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The cornea is the clear,
shiny membrane, which makes up the surface of the eyeball.
It is much like a clear window. To understand a corneal ulcer,
you must first understand how the cornea is constructed.
The cornea is comprised of three layers. The most superficial
layer is the epithelium. Actually, this layer is comprised
of many, very thin layers of cells. Below the epithelium is
the stroma, and the deepest layer is Descemet's membrane.
Because all of these layers are clear, it is not possible
to see them without special stains and a microscope.
An erosion through a few layers of the epithelium is called
a corneal erosion or a corneal abrasion. A corneal ulcer is
an erosion through the entire epithelium and into the stroma.
If the erosion goes through the epithelium and stroma to the
level of Descemet's membrane, a descemetocele exists. If Descemet's
membrane ruptures, the liquid inside the eyeball leaks out
and the eye collapses.
How does a corneal ulcer occur?
There are several causes for corneal ulcers in dogs. The most
common is trauma. An ulcer may result from blunt trauma, such
as a dog rubbing its eye on carpet, or due to a laceration,
such as a cat scratch. The second most common cause is chemical
burn of the cornea. This may happen when irritating shampoo
or dip gets in the eye.
Less common causes of corneal ulcers include bacterial infections,
viral infections, and other diseases. These may originate
in the eye or develop secondary to disease elsewhere in the
body. Examples of other diseases include Epithelial Dystrophy
(a softening of the cornea which is inherited in breeds such
as the Boxer), Keratoconjunctivitis Sicca (drying of the cornea
due to abnormal tear formation), and diseases of the endocrine
system (diabetes mellitus, hyperadrenocorticism, and hypothyroidism).
How does a corneal ulcer affect my dog?
A corneal ulcer is very painful. In response to pain, most
dogs rub the affected eye with a foot or on the carpet. To
protect the eye, they keep the lids tightly closed. Occasionally,
there will be a discharge that collects in the corner of the
eye or runs down the face.
How is a corneal ulcer diagnosed?
Superficial corneal abrasions are usually not visible. They
can be visualized with the use of fluorescein stain. A drop
of this stain is placed on the cornea. The dye will adhere
to an area of ulceration and is easily visualized with a special
black light called a Wood's light. This is the most basic
test performed and may be the only test needed if the ulcer
is acute and very superficial. If the ulcerated area is chronic
or very deep, samples are taken for culture and cell study
prior to applying the stain or any other medication.
How is a corneal ulcer treated?
Treatment depends on whether there is a corneal abrasion,
corneal ulcer, or descemetocele present.
Corneal abrasions generally heal within 3-5 days. Medication
is used to prevent bacterial infections (antibiotic ophthalmic
drops or ointment) and to relieve pain (atropine ophthalmic
drops or ointment). Antibiotic drops are only effective for
a few minutes so they must be applied frequently; ointments
last a bit longer but still require application every few
hours. It is suggested that an antibiotic preparation be instilled
in the eye 4 to 6 times per day. On the other hand, the effects
of atropine last many hours so this drug is only used twice
daily.
If a corneal ulcer or descemetocele is present, measures must
be taken to protect the eye and to promote healing. Since
dogs do not wear eye patches well, surgical techniques are
often used to close the eyelids and cover the ulcer or descemetocele.
These measures protect the eye for several days, then are
reversed so the dog can use the eye again.
Ulcers that do not heal well often have a build-up of dead
cells at the ulcer edge. These dead cells prevent normal cells
from the corneal surface from sliding over the ulcer edge
and filling in the defect. If this appears to be part of the
healing problem, the dead cells are removed from the edges
of the ulcer before the eyelids are surgically closed. In
some cases, removing the dead cells may be all that is needed
to start the healing process, so surgical closing of the eyelids
may not be necessary.
What if a corneal abrasion really turns out to be a corneal
ulcer?
This is a mistake that can happen because there is a judgment
call involved in differentiating the two. After 2-3 days of
treatment, your dog should be re-examined to be sure that
healing is progressing properly. If that does not happen,
a decision may be made to perform surgery.
Are there any side-effects from the eye medications?
Rarely, a dog will be allergic to an antibiotic that is instilled
in the eye. If your dog seems to be in more pain after the
medication is used, discontinue it and contact the veterinarian.
A dog with a corneal ulcer has quite a bit of pain in the
eye, so it keeps it tightly shut. Atropine is used to relieve
that pain. However, atropine also dilates the pupil widely.
This means that the dog is very sensitive to light in that
eye. Because of the light sensitivity, the eye will be held
closed in bright light.
Atropine's effects may last for several days after the drug
is discontinued. Do not be alarmed if the pupil stays dilated
for several days. Should you accidentally get atropine in
your eye, the same prolonged pupillary dilation will occur.
My dog began to drool excessively and paw at its mouth
after I administered the eye medications. Is that a reaction?
No. The tear ducts carry tears from the eyes to the back of
the nose. The eye medications may go through the tear ducts
and eventually reach the throat where they are tasted. Atropine
has a very bitter taste, which may cause drooling and pawing
at the mouth. You are seeing your dog's response to a bad
taste, not a drug reaction.
Since a corneal ulcer is painful, can I apply a topical
anaesthetic to the cornea?
A topical anaesthetic is often used to numb the cornea so
the diagnostic tests may be performed. However, these drugs
are toxic to the corneal epithelium and they prevent proper
healing. They are safe for one time use, but they should not
be used as part of the treatment.
How do I know when to discontinue medication?
The best way to tell that the cornea has healed is to repeat
the fluorescein stain test. This should be done after approximately
5-7 days of treatment.
There appear to be some red streaks near the ulcer. Is
that normal?
The normal cornea has no blood vessels going through it. However,
when a corneal ulcer or descemetocele occurs, the body senses
a need to increase its healing capabilities. New blood vessels
are created by a process called neovascularization. The new
vessels begin at the sclera (the white part of the eye) and
course their way to the ulcer.
Neovascularization is a good response because it hastens healing.
However, after the ulcer is healed, these vessels remain in
the cornea. They are not painful, but they do obstruct vision.
Therefore, it is desirable to remove them. This is done with
steroid (cortisone) ophthalmic drops or ointment. Cortisone
is used for a few days to several weeks, depending on how
many vessels exist.
It is important that steroids not be used in the eye too soon
because they will stop healing of a corneal ulcer and may
worsen it. Therefore, the fluorescein dye test should be performed
before beginning this type of medication. If steroids are
used and pain occurs in the eye again, discontinue the steroids
and have the eye rechecked.
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