Corneal ulcers in animals

Corneal ulcers are painful due to nerve exposure, and can cause tearing, squinting, and pawing at the eye. There may also be signs of anterior uveitis, such as miosis (small pupil), aqueous flare (protein in the aqueous humour), and redness of the eye. An axon reflex may be responsible for uveitis formation — stimulation of pain receptors in the cornea results in release inflammatory mediators such as prostaglandins, histamine, and acetylcholine.[4]

Diagnosis is through direct observation of the ulcer with the use of fluorescein stain, which is taken up by exposed corneal stroma and appears green (see photos above and below). With descemetoceles, Descemet's membrane will bulge forward and after staining will appear as a dark circle with a green boundary, because it does not absorb the stain. Other tests that may be necessary include a Schirmer's test for keratoconjunctivitis sicca and an analysis of facial nerve function for facial nerve paralysis.

Treatment of corneal ulcers includes topical antibiotic therapy to prevent infection, and pain medications, including topical atropine to stop spasms of the ciliary muscle. Atropine may decrease tear production and interfere with corneal healing.[5] Superficial ulcers usually heal in less than a week. Deep ulcers and descemetoceles may require corneal suturing, conjunctival grafts or conjunctival flaps, soft contact lenses, or corneal transplant. Topical corticosteroids and anesthetics should not be used on any type of corneal ulcer because they prevent healing and will often make them worse.

A new bioscaffold developed by TR BioSurgical is being evaluated for refractory corneal ulcers in dogs. The study is being conducted by board certified veterinary ophthalmologists and has shown promise in healing refractory ulcers that have failed conventional treatment .[6]

• Alaskan Malamute
• American Cocker Spaniel
• Boston Terrier
• Boxer
• Brussels Griffon
• Cairn Terrier
• Chesapeake Bay Retriever
• Chihuahua
• Chinese Shar Pei
• Dachshund
• Bulldog
• English Springer Spaniel
• German Shepherd Dog
• Golden Retriever
• Irish Setter
• Japanese Chin
• Lhasa Apso
• Maltese
• Pekingese
• Poodle
• Pug
• Rottweiler
• Samoyed
• Shih Tzu
• Silky Terrier
• Weimaraner
• Welsh Corgi
• Welsh Springer Spaniel
• West Highland White Terrier
• Wirehaired Fox Terrier

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Refractory corneal ulcers can take a long time to heal, sometimes months. Topical antibiotics are used continually to prevent infection. Pain medications are given as needed. Loose epithelium is removed with a dry cotton swab under topical anesthesia. This is in order to allow production of normal basement membrane and division of normal epithelium. Often further treatment is necessary, such as a keratotomy, which is superficial cutting or piercing of the cornea. There are two main types used in dogs: multiple punctate keratotomy (MPK) and grid keratotomy (GK). MPK involves making small superficial punctures into the cornea with a needle. GK is more commonly used and involves making parallel and perpendicular scratches in the corneal surface. Usually only topical anesthesia is necessary. By scoring the corneal surface, anchoring points are provided for attachment of new epithelium. Of course, these procedures should only be performed by a veterinarian, particularly one with some experience in this treatment. Complete healing takes about three to four weeks.[7] Keratotomies may lead to corneal sequestration in cats.[5] Other medications have been shown to be useful in topical treatment of refractory ulcers, including glycosaminoglycans such as sodium hyaluronate[8] and chondroitin sulfate,[9] aminocaproic acid,[10] and acetylcysteine.[11]

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 (inflamed cornea) 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 and 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 and fungal infection occurs readily after the initial ulceration, as disruption of the corneal epithelium allows attachment and colonisation of the underlying tissues by normal corneal commensals. Commonly isolated bacteria include Staphylococcus, Streptococcus and Pseudomonas, and empirical anti-microbial therapy should be effective against these bacteria.

• Ocular Pain
  • Blepharospasm
  • Increased lacrimation
  • Photophobia
• Corneal Oedema (1⁰/2⁰)
• Scleral injection
• Conjunctivitis

Differential diagnoses for the painful equine eye:

• Ulceration
• Uveitis
• Blepharitis
• Conjunctivitis
• Glaucoma
• Dachrocystitis

A full ocular exam should be performed for every case of eye pain. 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.

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, IOP changes.

Medical therapy should be based upon the severity of disease initially, and then by the response to therapy. The aims of the initial therapy are:

• Antibiosis
• Analgesia
• Anti-inflammatory
• Mydriatic

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 1.1 mg/kg, BID. 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.

In cases where the ulcer is not healing:

• Is there a persistent source of irritation?
  • Foreign body
  • Self traumatisation
  • Iatrogenic.
• Is it infected?
  • Review antibiotic therapy
  • C+S if not already performed.
• Immunosuppression?
  • Cushing's
  • Steroid therapy.
• Compliance?
  • Is what you've prescribed getting onto the eye?
• None of the above?
  • Abnormal epithelium may have formed, keratectomy may be appropriate.

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. 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.