Managing equine corneal disease (Proceedings)

Corneal disease is the most common reason for ophthalmic examination in the horse. Many corneal lesions are initially due to trauma.

Examination, Restraint and Nerve Blocks

A complete ophthalmic examination should always be completed to identify other ocular pathology which may be present or actually involved in initiating corneal lesions.

The cornea has very dense sensory innervations and corneal lesions are often painful. In order to examine the cornea adequately sedation is recommended in combination with local block of the auriculopalpebral nerve (to allow lid opening without pressure on the globe) and in some cases a block of the frontal nerve innervating the upper eyelid (if a subpalpebral lavage – SPL is to be placed).

Appearance, clinical signs, adnexal disease, other ocular signs

Using adequate magnification and a bright light source the contour of the cornea should be assessed for shape and diameter, corneal thinning (ulceration) or thickening (inflammatory infiltrate or neoplasia). Corneal opacification may be due to edema, cellular, lipid or mineral infiltrates vascularization or pigmentation. The eyelids should be examined for any interaction of the lid margin with the cornea (aberrant cilia, entropion, lid lacerations, lid neoplasia). The conjunctiva is examined for inflammatory or proliferative lesions. The conjunctival sac and third eyelid should be examined for foreign bodies. The anterior chamber, iris and lens may be involved in corneal disease (either in cases of uveitis secondary to corneal lesions, or as the primary site of the ocular disease – in recurrent uveitis and glaucoma).

Diagnostic Tests

The palpebral reflex should always be assessed to ensure adequate blinking and corneal protection. Schirmer values are rarely recorded in the horse, although qualitative tests of tear integrity may be more valuable. The cornea should always be stained with Fluorescein to diagnose corneal abrasions or ulcers. Rose Bengal staining may be indicated in some cases of tear mucin deficiency.

Culture and sensitivity testing and corneal cytology are important parts of the diagnostic process in horses with any ulcerative corneal disease and may even be used by the specialist in cases of non-ulcerative keratopathies. Although culture is rarely indicated in cases of superficial ulceration, in any deep or persistent corneal ulceration this should be mandatory part of the examination. Cytology (Diff-Quik) can identify cells types and the presence of bacteria or fungal hyphae. Gram staining is particularly useful in selection of antibiotics in suspected bacterial dermatitis. Cytology submitted for fungal staining is especially important in the more southerly parts of the USA.

Keratitis – Ulcerative

Corneal abrasions and ulcers are initiated in most cases by ocular trauma (either injuries or irritation from foreign bodies, eyelid cilia). In horses although primary keratoconjunctivitis sicca is not common, facial paralysis can result in corneal exposure and epithelial desiccation with subsequent ulceration.

Superficial acute abrasions and ulcers are treated empirically topical therapy with broad spectrum antibiotics (neomycin, polymixin and gramicidin or bacitracin). This is often effective in allowing healing by sliding of the adjacent epithelial cells. Painful eyes can be treated with a cycloplegic agent (atropine) to effect (just enough for pupil mydriasis). A small corneal ulcer should be able to re-epithelialize in 1-3 days. In horses it is recommended that even superficial ulcers are re-checked frequently due to their tendency to develop more severe problems (keratomalacia and fungal infections)

A superficial ulcer that fails to re-epithelialize may have a defect in the healing mechanisms with failure of epithelial cells to adhere to the corneal surface. Alternatively the initiating cause may still be present (poorly repaired lid laceration, facial paralysis, ectopic cilia, conjunctival or third eyelid foreign body and must e ruled out. In cases where there is no continued corneal trauma 9ie genuine indolent superficial ulcer) epithelial debridement may stimulate healing. In more refractory cases and in the absence of cytology suggesting fungal infection a grid keratotomy may promote healing. Contact lenses with temporary tarsorraphy are sometimes used.

Infected bacterial ulcers (complicated corneal ulcers) may result in rapid enzymatic (proteases, hydrolases etc) breakdown of the corneal stromal collagen with the risk of corneal perforation. In these cases initial medical treatment should aim to control infection and reduced the effects of enzymes (and in particular proteases) causing the corneal tissue breakdown. The types of bacteria involved in these melting ulcers resulting in keratomalacia vary geographically and include Pseudomonas spp and beta-hemolytic Streptococcus.

Initial antibiotic selection for deep ulcers can be empirical or supported by cytology. Treatment with either topical fluoroquinolones (ofloxacin, moxifloxacin) or aminoglycosides (fortified amikacin, gentamicin) alternating with a cephalosporin (cefazolin in artificial tears) is good choices. These should be administered via SPL. Frequency of administration should be initially every 1- 2 hours until the melting has been controlled after which q6-4h is adequate. Ant proteases most commonly used are autologous serum or 1% EDTA solution. Atropine 1% is used as a cycloplegic (administered to effect preferably at q12h) and mydriatic in cases of severe secondary uveitis. Antibiotic choice may have to be adjusted based on the initial response to therapy or the results of culture and antibiotic sensitivity

In cases where deep corneal ulcers of descemetoceles develop it may be advised to treat the lesion surgically to provide support for the cornea while it heals. However in most cases it is important to control melting of corneal stroma before surgery if it is to be effective. Conjunctival grafts can be used (usually pedicle grafts) although these are likely to result in more scarring as the cornea heals. Other treatments include corneoconjunctival transpositions, amniotic membrane grafts or other types of xenograft. Corneal lacerations and perforated corneal ulcers are treated in the same manner. The equine cornea has tremendous capacity to heal even after large perforations if treated surgically and with vigorous medication.

Fungal keratitis/ulceration occur worldwide but is more commonly seen in humid and warm environments. Lesions present as white to gray superficial stromal infiltrates with epithelial ulceration or deep stromal (at Descemet's membrane) keratitis with marked uveitis. Superficial plaques can be debrided and treated with topical antifungal agents at least q4h. Miconazole, natamycin or voriconazole are suitable agents for many of the fungal pathogens seen. In cases which do not improve with medical therapy surgical treatment is indicated – this involves removing affected corneal stroma and applying a superficial graft for superficial ulcers and deep lamellar endothelial keratotomy in deep infection.

Keratitis – non-ulcerative

Stromal abscesses occur when bacteria or fungi are inoculated in the cornea stroma and covered by re-epithelialization. The appearance is of a yellow to white opacity deep in the cornea with peripheral vascularization. Treatment involves deriding the affected abscess and treating with antibiotics or antifungal agents. Cytology and culture should be performed to aid diagnose the etiology. Deep abscesses should be supported with corneal grafts of amniotic membrane.

Eosinophilic keratoconjunctivitis occurs in horses and is diagnosed by the appearance of infiltrative lesions (often multiple) and cytology. Treatment with topical corticosteroid is often effective at treating the disease.

Immune mediated keratitis (IMMK) presents as corneal opacification and vascularization at various levels within the cornea. Unlike ulcerative keratitis and corneal abscessation, and fungal keratitis these lesions regardless of depth are not painful or only mildly uncomfortable. The treatment depends on the level of the opacity and range from medical treatment with corticosteroid and cyclosporine to keratectomy to remove areas of opacification

Trauma – lacerations and blunt trauma

Corneal lacerations should be repaired as soon as possible after the injury to avoid wound retraction and infection. Referral to a veterinary; ophthalmologist is recommended in all cases. Sharp lacerations limited to the cornea generally have a better prognosis compared to blunt trauma which extends into the sclera. It is important to evaluate the status of the lens in all corneal injuries. Lens laceration and cortical prolapsed into the eye results in marked inflammation and requires lens removal at the time of corneal repair.

Neoplasia – SCC

Various tumors affect the conjunctiva and cornea. Squamous cell carcinoma is the most commonly seen especially in light colored eyes exposed to high levels of sunlight. Cytology of superficial biopsy of corneal lesions is indicated to confirm the diagnosis. Lesions may appear like diffuse flat superficial opacification d vascularization or develop into raised and papillomatous mass lesions. Treatment involves debulking the lesion and ancillary therapies which may include chemotherapeutic agents (mitomycin or fluorouracil), photodynamic laser therapy, beta or gamma irradiation or cryotherapy