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.
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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.