14 May 2012
Charlotte Maile
Job TitleCharlotte Maile begins by looking at common corneal ulcers in equines and goes on to rare and severe cases, offering diagnosis and therapy techniques
Equine corneal ulceration is a common, urgent problem in equine practice. Although a lot of cases are easy to treat medically, there are multiple surgical options for more severe cases. Thorough examination is required to guarantee a clear, clinical diagnosis and subsequent treatment. A systematic approach to diagnosis and treatment is required and is discussed here.
CORNEAL ulcers are relatively common in horses and are probably related to the prominence of the equine globe. Horses presenting with symptoms of corneal ulceration should be treated as an emergency to prevent progression to a more severe, deep ulcer.
The most common cause of corneal ulceration is trauma to the eye, which may be due to external sources, for example branches and fence posts, or endogenous causes, such as entropion. The cornea is usually a very good barrier to bacteria and fungi, however, when there is trauma that damages this barrier, bacteria or fungi can adhere to the surface and initiate infection. Therefore, common causes of corneal ulceration are bacteria or fungi commonly found on the surface of the eye, such as Staphylococcus, Streptococcus and Aspergillus species (Brooks, 2002). However, fungal causes are regarded as very rare in the UK, but a major problem in the US (Sansom, Featherstone et al, 2005).
Ulceration commonly results in the clinical signs of blepharospasm, epiphora, reddened conjunctiva, corneal oedema (seen as a cloudy area of cornea) and swollen eyelids. It is often accompanied by a constricted pupil (miosis) and photophobia due to anterior uveitis.
Diagnosis of corneal ulcers involves performing a full eye examination using an ophthalmoscope, and the use of fluorescein dye is essential to locate the ulcer.
Fluorescein dye is a bright green stain that adheres to any areas where the corneal epithelium is damaged and the underlying stroma revealed. This confirms the presence of a corneal ulcer, but also helps to diagnose a globe that is at high risk of rupture. If the dye adheres to the perimeter of the ulcer, but not the centre, this means the Descemet’s membrane is exposed and the ulcer has progressed to form a descemetocele and, therefore, has a high risk of rupturing.
Rose bengal stain is used to detect fungal ulcers as it indicates a defect in the mucin layer of the tear film, which occurs with fungal infections (Brooks, 2002). Bacterial and fungal culture is advised in cases of either rapidly progressing, deep corneal ulcers or persistent ulcers.
Secondary anterior uveitis is a common finding alongside corneal ulceration and presents as a constricted, unresponsive pupil and aqueous flare (a build-up of protein/fibrin and cells in the anterior chamber). It is important to treat the anterior uveitis along with the ulceration.
A thorough examination is essential as many types of corneal ulcer exist and different types require different treatments. Superficial (simple) corneal ulcers are the most common and easily treated if caught early. These normally heal rapidly and have no longterm effects. More serious forms of ulcers include melting ulcers, indolent ulcers and mycotic ulcers (rare in UK).
Melting ulcers are due to excessive levels of proteinases in the tear film that lead to degeneration of the stroma. This degeneration gives the stroma a liquefied appearance – hence the term “melting” (Brooks, 2002).
Indolent ulcers are also known as superficial, non-healing ulcers. These are superficial ulcers that do not heal, resulting in a redundant epithelial border where the epithelium has not adhered to the underlying stroma (Michau, Schwabenton et al, 2003).
Mycotic (fungal) ulcers are rarely seen in the UK, but are relatively common in more humid countries, such as the US. These ulcers classically present with a cake frosting appearance or punctate lesions of the epithelium (Sansom, Featherstone et al, 2005). Cultures should be taken for fungal culture if suspected and a corneal biopsy may reveal fungal elements.
Viral keratitis is a very rare cause of corneal ulceration resulting in superficial punctate lesions. In these cases some fluorescein or rose bengal dye retention may be present, but clinical signs are intermittent. It is thought to be linked to equine herpesvirus type two (Kershaw, von Oppen et al, 2001).
The treatment of uncomplicated superficial ulcers is relatively straightforward. Treatment regimes should include a topical, broad-spectrum antibiotic as prophylaxis against bacterial infection, analgesia and treatment of secondary anterior uveitis.
Commonly used antibiotic preparations include chloramphenicol, ciprofloxacin and gentamicin. Ideally, these preparations should be applied multiple times throughout the day (every six to eight hours).
In cases of fungal corneal ulcers, a suitable antifungal drug should be used (depending on culture results).
Systemic NSAIDs are recommended to relieve ocular pain and flunixin is thought to be the best preparation to use.
A mydriatic drug should be used to combat the miotic pupil. Atropine drops should be used once daily (application should be kept to a minimum so only use it as needed). Horses treated with atropine lose the ability to constrict their pupil normally in response to bright light, therefore these horses should be kept in a darkened stable and out of bright daylight.
In more severe superficial ulcers, anticollagenase treatment is recommended. Corneal cells produce proteolytic enzymes that prevent re-epithelialisation, and thus delay healing of some corneal ulcers.
Topical serum or ethylenediamine tetra-acetic acid (EDTA) can be used to reduce the action of these enzymes and thereby speed healing of the lesion. Serum can be applied as often as possible, and in more severe cases where EDTA is used, this should be administered hourly (Brooks, 2002).
Rather unsurprisingly, horses with painful eyes are not keen on repeated topical applications, therefore, in some cases – typically uncooperative patients or those with deeper, more severe ulcers – hospitalisation may be required. In these cases the use of a subpalpebral lavage (SPL) system can be used to allow frequent, stress-free treatment (Giuliano, Maggs et al, 2000).
This system involves a length of tubing that allows medications to be administered without touching the painful eye. Typically, a small catheter is placed through the upper eyelid under local anaesthetic and sedation. The end of the catheter must be smooth and not cause any irritation to the cornea or this may result in additional corneal irritation and ulceration.
The catheter should then have a length of thin tubing attached and this most commonly runs down the crest of the horse’s neck, so the port for administering medication is away from the eye. Care must be taken to ensure the tubing is firmly attached and will not easily by rubbed or pulled off by the patient.
Some debridement of the ulcer can aid healing by removing the necrotic tissue at the margins. This helps to reduce scar formation and can be done under local anaesthetic using a sterile cotton bud.
A keratotomy may be performed by making small incisions using a needle in either a punctate or grid pattern over the damaged cornea. This helps new migrating epithelial cells and encourages adherence to the corneal surface (Brünott, Boevé et al, 2007).
In more severe cases of corneal ulceration, surgical intervention may be required. There are several surgical options – depending on the severity, type and cause of the ulcer. Third eyelid flaps were previously used so the third eyelid acts as a bandage for the cornea; however, conjunctival grafts are now the preferred treatment.
Conjunctival pedicle grafts involve suturing a portion of conjunctiva over the ulcer site (Brooks, 2002). These flaps help by not only protecting the ulceration site from exposure, but also by improving blood supply to the region. The main disadvantage to this technique is corneal scarring, but this is a relatively small price to pay in cases where enucleation may be the alternative option.
Amniotic membrane flaps can also be used. This involves using small pieces of amniotic membrane to repair the defect in the cornea. This technique may also be used alongside a conjunctival graft.
The amniotic membrane is believed to have additional benefits over a simple conjunctival graft as it has antiprotease and antifibrotic properties that encourage rapid corneal healing (Lassaline, Brooks et al, 2005). The amniotic membrane is very strong and closely resembles the composition of the conjunctiva, making it an ideal substitute for a conjunctival graft (Plummer, 2009).
Contact lenses have been reported to be used in cases of ulcers that are refractory to topical treatment and where surgical intervention is not suitable (Wada, Yoshinari et al, 2000).
Contact lenses help protect the new epithelium and are thought to reduce pain in horses. The main problem with this method is in retaining the lens as they quite commonly fall out of the patient’s eye.
Corneal transplantation is a highly successful technique that may be used in horses with severe ulcers. It provides both a therapeutic solution and reduced scarring as the transplant helps to protect the underlying corneal surface and stimulate healing (Brooks, Plummer et al, 2008).
According to Brooks et al, donor corneas should be harvested from fresh equine eyes within 24 hours of death and can be frozen for storage. During surgery, the diseased area of the cornea should be removed without penetrating Descemet’s membrane and the donor cornea sutured over the defect with a one millimetre overlap at the margins.
In cases of melting or severely infected ulcers, a conjunctival pedicle or amniotic membrane graft may then be sutured over the transplant site to encourage acceptance of the graft into the cornea.
In cases of perforated ulcers that have been unresponsive to treatment, enucleation is often the only option.
Corneal ulcers often provide very rewarding cases for the equine practitioner and should not be looked upon with dread.
Charlotte Maile
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