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.
The Schirmer tear test (STT) is used to confirm the diagnosis. The Schirmer tear test-1 is most often used and is performed
prior to application of diagnostic reagents to the eye. The STT-1 measures both reflex and basal tear production. The Schirmer
tear test-2 is performed after topical anesthetic has been applied to the eye, and thus, measures primarily basal tear production.
The STT-2 offers little benefit for veterinary use. The normal STT-1 value is >15 mm of wetting after one minute for both
dogs and cats. The significance of STT-1 values between 10-14 mm/minute should be interpreted with clinical signs, but values
<10 mm/minute usually denote clinical KCS. Cats are an exception to this latter point. Low STT-1 values are common in cats
with non-inflamed and otherwise normal eyes. Therefore, the difference in STT-1 values between the eyes may be more important
for interpretation in cats than the absolute value. The STT-1 should be performed on any pet with an irritated eye, ocular
discharge, or corneal disease of undetermined cause.
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