Everything about Corneal Ulcers In Animals totally explained
A
corneal ulcer, or
ulcerative keratitis, is an inflammatory condition of the
cornea involving loss of its outer layer. It is very common in
dogs and is sometimes seen in
cats. In
veterinary medicine, the term
corneal ulcer is a generic name for any condition involving the loss of the outer layer of the cornea, and as such is used to describe conditions with both inflammatory and traumatic causes.
Corneal anatomy of the dog and cat
The cornea is a transparent structure that's part of the outer layer of the
eye. It
refracts light and protects the contents of the eye. The cornea is about one-half to one millimeter thick in the dog and cat. The
trigeminal nerve supplies the cornea via the
long ciliary nerves. There are
pain receptors in the outer layers and
pressure receptors deeper.
Transparency is achieved through a lack of blood vessels, pigmentation, and
keratin, and through the organization of the
collagen fibers. The collagen fibers cross the full diameter of the cornea in a strictly parallel fashion and allow 99 percent of the light to pass through without
scattering.
There are four important layers in the dog and cat cornea. The outer layer is the
epithelium, which is 25 to 40
micrometers and five to seven
cell layers thick.
Superficial ulcers involve a loss of part of the epithelium. Deep ulcers extend into or through the stroma and can result in severe scarring and corneal perforation.
Descemetoceles occur when the ulcer extends through the stroma. This type of ulcer is especially dangerous and can result in perforation.
The location of the ulcer depends somewhat on the cause. Central ulcers are typically caused by trauma, dry eye, or exposure from
facial nerve paralysis or
exophthalmos. Ulcers in the inferior nasal cornea may be caused by foreign material trapped under the
third eyelid. Entropion or distichiae may cause ulceration of the peripheral cornea. Immune-mediated eye disease can cause ulcers at the border of the cornea and
sclera.
Symptoms
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.
Diagnosis
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. With descemetoceles, Descemet's membrane will bulge forward and after staining will appear as a dark circle with a green boundary, because it doesn't 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
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. 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 shouldn't be used on any type of corneal ulcer because they prevent healing and will often make them worse.
Refractory corneal ulcers
(See also: Recurrent corneal erosion for the condition in humans)
Refractory corneal ulcers are superficial ulcers that heal poorly and tend to recur. They are also known as
indolent ulcers or
Boxer ulcers. They are believed to be caused by a defect in the basement membrane and a lack of
hemidesmosomal attachments. They are recognized by undermined epithelium that surrounds the ulcer and easily peels back. Refractory corneal ulcers are most commonly seen in middle aged or older dogs and often occur in the other eye later. They are similar to
Cogan's cystic dystrophy in humans.
Commonly affected breeds
- Alaskan Malamute
- American Cocker Spaniel
- Boston Terrier
- Boxer
- Brussels Griffon
- Cairn Terrier
- Chesapeake Bay Retriever
- Dachshund
- Bulldog
- German Shepherd Dog
- Golden Retriever
- Irish Setter
- Pekingese
- Poodle
- Rottweiler
- Samoyed
- Silky Terrier
- Springer Spaniel
- Weimaraner
- Welsh Corgi
- West Highland White Terrier
- Wirehaired Fox Terrier Keratotomies may lead to corneal sequestration in cats. and chondroitin sulfate, aminocaproic acid, and acetylcysteine.
Melting ulcers
Melting ulcers are a type of corneal ulcer involving progressive loss of stroma in a dissolving fashion. This is most commonly seen in Pseudomonas infection, but it can be caused by other types of bacteria or fungi. These infectious agents produce proteases and collagenases which break down the corneal stroma. Complete loss of the stroma can occur within 24 hours. Treatment includes antibiotics and collagenase inhibitors such as acetylcysteine and blood serum. Surgery may be necessary.
Corneal ulcers in cats
Corneal ulcers in cats can be caused by trauma, detergent burns, infections, and other eye diseases. One common cause not seen in dogs is infection with feline herpesvirus 1 (FHV-1). FHV-1 causes ulceration by direct infection of the epithelial cells. Lesions appear as round or dendritic (branching) ulcers. FHV-1 also suppresses healing of the cornea. Symptoms include conjunctivitis, squinting, eye discharge, and blood vessels on the cornea. It can cause severe scarring. Treatment is with topical antiviral drugs and antibiotics, and oral L-lysine, which competes with arginine and inhibits viral replication.[Further Information]
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