Dog Owner's Home Veterinary Handbook.pdf - Mr. Walnuts

glands. The inner layer is composed of mucus secreted by the conjunctiva.
Mucus is a wetting agent and holds the aqueous layer against the surface of
the eye.
Diseases of the tearing mechanism produce either a dry eye or a watery eye.
KERATOCONJUNCTIVITIS SICCA (DRY EYE)
THE EYES • 187
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 test involves 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