Causes of Dry Eye
Dry eye (keratoconjunctivitis sicca or “KCS”) can occur in any dog. Most cases of KCS are thought to be caused by an auto-immune or immune-mediated disorder, which means that the dog’s immune system is targeting, attacking and damaging the functional tissue of its own tear glands. How or why this happens in dogs with dry eye is not well-understood. The mechanisms of the immune system are extraordinarily complex, and research is constantly underway to try and unravel its mysteries.
Other things that reportedly have caused dry eye in dogs include:
- Administration of general anesthesia
- Application of topical anesthetics or other medications to the surface of the eye, especially sulfonamide drugs which tend to be toxic to the tear glands
- Trauma to the eye
- Surgical removal of the gland of the third eyelid, usually done to treat “cherry eye”
- Systemic diseases, such as hypothyroidism, canine distemper, Cushing’s disease (hyperadrenocorticism), Addison’s disease (hypoadrenocorticism), rheumatoid arthritis and diabetes mellitus
- Damage to the facial nerve (cranial nerve VII)
- Chronic conjunctivitis (persistent inflammation of the sensitive lining of the eyelids)
- Bacterial or viral eye infections
- Middle ear infections
- Hereditary breed predispositions
A branch of the facial nerve innervates the tear glands of dogs. On its way from the brain to the eye area, this nerve passes through the dog’s middle ear. Middle ear infections, called “otitis media,” can cause dry eye, because they can damage this important branch of the facial nerve. This can happen in one or both eyes, depending on whether the infection affects one or both of the dog’s ears. Usually, when the facial nerve is injured, some of the muscles of the face will also be affected. In rare cases, especially in small breeds, dogs may be born without tear glands. These animals will never be able to produce tears. This congenital defect (meaning one that is present at birth) will cause profound dry eye in affected animals.
Unfortunately, in many cases, the underlying cause of dry eye is never determined. In medical circles, this is referred to as “idiopathic keratoconjunctivitis sicca.” While this may sound fancy, “idiopathic” simply means that the cause of the condition has not yet been figured out by the scientific community. As you might imagine, there are lots of idiopathic illnesses in our canine companions.
Preventing Dry Eye
There is no present way to prevent dry eye that is caused by an immune-mediated abnormality. Taking steps to avoid or prevent the other potential causes of dry eye certainly will help, such as keeping the ear canals clean to prevent ear infections, avoiding trauma to the face so that the tear glands and facial nerve are not directly damaged, and managing any systemic diseases that the dog may develop. Many experts feel that affected animals probably should not be bred, because dry eye is thought to have a hereditary component.
Immune-mediated dry eye is a chronic condition that is not curable but usually can be managed with lifelong treatment. Good hygiene and a high-quality diet are always important to prevent or manage any type of eye problem in companion animals.