Abnormalities of the lacrimal system are common and generally divided into those that affect the secretory system (e.g., dry eye) or the excretory system (e.g., blocked tear duct). The more common conditions are discussed.

Keratoconjunctivitis sicca (KCS) - dry eye

Keratoconjunctivitis sicca (KCS), or dry eye, is common in the dog and relatively uncommon in other domestic animals. It represents a quantitative tear abnormality with deficiency of aqueous tears. Clinical signs can include blepharospasm, mucoid to mucopurulent ocular discharge, conjunctival hyperemia, and varying degrees of corneal vessels, pigment, and scarring. Corneal ulcers can develop concurrent with KCS. In chronic or refractory cases, blindness is possible from severe corneal scarring or pigment, or progressive corneal ulceration. KCS is most often a disease of middle-aged or older dogs that develops gradually in one or both eyes, but it can occur acutely and in young dogs. Commonly affected (or predisposed) breeds include the Cavalier King Charles Spaniel, West Highland white terrier, Yorkshire terrier, American Cocker Spaniel, English bulldog, Shih Tzu and Lhasa apso, but any breed can be affected.

Canine KCS is most often due to immune-mediated disease directed against the orbital and nictitans lacrimal glands. Histopathology reveals lymphocytic-plasmacytic glandular infiltrates, and this is the basis for immunomodulating treatments. Other causes of KCS include lacrimal gland aplasia or hypoplasia, infection (e.g., distemper virus), neurogenic (e.g., trigeminal neuropathy), neuroparalytic (e.g., facial neuropathy), trauma, iatrogenic (e.g., nictitans gland excision), and drug-induced. Drugs with potential to reduce tear production include topical or systemic atropine, oral sulfonamides (e.g., azulfidine or sulfadiazine), etogesic, and certain glaucoma drugs (e.g., timolol). Conditions such as diabetes mellitus, hypothyroidism, and dehydration, may reduce tear production. Topical and general anesthesia reduces tear production, and pets under general anesthesia should have an artificial tear ointment applied to the eyes. Application of an antibiotic ointment during anesthesia is not advised because of greater potential for corneal toxicity and allergic reactions (especially in cats).

Effective dry eye treatment depends on an accurate assessment of the cause, correction of complicating factors (e.g., lagophthalmos), and owner compliance. Initial treatment usually requires a combination of tear stimulants and lubricants, topical antibiotic, and possibly topical steroid. Treatments are tapered or modified depending on patient response.

Lacrimomimetics are agents that imitate natural tears or tear components and are synonymous with artificial tears. Normal tears are trilaminar with lipid, aqueous, and mucin components, and each layer has a specific function. Artificial tear preparations are formulated to simulate one or more components of the tear film, and most are available over-the-counter as solutions, gels, or ointments. A combination of products can be helpful in severely affected patients, in part, because of different ocular retention times. Artificial tear solutions more closely resemble aqueous tears and are helpful for loosening ocular mucous and debris, but they have the shortest retention times. Gels and ointments have substantially longer contact times, thus providing justification for combination treatment. Mucinomimetic agents are those that simulate tear mucin. They tend to be viscous and consist of polymers such as carboxymethylcellulose (Celluvisc), hydroxypropyl methylcellulose (GenTeal Gel), carbomer (Lubrithal, Optixcare), and sodium hyaluronate (I-Drop Vet Plus). Celluvisc and GenTeal gel (not to be confused with GenTeal drops) are available OTC at most pharmacies, whereas the other aforementioned mucinomimetic agents are exclusive veterinary products. Artificial tear ointments usually combine petrolatum and mineral oil, and common OTC brands are Puralube, Lacrilube, Refresh PM, Tears Naturale PM, etc. The ointments simulate tear lipid, have the longest retention times, and help to prevent evaporation of tears. Most artificial tear preparations are intended for repeated daily application and are well tolerated. However, pets can develop sensitivity to preservatives contained in these preparations, and preservative-free preparations (designated PF) are preferred in these instances. GenTeal gel has hydrogen peroxide as a preservative that is cleaved into water and oxygen immediately after application (the so-called disappearing preservative).

Lacrimostimulants (or lacrimogenics) are agents that increase or stimulate natural tear production. Pilocarpine has historically been used for this purpose. Two percent pilocarpine at a dosage of 1-2 drops/25 pounds body weight BID in the food is a good starting dose. The dosage is gradually increased until a response is observed or signs of toxicity develop. Toxic signs include salivation, bradycardia, vomiting, or diarrhea. Dilute topical pilocarpine solution may be used as an alternative to oral therapy. Concentrations of 0.125%-0.25% are recommended. Using a stock solution of 2%, these may be prepared as follows: 0.125% prepared by adding 1 ml of stock solution to 15 ml of artificial tears; 0.25% prepared by adding 2 ml stock solution to 14 ml artificial tears. Pilocarpine is mentioned here for completeness, but since cyclosporine was introduced, it has declined in popularity.