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Basics ⬇

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BASICS

Definition!!navigator!!

  • Corneal ulceration is a sight-threatening disease requiring early clinical diagnosis, laboratory confirmation, and appropriate medical and surgical therapy
  • Ulcers can range from simple, superficial breaks or abrasions in the corneal epithelium to full-thickness corneal perforations with iris prolapse
  • Prompt therapy is necessary to prevent progression and sight- and globe-threatening complications

Pathophysiology!!navigator!!

  • The thickness of the equine cornea is 0.9–1.2 mm
  • The normal equine corneal epithelium is 8–10 cell layers thick but increases to 10–15 cell layers following corneal injury. The epithelial basement membrane is not completely formed 6 weeks following corneal injury, in spite of the epithelium completely covering the ulcer site. Healing time of a 7 mm diameter, midstromal depth corneal trephine wound was nearly 12 days in noninfected wounds
  • Bacterial and fungal organisms normally found in the horse conjunctival flora are potential ocular pathogens. Staphylococcus, Streptococcus, Pseudomonas, Aspergillus, and Fusarium spp. are commonly isolated from equine corneal ulcers
  • A defect in the tear film or corneal epithelium allows bacteria or fungi to adhere to the cornea and to initiate infection
  • Tear film neutrophils and some bacteria and fungi are associated with highly destructive protease and collagenase enzymes that can result in rapid corneal stromal degradation and corneal thinning in the horse. Excessive protease activity is termed “melting” and results in a gelatinous appearance of the stroma. Severely affected corneas are at great risk of perforation
  • Horse corneas demonstrate a strong fibrovascular healing response

Systems Affected!!navigator!!

Ophthalmic

Incidence/Prevalence!!navigator!!

Common and may be associated with bacterial and fungal infection.

Signalment!!navigator!!

All ages and breeds.

Signs!!navigator!!

  • The eye may be cloudy, red, and painful
  • Blepharospasm and epiphora
  • A corneal defect may be obvious or may require the application of topical fluorescein to highlight its margins
  • Corneal edema may surround the ulcer or involve the entire cornea
  • White- to cream-colored infiltrate may be present in the stroma in or around a corneal ulcer and is usually indicative of the presence of an infectious agent
  • Signs of anterior uveitis are found to some extent with every corneal ulcer and include miosis, aqueous flare, fibrin, hyphema, or hypopyon

Causes!!navigator!!

  • Trauma
  • Infection should be considered possible in every corneal ulcer in the horse. Infectious keratitis develops in eyes with traumatic corneal abrasions, and eyes with epithelial defects due to chronic edema, keratoconjunctivitis sicca, exposure keratitis, neurotrophic keratitis, and neuroparalytic keratitis. Immunosuppressive conditions and topical steroids predispose to the development of corneal infections. Fungal involvement should be suspected if there is a history of corneal injury with vegetative material, or if a corneal ulcer has received prolonged antibiotic and/or corticosteroid therapy with minimal or no improvement
  • Foreign bodies, chemical burns, and immune mechanisms may also cause corneal ulceration
  • Persistent superficial ulcers may become indolent due to hyaline membrane formation on the ulcer bed

Risk Factors!!navigator!!

  • The prominent eye of the horse may predispose to injury
  • Tear film proteases are elevated in both eyes of a horse with an ulcer in 1 eye
  • Healing of ulcers does not occur until tear film proteases are reduced to baseline levels

Diagnosis ⬆ ⬇

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DIAGNOSIS

Differential Diagnosis!!navigator!!

  • Fluorescein dye (undiluted) retention is diagnostic of a corneal ulcer
  • Uveitis, blepharitis, conjunctivitis, glaucoma, and dacryocystitis must be considered in the differential for a painful eye

Laboratory Tests!!navigator!!

  • Microbial culture and susceptibility for bacteria and fungi are recommended with rapidly progressive and deep corneal ulcers
  • Corneal cultures should be obtained first, followed by corneal scrapings for cytology
  • Mixed bacterial and fungal infections can be present
  • Vigorous corneal scrapings, at the edge and base of the lesion, to detect bacteria and deep hyphal elements can be obtained following application of a topical anesthetic with the handle end of a sterile scalpel blade or a cytology brush. Superficial swabbing may not always yield organisms. Stain cytologic specimens with Gram, Giemsa, or Wright's stain

Diagnostic Procedures!!navigator!!

  • All corneal injuries should be fluorescein stained to detect corneal ulcers. Small corneal abrasions are detected through the use of oblique transillumination and fluorescein dye retention
  • A “crater-like” defect that retains fluorescein dye at its periphery and is clear in the center is a descemetocele, and indicates the globe is at high risk of rupture. Descemet's membrane in the horse is 21 μm thick

Pathologic Findings!!navigator!!

  • Many early cases present initially as minor corneal epithelial ulcers or infiltrates, with blepharospasm, epiphora, and photophobia
  • At first, anterior uveitis and corneal vascularization may not be clinically pronounced
  • Superficial and deep corneal vascularization and painful uveitis may occur
  • Extensive stromal lesions, vascularization, conjunctival injection, and corneal edema may then become evident
  • Corneal collagen breakdown or “melting” appears as a gelatinous, gray opacity to the margins and/or central regions of an ulcer. Melting corneal collagen may appear to drip off the surface of the eye
  • Cellular infiltrate may develop rapidly and appears as white-to-yellow corneal opacities
  • A descemetocele can be recognized as a clearing at the bottom of a deep ulcer. It does not retain fluorescein dye, whereas deep ulcers (with some remaining corneal collagen) retain fluorescein
  • Deep penetration of the stroma to Descemet's membrane with perforation of the cornea is a possible sequela to all corneal ulcers in horses

Treatment ⬆ ⬇

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TREATMENT

Appropriate Health Care!!navigator!!

  • Corneal ulceration should always be considered an emergency
  • The horse cornea can rapidly deteriorate if ulcerated and is prone to infection
  • Subpalpebral lavage treatment systems are used to treat a fractious horse or for frequent therapy

Activity!!navigator!!

Horses should be stall rested until the condition is healed.

Client Education!!navigator!!

  • A slowly progressive, indolent course often belies the seriousness of the ulcer
  • Corneal ulcers in horses may rapidly progress to descemetoceles or perforations
  • Corneal ulcers in horses are often very slow to heal
  • Anterior uveitis may be difficult to control
  • Scarring and vascularization of the cornea are common to the horse following ulceration

Surgical Considerations!!navigator!!

  • Surgical placement of a conjunctival flap, corneoconjunctival transposition, or corneal transplantation may be indicated for rapidly progressive and deep corneal ulcers
  • Removing necrotic tissue by keratectomy speeds healing, minimizes scarring, and decreases the stimulus for iridocyclitis
  • Conjunctival grafts or flaps are used for the clinical management of deep, melting, and large corneal ulcers, descemetoceles, and for perforated corneal ulcers with and without iris prolapse
  • Amniotic membrane grafts or synthetic collagen grafts are used to facilitate healing and decrease scar tissue development in large or melting corneal ulcers
  • Panophthalmitis following perforation through a corneal stromal ulcer has a grave prognosis and enucleation may be considered
  • Persistent ulcers may need surgical debridement with a diamond burr or a superficial keratectomy to remove the hyaline membrane slowing healing

Medications ⬆ ⬇

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MEDICATIONS

Drug(s) of Choice!!navigator!!

  • Topically applied antibiotics, such as neomycin–polymyxin B–bacitracin, gramicidin, chloramphenicol, gentamicin, ofloxacin, ciprofloxacin, or tobramycin ophthalmic solutions, may be used for bacterial ulcers. Amikacin (10 mg/mL) may also be used topically. Frequency of medication varies from every 1 h to every 8 h
  • Topical 1% atropine (every 4 h, then decrease the frequency of administration as soon as the pupil dilates) to stabilize the blood–aqueous barrier, to minimize pain from ciliary muscle spasm, and to cause pupillary dilatation
  • Topically administered autogenous serum is used in ulcers with evidence of collagenolysis, infection, or chronicity. The serum can be administered topically as often as possible. Acetylcysteine (5%) or sodium EDTA (0.2–1.0%) can also be administered until stromal liquefaction diminishes. Multiple anticollagenase medications may be needed to arrest melting in some horse eyes
  • Systemic and topically administered NSAIDs such as phenylbutazone (1 g BID PO) or flunixin meglumine (1 mg/kg BID IV, IM, or PO) to reduce uveal exudation and relieve ocular discomfort

Contraindications!!navigator!!

Topical corticosteroids may encourage growth of bacterial and fungal opportunists and also impair collagen production and epithelial migration and adhesion.

Precautions!!navigator!!

Horses receiving topically administered atropine should be monitored for signs of colic.

Alternative Drugs!!navigator!!

Topical autogenous serum can reduce tear film and corneal protease activity in corneal ulcers. It can be stored at room temperature; however, refrigeration is recommended. Replace the serum with fresh every 8 days.

Follow-up ⬆ ⬇

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FOLLOW-UP

Patient Monitoring!!navigator!!

  • The clarity of the cornea, the depth and size of the ulcer, the degree of corneal vascularization, the amount of tearing, the pupil size, and intensity of the anterior uveitis should be monitored. Serial fluorescein staining of the ulcer is indicated to assess re-epithelialization
  • As the cornea heals, the stimulus for the uveitis will diminish, and the pupil will dilate with less frequent atropine administration
  • Self-trauma should be reduced with hard- or soft-cup hoods

Prevention/Avoidance!!navigator!!

Corneal ulcers in horses should be aggressively treated no matter how small or superficial they may be. Progression is common and consequences can be severe.

Possible Complications!!navigator!!

Globe rupture, phthisis bulbi, and blindness are possible sequelae.

Expected Course and Prognosis!!navigator!!

  • Corneal ulcer often heals slowly and with scarring. There is a strong tendency to vascularize
  • If the replication and spread of bacteria are not halted, the process of stromal degradation and “melting” ultimately leads to total loss of stromal tissue and corneal perforation
  • Conjunctival flaps or corneal grafts may be necessary to save the globe and vision but are associated with scarring of the ulcer site

Miscellaneous ⬆ ⬇

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MISCELLANEOUS

Associated Conditions!!navigator!!

  • Corneal infection and iridocyclitis are always major concerns for even the slightest corneal ulcerations
  • Iridocyclitis or uveitis is present in all types of corneal ulcers and must be treated in order to preserve vision

Abbreviations!!navigator!!

NSAID = nonsteroidal anti-inflammatory drug

Suggested Reading ⬆ ⬇

Brooks DE. Ophthalmology for the Equine Practitioner, 2e. Jackson, WY: Teton NewMedia, 2008.

Gilger BC, ed. Equine Ophthalmology, 3e. Philadelphia, PA: WB Saunders, 2017.

Author(s) ⬆

Author: Caryn E. Plummer

Consulting Editor: Caryn E. Plummer

Acknowledgment: The author/editor acknowledges the prior contribution of Dennis E. Brooks.