Clinical management of the corneal wound in horses
Introduction
Corneal disease is one the most common ophthalmic problem in horses. Ulcerative keratitis is a potentially vision – theartening disease, as defects of the intact corneal epithelium can result in secondary microbial infection, progressive stromal loss and corneal perforation. Although infectious keratitis is one of the most common corneal diseases in the horse, other types of ulcerative keratitis including eosinophilic keratitis, foreign body associated keratitis, traumatic keratitis, superficial corneal erosions with anterior stromal sequestration, and superficial, nonhealing corneal ulceration have been described. Superficial, nonhealing corneal ulcers, or refractory ulcers, are chronic, nonhealing, noninfected ulcers that occur frequently in dogs, and have been reported in cats and horses. These corneal ulcers have a characteristic redudant nonadhered epithelial border, and the lesion is limited to epithelial loss with little to no corneal involvement. Superficial, nonhealing corneal ulcers have also been referred to as refractory epithelial erosions, persistent corneal ulcers, recurrent corneal erosion syndrome, recurrent erosion, indolent ulcers, chronic erosions, indolent-like corneal ulcers, refractory ulcers, spontaneous chronic corneal epithelial defects, and Boxer ulcers.
Ethiopathogenesis of indolent ulcer
The ethiopathogenesis of indolent, superficial corneal ulcers has been described to be an alteration in the function of the corneal basement membrane. The adherence of the epithelium to the stroma is disrupted and there are everting, loosened edges of the epithelium around the ulcer. The disrupted adherence between epithelium and stroma can be due to following mechanisms: formation of an abnormal membrane between the stroma and epithelium, alteration in the homeostasis of growth factors and neurotransmitters, and a disrupted balance of degradative enzymes.
Pathogenesis
There are two different pathophysiolgies for superficial nonhealing ulcers :1, a thin acellular layer of hyaline work as a barrier for epithelial healing; 2, the “normal” healing of a corneal defect starts with preparation of the epithelial cells for migration. Fibronectin, fibrin, laminin and other extracellular matrix proteins form a matrix creating a scaffold for the migrating epithelial cells. During this migration, these epithelial cells release plasminogen activator in the presence of fibronectin and fibrin. Plasminogen is then converted into plasmin, causing the newly formed scaffold to detach from its surrounding. This enables the epithelial cells to form permanent anchoring fibrils to the underlying basement membrane. This cascade continues until the defect is covered with firmly attached epithelial cells. In this situation the healing process is hindered and consequently a superficial nonhealing ulcer is the result.
Differential diagnoses
Indolent ulcers are devoid of stromal involvement, microbial infection, inflammation, and an underlying mechanical cause. Clinical examination is require to rule out presence of foreing bodies, including aberrant hairs, and eyelid margin defects that may be producing persistent mechanical erosion of the epithelium.
Treatment
Many therapeutic options have been described. The therapy of choice is based on clinical sings, the experiences of the veterinarian, the efforts to be made by the owner, and financial aspects. Medical therapy is often the first choice. As medical treatments generally give disappointing results with superficial nonhealing corneal ulcers, the absence of response after a period of time may be a sign of the presence of such ulcers. A soft contact lens can be used to protect the cornea. In a horse that was medically treated because of an indolent ulcer, the affected eye became less painful as soon as the soft contact lens was applied.
When loosened epithelial margins are present, debridement is advised using a cotton tip or small curette. Debridement of all redundant tissue is the recommended treatment on initial presentation of an indolent ulcer. Topical anesthetic is applied, and auriculopalpebral nerve block may be performed. Sterile, dry, cotton – tipped applicators are then used to manually debride the unattached and loosely attached epithelium. This procedure removes abnormal epithelium and may stimulate migration of adjacent epithelial cells.
When a superficial corneal ulcers is present for more than 2 weeks (despite medical treatment and/or debridement) superficial keratectomy or grid keratotomy is indicated. A superficial keratectomy requires general anesthesia, a well – experienced ophthalmic surgeon and a well – equipped surgery room. A grid keratotomy is easier to perform; only simple equipment is needed, and the procedure can be performed in the standing horse. The results of both procedures are favourable, but the mean healing after grid keratotomy is shorter than that after superficial keratectomy. Because of the simplicity of the procedure, the shorter healing time, the smell amount of discomfort after surgery, the slight risk of serious complications and the minimal scar formation, grid keratotomy is preferred to superficial keratectomy in superficial indolent ulcers.
Punctate keratectomy is a variant of grid keratotomy. Using a hypodermic needle small punctures are made in the cornea at the site of the ulcer, as well as in the healthy cornea around the lesion.
Prognosis
The therapy may be prolonged in some affected horses, intervention with debridement, grid keratotomy, thermal cautery, or superficial keratectomy improves the chance for more rapid resolution.