Corneal diseases in horses (Proceedings)
Equine Corneal Diseases:
The cornea is the front layer of the fibrous tunic of the eye; it is composed of three distinct layers and one distinct membrane. The outer epithelial layer is approximately five to ten cells thick. The middle stromal layer comprises about 95% of the cornea, and the inside layer is the endothelial layer, and its basement membrane is Descemet's membrane. The epithelial layer is hydrophobic and does not retain water-soluable dyes such as fluorescein. The stromal layer is hydrophilic and will retain these dyes if Descemet's membrane is hydrophobic. The equine cornea is approximately 700 µ thick centrally and 800 µ thick peripherally. The junction between the sclera and the cornea is referred to as the limbus, and in the horse, has a distinct coloration pattern.
Corneal Ulceration:Ulcerations are defined as a break in the epithelial layer, and they may continue into the stroma and threaten the integrity of the cornea. Often, the epithelial layer will cover a stromal defect, resulting in a condition known as stromal abscessation. Diagnostic procedures that are helpful in corneal ulceration include cytology, culture and sensitivity, and various dye tests. Cultures and sensitivities should be obtained first. Cytology, however, can be the most helpful in determining the initial therapy route. Exfoliative cytologies can be obtained with surgical blades, Kimura spatulas, and Cytobrushes. At least three to five specimens should be collected, the first of which would be evaluated by Difquick-type staining to assess the presence of bacteria, fungal elements, and the general cell population.
Various antibiotic therapies are available for managing corneal ulcerations; the most common of which is the triple antibiotic of bacitracin, neomycin, and polymyxin. Based on cytology results, we recommend choosing chloramphenicol for gram-positive cocci, which often are beta-hemolytic streps, and an aminoglycoside such as gentamicin or tobramycin for gram-negative rods, which often are pseudomonas. New-generation fluoroquinolones, including Vigamox and Zymar, are available and provide a broadspectrum of activity for which many of the common bacteria show good resistance patterns. Other methods of providing more aggressive medical therapy include subconjunctival injections and fortification of standard ophthalmic preparations. Antifungal agents include miconazole, itraconazole, and voriconazole; our preference at this time is itraconazole plus/minus DMSO. If the cornea is showing signs of "melting," anticollagenase products are available, and they include acetylcysteine, potassium EDTA, and autologus serum. Mydriatic agents may be helpful, and the most common therapeutic one is atropine; caution should be given in application of atropine to avoid colic. Hypertonic ointments can help reduce corneal edema and create a better environment for healing; the most common product is 5% sodium chloride ointment. For aggressive ulcers, surgical intervention may be necessary. Following debridement, a simple procedure such as a third eyelid flap and temporary tarsorrhaphy provide some protection. Advancement/hood grafts are relatively easy to perform with minimal specialized equipment. Specialized procedures include conjunctival pedicle grafting and amniotic grafting. Grafting procedures are aimed at providing tectonic support for the ulcer while promoting active healing. Amniotic and conjunctival pedicle grafts provide a good vascular supply to the ulcer and have an excellent rate of success in managing aggressive corneal ulcers. Conjunctival grafting involves harvesting a strap of conjunctiva; the width of which is designed to cover the entire defect, with some excessive tissue to provide trimming. We first measure the size of the ulcer and then harvest an appropriate graft, typically from the lateral scleral area. Once the graft has been harvested and shaped, the ulcer is carefully debrided. Specimens can be obtained at this time for further culture and cytology or histopathology. Two to four cardinal sutures are placed with 7-0 Vicryl to secure the graft in position, and then simple interrupted patterns are used to finish placement of the graft. The graft site is then closed in a continuous pattern, and subconjunctival injections are given; at that time, a subpalpebral lavage system may be placed. A temporary tarsorrhaphy is used to protect the more delicate surgical site.