Clinical management of the corneal wound in horses
Introduction
Ocular trauma is a relatively common occurrence in the equine. Ocular – corneal injury occurs because of the fractious or excitable nature of horses and the force with which they react to unexpected or unusual situations, whether restrained in stocks, trailers, or simply left to their own devices in a stable or field. Many injuries can have devastating consequences for vision if not treated effectively in a timely fashion.
Many ocular injuries look worst when seen soon after the traumatic incident occurs. Thorough examination of the eye and adnexa at the time of the injury as well as during the ensuing few days is important. Factors that suggest an eye is beyond repair include severe lacerations that extend a significant way into the sclera, loss of ocular contents (lens, vitreous), intraocular infection, retinal detachment, and ocular prolapsed with severe damage to the extraocular muscles and optic nerve.
General ocular examination
Acute injuries in the horse being painful and resentful of manipulations around the effected eye. Maze testing in not recommended because a horse with visual impairment is likely to incur further injury by blundering into unseen objects. The pupillary light reflex may be difficult to assess, particularly in an eye that has a miotic pupil associated with uveitis. A dilated pupil in a traumatized eye suggests severe ocular damage and poor prognosis.
It is essential that the animal be adequately sedated for ocular examination of the eye. This may be not necessary for animals with minimal painful injuries. Regional motor nerve blocks facilitate safe examination, especially if any injury has weakened cornea or sclera. A block of the auriculopalpebral nerve can be performed along the zygomatic arch, halfway between the lateral canthus of the eye and the base of the ear. The injured eye is frequently difficult to examine soon after trauma due to massive tissue swelling (blepharoedema and chemosis).
The ocular examination should proceed from the anterior to posterior segments, examining lids, orbital structures, conjunctiva, cornea and sclera, anterior chamber, lens, and retina.
Corneal trauma
The cornea should be examined for any change in clarity or surface contour. Any evidence of corneal irregularity should be evaluated by fluorescein staining to determine the presence and extent of corneal abrasions and lacerations.
Superficial corneal lacerations may consist of an area of surface irregularity or a flap of corneal tissue. With deep nonperforating injuries, the edges of the laceration gape and adjacent cornea is thickened by corneal edema. Superficial corneal abrasions involve epithelial loss and, occasionally, some corneal stromal damage. The eye is painful and characterized by conjunctival injection, chemosis, corneal irregularity, and corneal opacification due to edema. Uveitis occurs because of an axon reflex between the densely innervated cornea and the anterior uvea.
Acute corneal abrasions can be treated with topical antibacterial agents. Atropine should be applied two to four times daily to achieve mydriasis and cycloplegia. Systemic NSAIDs reduce pain and help control uveitis.
In some corneal injuries, a flap of corneal stroma and epithelium may be displaced. Sutures must be anchored to healthy or vascularized stroma. Corneal lacerations should be sutured using general anesthesia. Lacerations of the corneal stroma that penetrate the stroma but do not perforate the cornea are sutured with simple interrupted sutures. The depth of the sutures is determined by the depth of the lacerations, but should not incorporate the inner endothelial corneal surface. Sutures should be placed 1 to 2 mm apart.
Perforating lacerations of the cornea typically have associated iris prolapse, anterior chamber collapse, and severe iridocyclitis. At the moment of perforation, the iris is carried into the wound by escaping aqueous humor. Tissue damage causes breakdown of the blood – aqueous barrier and massive influx of protein, including fibrinogen, into the anterior chamber. The amount of the fibrinous aqueous on the corneal surface is a poor guide to the lesion´s severity. Preoperative medications to treat the keratitis and uveitis include topical and systemic broad – spectrum antibiotics, 1 % topical atropine, and systemic NSAIDs. To repair a perforating corneal laceration, the iris should be separated from the margins of the laceration and repositioned in the anterior chamber, and the cornea repaired. If the lesion is less than 24 hours old, the iris most likely can be replaced intact. If the lesion is older than this, any devitalized tissue should be trimmed carefully with scissors until the hemorrhage is detected and then the iris replaced in the eye. After the closure of the corneal wound, reforming of anterior chamber with lactated Ringer´s solution, viscoelastic materials or sodium hyaluronate is done. Postoperative therapy should continue as before surgery. At the time of corneal repair, a subpalpebral lavage tube placed through the upper (lower) eyelid facilitate topical applications.
Foreing bodies
Ocular trauma may result in foreing bodies becoming longer in the eye. Penetrating and perforating corneal foreing bodies should be removed and defect sutured. Medical therapy should include topical and systemical antiobiotics, a mydriatic/cycloplegic drug, and NSAIDs systemically. In the view of the potential risk of the bacterial or fungal organisms being inoculated into the cornea or globe with the foreing body, corticosteroids should not be used in such cases. Both metallic and nonmetallic foreing bodies can be located with B – mode ultrasonography using 7,5 or 10 – MHz transducer probes. A foreing body within within the anterior segment can usually be removed surgically without complication. Foreing bodies located in the vitreous can be removed through the ciliary body pars plana (this is a difficult technique, and require specialized training and instrumentation).
Prognosis
Negative prognostic indicators include corneal lacerations of more than 15 mm in length; lacerations that extend to, along, or past the limbus; presence of keratomalacia and hyphema. positive prognostic indicators for maintenance of the eye and vision include corneal lacerations of less than 15 mm in length. Poor prognosis is associated with long – standing wounds, large wounds, lens capsule rupture, lens luxation, hemorrhage, and complete hyphema. Guarded prognosis is associated with acute injuries, small wounds, no intraocular damage, sealed wounds, and small amounts of hyphema.