Keratoconjunctivitis Sicca (Dry Eye)

Signs usually affect both eyes. Owners notice red eyes, squinting, excessive blinking, pawing at the eyes and a thick mucoid to mucopurulent discharge that is most evident in the morning. Acute cases have a very dry, painful cornea and conjunctiva. With chronicity the cornea develops superficial blood vessels, pigment and fibrosis, and ulcers may form. A dry eye on one side with a dry nostril on the same side indicates neurogenic disease. On the Schirmer tear test, 15 mm/min or more is normal, 11 to 14 early, 6 to 10 mild to moderate, and 5 or less severe.

Keratoconjunctivitis sicca, or dry eye, is a deficiency of the aqueous part of the tear film that leaves the cornea and conjunctiva dry and inflamed. In dogs the commonest form is immune-mediated: lymphocytes infiltrate and destroy the lacrimal gland and the gland of the third eyelid, a process with features shared with human Sjogren syndrome. Other causes include canine distemper, systemic sulfonamides, hereditary or congenitally small glands, trauma, loss of nerve supply and removal of the third-eyelid gland. Tear hyperosmolarity and inflammation trigger protease release, cytokines and inflammatory cell infiltration, which damage the epithelium and destabilise the tear film. The desiccated cornea becomes vascularised, pigmented and scarred, may ulcerate, and vision can be lost.

Keratoconjunctivitis sicca occurs primarily in dogs, rarely in cats, where chronic feline herpesvirus 1 infection can scar the conjunctiva and block tear flow, and is extremely rare in horses, where head trauma is a cause. A University of Illinois veterinary ophthalmologist describes dry eye as the most common condition she sees in dogs, typically at 4 to 6 years of age although any age can be affected. Flat-faced breeds such as shih tzus, pugs and Boston terriers are especially susceptible. Complete excision of a prolapsed third-eyelid gland predisposes 30 to 40% of dogs to the disease in later life; even successful surgical repositioning leaves about 20% at risk. It is described as a major cause of chronic ocular surface disease in dogs.

Qualitative tear film deficiency produces similar surface disease despite normal aqueous production; it is recognised by rapid tear film break-up time or vital-dye staining. Neurogenic keratoconjunctivitis sicca is usually unilateral with a dry ipsilateral nostril. In cats, chronic herpesvirus-associated conjunctival scarring should be considered. Measuring aqueous tear production with the Schirmer tear test confirms quantitative dry eye.

Eye: lacrimal gland, third-eyelid gland, tear film, cornea, conjunctiva; nerve supply to the lacrimal gland (neurogenic form)

Canine disease usually needs lifelong topical treatment; cyclosporine raises tear production in about 80% of dogs with Schirmer 2 mm/min or more. Early treatment gives a good chance of keeping vision; untreated eyes risk ulcers, rupture and blindness.