Keratoconjunctivitis sicca (KCS) results from inadequate aqueous tears. To understand the “dry eye” syndrome, it is necessary to understand the normal health of the cornea as it relates to the tear film. The cornea is the clear, outer windshield of the eye. Like all living tissues, it requires oxygen and nutrients to remain healthy. These are not supplied through blood vessels as in other parts of the body, but through the fluid inside the eye and the three-layered tear film on the surface. The outermost layer is an oily layer supplied by glands in the eyelids. The middle layer is the watery (aqueous) layer produced by the lacrimal glands that are located in the upper eyelid and in the third eyelid. This is the layer affected in KCS. The innermost layer of the tear film that is in direct contact with the cornea is a mucous layer produced by glands located in the conjunctival tissue around the eye. Keratoconjunctivitis sicca is due to dysfunction in the corneal tear film, and it results in patchy, dry areas across the corneal surface. In more advanced cases, widespread and severe corneal drying can occur and lead to permanent damage. The dried cornea, deprived of oxygen and nutrients through the tear film, rapidly undergoes destructive changes. Additionally, the mucous layer tries to make up for the lack of watery tears and an accumulation of mucus can be present on the surface of the cornea. Mucus can provide an excellent home for bacteria, which can lead to ocular infections. Corneal drying can lead to significant patient discomfort, corneal vascularization, corneal pigmentation, corneal scarring, corneal mineralization, corneal ulcers and even corneal rupture.
Diagnosis is based on history, clinical signs and a number of diagnostic procedures. The most important test is the Schirmer Tear Testing (STT), which measures watery tear production. This involves placing a soft strip of paper in your pet’s lower eyelid for 1 minute. Also, fluorescein stain (a bright green stain) is used to define breaks in the corneal surface and to assess the rate of tear film breakup. Sometimes Rose Bengal Stain (a reddish pink stain) may be used to evaluate the health of the outer layer of the cornea called the epithelium. Cytology and/or culture may also be recommended to quantify the health of the conjunctival cells and look for infectious organisms.
A number of causes have been documented to play a role in the development of KCS. These include diabetes, hypothyroidism, infections of the eye and lacrimal glands (e.g.: canine distemper virus), neurologic conditions, birth defects, and immune-mediated disease that negatively impacts the tear secreting glands. Immune-mediated KCS is the most common form in the canine patient. The forms of KCS associated with systemic disease, such as low thyroid hormone, require treatment of the underlying condition, in addition to eye-specific treatments, and may require some routine blood work for diagnosis.
Another cause of keratoconjunctivitis sicca is a toxic effect produced by some sulfa-containing antibiotics and non-steroidal anti-inﬂammatory drugs. Because some of these drugs may be necessary for the treatment of other diseases, it may not be possible to change the patient’s medications. It is very important that you tell your veterinary ophthalmologist about all medications and supplements that your pet currently takes – as well as medications take in the few months before the problem began.
There are several areas to address when treating keratoconjunctivitis sicca. A primary goal is to reduce the overgrowth of bacteria commonly associated with the dry eye syndrome. Topical anti-inﬂammatories are indicated when corneal staining shows no ulceration. This medication helps reduce inﬂammation of the conjunctival and corneal surfaces and reduces the long-term scarring effects. Corticosteroids cannot be used in the face of ulcers because they may decrease healing and increase the spread of infection. Artificial tear preparations are often indicated to supplement the deficient tear film and make your pet more comfortable. In addition to drops or gel preparations, artificial tear ointments are sometimes used to provide prolonged corneal contact overnight and during times when the patient cannot be treated frequently.
The most important medications for immune-mediated KCS are the immunomodulating agents such as cyclosporine (Optimmune) and tacrolimus. These medications reduce inflammation and stimulate a patients’ own lacrimal glands to produce tears. This is important because artificial tears do not contain the nutrients and enzymes that are present in natural tears. The good news is that these medications work in about 80 to 90% of dogs with maximal tear production occurring in 4 to 6 weeks. The bad news is that these medications must be given lifelong to control the dry eye symptoms. We will work to minimize the frequency of medication but many patients require daily treatment. Life- long topical medications seldom give rise to complications, which occur in less than 5% of these cases. Complications of cyclosporine and tacrolimus are typically limited to inﬂammation of the pink tissue (conjunctivitis) as a result of a local allergic reaction to the medication or the preservatives in the formulation. Some newer studies have tried to draw a link between the use of these medications and some cancers. It is unclear, however, whether the medication, the underlying dry eye disease, or some other cause in the cases where neoplastic changes have occurred. Most often the reports are in patients (usually humans) that take the medications by mouth – particularly after organ transplants. Regardless the cause, this is an extremely rare side effect with topical use of these medications. Please do not hesitate to discuss this with the veterinarian, if you have any other concerns.
Your awareness of your pet’s symptoms and compliance with recommendations for medication and recheck examinations will greatly help control the disease and improve your pet’s outcome.
Most patients with keratoconjunctivitis sicca do well if medications are administered on a timely basis. In cases where medicines cannot be given regularly or where medications are not effective, surgical techniques, such as a parotid duct transposition or others may be recommended. Any surgical procedures carry a risk of complication, including potential anesthetic risks, breakdown of the tissue or suture (wound dehiscence), obstruction or deterioration of the salivary gland or duct, salivary stone formation, facial nerve damage, infections at the surgical site, severe periocular wetting, corneal ulcerations, corneal scarring or vascularization, and others. Some of these complications can lead to blindness.