Tears are made up of three main components. The outer layer of tears is secreted by glands in the eyelid margins. This layer is made up of a fatty material that limits evaporation of the tears. The middle layer of the tears is the watery secretion that comprises the majority of the tears. The watery secretion of the tears (aqueous) is produced by two glands surrounding the eye. The lacrimal gland, located above the eye, produces 50%-70% of the aqueous secretion. The remaining 30%-50% is produced by the gland of the third eyelid, located beneath the eye. The inner layer of the tears is mucus, which adheres the aqueous and lipid portion of the tears to the corneal surface. 

Tear production may be measured by performing a Schirmer tear test. This involves placing a specially prepared test strip next to the eye for one minute. The tears will wick onto the paper, and the length of strip showing absorption is measured in millimeters. Normal tear production is greater than 15 mm of moisture in one minute. If tearing is less than 5 mm per minute, this is diagnostic for dry eye. If tearing is 5-15 millimeters per minute, this is a suspicious area indicating early or subclinical dry eye.

Dry eye or keratoconjunctivitis sicca (KCS), is caused by an inadequate secretion of tears, specifically the aqueous portion of the tears. Problems you may note include yellow-green mucus from the eye that tends to have a thick and ropy consistency, redness to the white part of the eye, squinting, or cloudiness of the cornea. Bacterial infections due to inadequate flushing of the surface of the eye, corneal ulcers (scratch on the eye), and corneal scarring are complications associated with dry eye. 

Potential causes for dry eye include hereditary, immune-mediated, drug-induced, surgically induced, or chronic allergic eyelid disease. Other less common causes include the distemper virus, radiation therapy around the eye, aging, nerve palsy or trauma. Some breeds that are predisposed to dry eye include English bulldogs, Lhasa apso, West Highland terrier, cocker spaniels, schnauzers, and Samoyeds. Immune-mediated disease is one of the most common causes for dry eye in the dog. The animal's own immune system attacks the glands producing the tears. The resultant glandular inflammation causes a reduction in tear production. Drugs implicated in dry eye include some antibiotics (specifically sulfa drugs), Etogesic, and atropine. Surgically induced dry eye may be caused by the removal of the gland of the third eyelid. This gland may protrude from the inner corner of the eye, resulting in a condition known as cherry eye. In the past, removal of the gland was considered the treatment of choice. It is now documented that removal of the gland will significantly increase the risk of an animal developing dry eye in the future, especially in breeds that are predisposed. Surgical replacement of the gland is the best option to limit the development of dry eye. Chronic allergies may result in inflammation of the eyelids and tissue around the eye, lowering tear production.

If a correctable cause for the dry eye can be identified, i.e., drug toxicity, immediate action should be taken to remove the offending agent. Topical eye medication is the mainstay of therapy for dogs with dry eye. Cyclosporin has been used topically. The response to this drug has revolutionized the approach to managing dry eye cases. Cyclosporin will stimulate the lacrimal glands, increasing tear production in approximately 85% of cases. The response is better in dogs that have some remaining tear production. Dogs with tear production of less than 2 mm per minute seem to respond poorly. The response to Cyclosporin may take several weeks. Therefore, during the interim, frequent supplementation with artificial tears and topical antibiotics may be necessary for comfort, and to limit corneal scarring and control infection. If the eyes respond well to Cyclosporin, as evidenced by an increase in tear production, continued use of it is necessary to maintain tear production. If Cyclosporin therapy is discontinued, tear values will revert to the previous values. If there is no response to treatment within 8 weeks, there are few other proven medical avenues that have a significant effect on tear production. If the dry eye is threatening the vision or the eyes are painful, the only other alternative is surgery. The procedure involves transplanting one of the salivary ducts to empty on the eye. Although saliva is not exactly like tears, it is a good substitute. The surgical procedure performed is not a panacea, but will keep the eyes comfortable and limit the chances for blindness. 

Dry eye is a potentially vision-threatening disease that, if identified and managed appropriately, generally can be controlled with medication. If you note any of the above-mentioned clinical signs in your pet, evaluation by your veterinarian should be considered for the health of your pet's eyes.

-Susan M. Carastro, D.V.M., M.S., Diplomate, American College of Veterinary Ophthalmologist, practices at the following locations: Animal Eye Specialty Clinic, 372 S. Power Line Rd., Deerfield Beach, FL 3344,2 tel: (954) 421-5099; 3421 Forest Hill Blvd, W. Palm Beach, FL 33406, tel: (561) 967-5966; and at 2399 S. Kanner Hwy, Stuart, FL 34994, tel: (561) 220-8485.