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Anyone
who has ever experienced having an eyelash trapped under a contact lens
or a grain of sand blown into their eye quickly discovers that the
cornea is loaded with pain receptors.
A corneal ulcer – a
scratch or scrape involving the cornea – is a relatively common,
potentially vision-threatening disease of the cornea in cats.
Cornea 101 – the basics
The cornea is the clear membrane that covers
the surface of the eyeball.
Anatomically, it is composed of several layers.
The outer surface layer is called the
epithelium.
Just beneath the epithelium is
the stroma.
The innermost layer is called
Descemet’s
membrane.
“All cat breeds are potentially at risk of
developing corneal ulcers”, warns Dr. Chris Pirie, a board-certified
veterinary ophthalmologist at the Cummings School of Veterinary Medicine
at Tufts university. However, cats in general have pretty refined
defense mechanisms to prevent damage to their corneas. They have
vibrissae (specialized
“whiskers” above their upper eyelids) that can detect objects that
approach their eyes, allowing them to take evasive action.
They also have a well-developed blink response.
Attached to the back of the eyeball is a muscle called the
retractor bulbi muscle.
When this muscle contracts, the eyeball is pulled back into the
socket. This retraction of the
eye allows the nictitating
membrane – sometimes called the “third eyelid” – to elevate,
protecting the cornea. Despite these sophisticated mechanisms, cats will
occasionally suffer trauma to the cornea, and an erosion occurs on the
corneal surface. If the
erosion goes through the entire epithelium into the stroma, this erosion
is called a corneal ulcer. If
the ulcer goes deep into the stroma all the way down to Descemet’s
membrane, the condition is called a
descemetocele (pronounced “dess-a-met-a-seal”).
If the ulcer goes deeper, through Descemet’s membrane, the fluid
inside the eyeball flows out and the eye collapses.
The latin root for cornea is
kerat. When the cornea becomes
inflamed, the medical term for this is
keratitis.
If an ulcer is the cause of the inflammation, the condition is
called ulcerative keratitis.
The causes
“Corneal ulcers in cats are typically secondary to a
primary ocular disorder” says Dr. Pirie. “The most common of these would
include feline herpes virus.” Trauma is another
common cause of corneal ulcers in
cats. Although a scratch
from another cat is a frequent cause of corneal damage, there are more
innocent causes of trauma, such as rubbing the eye on the carpet, or an
unexpected interaction with a plant or tree branch.
Chemical burns, such as that caused by shampoo can cause corneal
ulcers if the shampoo gets into the eye.
Decreased tear production – a condition called
keratoconjunctivitis sicca or
“dry eye” – can predispose cats to corneal ulcers.
This condition is more common in dogs than cats.
“Conformation plays a significant role” adds Dr. Pirie. “Breeds
at risk, based on conformation alone, would include any of the
brachycephalic (flat-faced) breeds, for example Persians
and Himalayans. These breeds often have macroblepharon (wide
eyelid openings) and lagophthalmos (inability to completely close the
eyelids), both of which contribute to premature drying of the cornea and
disruption of the normal tear film.”
Eyelash and eyelid disorders can also lead
to corneal trauma. Eyelash disorders such as
distichia (extra eyelashes) or
ectopic cilia (misdirected
eyelashes) can lead to corneal trauma as the abnormal lashes rub against
the cornea. Fortunately,
eyelash disorders are rare in cats. Eyelid disorders are more common.
Entropion
is a condition in which the eyelid rolls inward, causing hair near the
eyelid margin to contact the cornea.
As the hair continually rubs against the cornea, an ulcer can
form.
Julie is an 18 year-old domestic shorthaired cat, one
of six cats owned by Mary Baysinger. In 2007, Julie was diagnosed with
intestinal lymphoma. Julie
responded well to chemotherapy and is doing well nearly 3 years after
her diagnosis. However,
Julie lost almost 3 pounds early in the course of her illness, causing
her eyes to become sunken.
When her eyes retracted into their sockets, she developed bilateral
entropion – both lower eyelids rolled inward, toward the eyeball.
Poor Julie developed corneal ulcers on both eyes.
At age 18 and with her cancer in remission, Ms. Baysinger was
reluctant to have Julie undergo general anesthesia and corrective
surgery. “Julie has done so well, despite her age and her intestinal
cancer. If something happened
to her while under anesthesia, I don’t think I’d forgive myself”,
said Ms. Baysinger. Fortunately,
she has been able to successfully manage Julie’s ulcers with frequent
applications of antibiotic eye ointments that form a protective barrier
between the hairs and the cornea. “These eye ointments have made her
much more comfortable. If she were younger and healthier, I’d do the
surgery right away. For now, she seems happy and comfortable”, she adds.
How can you tell?
Corneal ulcers are painful, and most cats
with corneal ulcers will exhibit some signs of discomfort, such as
tearing, rubbing the eye, and keeping the affected eye partly or
completely closed. To prove
that an ulcer is the cause of the discomfort, a
fluorescein stain is usually performed.
To perform this test, a drop of a fluorescent orange-colored
liquid is applied to the cornea.
If the cornea is intact, the dye washes smoothly over the corneal
surface. If an erosion or
ulcer is present, however, the dye will adhere to the exposed stroma and
can be easily detected using a black light.
Treatment
Treatment of corneal ulcers varies,
depending on the depth and severity of the ulcer.
Antibiotic drops or
ointment is applied to the cornea several times a day, to prevent an
infection from occurring, and
superficial ulcers typically heal in 3 to 5 days.
Irritation of the cornea often leads to spasm of a muscle inside
the eye called the ciliary muscle. When
this muscle spasms, it causes pain for the cat. Atropine drops or
ointment, applied to the affected eye, causes paralysis of the ciliary
muscle and reducing pain and discomfort.
Atropine will cause the pupil to dilate widely, making the
affected eye very sensitive to light and causing squinting, especially
in bright light. “If herpes is
suspected, antiviral medicine is warranted” notes Dr. Pirie. Ulcers
caused by feline herpesvirus-1 take longer to heal than superficial
ulcers caused by trauma. Cats who rub at their eye a lot may need to be
fitted with an Elizabethan collar to prevent further trauma.
Occasionally,
a superficial ulcer is slow to heal because the edges of the ulcer
contain dead cells. The dead
cells prevent a new layer of epithelium from growing across the ulcer
and filling in the defect. In these cases, removing the dead cells from
the edges of the ulcer with a sterile cotton swab may be necessary to
start the healing process. After a few (5 to 7) days of treatment, the
fluorescein stain test is performed again. If the cornea does not take
up any stain, it is considered to be healed.
Deep ulcers and descemetoceles that are at
risk for perforating require more aggressive therapy, such as applying a
soft contact lens to the affected cornea, or some type of surgical
technique designed to cover the ulcer or descemetocele .
A common surgical procedure is a
conjunctival graft.
In this procedure, a small piece of tissue adjacent to the cornea
is sutured over the ulcer. This allows blood vessels to deliver
nutrients, antibodies, and infection-fighting cells to the damaged
cornea, as well as providing mechanical support, in a similar fashion to
how a skin graft promotes healing of a severe burn.
Another common procedure is
partial tarsorrhaphy (pronounced tar-sor-a-fee),
in which the eyelids are temporarily sutured together. The partial
closure of the lids protects the cornea. Once the cornea has healed, the
tarsorrhaphy is reversed so that the cat can use the eye again. If
special techniques, diagnostic equipment or surgical skill is required,
your veterinarian may suggest referral to a board-certified veterinary
ophthalmologist.
Ulcerative keratitis is a potentially
vision-threatening condition.
Prompt diagnosis and aggressive treatment is required to preserve
vision and keep cats comfortable.
Sidebar – administering eye medications
Administering eye medication
“We often suggest owners try to have the animal learn a positive
association (i.e. receiving a treat) with the application of a
medication. However, this is often more effective with dogs”, says Dr.
Chris Pirie, a veterinary ophthalmologist at Tufts. “Regarding the
application of the medication (i.e. drops), we generally suggest the
owner rest their right hand (if right-handed) on top of the animals
head, while holding the bottle using their thumb and forefinger. The
palm and pinky finger are concurrently used to elevate the upper eyelid
to allow application of the drop from above.
During all of this, the left hand is used to stabilize the
animals head and the owner’s body (animal sitting on their lap) to
prevent the animal from backing up and getting away. It is important to
note that the hand applying the medication is resting on the animals
head, as if the animal moves, so does the dropper , which reduces the
risk of the dropper tip hitting the globe and causing further injury.”
Updated 1/27/12 |
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