The purpose of tears
Tears primary role is to provide lubrication to the eye but they also contain anti-bacterial proteins, salts, sugars, and even oxygen to nourish the eye. Tears flush away irritants and infectious agents that are constantly getting in our eyes. Since the outer portions of the eye do not have a blood supply, the tears must bring sugars and oxygen and must remove metabolic waste.
Tears consist mostly of water, but also of oil secreted by the eyelid glands, and mucus. They are secreted by two lacrimal glands in dogs and cats: one just above the eye and another in the third eyelid (or so-called nictitating membrane).
Without tears, eyes become irritated, the conjunctival tissues around the eyes get red, the cornea itself in time will turn brown to protect the eye, and a gooey, yellow discharge predominates. Blindness can result.
Dry eye is otherwise known as Keratoconjunctivitis sicca (or KCS) which means inflamed, dry cornea and conjunctiva. This occurs when there is a deficiency in the water portion of the tear film, which normally accounts for 95% of the tear volume. Without the water the film becomes primarily oil and mucous which cause the classic presentation of yellow gooey discharge.
Why do Eyes Become this Dry?
The most common cause of Dry eye appears to be immune-mediated destruction of the tear-producing gland tissue. We do not know what causes this type of inflammatory reaction but certain breeds are predisposed: the American Cocker Spaniel, the Miniature Schnauzer, and the West Highland white terrier.
Other causes of Dry Eye:
- Distemper virus (dogs)
- Herpesvirus (cats)
- Congenital (e.g. some lines of Yorkshire terriers)
- Exposure to sulfa-containing antibiotics.
- General anaesthetic
- Third eyelid gland removal
- Trauma to the head
How we make the Dry eye diagnosis
Tear production is measured in consultation using a Schirmer Tear test. This is performed conscious with a strip of specific paper that is put just inside the lower eyelid in the outer corner of the eye and left for 60 seconds. The moisture of the eye will wet the paper. At the end of the 60-second period, the length of the moistened area on the paper is measured. A length of 15mm or more is normal. A length 11 to 14mm is a borderline result. A height of less than 10mm is dry. A height less than 5mm is severely dry.
Cyclosporine suppresses the immune destruction that is the most common cause of KCS, and tear production is restored. The success of this treatment plus its convenient dosing interval of once or twice a day has made this medication the primary treatment for dry eye.
After beginning cyclosporine eye drops or ointment, a recheck in three weeks or so is a good idea to check for improvement. If the Schirmer tear test is still showing poor results, the medication can be given three times a day; if excellent results are seen, the medication can be dropped to once a day.
Dogs with Schirmer tear tests as low as 2mm still have an 80% chance of responding to cyclosporine. This medication has been a miraculous breakthrough in the treatment of dry eye. Unfortunately, it is relatively expensive as eye medication goes but after messing around with less effective treatments requiring more frequent administration for less predictable results, cyclosporine is probably worth it.
Occasionally patients simply do not show a good response to cyclosporine ointment but will respond when the concentration is increased. Higher concentration products can easily be formulated by compounding pharmacies.
Tacrolimus is another medication able to locally suppress immunity. This product has recently gained popularity in human medication as a topical anti-inflammatory treatment that is cortisone-free. It does not come in a formulation appropriate for eyes but can be made into one by a compounding pharmacy. It is used in a manner like cyclosporine and is generally of similar cost.
Pilocarpine is what is called a cholinergic drug, which means it works on the autonomic nervous system (the part that controls automatic functions such as gland secretion). This medication can be given in the eye or orally to stimulate tear production. To use this medication orally, the eye drops are given at an increasing dose until side effects are seen (diarrhea, drooling, vomiting or drop in heart rate). At that point the dose is reduced and continued indefinitely, usually twice a day. Alternatively the drops can be given in the eyes. Recent studies have shown that pilocarpine does not increase tear production in normal dogs so there is some question over how well this method works.
Artificial tears can be purchased in most drug stores. These can be combined with other therapies and are certainly very soothing. The problem is that they are typically recommended for use 4 to 6 times a day.
Antibiotic products are often needed, especially when starting treatment for KCS because secondary infections are common when there are inadequate tears to wash infectious agents away. These products do not increase tear production but may be important, especially early in therapy.
Mucomyst® Eye Drops are made from a respiratory product used to dissolve thick mucus. In an eye formula, Mucomyst (active ingredient acetylcysteine) helps remove the thick eye discharge that accompanies dry eye.
Your vet will help determine the best eye treatment for your pet based on their eye requirements and within your financial means.
There is a surgical option to treat dry eye which is performed by veterinary ophthalmologists. This is called the parotid duct transposition. The parotid duct is the salivary gland on either side of the facial cheek. It produces saliva that is carried to the mouth via a long duct. This duct can be carefully dissected out and moved so as to deliver saliva over the eye. Saliva actually makes a reasonable substitute for tears though in time some mineral deposits will form on the eye surface and eye drops may be needed to control this. The dog’s eyes will water when he is fed, and facial wetting may be objectionable.