Keratoconjunctivitis sicca, or dry eye, is a condition in which there is an inadequate production of tears.

The disease involves a disorder of the lacrimal, or tear-producing, glands.  Signs of the disease can include a mucoid discharge, redness, squinting, progressive loss of vision, and development of corneal ulcers. 

Diagnosis by a veterinarian is accomplished using several procedures, and treatment is directed at attempting to restore normal tear production by eliminating any underlying causes, attempting to stimulate the animal's own tear production, and artificially replacing tears for as long as necessary. 

Introduction

• Keratoconjunctivitis sicca, or dry eye, is a disorder of tear production commonly seen in the dog, and less often in the cat.  The disease involves a disorder of the lacrimal, or tear-producing, glands.  Signs of the disease can include a mucoid discharge, redness, squinting, progressive loss of vision, and development of corneal ulcers.  Diagnosis by a veterinarian is accomplished using several procedures, and treatment is directed at attempting to restore normal tear production by eliminating any underlying causes, attempting to stimulate the animal's own tear production, and artificially replacing tears for as long as necessary.  Certain cases which do not respond to medical therapy may require a surgical procedure called a parotid duct transposition. 
• Keratoconjunctivitis sicca (KCS), or "dry eye", is a common condition seen in dogs of every age, although more often seen in older dogs, and virtually every breed.  Breeds that appear to be especially prone to the disease include the American Cocker Spaniel, Miniature Dachshund, English Bulldog, Lhasa Apso, Miniature Schnauzer, Chinese Pug, Chinese Shar Pei, Shih Tzu, West Highland White Terrier, and the Yorkshire Terrier.
• "Many cases of KCS in dogs appear to be immune-mediated"
  
• In cats, the condition is most commonly initiated by viral infections, most commonly Herpes virus infection.  The condition is a result of decreased tear production and changes in the composition of the tear film.
• The tear film is extremely important to the eye in several respects.  Its functions include 1) providing oxygen and nutrition to the cornea (the clear outer "windshield" of the eye), 2) removing waste products and debris, 3) providing lubrication to the eyelids, 4) maintaining a smooth optical surface by compensating for any small defects which may be present on the cornea, and 5) providing an antibacterial "screen" by virtue of its enzymes and antibody components.
• The tear film itself is made up of three layers.  The outermost layer is an oily layer, produced by the meibomian glands of the upper and lower eyelids.  This outer oil layer helps to slow evaporation of the middle, aqueous layer.  The aqueous layer is produced by the lacrimal gland, which lies just above the eye, and partly by the gland of the third eyelid.  The third, or inner layer, is a mucous layer produced primarily by "goblet cells" in the conjunctiva (the thin membrane which covers the white of the eye and inner eyelids).  It is usually a disease of the lacrimal gland, with a subsequent decrease in the middle aqueous layer, which results in keratoconjunctivitis sicca.

Clinical Signs

• The clinical signs of KCS will vary depending on the duration and severity of the disease.  These include;
  1. an increased amount of mucous discharge from the eye or eyes (the disease may affect only one eye or both).  This increase in mucous is thought to be nature's attempt to compensate for the loss of the watery layer
  2. increased squinting, often pawing at the eyes as if they are painful,
  3. redness and inflammation around the eyes,
  4. a dull, irregular appearance to the corneal surface,
  5. progressive loss of vision, due to corneal scarring and pigmentation
  6. development of corneal ulcers which can eventually perforate, resulting in collapse and loss of the eye. 
     

Causes of KCS

• There are a number of potential causes of KCS although often the exact cause is never determined.  Potential causes may fall into one of several categories.  These include:
  1. Congenital - i.e., existing at birth (but not necessarily inherited).  A congenital lack of lacrimal activity, may be seen in young Pugs, is often outgrown as the pup matures
  2. Inherited - There may be a hereditary component, especially in those susceptible breeds mentioned earlier
  3. Drug-induced - Transient KCS is sometimes noted following anesthesia and surgery; it is hard to discern whether this is due to the anesthetic itself, or possibly to circulatory disturbance during surgery
  4. Drug Toxicity - It has been found that certain drugs, most notably the sulfa-containing medications, can induce KCS, depending on the susceptibility of the animal
  5. Auto-immune - It is now felt that a large percentage of KCS cases are due to immune-mediated disease, whereby antibodies are produced which actually destroy the lacrimal gland
  6. Neurologic - The facial nerve stimulates the lacrimal gland to produce tears and damage to this nerve, either through trauma or disease, can result in KCS
  7. Infection - The canine distemper virus is one agent which can directly infect the lacrimal gland, resulting in the classic picture of the lack-lustre, mucousy-eyed sick little puppy.  In cats, viral infections, especially with Herpes virus is the most common cause of keratoconjunctivitis sicca
  8. Hormonal imbalance - Certain cases of KCS appear to be related to hypothyroidism, or an underactive thyroid gland.  These dogs often show other symptoms as well, such as weight gain, lethargy, and changes in the skin and haircoat.

Diagnosis

• Diagnosis and prompt treatment by a veterinarian are obviously quite important.  Often the diagnosis is relatively simple.  The signs are often suggestive, depending on the severity of the condition.  The condition must, however, be distinguished from allergic or primary infectious conjunctivitis. 
• Confirmation of the diagnosis is achieved utilizing the Schirmer Tear Test, which uses a small strip of sterile filter paper to measure tear flow.  The strip is placed under the lower eyelid for one minute.  Normal tear production during this time is generally considered to be between 15 and 25 mm.  In certain cases where the tear production appears to be borderline or questionable, other tests may be of value - these tests include specific staining procedures.  Cases which appear to have secondary bacterial infection may require examination of the bacteria under a microscope, or even a bacterial culture. 
  
• For those cases in which hypothyroidism is suspected to be a contributing factor, appropriate testing may be necessary.

Treatment

• The ultimate goal of therapy is the return of normal tear function.  To achieve this may involve, first of all, eliminating any possible underlying causes for the condition.  This might include discontinuing any medications which could be contributing to the problem, or placing the hypothyroid dog on a thyroid supplement.
• Secondly will be an attempt to stimulate normal tear production through the use of various medications.  This may include pilocarpine, a drug which stimulates the sympathetic nervous system to produce tears.  A new drug, which is still in the experimental stages, is cyclosporine A.  This immunosuppressive drug, which originally was used in heart transplant patients, is showing extremely promising results.  It appears to act by suppressing the autoimmune response which is thought to be involved in so many cases.
• Thirdly, until the goal of normal tear production is achieved, the animal is placed on an artificial tear preparation.  These often need to be instilled into the eye many times throughout the day in order to have any real benefit.  Other medications which may be used in conjunction with the above include antibiotics, to control secondary bacterial infections, and corticosteroids, to reduce inflammation, pigmentation, and scarring.  Corticosteroids, however, must always be used with caution, because of their tendency to inhibit the healing process.  They can be especially devastating in the face of corneal ulcers.  They must therefore only be used under the supervision of a veterinarian.           
• Any corneal erosions, or ulcers, which may have developed due to the KCS must be treated appropriately.  Depending on the severity, this may necessitate surgical intervention in order to save the eye.
• Although the majority of KCS cases seem to respond quite well to appropriate medical therapy, the occasional case does not.  For these patients, a surgical approach which produces artificial "Tears" may be indicated.  The procedure, a parotid duct transposition, involves rerouting the duct of the parotid salivary gland, so that the secretions, (saliva), enter at the lower eyelid rather than the oral cavity.  This is done by cutting the duct where is normally enters the mouth, tunneling beneath the facial skin and reattaching the duct at the lining of the eye.  The procedure, which was first performed in man in 1950, has an extremely high success rate.  The secretion becomes quite profuse, whenever the animal eats and produces saliva, which is preferably several times throughout the day.  Dogs which have had this procedure generally require very little medication afterward.
  

Conclusion

• Keratoconjunctivitis sicca, or the lack of normal tear production, can be a devastating disease.  The necessity for appropriate medical treatment for KCS cannot be overemphasized.  Early recognition and treatment of the condition can often prevent the more serious consequences from occurring.  It is therefore important that the pet owner or breeder be aware of what the symptoms are, especially in those breeds which are particularly susceptible to this condition.

References

• PET FOCUS 1990, 2(2): 21-23.