Dry eye is a condition in which the dog does not produce enough tears. (Dry eye is exclusively a canine disease; we never see spontaneous dry eye in cats, although it can develop after certain traumatic or surgical episodes.) When this happens the eye presents with a dull, sticky, gooey appearance. Often there is a green, mucousy discharge on the eye, and the eye may be red, inflamed, and sometimes develop a dark cast to the cornea.
Technically, Dry Eye is called Kerato-Conjunctivitis Sicca, or KCS, and it leads to a whole host of secondary problems. Eye infections are very, very common in dry eye dogs, and these can sometimes be severe. Corneal ulcers are also very common due to the lack of lubrication in the eye. The dark cast to the cornea is due to corneal precipitates, or scarring, as the body reacts to the chronic corneal inflammation by trying to protect it.
The Normal Tear Film
Like humans, dogs and cats produce a tear film naturally at all times to help protect the eyes. This tear film serves three purposes:
1.) The tear film helps to lubricate the eye to aid with blinking and comfort of the eye;
2.) It serves in an immune system capacity to help prevent infections of the eye; and
3.) It washes away dirt and debris that would otherwise accumulate on the surface of the eye.
There are three components to the tear film in dogs: a mucous layer, a lipid – or fatty – layer, and a watery, or aqueous layer. The aqueous layer is the largest component in terms of tear volume. In the vast majority of dry eye cases the culprit is inadequate production of the aqueous layer. This leaves the mucous and lipid layers behind, which is what produces the gooey, sticky appearance.
In addition to having the mucous layer more prominent, dry eye dogs are much more prone to secondary eye infections, and that is what produces the green mucousy discharge. Dry eye most commonly affects both eyes, but it can present unilaterally in some cases.
Diagnosing Dry Eye
A presumptive diagnosis of dry eye can usually be made just by observation of the signs listed above. Testing for dry eye is a simple procedure called a Schirmer Tear Test (STT). To perform the STT a small piece of specially calibrated paper is inserted inside the lower lid of each eye, and left in for 60 seconds.
This paper stimulates the eye to produce tears, and that tear film then runs up the paper a given distance. For normal dogs, we like to see the tears run up 20mm or more. Dogs with an STT of less than 10mm are clearly dry eye, and those in between are a grey zone, and treated on a case by case basis.
Dry eye is much more common in small breed dogs than large breeds, and the brachycephalic (smashed nose) breeds are overrepresented. Pugs, shih tzus, Pekingese, Lhasa apsos, and all the bulldog breeds can all very commonly develop dry eye.
Treatment for Dry Eye
Up until the mid-1990’s there was no effective treatment for dry eye, and KCS dogs often went blind from the chronic irritation in the cornea. Starting in the late 90’s a newer drug was developed, called cyclosporine, which when applied religiously to the eye will produce a healthy tear film in about 80% of dry eye dogs.
Recently, a second drug called Tacrolimus has come on the market for use in dry eye. Between it and Cyclosporine we can now provide relief to well over 90% of dry eye dogs. However, no matter which drug is used, it must be continued for life to prevent the dog from reverting back to a dry eye condition. Additionally, it’s very common that antibiotic eye drops must be used periodically to clear any secondary eye infections.