The Schirmer Tear Test (STT) is a fundamental diagnostic tool in veterinary ophthalmology, primarily used to assess tear production in dogs. Tears play a crucial role in maintaining ocular health, ensuring adequate lubrication, protecting against environmental irritants, facilitating clear vision, and promoting corneal integrity. When tear production is insufficient, it can lead to a serious condition known as Keratoconjunctivitis Sicca (KCS), or dry eye, which is particularly prevalent in certain dog breeds and poses significant risks to vision if left untreated.
First introduced in the 1960s by Dr. Otto Schirmer, the Schirmer Tear Test remains a simple, cost-effective, and reliable method for quantifying tear volume over a specific time period. This guide explores the STT in exhaustive detail, covering its physiological basis, the procedure itself, interpretation of results, clinical significance in different breeds, breed predispositions, common causes of abnormal tear production, treatment strategies, and implications for long-term ocular health. Additionally, we discuss special considerations in clinical settings, advances in STT methodology, and the role of this test in preventive veterinary care.
Understanding Tear Production and Ocular Surface Health
Before delving into the technical aspects of the Schirmer Tear Test, it is essential to understand the physiology of tear production in dogs. Healthy eyes rely on a three-layered tear film to function properly:
- Mucin Layer (Innermost): Secreted by goblet cells in the conjunctiva, this layer helps the tear film adhere to the corneal surface and ensures even spreading.
- Aqueous Layer (Middle): Produced primarily by the lacrimal and accessory lacrimal glands, this layer forms the bulk of the tear film and contains water, electrolytes, proteins, and antibodies that nourish and protect the eye.
- Lipid Layer (Outermost): Secreted by the meibomian glands in the eyelids, this oily layer prevents rapid evaporation of tears.
Disruption in any of these layers—especially the aqueous layer—can result in dry eye syndrome. In dogs, aqueous tear deficiency is most commonly linked to KCS, an immune-mediated condition where the lacrimal glands are damaged, leading to reduced tear production.
What is the Schirmer Tear Test (STT)?
The Schirmer Tear Test is designed to measure the aqueous component of the tear film. It is performed using a small, sterile, filter paper strip impregnated with a non-irritating dye (usually methylcellulose-based). The strip is placed in the ventral conjunctival sac (between the lower eyelid and the eyeball), typically at the junction of the middle and lateral third of the lower lid, and left in place for one minute. The amount of moisture absorbed by the strip is measured in millimeters (mm) of wetting per minute.
Two types of Schirmer Tear Test are commonly used:
- STT Type I (Basal and Reflex Tear Production): Measures total tear production, including both basal (constant) and reflex (stimulated) tears. This is the most commonly used version in veterinary practice.
- STT Type II (Basal Tear Production): Measures only basal tear production by first anesthetizing the eye and then blocking reflex tearing. This is less frequently used in routine clinics due to its complexity and limited additional clinical value over STT I.
In most canine cases, the STT Type I is preferred due to its simplicity, speed, and direct relevance to diagnosing KCS.
Step-by-Step Procedure of Performing STT in Dogs
Accurate execution of the Schirmer Tear Test is essential for reliable results. The following step-by-step protocol is standard in veterinary clinics:
- Gather Equipment:
- Sterile Schirmer Tear Test strips.
- Clean gloves.
- Tissue or gauze (to wipe nasal secretions if needed).
- A timer.
- Restrain the Dog:
- Gently restrain the dog to minimize movement. For anxious dogs, mild physical restraint or the help of an assistant is advised. The use of sedation is rarely necessary for STT unless the dog is extremely uncooperative or has concurrent eye pain.
- Prepare the Eye:
- Ensure there are no topical medications, especially anesthetics or artificial tears, administered prior to the test, as these can artificially alter results.
- Wipe away excess tears or discharge gently if visible, but do not stimulate tear production.
- Insert the Strip:
- Bend the notch at the top of the strip at a right angle to hook it under the lower eyelid.
- Place the strip in the ventral conjunctival sac, avoiding contact with the cornea to prevent irritation.
- Position the strip vertically, ensuring it is not twisted or bent.
- Start the Timer:
- Immediately start a one-minute timer once the strip is in place.
- Minimize Distractions:
- Keep the dog calm and avoid talking loudly or making sudden movements that could startle the animal and trigger reflex tearing.
- Read the Result:
- After one minute, carefully remove the strip.
- Read the millimeter scale where the wetting ends. The result should be recorded in mm/min.
- Interpret the Reading:
- Compare the measurement against standard reference ranges (see next section).
- Repeat the test on the contralateral eye, as KCS can be unilateral or bilateral.
Normal and Abnormal STT Values in Dogs
Interpretation of STT results is based on established reference values:
- Normal Tear Production: ≥ 15 mm/min
- Mild or Questionable Tear Deficiency: 11–14 mm/min
- Moderate to Severe Tear Deficiency (KCS): ≤ 10 mm/min
It is important to note that baseline STT values can vary slightly depending on breed, age, and environmental factors. For example, brachycephalic breeds (e.g., Pugs, Shih Tzus) may have lower normal values due to anatomical differences. Similarly, older dogs may show decreased tear production due to age-related gland atrophy.
Additionally, transient decreases in tear production can occur due to anesthesia, recent ocular surgery, corneal irritation, or systemic illness. Therefore, a single low reading should not be the sole basis for diagnosing KCS; clinical signs must be correlated.
Breed Predisposition to Low Tear Production and KCS
Certain dog breeds are genetically predisposed to aqueous tear deficiency, making routine STT screening particularly important in these populations. Breeds with high susceptibility to KCS include:
- Cocker Spaniels (American and English): One of the most commonly affected breeds due to immune-mediated lacrimal gland destruction.
- Bulldogs (French and English): Their facial conformation leads to chronic eye irritation and secondary KCS.
- Shih Tzus: Prone to both KCS and exposure keratitis due to exophthalmos.
- Pugs: Similar to Shih Tzus, increased risk due to shallow orbits.
- West Highland White Terriers: Often exhibit immune-mediated KCS.
- Lhasa Apsos: High incidence of lacrimal gland disorders.
- Yorkshire Terriers: Frequently develop dry eye, especially in middle to old age.
- Pekingese: Anatomical challenges predispose to tear film instability.
- Samoyeds: Known for inherited lacrimal gland aplasia.
- Chihuahuas: Small eyes with high surface exposure.
In these breeds, regular STT monitoring—even in the absence of symptoms—can aid in early diagnosis and prevention of corneal damage.
Causes of Abnormal STT Results
Abnormal STT values, particularly low readings, can stem from a variety of underlying causes. These can be categorized into primary and secondary etiologies:
1. Immune-Mediated KCS (Primary KCS):
- Autoimmune destruction of lacrimal glands.
- Most common cause, especially in predisposed breeds.
- Often associated with other autoimmune conditions like sebaceous adenitis.
2. Neurogenic KCS:
- Damage to the facial nerve (cranial nerve VII) or trigeminal nerve (cranial nerve V), disrupting the neural reflex arc for tear production.
- Can result from otitis media/interna, trauma, or neoplasia.
3. Drug-Induced KCS:
- Certain medications can suppress tear production:
- Sulfa antibiotics (trimethoprim-sulfadiazine) – most common.
- Antihistamines, atropine, systemic chemotherapy agents.
4. Anesthetic or Sedative-Related KCS:
- Anesthetic drugs, particularly alpha-2 agonists like dexmedetomidine, can transiently reduce tear production.
- Always consider timing when STT follows anesthesia.
5. Infectious Causes:
- Canine distemper virus can lead to permanent lacrimal gland damage.
- Chronic conjunctivitis may impair gland function over time.
6. Congenital or Anatomical Abnormalities:
- Lacrimal gland aplasia (e.g., in Miniature Schnauzers).
- Ectopic cilia, distichiasis, or entropion causing chronic irritation and secondary gland dysfunction.
7. Systemic Diseases:
- Hypothyroidism.
- Diabetes mellitus.
- Lymphoplasmacytic inflammation (seen in immune-mediated polyglandular syndromes).
8. Age-Related Changes:
- Glandular atrophy in senior dogs can lead to decreased tear production.
Clinical Signs of Keratoconjunctivitis Sicca (Dry Eye)
Dogs with low STT values often exhibit visible and behavioral symptoms. Recognizing these early signs can lead to prompt diagnosis and treatment:
- Chronic Conjunctivitis: Persistent redness of the conjunctiva.
- Mucoid or Mucopurulent Discharge: Thick, yellow to greenish discharge due to secondary bacterial infection.
- Corneal Opacity or Pigmentation: Chronic dryness leads to corneal vascularization, melanin deposition, and scarring.
- Corneal Ulcers: Due to poor healing and lack of protective tear film.
- Squinting or Blepharospasm: Pain or discomfort from corneal irritation.
- Excessive Blinking or Rubbing: Dogs may paw at their eyes due to irritation.
- Dull or Hazy Cornea: Loss of normal corneal luster.
- Reduced Visual Acuity: In severe, long-standing cases.
Owners may initially mistake these symptoms for simple allergies or eye infections. However, untreated KCS can lead to irreversible blindness, making early detection through STT critical.
Differentiating KCS from Other Ocular Conditions
Not all dogs with ocular discharge or redness have KCS. It is vital to differentiate dry eye from other common eye problems:
- Conjunctivitis: Can be allergic, bacterial, viral, or irritant-induced. STT values are typically normal or even elevated due to reflex tearing.
- Corneal Ulcers: Often associated with trauma. STT may be normal, but fluorescein staining confirms corneal defects.
- Dry Eye with Reflex Tearing: Some dogs with partial KCS may have fluctuating tear production, causing intermittent watering (paradoxical tearing), which can confuse diagnosis.
- Nasolacrimal Duct Obstruction: Results in epiphora (excessive tearing), not dry eye. Dye disappearance tests (e.g., fluorescein flush test) help diagnose duct issues.
Thus, STT is a cornerstone in differential diagnosis and should be performed whenever chronic ocular discomfort or abnormal discharge is present.
Importance of STT in Routine Eye Exams and Breeding Programs
The Schirmer Tear Test is not only a diagnostic tool but also a preventive measure. Integrating STT into regular wellness checkups, especially for high-risk breeds, helps identify subclinical KCS before irreversible damage occurs.
In breeding programs, STT screening is highly recommended. Reputable breeders often test adult dogs before breeding to ensure they do not carry traits linked to lacrimal gland dysfunction. This practice helps reduce the prevalence of inherited KCS in future generations.
Additionally, ophthalmic screening programs such as those run by organizations like the Canine Eye Registration Foundation (CERF) or the Orthopedic Foundation for Animals (OFA) often include STT as part of comprehensive eye evaluations.
Treatment and Management of KCS in Dogs
Once KCS is diagnosed via STT, a tailored treatment plan is essential. The goals of therapy are to:
- Stimulate natural tear production.
- Replace tears artificially.
- Control inflammation.
- Prevent secondary infections.
- Promote corneal healing.
Common treatment modalities include:
1. Topical Cyclosporine:
- First-line treatment for immune-mediated KCS.
- Available as 0.2% or 1% ophthalmic ointment (e.g., Optimmune®).
- Works by suppressing T-cell-mediated inflammation and stimulating lacrimal gland function.
- Typically administered 1–2 times daily.
- Signs of improvement usually seen within 2–6 weeks.
- Long-term use required in most cases.
2. Topical Tacrolimus:
- Alternative immunomodulator, especially for non-responders to cyclosporine.
- More potent anti-inflammatory effects.
- Often used at 0.02% or 0.03% concentration.
- Considered in refractory cases.
3. Artificial Tear Preparations:
- Provide temporary lubrication.
- Available as drops, gels, or ointments.
- Must be used frequently (4–6 times daily) due to short residence time.
- Not curative but essential adjunct therapy.
4. Antibiotics and Anti-Inflammatory Agents:
- If bacterial infection or severe inflammation is present, topical antibiotics (e.g., chloramphenicol) or corticosteroids may be used—with caution, as steroids can worsen corneal ulcers.
5. Pilocarpine (Systemic):
- Rarely used; may stimulate tear production via parasympathetic activation.
- Risk of systemic side effects (salivation, vomiting, diarrhea).
- Used only when neurogenic KCS is suspected.
6. Surgical Options (Parotid Duct Transposition):
- In cases unresponsive to medical therapy.
- Redirects salivary duct from the parotid gland to the conjunctival sac.
- Provides moisture, but saliva is not an ideal substitute for tears.
- Risk of complications: mineral deposition on cornea, chronic irritation.
- Considered a last resort.
Regular STT monitoring during treatment is essential to assess response. Successful therapy often leads to progressive increases in STT values over weeks to months.
Special Considerations in STT Administration
While the STT is generally straightforward, several nuances must be considered for accurate results:
1. Timing with Other Tests:
- STT should be performed before any other ophthalmic tests (fluorescein staining, tonometry) to avoid contamination or reflex tearing from anesthetic drops.
2. Environmental Influence:
- Drafts, bright lights, or loud noises may stress the dog, altering tear production.
- Conduct the test in a quiet, dimly lit, comfortable environment.
3. Technique Errors:
- Incorrect placement (e.g., touching the cornea) can cause irritation and stimulate reflex tearing.
- Bent or compressed strips may absorb tears unevenly.
- Ensure the strip is vertical and fully in contact with the conjunctiva.
4. Interpreting Fluctuating Values:
- Some dogs exhibit diurnal variation in tear production.
- Repeat testing may be necessary if results are borderline.
5. Testing Both Eyes:
- Always test both eyes independently.
- Asymmetric results may indicate unilateral pathology (e.g., trauma, neurogenic causes).
6. Pediatric and Geriatric Dogs:
- Puppies may have lower baseline STT due to immature glands.
- Senior dogs often show age-related decreases; monitor trends rather than absolute values.
Advancements and Limitations of the Schirmer Tear Test
Despite its widespread use, the STT has limitations:
- Operator Dependence: Results can vary based on technique.
- Reflex Tearing Influence: STT I includes reflex tears, which may mask true basal deficiency.
- Does Not Assess Tear Film Quality: The test measures quantity, not quality (mucin or lipid layers).
- Short Test Duration: One minute may not reflect long-term tear stability.
- Uncomfortable for Some Dogs: May not be tolerated well in painful or aggressive animals.
To address these limitations, newer diagnostic approaches are being explored:
- Phenol Red Thread Test (PRTT): A faster (15–30 seconds), less invasive alternative using pH-sensitive thread. Gaining popularity in small-breed dogs.
- Tear Break-Up Time (TBUT): Measures tear film stability by assessing how long it takes for dry spots to appear after a blink. Often used alongside STT.
- Ocular Surface Staining (e.g., Lissamine Green, Rose Bengal): Highlights damaged corneal and conjunctival cells.
- Meibography and Infrared Imaging: Evaluates meibomian gland structure and function.
- Tear Osmolarity Measurement: An emerging tool to assess tear composition, though not yet widely available in veterinary practice.
While these methods offer deeper insights, the STT remains the gold standard due to its accessibility, affordability, and proven efficacy.
STT in Emergency and Critical Care Settings
In emergency scenarios—such as trauma, neurological disorders, or systemic illness—assessing ocular health is often overlooked. However, STT can provide valuable information:
- Dogs under general anesthesia are at risk for corneal exposure and drying.
- Comatose or neurologically impaired dogs may have absent blink reflexes, increasing ulcer risk.
- STT helps determine need for artificial tear application or temporary tarsorrhaphy (surgical eyelid closure).
Integrating STT into emergency protocols enhances comprehensive patient care.
Owner Education and Home Monitoring
Educating dog owners about KCS and the role of STT is pivotal for long-term success:
- Recognizing Early Signs: Owners should monitor for redness, discharge, or squinting.
- Medication Compliance: Topical treatments require daily, long-term application.
- Scheduled Rechecks: Regular vet visits with repeat STT to monitor progress.
- Avoiding Risk Factors: Discourage use of sulfa drugs in predisposed breeds.
- Breed-Specific Precautions: Inform owners of genetic risks in their dog’s breed.
Some owners may perform at-home monitoring using artificial tear response observation, though STT must remain a professional diagnostic tool.
Prognosis and Long-Term Outlook
With early diagnosis and consistent treatment, the prognosis for dogs with KCS is generally favorable:
- Most dogs respond well to cyclosporine or tacrolimus.
- STT values often improve from <5 mm/min to >15 mm/min within months.
- Clinical signs typically resolve with proper management.
- Lifelong therapy is usually required, but quality of life remains high.
Untreated KCS, however, leads to progressive corneal scarring, pigmentary keratitis, ulcers, and ultimately, blindness. Hence, the STT is not just a diagnostic step—it is a gateway to preserving vision.
Conclusion
The Schirmer Tear Test is an indispensable tool in veterinary ophthalmology, offering a simple yet powerful means of evaluating tear production in dogs. Its role in diagnosing and managing Keratoconjunctivitis Sicca cannot be overstated. From routine wellness exams in high-risk breeds to emergency evaluations in critical care, the STT provides actionable insights that directly influence treatment outcomes.
Veterinarians, technicians, and pet owners alike must appreciate the significance of this test. Through proper technique, interpretation, and integration into preventive care, the Schirmer Tear Test helps safeguard ocular health and maintain the clarity of vision that dogs rely on to navigate their world.
Early detection, accurate diagnosis, and consistent management—supported by the STT—ensure that even dogs prone to dry eye can live comfortably and see clearly for years to come.
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