What is the tear film, and why is it important?

Keratoconjunctivitis sicca (KCS), or dry eye, is an ocular condition commonly diagnosed in dogs. It is less common in other species. Keratoconjunctivitis sicca results most often from an inadequate quantity of tears or a deficient quality of tears. Tears are produced by the lacrimal, or tear gland, and the gland of the third eyelid. Tears are needed to provide lubrication and nutrition to the cornea, as well as remove debris and/or infectious agents from the eye.

The most common cause of KCS in the dog is immune mediated inflammation of the tear glands. Other causes of KCS include but are not limited to:

• Congenital disease, such as small or absent lacrimal glands
• Infectious disease, such as canine distemper virus
• Neurologic deficiency, such as loss of nerve innervation to the eye
• Endocrine disease, including hypothyroidism, Cushing’s disease, and diabetes mellitus
• Prolapsed gland of the third eyelid (“cherry eye”) and/or removal of the gland of the third eyelid
• Radiation therapy near the eye
• Drug toxicity, including use of sulfa derivative medications

Certain breeds are more likely to develop KCS, suggesting there is a genetic basis. Commonly affected breeds include the Cavalier King Charles Spaniel, English Bulldog, Lhasa Apso, Pug, Shih Tzu, and West Highland White Terrier. However, regardless of breed, any dog can be affected with KCS.

The most common clinical signs of KCS include painful, red eyes, with thick mucoid discharge. Dry eye most commonly occurs in both eyes, and some animals may develop secondary corneal ulceration or bacterial conjunctivitis. Corneal ulceration with secondary infection can lead to loss of an eye. Chronic, uncontrolled dry eye may also lead to corneal pigmentation, vascularization, and scarring, which may lead to visual impairment.

Several important diagnostic tests are involved in diagnosis of KCS. The most important test involves looking at the corneal surface cells and tear film with an instrument called a biomicroscope. Paper test strips called Schirmer Tear Test strips may also be utilized to quantify tear production from both eyes. If your pet has a normal tear quantity but has clinical signs of KCS, your veterinary ophthalmologist may also perform a tear film break-up time test to support a diagnosis of a qualitative tear film deficiency.

Treatment of KCS includes daily lifelong administration of topical tear stimulant medication. These medications reduce inflammation, as well as stimulate natural tear production. They are typically administered two to three times daily and are safe to give long term. Specific dosing instructions will be made by your veterinary ophthalmologist. Concurrent use of tear replacement lubricating drops may help improve comfort for many pets. 

The majority of dogs respond favorably to drops, but in severe cases of KCS that are poorly responsive to medical management, a surgical procedure called a parotid duct transposition may be recommended. The procedure involves redirecting the parotid salivary duct from the mouth to the eye in order to provide salivary secretions to the cornea. Parotid duct transposition often results in a more comfortable patient with less chance of corneal ulceration.

Early diagnosis with lifelong treatment, as well as routine follow-up examinations is of paramount importance for patients with KCS.  In the majority of dogs, prognosis can be excellent for long-term comfort and maintenance of vision.