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Corneal Ulcer - Horse (view source)
Revision as of 18:59, 27 September 2012
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Also known as: '''''Equine Corneal Ulcers'''''
==Introduction==
Corneal ulceration is a '''very common disease of the equine eye''' and can have sight threatening consequences. '''Aggressive treatment''' is always indicated, as even apparently mild ulcers can progress quickly, causing serious complications.
==Aetiology==
A distinct cause for initial ulceration is not commonly found, although in many cases it can be assumed to be '''traumatic''' in origin. The horse’s eye is especially vulnerable to trauma due to its prominent position, compared with other species.
'''Exposure keratitis''' can occur in the horse, most commonly secondary to facial nerve paralysis. Hospitalised animals have been shown to have a decreased corneal reflex, and this corresponds to an increased incidence of ulcers in the hospitalised population. '''Foreign bodies''' embedded in the palpebral conjunctiva, or the nictitating membrane can cause persistent irritation and ulceration. Often the shape/distribution of the lesion is suggestive of this aetiology, but even in the absence of a characteristic lesion their presence should be considered and sought out.
'''Bacterial (+/- fungal) infection''' occurs readily after initial ulceration, as disruption of the corneal epithelium allows attachment and colonisation of the underlying tissues by normal corneal commensals. Commonly isolated bacteria include [[:Category:Staphylococcus species|''Staphylococcus'' spp.]], [[:Category:Streptococcus species|''Streptococcus'' spp.]] and [[:Category:Pseudomonas and Burkholderia species|''Pseudomonas'' spp.]], and empirical anti-microbial therapy should be effective against these bacteria.
==Clinical Signs==
These include: ocular pain manifested as blepharospasms, increased lacrimation and photophobia. Corneal oedema, scleral injection and conjunctivitis are also often present.
Signs of infection may include: necrotic edges, a cratered base, severe manifestations of pain and inflammation.
==Diagnostics==
Differential diagnoses for the painful equine eye include: ulceration, [[Equine Recurrent Uveitis|uveitis]], blepharitis, conjunctivitis, glaucoma, dacryocystitis.
A full ophthalmic exam should be performed.
'''Fluoroscein staining''' is usually diagnostic for corneal ulcers, although staining with '''Rose Bengal''' is also recommended as it can pick up early viral/fungal lesions, which will appear as multifocal disturbances to the tear film.
'''Cytology, culture and sensitivity''' is recommended for rapidly progressive or deep corneal ulcers. Cotton swabbing is often inadequate, and '''corneal scraping''', for example, with the blunt side of a scalpel blade is usually required. This can be greatly facilitated by the use of local nerve blocks and topical anaesthesia.
There is almost invariably a '''secondary uveitis''' present with corneal ulceration, and signs of this may also be seen: miosis, corneal oedema, aqueous flare, hypopyon, intra ocular pressure (IOP) changes.
==Treatment==
Medical therapy should be based upon the severity of disease initially, and then by the response to therapy.
The aims of initial therapy are:
:'''Antibiosis'''
:'''Analgesia'''
:'''Anti-inflammatory medication'''
:'''Mydriasis'''
The initial choice of antibiotic depends upon personal choice, experience and availability, but could include '''chloramphenicol, chlortetracycline''', bacitracin-neomycin-polymyxin (BNP), ciprofloxacin, ofloxacin and tobramycin. '''Topical gentamicin formulations''' are also available, but in the opinion of some, should be reserved for cases with stromal melting. Frequency of application can vary from q1h to q8h, depending on both the severity of the lesion, and the formulation used (ointment vs. drops).
Much of the pain associated with corneal ulceration is due to the secondary uveitis and miosis, and effective relief can often be gained with '''topical atropine'''(1%), leading to mydriasis. Dosing is generally q4h initially, and then as required to maintain dilation. Mydriasis is also important to avoid some of the complications associated with uveitis, such as '''synechiae formation''' and glaucoma. Pain is also associated with inflammatory response occurring in the adjacent sclera and conjunctiva, and systemic analgesia in the form of [[NSAIDs]] is usually indicated, for example, '''flunixin''' meglumin. '''Topical NSAIDs''' are available (diclofenac, flurbiprofen) and effective, but have been shown to increase corneal healing time.
In horses that are difficult to treat, or in cases that require very frequent treatment, then placement of a '''sub-palpebral lavage system''' can be very useful. As an adjunct to therapy, physical protection of the eye may be required, in the form of a '''mask'''. Some horses will rub their eyes in response to pain, and this can cause further corneal damage. '''Box rest''' is also vital, as over-exertion has been linked to intra-ocular haemorrhage and increased severity of uveitis.
Success in your therapeutic regime can be judged by a reduction in pain, and a decrease in size of the ulcer. '''Healing generally occurs rapidly at first''', followed by a slowing after 5-7 days. As a rough guide, a non-infected ulcer can be expected to heal at approximately 0.6mm/day.
==Complications==
===Non-healing ulcer===
This may be due to the following causes:
'''Persistent source of irritation''': such as a foreign body, self trauma, iatrogenic
'''Infection''': antibiotic therapy should be reviewed, culture and sensitivity performed
'''Immunosuppression''': due to [[Equine Pituitary Pars Intermedia Dysfunction|Cushing’s]], steroid therapy
'''Compliance''': check that treatment is being administered effectively
None of the above: abnormal epithelium may have formed, keratectomy may be appropriate.
===Melting Ulcer===
Melting ulcers reflect '''inappropriate collagenolysis''' of the corneal stroma, by '''matrix-metalloproteinases''' (MMPs). Bacterial pathogens (especially ''Pseudomonas'' and β-haemolytic ''Streptococcus'') induce the corneal epithelial cells and resident leucocytes to upregulate pro-inflammatory, and MMP-activating cytokines (IL-1,-6 and -8). These bacteria can also produce their own proteinases. The combination of exogenous, and upregulated endogenous, proteinases leads to a rapid breakdown of collagen, with the characteristic melting appearance. Untreated, this can lead to '''perforation within 12 hours''' (so act hard and fast!). There are several therapeutic options for inhibiting MMPs:
:'''Autogenous serum''' – administer topically as often as possible. Keep refrigerated and change every 8 days.
:'''EDTA''' - 0.05% q1h
:'''Acetylcysteine''' – 5-10% q1h
:'''Tetracyclines''' – Doxycycline especially has been shown to have anti-MMP effects
:'''Tetanus antitoxin''' – can be delivered sub-conjunctivally. It contains macroglobulins with anti-collagenase effects.
A combination of the above may be necessary early in the disease course. '''Effective antibiosis''' is also paramount, and gentamicin is a good empirical choice (although there are some reports of gentamicin-resistant ''Pseudomonas'' species). Obviously, treating an eye this frequently in practice will be difficult, so '''referral''' is probably the best option.
MMPs can be further '''activated iatrogenically''', by treatment (topical or systemic) with corticosteroids. Steroids also reduce the immune defences of the eye, so just in case you forgot...
'''Steroids are contraindicated in the treatment of corneal ulcers!'''
===Corneal Rupture===
The appearance of a '''descematocoele''' should alert you to the danger of the eye rupturing. Put a donut '''bandage over the eye''', and give some '''broad spectrum antibiosis''' in case it ruptures on the way to the referral centre! '''Enrofloxacin''' is probably a good choice as it has good ocular penetration.
{{Learning
|flashcards = [[Equine Internal Medicine Q&A 04]]
}}
{{Chapter}}
{{Mansonchapter
|chapterlink = http://www.mansonpublishing.co.uk/book-images/9781591610366_sample.pdf
|chaptername = Corneal Ulceration
|book = Ophthalmology for the Equine Practitioner
|author = Dennis Brooks
|isbn =9781591610403
}}
{{review}}
{{OpenPages}}
[[Category:Expert Review - Horse]]
[[Category:Neurological Diseases - Horse]]