WHAT IS A DRY EYE OR
KERATOCONJUNCITIVITIS SICCA?
Dry eye is a complex disease in both animals and people. It is a disease of
the ocular surfaces due to deficient tear production, production of poor quality tears, or
poor tear distribution.
ARE SOME PETS MORE LIKELY TO HAVE DRY
EYES?
Dry eyes can affect any animal. Dogs are more commonly affected than cats.
Dogs with shortened snouts and prominent eyes (English Bulldogs, Lhasa Apso, Shih
Tzu, etc) are much more likely to develop dry eyes than other breeds. Nevertheless,
dry eye is probably the most common eye disease of dogs.
WHY ARE TEARS IMPORTANT?
Tears have several very important functions. Tears are necessary to smooth
out microscopic imperfections that are normally present on the surface of the eye.
This smooth surface provides a clear image to the inner portions of the eye so that vision
is not distorted or blurred.
The normal surface of the eye does not contain
any blood vessels in most common animal species. The tears take the place of blood
and provide many of the important nutrients to the cells on the surface of the eye.
Tear deficiency may lead to blood vessels growing on the surface of the eye to
provide additional nutrition to the eye. Other cells that cause pigment (dark
colored tissue) and scarring may accompany the blood vessels as they grow out on the eye.
Visual impairment or blindness can occur when the surface of the eye is covered
with this abnormal tissue growth.
Tears contain many immune factors which prevent
common bacteria, fungi and viruses from growing on the eye and causing infections.
Any eye that does not have a sufficient supply
of tears will usually be infected. Chronic ulcers, perforations and blindness can
result from uncontrolled infections in dry eye patients.
The eyelids rub against the
surface of the eye just like windshield wipers clear the windshield. Tears provide
lubrication so the eyelids can move smoothly across the surface of the eye. Dry eyes
can be very uncomfortable if the eyelids drag across a dry surface.
Tearing is an important method
for the eye to flush foreign debris from the eye. If the eye cannot produce enough
tears, dirt and debris can remain on the surface or under the eyelids. The tear
fluid will help to "flush" debris from the eye.
WHERE DO TEARS COME
FROM?
The tears originate from several types of glands around the eye. One large
gland is located just above the upper eyelid. Another gland is located at the base
of the third eyelid near the inside corner of the eye. Thousands of microscopic
glands are located in the conjunctival tissue (white of the eye). The margins of the
upper and lower eyelids are lined with glands that secrete on to the surface of the eye.
All of these glands secrete different ingredients that mix together to form the
tears.
WHAT CAN CAUSE A TEAR
DEFICIENCY?
Tear deficiencies are caused by many conditions; however, the vast majority of
causes remain unidentified due to the lack of knowledge about the tear glands and how they
are controlled. Injury to the head many damage the tear producing glands, or it can
damage the normal nerve control of the glands. Infections such as canine distemper
can affect the tear glands and destroy their ability to produce tears. Immune
mediated diseases and skin allergies are often accompanied by tear abnormalities because
the tear producing glands are simply modified skin glands. Hormonal disorders such
as hypothyroidism may interfere with normal tear productions. Drugs and toxins may
cause temporary or permanent tear deficiencies. These are just a few of the known
causes of dry eyes. A wide variety of other mechanisms may lead to tear
abnormalities.
HOW ARE DRY EYES
TREATED?
There are three general approaches to the treatment of dry eyes: (1) treat
the primary disease that is interfering with normal tear production, (2) stimulate the
tear glands to produce tears, and (3) supplement the eyes with artificial tears and
lubricants. Unfortunately, only a few patients will resume normal tear production by
treatment of their primary disease. Most patients will require additional therapy
for their dry eyes.
Two drugs, pilocarpine and cyclosporine, are known to increase tear production;
however, not all patients will respond to these drugs. Pilocarpine can be
administered as a topical and as a systemic (oral) medication to dry eye patients.
Pilocarpine is usually mixed with antibiotics, mucolytic agents and artificial tears for
topical use. This combination product must be administered at least four times each
day. Cyclosporine has the advantage that it only requires twice daily
administration. It cannot be mixed in the same bottle with antibiotics or other
medications, so other additional topical treatments are occasionally necessary.
Approximately one-half of dry eye patients will not be able to produce sufficient
tear volumes regardless of stimulation therapy with pilocarpine or cyclosporine.
These patients require additional topical therapies to control their eye diseases.
Additional medications include artificial tears, ocular lubricants, vitamin A,
corticosteroids, antibiotics and hormones.
Each patient is an individual that requires "customized" therapy
depending upon the ocular needs of that patient. For this reason, it is important
that the ophthalmologist reexamine a patient after treatments are begun. By
reexamining the eyes and measuring tear production, the ophthalmologist can determine if
progress is being made with the particular medical therapy. If not, then adjustments
in the therapy can be made.
Patients that cannot receive frequent medical therapy or do not respond to medical
therapy may be recommended to have a surgical procedure to improve their condition.
A duct from one of the salivary glands may be transferred from the mouth to beneath the
eyelid so saliva will flow across the eye. This "natural" artificial tear
will help keep the eye moistened. In another surgical treatment, the eyelids maybe
slightly shortened to reduce exposure of the eye to the environment. This will slow
evaporation of moisture from the surface of the eye.
Additional care for the dry eye patient includes daily cleansing of the eye and
eyelids. This accumulation of mucus and dry debris should be washed using plain
water or saline. A home made saline solution can be made by adding one teaspoon of
table salt to one pint of water. This dilute salt water will soften dry debris and
make it more easily removed from the eyelids.
CAN DRY EYES BE CURED?
A few dry eye patients will resume normal tear production, and medications may be
discontinued. Most patients affected with dry eyes will require the application of
medicine to the eyes each day. The frequency of the treatments will depend upon the
patient's ability to produce tears. Both the ophthalmologist and client working
together can help the patient maintain visual and comfortable eyes.
SIGNS OF DRY EYE
The clinical signs that may occur in an eye that lacks tear production are:
1. Accumulation of mucus in the corners of the eye
2. Redness/swelling
3. Dry, dull cornea
4. Squinting and rubbing the eye
5. Sealed appearance of the eye, in severe cases
6. Loss of corneal transparency
7. Possible presence of corneal ulcers
SCHIRMER TEAR TEST
A diagnosis of KCS is based on clinical signs and the results of the Schirmer tear
test - an easy method of measuring tear production. Your veterinarian will place a
small strip of specialized filter paper on the edge of your pet's lower eyelid.
After one minute, the amount of moisture absorbed by the filter paper is measured.
The measurement is then compared to predetermined standards to identify if tear production
is below normal. The test takes minutes and is totally painless.
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