Keratoconjunctivitis sicca (KCS), commonly known as "dry eye," is a chronic inflammatory condition of the cornea and conjunctiva resulting from deficient tear production.
Overview and Clinical Importance
Keratoconjunctivitis sicca (KCS), commonly known as "dry eye," is a chronic inflammatory condition of the cornea and conjunctiva resulting from deficient tear production. KCS is one of the most common ocular conditions in dogs with a reported annual incidence of 0.3% to 1.5% in North America. The term derives from Greek and Latin roots: "kerato" (cornea), "conjunctivitis" (inflammation of conjunctiva), and "sicca" (dry).
Without adequate aqueous tears, the cornea and conjunctiva become dry, inflamed, and susceptible to secondary bacterial infection, ulceration, and progressive scarring. If untreated, KCS can lead to corneal pigmentation, neovascularization, and potentially permanent vision loss. Early diagnosis and lifelong management are critical for maintaining ocular comfort and preserving vision.
High-YieldKCS is frequently misdiagnosed as bacterial conjunctivitis. Always perform a Schirmer tear test (STT) in any dog presenting with mucoid ocular discharge, conjunctivitis, or recurrent corneal ulcers!
| Layer |
Source |
Function |
| Lipid (outer) |
Meibomian glands |
Prevents tear evaporation; stabilizes tear film |
| Aqueous (middle) |
Lacrimal gland (70%), Third eyelid gland (30%) |
Provides 95% of tear volume; oxygen, nutrients, antimicrobials |
| Mucin (inner) |
Conjunctival goblet cells |
Allows tear adherence to hydrophobic corneal epithelium |
Anatomy and Physiology of the Tear Film
The precorneal tear film (PTF) is a thin, complex layer essential for corneal health. It provides lubrication, delivers oxygen and nutrients to the avascular cornea, removes debris and pathogens, and creates a smooth optical surface for light refraction.
Three Layers of the Tear Film
High-YieldQuantitative KCS = deficiency in aqueous layer (most common). Qualitative KCS = deficiency in lipid or mucin layers. The NAVLE primarily tests on quantitative KCS.
| Cause |
Key Features and Clinical Relevance |
| Immune-mediated |
MOST COMMON cause (greater than 80%). Lymphocytic-plasmacytic infiltration destroys lacrimal gland acinar cells. Bilateral, progressive, lifelong management required. |
| Drug-induced |
Sulfonamides (trimethoprim-sulfa): Can cause permanent KCS. Etodolac (NSAID): ~15% incidence. Phenazopyridine. May be temporary or permanent. |
| Neurogenic |
Loss of CN VII parasympathetic innervation to lacrimal gland. Often UNILATERAL with ipsilateral dry nose (xeromycteria). Causes: otitis media/interna, trauma, neoplasia. |
| Congenital |
Lacrimal gland aplasia/hypoplasia. Yorkshire Terrier, Cavalier King Charles Spaniel (dry eye curly coat syndrome), Bedlington Terrier. |
| Infectious |
Canine distemper virus: systemic disease with multiorgan involvement. Leishmaniasis in endemic areas. |
| Metabolic |
Diabetes mellitus, hypothyroidism, hyperadrenocorticism. Usually bilateral. Look for concurrent systemic signs. |
| Iatrogenic |
Surgical removal of third eyelid gland ("cherry eye"). Third eyelid gland produces 30% of tears - excision predisposes to KCS. Radiation therapy. |
Etiology and Classification
NAVLE TipWhen you see "UNILATERAL dry eye + dry nose on SAME SIDE" = think NEUROGENIC KCS. When you see "dog on trimethoprim-sulfa develops dry eye" = drug-induced KCS!
Breed Predispositions
KCS has a strong breed predisposition, particularly in brachycephalic breeds and spaniels. Middle-aged to older dogs are most commonly affected, and spayed females may be overrepresented.
High-risk breeds: American Cocker Spaniel, English Bulldog, Cavalier King Charles Spaniel, West Highland White Terrier, Shih Tzu, Lhasa Apso, Pug, Pekingese, Boston Terrier, Miniature Schnauzer, Yorkshire Terrier, English Springer Spaniel, Bloodhound, Samoyed.
| Clinical Sign |
Pathophysiology and Characteristics |
| Mucoid/Mucopurulent Discharge |
MOST CONSISTENT sign. Thick, ropy, yellow-green discharge adheres to cornea. Results from decreased aqueous with compensatory mucin overproduction. |
| Blepharospasm |
Squinting and excessive blinking due to ocular pain from frictional irritation. More prominent in acute cases; may diminish with chronicity. |
| Conjunctival Hyperemia |
Redness of palpebral and bulbar conjunctiva due to inflammation. Often confused with bacterial conjunctivitis. |
| Dull, Lusterless Cornea |
Loss of normal corneal shine due to tear film deficiency. The cornea appears dry and lacks its normal bright, clear appearance. |
| Corneal Neovascularization |
Superficial blood vessel ingrowth. Chronic response to corneal hypoxia and inflammation. The cornea attempts to replace metabolic support normally provided by tears. |
| Corneal Pigmentation |
Dark brown/black melanin deposition (hyperpigmentation). Chronic change indicating long-standing disease. May cause vision impairment if extensive. |
| Corneal Ulceration |
Increased friction and reduced antimicrobial protection predispose to epithelial breakdown. Recurrent ulcers are common. Fluorescein positive. |
| Xeromycteria |
Dry nose on SAME SIDE as affected eye. PATHOGNOMONIC for neurogenic KCS. CN VII also innervates lateral nasal glands. |
Clinical Signs and Physical Examination Findings
| STT Value (mm/min) |
Interpretation |
Clinical Action |
| ≥15 mm/min |
Normal |
Consider qualitative KCS if clinical signs present |
| 10-14 mm/min |
Borderline / Early KCS |
Suspicious - correlate with clinical signs; retest frequently |
| 5-10 mm/min |
Moderate KCS |
Initiate treatment; good prognosis with therapy |
| Less than 5 mm/min |
Severe KCS |
Aggressive treatment required; guarded prognosis |
Diagnosis
Schirmer Tear Test (STT)
The Schirmer Tear Test I (STT I) is the CORNERSTONE diagnostic test for quantitative KCS. It measures combined basal and reflex tear production by placing a standardized filter paper strip in the lower conjunctival fornix for exactly 60 seconds.
STT Procedure
- Perform BEFORE any topical medications or fluorescein staining
- Bend strip at notch while still in packaging
- Place notched end in central-lateral lower conjunctival fornix
- Ensure strip contacts cornea (stimulates reflex tearing)
- Leave in place for exactly 60 seconds
- Read measurement immediately after removal
STT I Interpretation in Dogs
High-YieldDogs with STT as low as 2 mm/min still have ~80% chance of responding to cyclosporine therapy! Don't give up on medical management too quickly.
Additional Diagnostic Tests
| Test |
Purpose and Interpretation |
| Fluorescein Stain |
Detects corneal ulceration. Hydrophilic dye adheres to exposed stroma. Perform AFTER STT to avoid interference. |
| Tear Film Break-Up Time (TBUT) |
Assesses tear film stability. Normal: ≥20 seconds. Less than 5 seconds = unstable PTF. Used for qualitative KCS diagnosis. |
| Rose Bengal Stain |
Stains devitalized or dead epithelial cells lacking mucin protection. Useful for detecting early ocular surface damage. |
| Tonometry |
Measures intraocular pressure. Rule out concurrent glaucoma, especially in predisposed breeds. |
| Culture and Sensitivity |
If secondary bacterial infection suspected, especially with corneal ulceration. Perform before fluorescein application. |
Treatment
Treatment goals include: (1) Stimulating natural tear production, (2) Replacing/supplementing tear film, (3) Reducing inflammation, and (4) Managing secondary complications. Treatment is LIFELONG in most cases.
Medical Treatment Options
NAVLE TipCyclosporine = first-line for immune-mediated KCS. Tacrolimus = second-line for cyclosporine failures. Pilocarpine = ONLY for neurogenic KCS (cholinergic stimulation of lacrimal gland). These drug indications are HIGH-YIELD!
Mechanism of Action: Immunomodulators
Cyclosporine and tacrolimus are calcineurin inhibitors that reversibly inhibit T-helper cell proliferation and prevent release of proinflammatory cytokines (especially IL-2). By suppressing the immune-mediated destruction of lacrimal gland tissue, tear production is restored. Additionally, these drugs may have direct lacrimostimulatory effects independent of their immunomodulatory action.
Surgical Treatment: Parotid Duct Transposition (PDT)
Indications: Reserved for dogs that fail to respond to 3-6 months of aggressive medical therapy and remain persistently uncomfortable. The procedure reroutes the parotid salivary duct from the mouth to the lower conjunctival fornix, providing salivary secretions to lubricate the eye.
Success rate: ~92% surgical success. 90% of owners would proceed with surgery again.
Complications (50% rate): Saliva intolerance, excessive salivation with facial dermatitis, calcium/mineral deposition on cornea (requires ongoing management), duct occlusion, inadequate salivary flow.
High-YieldParotid duct transposition is NOT first-line treatment. Most dogs (greater than 80%) respond to medical therapy. Surgery is only for refractory cases. Post-surgical dogs often still require topical medications to manage complications.
| Drug |
Concentration |
Dosing |
Notes |
| Cyclosporine (Optimmune) |
0.2% ointment (commercial); 0.5-2% compounded |
Q12H topically |
FIRST-LINE. FDA approved. Response in 30-45 days. Calcineurin inhibitor. |
| Tacrolimus |
0.02-0.03% compounded |
Q12H topically |
10-100x more potent than CsA. Second-line for CsA non-responders. Off-label. |
| Pilocarpine |
1-2% ophthalmic (oral use) |
Oral: titrate to effect |
For NEUROGENIC KCS only. Parasympathomimetic. Side effects: drooling, diarrhea, vomiting. |
| Artificial Tears |
Various (viscous preferred) |
Q2-6H as needed |
Adjunct therapy. Critical early while awaiting CsA/tacrolimus effect. Not monotherapy. |
| Topical Antibiotics |
Various |
Q6-12H |
For secondary bacterial infection. Often used initially. Avoid aminoglycosides if ulcer present. |
Prognosis and Monitoring
Prognosis: Good to excellent with early diagnosis and consistent treatment. Dogs with higher STT values at diagnosis and those without extensive corneal changes tend to respond better. Approximately 80% of dogs respond to immunomodulatory therapy even with severe disease.
Timeline for improvement: Clinical signs typically improve within 2-4 weeks of initiating treatment. Full response to cyclosporine/tacrolimus may take 30-45 days. Some dogs may take up to 12 weeks to show significant improvement.
Monitoring: Recheck with STT in 4 weeks after initiating therapy. Once stable, monitor every 6-12 months. Dogs achieving STT greater than 20 mm/min may reduce to once daily maintenance therapy.
Poor prognostic indicators: STT of 0 mm/min, extensive corneal scarring/pigmentation at presentation, congenital alacrima, complete glandular destruction.
Memory Aids for NAVLE Success
"DRY EYE - STT" Mnemonic
D - Discharge (thick, mucoid, ropy)
R - Redness (conjunctival hyperemia)
Y - Yellow-green mucus accumulation
E - Eye pain (blepharospasm)
Y - Yet looks like conjunctivitis (often misdiagnosed)
E - Every case needs STT!
STT - Schirmer Tear Test: less than 15 = suspicious, less than 10 = KCS, less than 5 = severe
Drug Selection Quick Guide
"CTP" = Cyclosporine First, Tacrolimus Second, Pilocarpine for Nerves
Cyclosporine 0.2% (Optimmune) = FDA-approved, first-line
Tacrolimus = More potent, for cyclosporine failures
Pilocarpine = ONLY for neurogenic KCS (stimulates parasympathetic pathway)