Dry Eye (Keratoconjunctivitis Sicca) in Dogs
Keratoconjunctivitis sicca is a disorder of the tear glands that results in insufficient aqueous tear production and a correspondingly dry cornea. The tear film contains less of the aqueous layer and more of the mucus layer. In consequence, the classic sign of dry eye is a thick, stringy, mucoid to mucopurulent discharge.Since this type of discharge can also be seen with conjunctivitis, dogs with dry eye may be mistakenly treated for chronic conjunctivitis for long periods with little or no improvement.
In a dog with dry eye, the bright, glistening sheen normally seen in the eye is replaced by a lackluster appearance in which the cornea is dry, dull, and opaque. Recurrent bouts of conjunctivitis are typical. Eventually the cornea becomes ulcerated or develops keratitis. Blindness may ensue.
Dry eye can have several causes. Immune-mediated diseases appear to play a major role. Other cases are idiopathic-that is, the cause is not known. Breeds predisposed to dry eye include Bulldogs, Cocker Spaniels, Lhasa Apsos, West Highland White Terriers, and others.
Some specific conditions that predispose a dog to dry eye include:
- Injury to the nerves that innervate the lacrimal glands. A branch of the facial nerve that activates the tear glands passes through the middle ear. Infections in the middle ear can damage this branch, affecting the tear glands as well as the muscles on that side of the face. In this case, the opposite eye is not affected.
- Injury to the tear glands themselves. Partial or complete destruction of tear glands can follow systemic diseases such as canine distemper, Addison's disease, and immune-mediated diseases such as rheumatoid arthritis. Bacterial blepharitis or conjunctivitis can destroy the tear glands or occlude the small ducts that carry the tears into the eye. A number of sulfonamide drugs are toxic to tear glands. Tear gland injuries may be partially reversible if the underlying cause is eliminated.
- Congenital absence of the tear glands is rare, but may occur in the smaller breeds.
- Removal of the third eyelid or the lacrimal gland attached to it.
The diagnosis of dry eye is made by measuring the volume of tears. The Schirmer tear testinvolves placing a commercial filter paper strip into the tear pool at the inner corner of the dog's eye and leaving it for one minute to see how much of the strip is wetted. Normally the strip should be wet to a distance of 20mm. In dogs with dry eye, the strip wets less than 10mm (often less than 5mm).
Treatment: For many years, the frequent application of artificial tears was the only treatment available for dry eye. But FDA approval of ophthalmic cyclosporin has revolutionized treatment and greatly improved results. Cyclosporin is an immunosuppressive drug that reverses immune-mediated destruction of the lacrimal glands.
Cyclosporin ointment is applied to the surface of the involved eye. The frequency of application must be determined by your veterinarian. The effect is not immediate. Artificial tears and topical antibiotics should be continued until the Schirmer tear test indicates that the volume of tears is adequate.
Treatment is life-long. Interrupting cyclosporin for as little as 24 hours causes symptoms to reappear in 90 percent of dogs. This can be reversed by resuming treatment.
When damage to the lacrimal glands leaves little or no functioning tissue, cyclosporin is not likely to be effective. Artificial tears (drops and ointments) prescribed by your veterinarian must then be instilled into the dog's eyes several times a day for life. Ointments are less expensive and do not need to be applied as frequently as drops. Saline drops should not be used because they aggravate the problem by washing away the lipid layer of the tear film.
A topical mucolytic agent containing acetylcystine may be recommended to reduce heavy mucus accumulation. Topical antibiotics are indicated whenever the mucus discharge becomes purulent. Topical corticosteroids may be prescribed by your veterinarian in some circumstances to decrease inflammation. In the presence of corneal ulceration, corticosteroids are contraindicated because they predispose a dog to rupture of the cornea.
Surgical treatment can be considered as a last resort when medical management fails. The operation involves transplanting the duct of the parotid salivary gland up into the corner of the eye. The saliva takes the place of the tears. The operation has several significant disadvantages. One is that the volume of tears may be more than the drainage system can handle. This can result in a watery eye and the accumulation of mineral deposits on the cornea and face.
This article is excerpted from “Dog Owner’s Home Veterinary Handbook” with permission from Wiley Publishing, Inc.
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