OCULAR EMERGENCIES OF THE HORSE
Dennis E. Brooks, DVM, PhD
Diplomate, American College of Veterinary Ophthalmologists
Professor of Ophthalmology
Ophthalmology Service Chief
University of Florida
EQUINE CORNEAL ULCERATION
Equine corneal ulceration is very common in horses and 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. The prominent eye of the horse may predispose to traumatic corneal injury. Both bacterial and fungal keratitis in horses may present with a mild, early clinical course, but require prompt therapy if serious ocular complications are to be avoided.
Corneal ulcers in horses should be aggressively treated no matter how small or superficial they may be. 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.
Bacterial and fungal growth must be halted and the microbes rendered non-viable. Broad- spectrum topical antibiotics are usually administered with culture and sensitivity tests aiding selection. Topical antibiotic solutions interfere with corneal epithelial healing less than ointments. Gentamicin should be used in ulcers with evidence of stromal melting only.
Topically applied antibiotics, such as bacitracin-neomycin-polymyxin B, gentamicin, ciprofloxacin, or tobramycin ophthalmic solutions may be utilized to treat bacterial ulcers. Natamycin, voriconazole, miconazole, itraconazole/ DMSO, fluconazole, amphotericin B, betadine solution, chlorhexidine gluconate, posaconazole, voriconazole, and silver sulfadiazine can be utilized topically for fungal ulcers. Frequency of medication varies from q2h to q8h. Cefazolin (55mg/ml), bacitracin, and carbenicillin are effective against beta hemolytic Streptococcus. Ciloxan (ciprofloxacin), amikacin (10 mg/ml), and polymyxin B (0.25% IV solution) may be used topically for gentamicin resistant Pseudomonas.
Collagenolysis prevention Collagenolysis prevention
Severe corneal inflammation secondary to bacterial (especially Pseudomonas and beta hemolytic Streptococcus) or, much less commonly, fungal infection, may result in sudden, rapid corneal liquefaction and perforation. Activation and/or production of proteolytic enzymes by corneal epithelial cells, leucocytes and microbial organisms are responsible for stromal collagenolysis or melting.
Serum is biologically nontoxic and contains an alpha-2 macroglobulin with antiproteinase activity. Autogenous serum administered topically can reduce tear film and corneal protease activity in corneal ulcers in horses. The serum can be administered topically as often as possible, and should be replaced by new serum every five-seven days. Five to 10 percent acetylcysteine, and/or 0.05% sodium EDTA can be instilled hourly, in addition to the other indicated drugs, for antimelting effect until stromal liquefaction ceases. It may be necessary to use serum, EDTA, and acetylcysteine simultaneously in severe cases.
Atropine sulfate is a common therapeutic agent for equine eye problems. Topically applied atropine (1%) is effective in stabilizing the blood-aqueous barrier, reducing vascular protein leakage, minimizing pain from ciliary muscle spasm, and reducing the chance of synechia formation by causing pupillary dilatation. Atropine may be utilized topically q4h to q6h with the frequency of administration reduced as soon as the pupil dilates. Systemically administered NSAIDs such as phenylbutazone (1 gm BID PO) or flunixin meglumine (1 mg/kg BID, IV, IM or PO) can be used orally or parenterally, and are effective in reducing uveal exudation and relieving ocular discomfort from the anterior uveitis in horses with ulcers.
Removing necrotic tissue and microbial debris by keratectomy speeds healing, minimizes scarring, and decreases the stimulus for iridocyclitis. Persistent superficial ulcers may need surgical debridement and keratotomy to remove the hyaline membrane slowing epithelial healing. Debridement to remove abnormal epithelium of refractory superficial erosions can be accomplished with topical anesthesia and a cotton-tipped applicator.
Conjunctival grafts or flaps are used frequently in equine ophthalmology for the clinical management of deep, melting, and large corneal ulcers, descemetoceles, and for perforated corneal ulcers with and without iris prolapse. To augment lost corneal thickness and strength, deep corneal ulcers threatening perforation may require conjunctival flap placement. Conjunctival flaps are associated with some scarring of the ulcer site. Coverage with a 360E, hood, island, pedicle, or bridge flap should be maintained for 4 to 12 weeks. Reoccurrence of the inflammation may occur following flap removal.
Amniotic Membrane Flaps
Amniotic membrane transplantation may provide decreased fibrosis, reduced vascularization of corneal ulcers, and faster reepithelialization in horses with superficial and/or deep corneal ulcers. They may be used alone or with conjunctival flaps.
Corneal Foreign Bodies
Penetrating and perforating corneal foreign bodies cause varying degrees of keratitis and uveitis and are common in horses. Superficial foreign bodies can be removed under topical anesthesia and the subsequent ulcer treated medically. Deep corneal and penetrating foreign bodies may cause severe uveitis/endophthalmitis and require more aggressive care.
Traumatic Corneal Edema
Blunt trauma can cause acute and severe corneal edema. Treat with topical 5% NaCl and topical NSAIDS.
Eyelid lacerations should be carefully repaired with a two-layer closure. Extra care should be directed at repositioning the eyelid margin.
THE EQUINE ORBIT
The orbit is composed of several bones forming a series of canals, fissures and foramina that contain the globe, orbital fascia, the optic nerve and other nerves, blood vessels, muscle, fat, and glands. Ultrasonography is a noninvasive, painless procedure that can qualitatively and quantitatively evaluate various orbital abnormalities. Diagnostic ultrasonography is indicated for evaluation of an exophthalmic globe or a globe obscured by opacities of the cornea, lens, or vitreous. This technique allows visualization of the retrobulbar soft tissue space previously only poorly visualized by other radiographic techniques. Differentiation of solid soft tissue masses versus cystic orbital masses, determination of the size of various globe or orbital components, and localization of foreign bodies is possible.
Orbital fractures can be identified by palpation, facial deformity, and radiography. Blepharedema, epistaxis, orbital emphysema, corneal ulcers, uveitis, and limitations of global motility due to entrapment by bone fragments may accompany orbital fractures. Orbital fractures can result in displacement of globe and have the potential for globe penetrating bone fragments.
Minor orbital rim fractures may not require surgical correction unless fracture fragments are impinging on the globe or perfect cosmesis is required. Serious periorbital fractures should be surgically repaired quickly as fibrous union of the fractured pieces begins within one week following the injury to make elevation and realignment very difficult. Interosseous wiring with stainless steel suture, bone plating and cancellous bone grafts may be necessary to immobilize and repair extensive orbital fractures.
Head trauma can cause globe proptosis. Proptosis is forward displacement of the eye from the orbit. It is seen commonly with retrobulbar hemorrhage and edema following penetrating orbital trauma. In cases of traumatic globe proptosis, careful ophthalmic exam will dictate viability of the eye. Lack of an indirect pupillary reflex to the normal eye, and miosis with severe hypotony and hyphema indicates severe trauma and poor visual prognosis. Temporary tarsorrhaphy is recommended for proptosis.
Treatment of Equine Orbital Disease
Systemically administered antibiotics and nonsteroidal anti-inflammatory agents are indicated to minimize infection and to reduce pain and eyelid swelling. Banamine or phenylbutazone can be given for pain associated with the orbital disease. Dimethyl sulfoxide can be applied topically to edematous eyelids to reduce lid swelling.
Conjunctivitis is inflammation of the mucous membrane that covers the posterior aspects of the eyelids (palpebral conjunctiva), the nictitans, and the sclera (bulbar conjunctiva). It is a nonspecific finding that indicates ocular inflammation, and may also be seen in systemic disease. Infectious and noninfectious diseases of the lids, cornea, sclera, anterior uvea, nasolacrimal system, and orbit can result in conjunctivitis. The conjunctiva is a mucous membrane that can reflect systemic dysfunction through color changes, as in anemia and jaundice.
Habronemiasis is a parasitic disease resulting in conjunctival and ocular granulomas. Onchocerciasis is a parasitic disease which can cause inflammation of the conjunctiva, cornea, and anterior uvea. Thelazia lacrymalis is a commensal parasite of the conjunctival fornices and nasolacrimal ducts of horses, and can incite conjunctivitis, superficial keratitis, dacryocystitis, and mild eyelid swelling. Foal conjunctivitis is associated with neonatal maladjustment syndrome, septicemia, immune-mediated hemolytic anemia, environmental allergens and irritants, dermoids, and subconjunctival or episcleral hemorrhages secondary to birth trauma.
Bacterial conjunctivitis is treated with topical broad-spectrum antibiotic initially (triple antibiotic is usually appropriate), which may change after results of bacterial culture and sensitivity. Treat every 6 hours to 12 hours depending on severity of disease. Topical corticosteroids are indicated following resolution of the infection.
Allergic conjunctivitis is treated with topical corticosteroids and is often difficult to eliminate completely due to the nature of the horse's environment. The prognosis associated with conjunctivitis secondary to systemic or complicated ocular disease varies with the specific disease. Eosinophils may invade the conjunctiva.
EQUINE RECURRENT UVEITIS (Periodic ophthalmia, moon blindness, iridocyclitis)
Equine recurrent uveitis (ERU) is a common cause of blindness in horses. It is a group of immune-mediated diseases of multiple origins.
The major goals of treatment of ERU are to preserve vision, decrease pain, and prevent or minimize the recurrence of attacks of uveitis. Specific prevention and therapy is often difficult, as the etiology is not identified in each case. Prednisolone acetate or dexamethasone should be applied initially. The nonsteroidal anti-inflammatory drugs (NSAID) can provide additive anti-inflammatory effects to the corticosteroids, and are effective at reducing the intraocular inflammation when a corneal ulcer is present. Cyclosporine A, an immunosuppressive drug, can be effective topically for ERU. Flunixin meglumine, phenylbutazone, or aspirin (10 mg/kg BID PO) are frequently used systemically to control intraocular inflammation. Some horses become refractory to the beneficial effects of these medications, and it may be necessary to switch to one of the other NSAID to ameliorate the clinical signs of ERU. Topical atropine minimizes synechiae formation by inducing mydriasis, and alleviates some of the pain of ERU by relieving spasm of ciliary body muscles. It also narrows the capillary inter-endothelial cell junctions to reduce capillary plasma leakage.
The glaucomas are a group of diseases resulting from alterations of aqueous humor dynamics that cause an intraocular pressure (IOP) increase above that which is compatible with normal function of the retinal ganglion cells and optic nerve. Horses with previous or concurrent uveitis, aged horses, and Appaloosas are at increased risk for the development of glaucoma.
The systemically administered carbonic anhydrase inhibitors acetazolamide (1-3 mg/kg QD, PO), the topical carbonic anhydrase inhibitor dorzolamide (2% TID), and the beta-blocker timolol maleate (0.5 % BID) have been utilized to lower IOP in horses with varying degrees of success.