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.
Schirmer's Tear Test-1 (STT-1) is most often used to confirm the diagnosis and is performed prior to application of diagnostic
reagents to the eye. The STT-1 measures both reflex and basal tear production. Schirmer's Tear Test-2 is performed after
topical anesthetic has been applied to the eye, and thus, measures primarily basal tear production. The STT-2 is of limited
value for most veterinary patients. 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),
etodolac (Etogesic), and certain glaucoma drugs (e.g., timolol). Conditions such as diabetes mellitus, hypothyroidism, and
dehydration, may reduce tear production. Topical or general anesthesia will reduce tear production; 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.