Corneal anatomy of the dog and cat
The cornea is a transparent structure at the front 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. The epithelium holds the tear film in place and also prevents water from
invading the cornea and disrupting the collagen fibers. This prevents corneal
edema, which gives it a cloudy appearance. The epithelium sticks to the basement
membrane, which also separates the epithelium from the stroma. The corneal
stroma comprises 90 percent of the thickness of the cornea. It contains the
collagen fibers organized into lamellae. The lamellae are in sheets which
separate easily. Posterior to the stroma is Descemet's membrane, which is a
basement membrane for the corneal endothelium. The endothelium is a single cell
layer that separates the cornea from the aqueous humor.
Corneal healing
An ulcer of the cornea heals by two methods: migration of surrounding
epithelial cells followed by
mitosis (dividing) of the cells, and introduction of blood vessels from the
conjunctiva. Simple, small ulcers heal by the first method. However, larger or
deeper ulcers often require the presence of blood vessels to supply inflammatory
cells. White blood cells and fibroblasts produce granulation tissue and then scar tissue, effectively healing the cornea.
Superficial and deep corneal ulcers
Corneal ulcers are one of the most common eye diseases in dogs. They are
caused by trauma, detergent burns, and infections. Other eye conditions can
cause corneal ulcers, such as
entropion, distichia, corneal dystrophy, and
keratoconjunctivitis sicca. Superficial ulcers involve a loss of part of the
epithelium. Deep ulcers extend into or through the stroma and are 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. Corneal ulcers are painful due to nerve exposure, and
can cause tearing, squinting, and pawing at the eye.
Diagnosis
Diagnosis is through 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 doen not
absorb the stain.
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. 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 should never be used on any type of corneal ulcer because
they prevent healing and will often make them worse.
Refractory corneal ulcers
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 caused by a defect in the basement membrane. 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.
Treatment
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 perpindicular 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.
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. 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 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.
References
- Gelatt, Kirk N. (ed.)(1999). Veterinary Ophthalmology (3rd ed.).
Lippincott, Williams & Wilkins.
ISBN 0-683-30076-8
|
