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 in cats with conjunctivitis, cats with dry eye may be mistakenly treated for chronic conjunctivitis for long periods with little or no improvement.
Herpesvirus is considered to be a primary cause of dry eye in cats. Luckily, this disease syndrome is less common in cats than it is in dogs. A congenital form of this disease occurs in Burmese cats.
In a cat 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. Some specific conditions that predispose a cat to dry eye include the following:
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 the tear glands can follow systemic diseases. For example, feline herpes may block the glands. Bacterial blepharitis or conjunctivitis can destroy the tear glands or block 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.
The diagnosis of dry eye is made by measuring the volume of tears. The Schirmer tear test involves placing a commercial filter paper strip into the tear pool at the inner corner of the cat’ 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 length of 12 to 22 mm.