The tear film is made up of three ‘layers’, consisting of:

1. Lipid – reduces evaporation of tears and improves tear film stability
   
2. Aqueous – lubricates cornea, provides nutrition and protection
   
3. Mucin – improves the tear film stability

Tear film inadequacies can relate to insufficient quantity of tears being produced, known as ‘quantitative’ or insufficient quality of tear film, known as ‘qualitative’. 

‘Dry Eye’, Keratoconjunctivitis Sicca or KCS in the canine patient can be acute or chronic, and is most commonly due to a lack (quantitative) of aqueous production, which may be the consequence of trauma, Facial or Trigeminal nerve damage, or metabolic diseases, among others. In the majority of cases this inadequacy of the aqueous is caused by an immune-mediated condition, where destruction of the main glands which produce the aqueous (known as the Lacrimal Glands) are attacked by the body’s own immune system. The condition is progressive; over time, destruction of the glands is often complete and irreversible. For this reason, diagnosis and treatment is urgent. This condition will almost invariably affect both eyes. 

The result of the inadequate tear film is generally to cause inflammation of the eye surface and the eyelids, often with recurrent episodes of conjunctivitis. This can be painful, or uncomfortable. Any dog with recurrent conjunctivitis should be tested for KCS by performing a Schirmer Tear Test. Sticky green or yellow eye discharges that are difficult to bathe away are commonly seen. Corneal ulceration and pigment changes on the corneal surface result from chronic damage to the cornea resulting from a lack of tears, and can result in significant visual deficits. Certain breeds are predisposed to this condition, the West Highland White Terrier, English Bulldog, Shih Tsu and Cavalier King Charles Spaniel being of particular note.

A number of treatment options are available. All require acceptance of the life-long nature of the need for treatment. Lubrication of your dog’s eyes will assist in reducing the damage caused by inadequacy of natural tears, however, due to poorer persistence of these synthetic tears and absence of the nutrient provided by the aqueous, stimulating remaining tissue to produce natural tears is our goal. Caught early in the course of disease, the majority of patients will respond to twice daily application of cyclosporine in ointment form, known as ‘Optimmune’. If unresponsive, use of a human medicine ‘Protopic’ which contains tacrolimus, can be effective. It is important to note that this is an off-licence use of this medication, i.e. not specifically tested for safety of use in the species in question – for more information on this, see https://www.rcvs.org.uk/setting-standards/advice-and-guidance/code-of-professional-conduct-for-veterinary-surgeons/supporting-guidance/veterinary-medicines/ Section 4.14-20.

For patients that do not respond to either treatment, a surgical technique can be performed often with good results, known as ‘Parotid Duct Transposition’. This involves moving the duct of the parotid salivary gland so that saliva produced is released from the duct orifice into the area of the conjunctival sac, with the aim of keeping the cornea moist. Production of saliva is stimulated by feeding, so regular feeding of small meals assists in achieving this. A general anaesthetic is required for this to be done. Potential complications of this surgery include an excessively wet eye/face, and crystal formation due to the slightly differing compositions of the natural tear film and of saliva. Some cases will still need application of eye drops.

Other causes of dry eye exist, as previously noted, and should be discussed with your vet in the absence of a response to treatments. A full ophthalmic examination is always advised in these cases.