Dogs, like humans, produce tears that form an essential layer — the tear film — which covers the transparent cornea at the front of the eye. The tear film provides lubrication for the outer surface of the cornea. However, lubrication is not its only function. The tear film helps nourish and protect the cornea, and last but not least, has important optical properties. The film is so important to vision that it is actually considered the anatomical outer layer of the cornea, which accounts for more than two thirds of the eye’s optical power. So when a dog has dry eye because the tear film is compromised, not only will the dog feel uncomfortable. Vision can become impaired, too. How?
The cornea, together with the lens, bends light to create an image that is focused on the retina. “If the corneal structure is altered because of a disease affecting the composition and distribution of the tear film, light transmission and refraction by the cornea will be affected, and visual impairment will occur,” says Stefano Pizzirani, PhD, DVM, a veterinary ophthalmologist at the Cummings School.
The issue of corneal transparency is central here. Corneal transparency is an essential requisite for normal vision. Diseases affecting the tear film may lead to increased corneal thickness, to ingrowth of new blood vessels, and to subsequent invasion by pigmented cells, all of which mar the cornea’s transparency and compromise vision.
Corneal dryness occurs when the amount of tears is lower than normal. And the most common condition associated with tear deficiency in dogs is indeed “dry eye,” known in medical circles as Kerato-Conjunctivitis Sicca (KCS). Dogs predisposed to KCS include brachycephalic breeds like Lhasa apsos, shih tzus, English bull dogs, Cavalier King Charles spaniels, and pugs. American Cocker spaniels, West Highland white terriers, and Boston terriers are also affected commonly. The reasons for this, Dr. Pizzirani says, are that such breeds have very prominent eyes that are more exposed to tear evaporation; their third eyelid, a structure present in most animals and not in humans, has a reduced function; and these breeds often have a decreased blinking rate and/or incomplete blinking.
As a matter of fact, dolichocephalic breeds with longer noses — like German shepherds and collies (“dolichos” means long in Greek and “brachus” short) — are less prone to the condition. However, any dog of any breed, including mixed breed dogs, may be affected with dry eye. The underlying causes are several, and while the most common of those is considered to be an autoimmune disease that affects the lacrimal glands that produce tears, congenital malformations and toxic and neurological causes are also possible and do not target specific breeds.
The “early stages of dry eye may not be clearly evident to owners,” Dr. Pizzirani says. They include “redness, a little bit of squinting, and some mucous discharge from the eye.” This is not to be confused with the little amount of grey, jelly-like ooze that many owners find themselves wiping from just beneath their dog’s eye. The mucoid formations encountered in the case of dry eye are tenaciously attached to the cornea, Dr. Pizzirani points out, and “not easy to wipe off.” When the disease is more advanced, the signs will become more evident.
Fortunately, owners who take their dog for regular wellness exams will be able to learn of the disease in its early stages because measurement of the amount of tear production is a standard procedure and part of the overall ocular examination.
The veterinarian or the veterinary ophthalmologist uses a test called the Schirmer Tear Test. It involves inserting the sterile tip of a small, specially designed paper strip into the inside of the lower lid, where it is held for one minute. The strip becomes soaked with tears, and various amounts of tears will wet the strip to different lengths. The normal value is 15 millimeters per minute or more. “Any number below that has to be considered as a possible indication of ‘instability’ of the tear film,” Dr. Pizzirani says. “Early stages of dry eye are usually below 15 and above 10 millimeters per minute, which may not be very evident to most owners. When clinical signs are more evident, wetting values are below 10 millimeters per minute, and in the most severe cases, values drop to below 5 millimeters per minute.”
For the early stages of dry eye, and even for some of the more advanced cases, there are very effective medications. As mentioned above, the most common cause of dry eye is an autoimmune condition, and the drugs that are most effective are immune-suppressants. The first medication used specifically to treat dry eye was cyclosporine. This medication was developed and tested by veterinary ophthalmologists, and it is now also used in human cases. This is an excellent example of veterinary research translating into an effective treatment for people. A newer medication for the treatment of dry eye in dogs is tacrolimus. Both medications are in the form of drops or ointments that need to be applied two to three times a day. The therapy is typically needed throughout the lifetime of a dog.
Artificial tears with high viscosity should also be used. “High-viscosity drops are important because they stay on the eye longer than other formulations,” Dr. Pizzirani says. “One of our concerns with any long-term treatment is owner compliance. Some owners find it difficult to apply drops frequently. The high-viscosity tears are retained longer on the ocular surfaces and allow a less frequent application, although this depends to some degree on the severity of the disease.”
While artificial tears lubricate the cornea, the immune suppressants actually help correct the underlying problem if an autoimmune component is present, restoring much of the function of the lacrimal glands. “Medications can’t restore 100 percent of the function of the glands, and unfortunately, this disease is incurable. The amount and severity of the damage to the gland vary, but no matter how effective medications are, some glandular scarring will almost always be present before treatment begins,” Dr. Pizzirani notes. “But if the diagnosis is made in the early stages, the response to therapy is usually very satisfactory. However, if medications are discontinued, or the frequency of their application is reduced, the condition tends to worsen.”
A surgical approach
In some cases, the medications are ineffective and do not produce the hoped-for result. This may occur either because the underlying cause is not an autoimmune condition, or because the lacrimal gland is completely scarred. In these cases, to relieve the discomfort and to prevent further vision problems caused by scarring of the cornea, a surgical approach may be necessary. The operation is called “transposition of the parotid duct.” The parotid salivary gland is located behind the jaw and has a duct that carries saliva into the mouth. “We move that duct from the mouth to the inside of the lower eyelid,” Dr. Pizzirani explains. The eye is then lubricated with saliva, which replaces the missing tears. There are several salivary glands that continue to produce saliva, so the loss of the parotid gland’s saliva does not affect eating or digestion, even if the surgery is performed for both eyes.
It’s a reasonable solution, but “there are common side effects,” the doctor says. One comes from the fact that the composition of saliva and tears is quite different. The eye may develop a deposition of minerals, mostly calcium, on the cornea. That problem is treated with artificial tears that are acidic in nature, and a therapeutic diet available only by prescription. The diet reduces the alkalinity of the body’s secretions, including saliva, which helps cut down on the mineral deposits.
Fortunately, Dr. Pizzirani says, the surgery has become necessary less frequently since the advent of the drugs. “We can control most of our cases with medical therapy,” he says. And there’s a fairly wide window of opportunity to help a dog through the problem, since it tends to develop slowly over time. “Usually dry eye is a slow, progressive disease,” Dr. Pizzirani says. “The thickening of the cornea and the neovascularization [growth of new blood vessels] are gradual changes.” There are occasional instances in which the onset is acute, or sudden, and these cases are commonly caused by neurological problems. Years ago distemper was one of the possible causes. But in recent decades, the incidence of that disease has been dramatically reduced by vaccination.
The slow progression of dry eye; its simple and straightforward diagnosis; and the fact that cases found early respond best to therapy underlie the importance of preventive medicine and regular, yearly examinations.