Keratoconjunctivitis sica

There are three layers to the tear film that coats the eye. The layer closest to the eye is a mucus film that is responsible for binding the middle layer of the tear film. The middle layer is an aqueous (water based) tear fluid that is produced by the lacrimal gland (under the eyelid at the upper outside margin or the eye) and the nictitans gland (under the third eyelid). Finally the outer layer is an oil-based layer to prevent the aqueous layer from evaporating. Loss of the aqueous layer leads to the condition known as keratoconjunctivitis sicca, more commonly known as dry eye. Dry eye is rare in cats and very common in dogs, especially Miniature Schnauzers, West Highland White Terriers and Bull Terriers.
Signs of dry eye include sore eyes, chemosis (fluid accumulation in the eyelids), conjunctival hyperaemia (reddening), blepharospasm (spasm of eyelid) and mucoid to mucopurulent discharge (extra mucus +/- infection). The discharge is a result of extra production of the mucus and oil layers in an attempt to compensate for the loss of the aqueous layer. Chronic KCS can lead to ulceration and scarring of the cornea. The cornea becomes cloudy and blood vessels may be visible growing across the surface.

The most common cause of dry eye is immune mediated adenitis (the immune system attacks its own glands). Immune mediated conditions often have other associated signs such as atopy (hair loss), which can provide a clue to the cause. Dry eye has also been associated with hypothyroidism, canine distemper virus, and drugs that contain sulfa-compounds. Dry eye may also result if the nictitans gland (tear gland on third eyelid) is removed, as may be the case in the treatment of cherry eye (ref: Cherry eye). Cats may develop keratoconjunctivitis sicca due to chronic (long-term) herpes or chlamydial conjunctivitis.

Diagnosis of dry eye is relatively easy, but identifying the cause can be much more difficult. A Schirmer Tear test is performed in order to measure the volume of tears being produced, a decreased value confirming dry eye. Fluorescein is a dye used to identify any ulcers on the corneal surface that can arise secondary to the eye drying out.

There are several treatments available, but the choice will vary between individual circumstances. Artificial tears are available, but they need to be applied 6 times a day to be effective, which is impracticable for many people. In addition to the tears an eye lubricant should be applied. Cyclosporine drops are the current treatment of choice, acting by suppressing the immune system and thereby preventing it from further attacks upon the tear producing glands. These can usually be reduced to daily application or less for long term control. Antibiotics are used if an infection is present and anti-inflammatory drugs may be employed to reduce inflammation.