Freya M. Mowat
Management of Corneal Ulcers
A corneal ulcer is defined as an area of loss of the corneal epithelium and basement membrane. Ulcers can also extend into the corneal stroma, and if the deep layer of Descemet’s membrane is breached, the integrity of the globe is compromised, resulting in a full-thickness corneal perforation or rupture. Corneal ulcers are painful, and when severe they can lead to vision compromise and even blindness. Maintenance of the clarity and curvature or thickness of the cornea is vital to allow sufficient light to be focused on the retina. The goal in diagnosing and treating corneal ulcers is to promote quick healing with minimal scarring. Although primary infection is a rare cause of corneal ulceration, yeasts, fungi, and bacteria are considered normal resident flora in the conjunctival sac of healthy horses and mild corneal abrasions from trauma or other etiologies can therefore rapidly become infected. The eye has many passive and adaptive mechanisms to minimize the risk for ulceration and infection. The eyelids act as a physical barrier to trauma and help facilitate the passage of tears over the corneal surface and drainage down the nasolacrimal duct. Tears help to physically remove foreign material and contaminants from the corneal surface and also contain soluble antimicrobial components and immune cells. Epithelial healing in uncomplicated corneal wounds is rapid in horses, progressing at 0.6 mm per day. Stromal healing progresses more slowly and can be complicated by the presence of infectious organisms.
Diagnosis of Corneal Ulcers
Conducting the examination in an area where artificial or natural light can be eliminated is optimal. A focal light source is necessary, such as that provided by a direct ophthalmoscope or penlight. Examination is greatly enhanced by the use of intravenous sedatives, local nerve blocks, and topical analgesic application. Xylazine (0.2 to 1.1 mg/kg, IV) or detomidine (0.005 to 0.02 mg/kg, IV), with or without addition of butorphanol (0.033 to 0.066 mg/kg, IV), is commonly used to sedate horses for examination of a corneal ulcer. An auriculopalpebral nerve block with 1 to 1.5 mL of lidocaine can greatly enhance examination of the eye through paralysis of the upper eyelid muscles. Gentle examination is recommended where deep ulceration is suspected because even very minor pressure can cause a deep ulcer to rupture. Tonometry should be avoided in eyes with stromal ulcers. When the retina cannot be observed because of intraocular or corneal disease, the presence or absence of an indirect pupillary light reflex (PLR) and a dazzle reflex (squinting of the eye in response to a bright focal light source) can help the examiner give a prognosis, particularly in cases of suspected trauma. Use of the slit beam on a direct ophthalmoscope can help differentiate superficial from deep stromal ulcers because inward bending of the light beam into a “crater” in the cornea is indicative of stromal involvement and may warrant more aggressive diagnostics and therapy, as well as discussion of referral to an ophthalmologist. Fluorescein staining is helpful; a blue light source is required to visualize the staining, which will appear as a focal area of dense green fluorescent staining. The hydrophilic corneal stroma takes up fluorescein, and stain uptake indicates that the ulcer has full-thickness epithelial loss. Intact epithelium does not take up stain. A deep ulcer with fluorescein-positive “walls” may have a fluorescein-negative, clear center through which the intraocular structures can be observed; this is indicative of exposure of Descemet’s membrane (i.e., is a descemetocele; Descemet’s membrane does not bind fluorescein). If this is seen, immediate referral for surgical management should be offered. Fluorescein can also be used to evaluate a deep ulcer for potential aqueous leakage, with the Seidel test. Concentrated fluorescein on a dry fluorescein strip is applied directly to the lower edge of the ulcer site. When applied, it will appear orange, and if aqueous humor is leaking from the ulcer, the fluid will dilute the fluorescein, and a fluorescent green stream of fluid will flow down the cornea. A positive Seidel test indicates that the eye is ruptured, and options include referral for surgical correction or possible enucleation.
Corneal cytology and culture can be performed in a sedated patient, and an auriculopalpebral motor nerve block can help the veterinarian obtain a useful sample. Additional use of topical analgesics should be considered. These can be irrigated onto the corneal surface and include 0.5% proparacaine solution and 1% tetracaine (gel or drop forms available). The latter may provide a greater degree of analgesia, sufficient for deep sampling of corneal lesions. However, their benefit must be balanced with the fact that the preservatives in these preparations may be toxic to bacterial populations and may therefore compromise bacterial culture. Samples may be collected directly, by use of a moistened cotton-tip applicator or microbrush for cytology or a sterile swab for culture. However, deeper lesions can be sampled with a blunt-tipped blade (Kimura spatula or handle side of a sterile scalpel blade) because a greater quantity of material can be collected from a deeper stromal location. This should not be attempted in deep ulcers because of the risk for globe rupture. Although corneal cytology provides immediate information and positive identification of etiologic agents (e.g., intracellular bacteria or fungal elements) is diagnostic, the absence of visible etiologic agents does not necessarily correlate well with an absence of disease. Therefore both cytology and culture (bacterial and fungal) should be performed in any lesion that is considered to be at risk for infection, and negative results do not always eliminate the potential for infectious keratitis, which should be considered when a treatment regimen is planned. Bacterial drug sensitivity can also help to guide treatment; many laboratories offer an eye-specific drug panel for sensitivity testing. Corneal cytology can yield the diagnosis in horses with eosinophilic keratitis.
Superficial Corneal Ulceration
Superficial ulcers result from loss of the epithelium and epithelial basement membrane, exposing the most superficial layers of corneal stroma. Superficial ulceration is painful and will manifest as a combination of eyelid swelling, conjunctival hyperemia, and blepharospasm. The horse may rub the eye on the stall or a forelimb. Mild focal corneal edema (having a hazy blue or gray appearance) may be present around the area of ulceration and is related to superficial stromal hydration from the tear film secondary to focal loss of the hydrophobic epithelium. A mild “reflex” uveitis may be associated with superficial ulcers; little to no intraocular inflammation will be visible, but miosis and mild hypotony may be present. Discharge is usually serous in nature. Chronic superficial ulceration will cause superficial corneal vascularization, granulation tissue deposition, and fibrosis. Fluorescein staining reveals a well-demarcated area of uptake with no visible crater in the corneal surface (see Color Plate 143-1).
Superficial corneal ulcers can have a number of possible underlying etiologies. Although uncomplicated superficial traumatic abrasion is possible, further investigation for other possible etiologies should be performed. Conjunctival and eyelid foreign bodies from plant and other environmental materials are common. A careful visual examination should be performed and followed with careful palpation of the conjunctival surfaces, including the posterior side of the third eyelid. Congenital or acquired eyelid abnormalities, including distichiasis, entropion, colobomas, trauma, inflammation, neoplasia, and scarring, can result in corneal abrasion and should be addressed to limit corneal damage. Nonhealing superficial ulcers (those that do not heal as expected, usually within 7 days) should be investigated further, with some combination of cytology, bacterial culture, tear-film evaluation (e.g., with a Schirmer tear test or tear film breakup test), and biopsy. Other possible underlying causes for poor ulcer healing could include herpesvirus infection, corneal degeneration, corneal dystrophy, corneal mineralization (band keratopathy), and eosinophilic keratitis.
