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From Vetcentric.com

Something to Cry About:


April 13, 2006
Written by: Elizabeth L. DeLomba, DVM

We certainly take them for granted: those bright shiny eyes staring up at us in adulation. But it sure is sad when those same eyes are dull and coated with thick greenish mucus.

"Dry eye," or keratoconjunctivitis sicca (KCS), is actually one of the most common eye problems seen in veterinary medicine. It is also one of the more frustrating ones. Dry eye pains the dog, requires medication four to six times each day, and can cause gradual sight loss if owners forget to give the prescribed ointments or eye drops.

Simply put, KCS occurs when the eyes lack moisture due to a problem with tear production. Tears are actually more complex than most people realize. They are composed of three distinct layers. The outer layer is the lipid oily covering, which prevents evaporation of the underlying watery layer.

The middle layer is the aqueous layer. The aqueous is secreted by the lacrimal glands in the orbit and by the gland in the third eyelid. This layer actually makes up most of the tear volume and contains water, electrolytes, glucose, urea, polymers, glycoproteins, and tear proteins like globulins, albumin and lysozyme for the lubrication and hydration of the cornea.

The mucin layer is the deepest layer and is a hydrated glycoprotein made by the goblet cells in the conjunctiva. This mucin fills in any irregularities in the corneal surface to provide an optically smooth ocular surface. Additionally, bacteria and viruses get caught in the mucoproteins.

In addition to their protective role, tears function to carry oxygen and nutrients to the cornea. A healthy cornea is clear. Unlike other tissues, it lacks blood vessels to provide nutrients.

When the tear film is disrupted, the traumatized cornea may have pigment infiltration and blood vessel invasion. These structures obscure vision. Because of the lack of protection, the cornea is prone to injuries and ulcer formation. These can result in the formation of scar tissue, which further obscures vision.

KCS results from a deficiency in the aqueous tear film. It manifests as drying and inflammation of the cornea and conjunctiva and in ocular pain. Dogs typically present with conjunctivitis or pink eye, thick white to greenish mucoid ocular discharge, ocular discomfort, and, if it has been chronic, corneal vascularization, pigment deposition, and scarring. In most cases, the onset is gradual and the eyes appear red and inflamed.

There are several reasons that KCS occurs. The most common type of KCS is due to genetic predisposition. Breeds like American cocker spaniels, English bulldogs, West Highland white terriers, pugs, Yorkshire terriers, Pekinese, miniature schnauzers, and English springer spaniels, are most commonly affected.

Other causes of KCS include infection with the canine distemper virus, inflammation of the lacrimal glands, and a drug reaction -- especially after the use of sulfonamide medication. In addition, removal of the gland of the third eyelid, inflammation of the orbit, nerve damage, or even endocrine diseases such as Cushings disease, diabetes mellitus, or hypothyroidism, can cause "dry eye."

Your veterinarian may perform several tests to diagnose KCS. The most common is Schirmer's tear test (STT), which measures the ability of the eye to produce aqueous tears. The STT involves placing the end of a strip of special paper into eye and monitoring the amount of tears produced. In a normal dog, the paper quickly becomes wet with the tears produced; in those with severe KCS, no wetness may appear. There can be fluctuations in the STT values, but only weekly variations are considered significant. Dogs can also have deficiencies of the lipid and mucin portion of the tears and exhibit signs of KCS and yet have normal STT results.

Your veterinarian may also use stains to evaluate the health of the cornea. Fluorescein and Rose bengal stains are the most commonly used. They color devitalized cells or defects in the cornea.

Primary treatment of this disease is medical, and there are several different options. Most often, dogs with KCS are placed on tear supplementation. Artificial tears contain compounds that mimic both the aqueous and mucin components of tears. They moisten the eye and the ocular lubricants further protect the cornea. These medications can be effective, but their big drawback is that they must be applied four to six times a day.

Many dogs also are given antibiotic ointments or drops, which control the bacterial overgrowth that occurs with KCS due to the eye's lack of natural cleansing abilities.

Anti-inflammatory medications such as steroids are often used to minimize the conjunctivitis, to make the dog's eyes more comfortable, and help to reduce some of the corneal opacities associated with KCS. Because there are side effects to the long-term use of steroids, administration of these drugs must be monitored by your veterinarian. Steroids control the symptoms well, but they cannot be used if there is any ulceration of the cornea, a problem that commonly accompanies KCS.

One of the biggest breakthroughs in treatment of KCS is the use of cyclosporine. This medication is thought to modulate the immune system, but its method of action is poorly understood. A small amount of ointment is placed in the eyes twice daily. Treatment for several weeks is needed before results are observed, but the drug is reported to be about 75 percent effective. With this treatment, though, lifelong and consistent therapy is necessary. Use of cyclosporin can even help to reverse some of the pigment changes common in KCS eyes.

Other medical treatments include the use of oral pilocarpine, which stimulates tear production, and mucolytics to control the copious overproduction of mucus. Mucolytics decrease the amount of exudation and facilitate cleaning of the excess mucus that is not washed out the lacrimal system.

For those dogs that do not respond to medical treatment, there are surgical options, including a procedure called a parotid duct transposition. With this surgery, the duct from the salivary gland is actually relocated so that the saliva secreted bathes the eye. Because saliva and tears are actually similar fluids, this procedure can be very effective. One interesting side effect to this procedure is that when your dog gets hungry, his eyes water.

Another procedure is a permanent partial tarsorrhapy. With this method, by partially suturing the eyelids closed, exposure to evaporation and the elements decreases.

In most dogs, KCS is a controllable condition. With good ocular hygiene that keeps the eyes free of debris, appropriate medication, and help from a motivated owner, who knows? Maybe the stars will return to your dog's eyes.
 
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