Overview

• Deficiency of the aqueous layer of the precorneal tear film.
• Causes corneal/conjunctival drying and resultant surface inflammation.

Signalment

• Common in dog; rare in cat.
• Predisposed dog breeds-many brachycephalic and spaniel breeds, miniature schnauzers, poodles, bloodhounds, Samoyeds, West Highland white terriers, and Yorkshire terriers.
• Inheritance-undefined.
• Age of onset-variable and depends on the inciting cause.

Signs

• Conjunctival hyperemia.
• Mucoid to mucopurulent ocular discharge-intermittent to persistent depending on severity.
• Blepharospasm.
• Corneal changes-dryness, superficial vascularization, pigmentation, fibrosis, ulceration.
• Blepharitis due to ocular exudates.
• Severe disease-impaired vision or blindness.
• Cats are less symptomatic than dogs.

Causes & Risk Factors

• Immune-mediated/idiopathic-most common and possibly associated with other immune-mediated diseases (e.g., atopy).
• Infectious-canine distemper virus; chronic blepharoconjunctivitis (e.g., chronic herpes in cats).
• Iatrogenic-removal of the third eyelid gland (especially in at-risk breeds); radiation therapy.
• Congenital-Yorkshire terriers overrepresented.
• Neurologic-loss of parasympathetic innervation to lacrimal gland, trigeminal nerve deficit, or dysautonomia; neurogenic parasympathetic loss may have ipsilateral dry nose.
• Traumatic-after ocular proptosis or orbit inflammation.
• Systemic disease-diabetes mellitus, Cushing's disease, hypothyroidism or any debilitating disease.
• Drug-induced-systemic sulfonamides (e.g., trimethoprim-sulfadiazine) or etodolac.
• Transient KCS-general anesthesia and atropine.

Differential Diagnosis

• Often confused with allergic or bacterial conjunctivitis.
• Dogs with KCS may have concurrent secondary bacterial overgrowth.
• Differentiate with Schirmer tear test.

Diagnostic Procedures

• Schirmer tear test-decreased results diagnostic; normal value (dogs): at least 15 mm/min of wetting; symptomatic patients: usually <10 mm/min of wetting; difficult to interpret in cats.
• Fluorescein staining-corneal ulcers.
• Conjunctival cytology-may indicate the nature and degree of bacterial overgrowth.
• Aerobic bacterial culture and sensitivity if initial treatment is unsuccessful.

Drug(s)

• Lacrimostimulants: Cyclosporine 0.2% ointment or 1–2% compounded solution; tacrolimus 0.02–0.03% compounded solution or ointment-Therapy q12h recommended (q8h if severe or refractory).
• For neurogenic KCS-pilocarpine 0.2% topically q8h or very careful oral pilocarpine dosing regimen given narrow therapeutic window (see “Suggested Reading”).
• For feline KCS-antiviral therapy (see Conjunctivitis-Cats).
• Lacrimomimetics: artificial tears-help moisten the ocular surface to improve comfort and reduce signs; use viscous solutions or gels q2–12h depending on severity and ointment before bedtime; can reduce frequency once patient responds to lacrimostimulant therapy.
• Broad-spectrum antibiotics-topical ointment q6–8h for 3–4 weeks; indicated for secondary bacterial overgrowth.
• Ocular cleansing: use eye wash to remove discharge and debris prior to medications; if mucoid discharge is very tenacious 5% N-acetylcysteine can be used q6–12h as a mucinolytic agent prior to eye rinsing.
• Corticosteroids-topical; minimize inflammation; helpful in reducing corneal vascularization and pigmentation once aqueous tears improve; not commonly used due to corneal ulcer risk.

Contraindications/Possible Interactions

• Topical cyclosporine or tacrolimus-rarely irritating.
• Pilocarpine-initially irritating topically; systemic side effect risk.
• Topical corticosteroids-avoid with ulcerative keratitis or if severe KCS given ulcer predilection.

Abbreviation

KCS = keratoconjunctivitis sicca