Iodine, Phenol, Silver Nitrate, Copper Sulfate, Carbolic Acid and Trichloracetic acid have been used for corneal cautery. Most of these act by denaturing and precipitating the superficial corneal proteins. The selected agent is carefully absorbed onto a cotton swab then gently brushed against the affected portion of the well anesthetized cornea. Particular attention is paid to the margins of the lesion. Tincture of Iodine will cause slight discoloration of the lesion while the acid agents will generally cause the treated area to immediately turn greyish white. This affect is especially strong with phenol and trichloracetic acid and, at first, can be quite startling. After completing the cautery the cornea and conjunctiva should be flushed well with collyria or sterile saline to remove any remaining agent. The corneal haze caused by the cautery should clear in just a few days.
Currently, there are many other less hazardous and generally more effective techniques available for the treatment of corneal ulcers in animals. The use of chemical cautery should be reserved for the ophthalmic specialist or, if one is not available, for use only by a veterinarian experienced in the technique.
THIRD EYELID FLAPS
The third eyelid or nictitating membrane is well developed and mobile in domestic animals. It can be easily manipulated to create a bandage for an injured cornea. There are two forms of third eyelid flap. The first form secures the third eyelid to the upper eyelid. This method is easily performed under light sedation by even inexperienced surgeons. However, because the third eyelid is not secured to the globe the eye will be able to move independently of the flap and can actually interfere with healing. Alternatively, the third eyelid can be secured to the globe itself. Although this method is more difficult to perform and necessitates general anesthesia it does prevent rubbing of the cornea against the third eyelid. Both forms of third eyelid flap blind the animal and prevent the surgeon from observing the cornea. The surgeon must remove the flap to determine if the cornea has healed.
MULTIFOCAL SUPERFICIAL PUNCTATE KERATOTOMY
Refractory superficial ulcers (recurrent corneal erosions or Boxer Ulcers) fail to heal rapidly because of an abnormal attachment between the epithelial basement membrane and the underlying stroma. Multifocal superficial punctate keratotomy (MSPK) seems to speed healing by allowing new collagen scar tissue from the stroma to come to the surface. This new scar tissue provides an improved substrate for adherence of the basement membrane. If the punctures are kept shallow visible scarring is minimized.
Unless the doctor is experienced in this technique it should be done only under general anesthesia. After the cornea is thoroughly anesthetized and debrided, a 20-23 gauge needle is placed perpendicular to the cornea (some place the needle at a 30-45E slant). The needle is then advanced into the corneal tissue just enough to penetrate the anterior stroma. The punctures should be placed at 0.5 to 1.0 mm intervals and should cover all of the debrided area and 2-3mm beyond. Care should be taken not to puncture the full thickness of the cornea.
Grid keratotomy(GK) is a technique similar to the MSPK described above. In GK a needle is used to create a crosshatch pattern of superficial scarification in the affected cornea. After the cornea is completely debrided of abnormal tissue a 25-27 gauge needle is drawn across the debrided area in a series of horizontal, then vertical strokes, until a crosshatch pattern has been inscribed into the cornea. The needle tracks should be shallow and extend to no more than I/6 the corneal thickness. Because the needle is at a more acute angle to the cornea there is less danger of perforation and sedation or anesthesia of the animal is less often necessary. Because the scratches cut across the normal corneal fibers visible scarring is more likely.
SUPERFICIAL KERATECTOMY (LAMELLAR KERATOPLASTY )
With some particularly refractory superficial ulcers it may be necessary to actually remove the outer corneal layers to expose the underlying stroma. This allows for better epithelial- basement membrane adhesion and more rapid healing. The depth of the excision will vary with the depth of the corneal lesion. The keratectomy should be limited to the damaged cornea. Using an ophthalmic trephine or a #64 Beaver blade the corneal lesion is circumscribed. The edge of the incision is elevated and the damaged corneal tissue separated from the underlying stroma. The dissection should be carefully done with a Beaver blade or a corneal dissector so as to stay in a single parallel plane. After the dissection is complete the corneal tissue is removed leaving a carefully planned surgical ulcer.
Cyanoacrylate adhesive may be used to seal corneal defects or to support deep corneal ulcers during healing. Adhesive may also be used if primary surgical correction of a corneal defect is not possible. In many cases, adhesive may result in smaller and less opaque scars than would primary closure or conjunctival grafts. Adhesive should not be used if the cornea is infected or necrotic or if the wound is large and penetrates the full corneal thickness. Intraocular adhesive is toxic and may cause irreparable damage.
With the animal under heavy sedation or anesthesia, the eye is rotated so that the wound is horizontal and centrally placed between the eyelids. The wound is thoroughly debrided and completely dried as the adhesive will not adhere to intact epithelium or wet tissue. Only a very small amount of adhesive ( usually less than one drop ) is required to seal the wound. The adhesive should completely cover the wound and a little of the surrounding tissue. Excess adhesive is rough in texture and may rise above the normal corneal surface causing irritation to the eyelids. Adhesive applied in excess amounts or to a wet cornea will slough prematurely and may actually interfere with healing. A bandage contact lens may be used to cover the adhesive and protect the cornea. In some cases, the lens may be adhered to the adhesive for additional stability and protection.
Over the next few days or, in deep wounds far from the limbus, a few weeks the healing epithelium will undermine the adhesive and cause the glue plug to spontaneously slough. Occasionally, a small piece of adhesive may need to be manually removed. This usually causes no problem and healing will progress rapidly after the piece is removed.
CONJUNCTIVAL FLAPS AND GRAFTS
Because the bulbar conjunctiva, especially in dogs, is loosely attached to the eye it is easily separated from its connective tissue base and moved across the cornea to cover deep ulcers or other corneal defects. After separation the exposed underside of the conjunctiva provides a luxuriant vascular bed which, when placed in contact with a damaged cornea, can help to stop progression of the lesion and may enhance healing. Conjunctival flaps are useful for treatment of non-responsive superficial ulcers, deep ulcers or Descemetoceles as well as leaking or perforated ulcers where primary closure is not possible. Conjunctival flaps are more technically difficult to perform than third-eyelid flaps but have the advantage, if properly done, of being thin enough to preserve some vision and to allow examination of the eye by the veterinarian.
Advancement, or hood grafts, are useful for lesions of the dorsal and lateral cornea. With the animal under general anesthesia the bulbar conjunctiva is incised at the limbus and bluntly dissected toward the fornix. The separated tissue should be thin enough to see through and great care must be taken not to cut a hole in the flap. When enough conjunctival tissue has been separated to adequately cover the corneal lesion the conjunctiva is gently pulled over the lesion and stitched to the cornea with 6-0 to 7-0 interrupted absorbable sutures. Within a few weeks the conjunctiva will fuse with any exposed corneal stroma. The unattached portion of the flap will regress leaving an epithelialized scar.
For large central lesions requiring maximum support, 360E conjunctival flaps that cover the entire corneal surface are useful. 3600 flaps are performed in a manner similar to the 1800 flaps except that the dissection is around the entire limbus. Dissecting medially and around the third eyelid is particularly difficult. After dissection the conjunctiva is gently pulled over the cornea and sutured to itself with simple interrupted stitches. A conjunctival flap that is too thick will exert excessive tension on the sutures and may tend to separate prematurely and retract. After healing, the conjunctival tissue may need to be trimmed around the lesion. The severed conjunctiva will retract leaving an epithelialized scar.
CONJUNCTIVAL PEDICLE GRAFT
Conjunctival pedicle grafts can be very effective in treating deep stromal ulcers, Descemetoceles and perforated ulcers, especially those with associated keratomalacia. Using small ophthalmic scissors the bulbar conjunctiva is gently cut and undermined to free the graft tissue. The separated conjunctiva is then rotated not more than 45E across the corneal lesion and carefully sutured to the previously debrided cornea with 7-0 or 8-0 absorbable simple interrupted sutures. The graft should be as thin as possible and large enough to cover an area slightly bigger than the ulcer. A graft that is too thick or too small will tend to retract away from the cornea.
CONJUNCTIVAL ISLAND GRAFT
Free form conjunctival island grafts are also used to treat deep corneal ulcers, Descemetoceles and small perforations. With the animal under general anesthesia a round section of upper eyelid or anterior third eyelid conjunctiva and underlying connective tissue is removed. The graft should be slightly larger than the ulcer bed to accommodate post-operative tissue contraction. The graft is trimmed to the shape of the ulcer and sutured to the cornea with multiple 7-0 to 8-0 Vicryl simple interrupted sutures.
The transplanted island graft has no immediate connection to a vascular supply so it will quickly blanch white. The graft will eventually revascularize from the cornea. Corneal scarring is more dense with island grafts but there is no graft base to sever after healing. In many cases the graft epithelium will pigment and be very difficult for an untrained eye to see. In others, the graft will remain red and therefore easily seen.
Therapeutic soft contact lenses (TSCL) are frequently used to enhance treatment of chronic corneal erosions or superficial ulcerations. The lens acts as a transparent bandage that protects the corneal surface by providing a flexible but permeable barrier between the fragile healing epithelium and the eyelids and nictitating membrane. TSCL's can be applied to the injured eye without general anesthesia. The lenses are usually well tolerated and, in contrast to third-eyelid flaps, do not render the dog blind. The lenses have no corrective value and should not alter the animal's existing vision. Lens that are not extruded, by rubbing or action of the nictitans, can remain on the eye for several weeks without problems. Retained TSCL's may occasionally need to be removed from the eye and cleaned. After cleaning, the lens may be reinserted into the eye if healing is not complete.
Hydrophilic collagen shields are another form of ocular bandage. The shields can enhance corneal healing by hydrating and protecting the corneal surface and by guiding new corneal epithelial cells as they migrate into the wound. Collagen shields can also be used as a reservoir to deliver additional medication to the cornea. In contrast to TSCL's, collagen shields are designed to dissolve by enzymatic and mechanical action in 12, 24 or 72 hours.
Primary closure of the cornea is indicated when the corneal ulcer is deep and less than 3mm in diameter. With the animal under general anesthesia, the ulcer bed and edges are debrided to remove any abnormal tissue. From one to three 5-0 to 7-0 Vicryl sutures in a simple interrupted or horizontal mattress pattern should be inserted deep into the healthy tissue surrounding the ulcer. The cornea will distort as the sutures are tied leaving a football-like appearance to the corneal surface. Over the next few weeks the normal forces acting on the cornea will gradually return the cornea to normal or near normal curvature.
Each corneal ulcer has individual characteristic which, if properly evaluated, will allow the attendant veterinarian to select the specific treatment modality most likely to result in healing of the ulcer with a minimum of discomfort to the patient and a minimum of visual impediment. Any corneal ulcer that is not responding positively to treatment with two weeks should be referred to a veterinary ophthalmology for evaluation.