Tear Film Tests: Schirmer’s, TBUT, Tear Meniscus, and NITBUT
Introduction:
The tear film plays a critical role in maintaining ocular health and visual clarity. It provides lubrication, supplies nutrients, protects against infections, and creates a smooth refractive surface. Disorders of the tear film, especially dry eye disease (DED), are among the most common complaints in optometry and ophthalmology clinics worldwide.
Clinical evaluation of tear film stability and quantity is therefore essential in diagnosing, grading, and monitoring dry eye. Several tests are available, but the most commonly used include:
- Schirmer’s Test: Measures tear quantity.
- Tear Break-Up Time (TBUT): Measures tear film stability.
- Tear Meniscus Evaluation: Estimates tear volume clinically.
- Non-Invasive TBUT (NITBUT): Assesses tear film stability without fluorescein dye.
Anatomy and Physiology of Tear Film
The tear film is classically described as a trilaminar structure:
- Lipid layer: Secreted by meibomian glands; prevents evaporation.
- Aqueous layer: Secreted by lacrimal glands; provides volume, oxygen, nutrients.
- Mucin layer: Secreted by goblet cells; promotes wettability of corneal epithelium.
Modern concepts describe it as a complex gradient rather than strictly layered, but the classic model is still clinically useful.
1. Schirmer’s Test
Principle
Schirmer’s test measures the aqueous component of the tear film by using a filter paper strip inserted into the lower fornix of the eye. The length of wetting after five minutes indicates tear secretion.
Types of Schirmer’s Test
- Schirmer I (without anesthesia): Measures total tear secretion (basal + reflex).
- Schirmer I (with anesthesia): Measures basal secretion only.
- Schirmer II: Measures reflex secretion by stimulating nasal mucosa.
Procedure
- Use standardized Whatman filter paper strip (5 × 35 mm).
- Fold strip 5 mm from one end.
- Insert folded end into lower fornix at lateral one-third.
- Ask patient to keep eyes gently closed.
- After 5 minutes, measure length of wetting from the fold.
Interpretation
- Normal: >15 mm in 5 minutes.
- Borderline: 10–15 mm.
- Dry eye: <10 li="" mm.="">
- Severe deficiency: <5 li="" mm.="">
Advantages
- Simple, inexpensive, widely available.
- Provides quantitative estimate of tear secretion.
Limitations
- Invasive, uncomfortable, may induce reflex tearing.
- Results vary with environment and patient cooperation.
- Poor reproducibility.
Clinical Uses
- Diagnosis of aqueous-deficient dry eye (e.g., Sjögren’s syndrome).
- Screening in autoimmune diseases.
- Monitoring response to therapy in severe dry eye.
2. Tear Break-Up Time (TBUT)
Principle
TBUT measures tear film stability by recording the interval between a blink and the appearance of first dry spot on the cornea after fluorescein dye instillation.
Procedure
- Instill a small amount of fluorescein dye into the conjunctival sac.
- Ask patient to blink several times to distribute dye evenly.
- Examine under cobalt blue filter on slit lamp.
- Instruct patient to keep eyes open; measure interval until first dark spot appears on cornea.
- Repeat 2–3 times and average results.
Interpretation
- Normal: >15 seconds.
- Borderline: 10–15 seconds.
- Abnormal: <10 film="" indicates="" instability="" li="" seconds="" tear="">
Advantages
- Direct measure of tear stability.
- Useful for diagnosing evaporative dry eye (meibomian gland dysfunction).
Limitations
- Invasive due to fluorescein instillation.
- Subjective interpretation by examiner.
- Influenced by concentration of dye and illumination conditions.
Clinical Uses
- Diagnosis of evaporative dry eye.
- Assessment of meibomian gland dysfunction.
- Monitoring efficacy of artificial tears or punctal plugs.
3. Tear Meniscus Evaluation
Principle
The tear meniscus is the strip of tear fluid seen at the junction of the lower lid margin and the globe. Its height correlates with tear volume. Clinical observation with slit lamp can provide useful information about tear quantity.
Procedure
- Seat patient at slit lamp.
- Use diffuse or narrow illumination to examine inferior lid margin.
- Assess meniscus height, curvature, and regularity.
Interpretation
- Normal: Tear meniscus height ~0.3 mm.
- Reduced: Suggests aqueous deficiency.
- Absent or irregular: Seen in dry eye, conjunctival scarring, lid abnormalities.
Advantages
- Non-invasive and quick.
- No special equipment beyond slit lamp.
Limitations
- Subjective, depends on examiner judgment.
- May vary with blink or reflex tearing.
Clinical Uses
- Screening for dry eye disease.
- Monitoring tear quantity before and after treatment.
4. Non-Invasive Tear Break-Up Time (NITBUT)
Principle
NITBUT measures tear film stability without fluorescein dye. It uses reflection of grid patterns, Placido rings, or keratometer mires to detect disruption of tear film.
Procedure
- Patient fixates on a target in keratometer or topographer.
- Observe reflected mires or rings on corneal surface.
- Ask patient to blink, then keep eyes open.
- Note the time until distortion of mires occurs.
Interpretation
- Normal: >15–20 seconds.
- Borderline: 10–15 seconds.
- Abnormal: <10 film.="" li="" seconds="" tear="" unstable="">
Advantages
- Non-invasive, no dye instillation.
- More reproducible than TBUT.
- Preferred in research and advanced dry eye clinics.
Limitations
- Requires keratometer, corneal topographer, or specialized equipment.
- Less accessible in basic clinics.
Clinical Uses
- Dry eye disease diagnosis and grading.
- Pre- and post-contact lens fitting evaluation.
- Research in ocular surface disease.
Comparison of Tear Film Tests
| Test | Measures | Type | Advantages | Limitations |
|---|---|---|---|---|
| Schirmer’s | Tear quantity (aqueous) | Invasive | Simple, inexpensive | Variable, uncomfortable |
| TBUT | Tear stability | Invasive | Commonly used | Subjective, dye-dependent |
| Tear meniscus | Tear volume | Non-invasive | Quick, slit lamp based | Subjective |
| NITBUT | Tear stability | Non-invasive | Reproducible, research-friendly | Requires equipment |
Clinical Importance
Tear film tests are indispensable in diagnosing and managing dry eye disease. They help in distinguishing between aqueous-deficient and evaporative dry eye, monitoring treatment (artificial tears, punctal plugs, anti-inflammatory drugs), and assessing ocular surface health before contact lens fitting or refractive surgery. They are also critical in systemic diseases like Sjögren’s syndrome, rheumatoid arthritis, and diabetes.
Color Vision
Introduction:
Color vision is the ability of the visual system to perceive differences in the wavelength composition of light and interpret them as distinct colors. It is a complex function of the photoreceptors, retina, optic nerve, and visual cortex. The study of color vision is not only of physiological and psychological interest but also of enormous clinical importance. Many congenital and acquired ocular diseases present with impaired color vision, making it an essential part of a comprehensive eye examination.
Assessing color vision helps in screening for congenital defects (such as red-green color blindness), diagnosing acquired ocular diseases (optic neuropathy, macular disorders), and even determining occupational fitness in professions like aviation, navigation, military, and electrical work.
Physiology of Color Vision
Human color vision is trichromatic, based on three types of cone photoreceptors in the retina:
- S-cones: Sensitive to short wavelengths (blue, ~420 nm).
- M-cones: Sensitive to medium wavelengths (green, ~530 nm).
- L-cones: Sensitive to long wavelengths (red, ~560 nm).
The brain compares responses from these three cone types to perceive the full spectrum of colors. Rods, on the other hand, mediate scotopic (night) vision and do not contribute to color perception.
Theories of Color Vision
- Trichromatic Theory (Young–Helmholtz): Suggests three cone types each sensitive to one primary color (red, green, blue). Color perception arises from their combined stimulation.
- Opponent-Process Theory (Hering): Suggests color perception is based on opposing channels: red-green, blue-yellow, and black-white. Explains afterimages and color contrast.
- Modern understanding: Both theories complement each other and explain different aspects of color perception.
Classification of Color Vision Defects
1. Congenital Defects
Present from birth, usually bilateral, symmetrical, and non-progressive. More common in males due to X-linked inheritance.
- Anomalous trichromacy: All three cones present, but one type has altered sensitivity.
- Protanomaly – defect in L-cones (red).
- Deuteranomaly – defect in M-cones (green, most common).
- Tritanomaly – defect in S-cones (blue, rare).
- Dichromacy: One cone system absent.
- Protanopia – red cone absent.
- Deuteranopia – green cone absent.
- Tritanopia – blue cone absent.
- Monochromacy: Only one cone type functional (or none in achromatopsia).
2. Acquired Defects
Develop later in life, often asymmetric and progressive. Associated with ocular or systemic disease.
- Optic nerve diseases (toxic/nutritional optic neuropathy, glaucoma).
- Macular diseases (macular degeneration, diabetic maculopathy).
- Retinal diseases (cone dystrophy, retinitis pigmentosa).
- Drug toxicity (chloroquine, digoxin, ethambutol).
Rule of thumb: - Red-green defects → usually congenital. - Blue-yellow defects → usually acquired.
Tests for Color Vision
Several tests are available to detect and classify color vision defects. They are grouped into:
- Screening tests: Detect presence of defect (e.g., Ishihara plates).
- Quantitative tests: Grade severity (e.g., Farnsworth D-15).
- Diagnostic tests: Precisely classify type of defect (e.g., anomaloscope).
1. Pseudoisochromatic Plates (Ishihara Test)
Most widely used for screening red-green deficiencies. Consists of plates with colored dot patterns forming numbers or paths visible only to those with normal color vision.
- Advantages: Quick, simple, inexpensive, reliable for red-green defects.
- Limitations: Cannot detect blue-yellow defects; not quantitative.
2. Farnsworth D-15 Test
Consists of 15 colored caps to be arranged in correct hue sequence. Detects moderate to severe color vision defects and differentiates between red-green and blue-yellow anomalies.
3. Farnsworth–Munsell 100 Hue Test
A more detailed version with 85 caps arranged in four trays. Provides a quantitative score indicating severity and axis of color confusion.
4. Nagel Anomaloscope
Considered the gold standard for diagnosing red-green anomalies. Patient matches a yellow light with a mixture of red and green. Provides precise classification between protan and deutan defects, and between dichromats and anomalous trichromats.
5. Hardy–Rand–Rittler (HRR) Plates
Detect both red-green and blue-yellow defects. Useful in acquired disorders.
6. Lantern Tests
E.g., Farnsworth Lantern, Edridge–Green Lantern. Used in occupational screening (aviation, navy, railways) where recognition of signal lights is critical.
7. Modern Computerized Tests
Tablet- or screen-based applications that simulate standard color tests. Allow quick community screening, though calibration issues limit accuracy.
Procedure and Interpretation (Example: Ishihara Test)
- Ensure adequate natural daylight or equivalent illumination.
- Patient holds book at 75 cm, perpendicular to line of sight.
- Ask patient to read number within 3–5 seconds.
- Record errors and compare with scoring key.
Normal: All plates read correctly. Defective: Specific pattern of errors indicates protan or deutan deficiency.
Clinical Importance
- Early detection of congenital color blindness in children (affects learning, career choices).
- Diagnosis of acquired defects due to optic nerve or macular disease.
- Monitoring drug toxicity (e.g., ethambutol, hydroxychloroquine).
- Occupational screening for professions requiring accurate color discrimination (pilots, electricians, defense services).
Advantages of Color Vision Testing
- Simple, non-invasive, quick.
- Can detect early ocular and systemic disease.
- Helps in genetic counseling for congenital color blindness.
- Guides patient advice for occupational suitability.
Limitations
- Many tests are illumination dependent.
- Young children and illiterate patients may not cooperate fully.
- Screening tests do not quantify severity.
- Computerized versions may lack calibration accuracy.
Stereopsis
Introduction:
Stereopsis is the perception of depth and three-dimensional structure obtained from binocular vision. It is considered the highest grade of binocular vision, requiring precise alignment of both eyes and integration of visual information in the brain. The presence of stereopsis indicates that the two eyes are working together in a coordinated manner, allowing accurate depth judgment, spatial orientation, and fine motor tasks such as threading a needle, driving, or playing sports.
Clinically, stereopsis testing is a valuable tool for assessing binocular function, diagnosing strabismus, monitoring amblyopia therapy, and evaluating suitability for certain occupations. It also provides important prognostic information after strabismus surgery or vision therapy.
Physiology of Stereopsis
The human visual system achieves stereopsis through binocular disparity — the slight difference in images formed on the two retinas due to horizontal separation of the eyes (~6.5 cm interpupillary distance).
- Objects closer than fixation point → images fall on temporal retina (crossed disparity).
- Objects farther than fixation point → images fall on nasal retina (uncrossed disparity).
The brain fuses these disparities to create a perception of depth. The process occurs in the primary visual cortex (V1) and extrastriate areas.
Grades of Binocular Vision
- Simultaneous perception: Ability to perceive images from both eyes simultaneously.
- Fusion: Ability to merge images from both eyes into one single percept.
- Stereopsis: Highest grade — ability to perceive depth based on binocular disparity.
Types of Stereopsis
- Fine stereopsis: High-resolution depth perception under foveal fixation, measured in seconds of arc.
- Coarse stereopsis: Low-resolution depth perception using peripheral retina. Important in motion and large field depth perception.
Measurement of Stereopsis
Stereopsis is measured in seconds of arc, representing the smallest detectable disparity between images that can be appreciated as depth.
- Normal stereopsis: 40–60 arc seconds (fine stereoacuity).
- Poor stereopsis: >200 arc seconds.
- No stereopsis: Patient cannot detect depth differences.
Clinical Tests for Stereopsis
1. Titmus Fly Test
One of the most widely used tests, especially in children. Polarized spectacles are worn by the patient.
- Components: Large fly (gross stereopsis), animals (medium), circles (fine).
- Procedure: Patient with polarized glasses is asked to identify raised wings of fly, smaller animals, or circles appearing closer.
- Interpretation: Provides stereopsis measurement from 3000 to 40 arc seconds.
2. Randot Stereotest
Uses random dot patterns viewed with polarized glasses. No monocular cues are present, so it provides a pure test of stereopsis.
- Includes animal figures (for children) and circles (for adults).
- Ranges from 500 to 20 arc seconds.
3. TNO Test
Uses red-green anaglyph glasses to view random dot stereograms. Plates present hidden figures visible only if stereopsis is present.
- Range: 480 to 15 arc seconds.
- Useful for detecting subtle defects and assessing suppression.
4. Frisby Test
Consists of transparent plates with random dot patterns printed at different thicknesses. No glasses required.
- Naturalistic, real depth cues.
- Useful for young children as it is non-invasive and engaging.
5. Lang Stereotest
Random dot stereograms with cylindrical lens arrays embedded, requiring no glasses. Designed for preschool screening.
6. Digital/Computerized Tests
Modern apps and VR systems provide dynamic stereopsis testing, often used in research and vision therapy.
Procedure (Example: Titmus Test)
- Patient wears polarized glasses.
- Present test booklet at 40 cm under good illumination.
- Start with fly → ask patient to grasp raised wings (gross stereopsis).
- Proceed to animal figures and circle sets (fine stereopsis).
- Record the smallest disparity detected.
Interpretation of Results
- Normal: 40–60 arc seconds.
- Mildly reduced: 100–200 arc seconds.
- Severely reduced: >400 arc seconds.
- Absent: Patient unable to perceive depth cues.
Clinical Significance
- Confirms presence of functional binocular vision.
- Helps in diagnosing strabismus (absence or reduction of stereopsis).
- Evaluates amblyopia treatment (improvement of stereoacuity indicates progress).
- Guides strabismus surgery prognosis (better outcomes if stereopsis present).
- Occupational fitness testing (pilots, surgeons, sports professionals).
Causes of Abnormal Stereopsis
- Strabismus: Misalignment prevents fusion, reducing or abolishing stereopsis.
- Amblyopia: Suppression of one eye leads to poor stereopsis.
- Uncorrected anisometropia: Unequal image size (aniseikonia) prevents fusion.
- Optic nerve diseases: Optic neuritis, glaucoma.
- Macular disease: Age-related macular degeneration, diabetic maculopathy.
Advantages of Stereopsis Testing
- Non-invasive, safe, and quick.
- Provides objective evidence of binocular function.
- Useful in children and adults.
- Wide range of tests available for different age groups.
Limitations
- Requires patient cooperation and understanding.
- Some tests (Titmus) have monocular cues that may give false positives.
- Results may be influenced by refractive errors or poor illumination.
- Cannot distinguish between fine and coarse stereopsis in all tests.
Confrontation Visual Field Test
Introduction:
The visual field is the entire area that can be seen when the eye is fixed in one position, including both central and peripheral vision. Examination of the visual field is a critical part of ocular and neurological assessment because many diseases such as glaucoma, optic nerve disorders, retinal pathology, and intracranial lesions manifest as characteristic field defects.
The confrontation visual field test is the simplest and most widely used clinical method to assess visual fields at the bedside or in outpatient settings. It is a quick, cost-effective screening procedure that compares the patient’s field of vision to that of the examiner. Although not as precise as automated perimetry, it remains invaluable for detecting gross field defects and guiding further investigation.
Anatomy and Physiology of the Visual Field
The normal monocular visual field extends approximately:
- 60° nasally
- 100–110° temporally
- 60° superiorly
- 75° inferiorly
The binocular field of vision is the overlapping area seen by both eyes, essential for binocular vision and stereopsis. The integrity of the visual field depends on healthy functioning of the retina, optic nerve, chiasm, optic tracts, lateral geniculate body, and visual cortex.
Principle of Confrontation Test
The test is based on the assumption that the examiner’s visual fields are normal. By comparing the patient’s ability to perceive stimuli presented in the periphery with the examiner’s perception, defects can be detected. The examiner and patient face each other at eye level, and stimuli such as fingers, hand movement, or colored objects are introduced from the periphery toward the center.
Objectives
- To screen for gross visual field defects.
- To localize lesions in the visual pathway (retina to cortex).
- To detect neurological abnormalities affecting vision.
- To guide referral for detailed perimetry when needed.
Procedure of Confrontation Visual Field Test
- Seat the patient at eye level facing the examiner, about 1 meter apart.
- Ask the patient to cover one eye while fixing gaze on the examiner’s opposite eye.
- The examiner closes their corresponding eye to simulate monocular comparison.
- Examiner introduces a target (moving fingers, small object, or colored tip) from periphery toward the center in different quadrants (superior, inferior, nasal, temporal).
- Ask the patient to report when they first detect the stimulus.
- Compare patient’s response with examiner’s perception.
- Repeat for the other eye and all quadrants.
Variations of the Test
- Counting fingers: Patient states number of fingers shown in periphery.
- Hand movement: Patient identifies direction of moving hand.
- Color confrontation: Patient identifies colored object (detects acquired color defects).
- Red desaturation test: Patient compares intensity of red object between eyes (useful in optic nerve disease).
- Amsler grid: Though primarily central field, can complement confrontation for macular function.
Interpretation
Normal response: Patient detects stimulus at same time and position as examiner. Abnormal response: Patient fails to detect stimulus in certain areas, suggesting visual field loss.
Common Visual Field Defects Detected
- Central scotoma: Seen in optic neuritis, macular disease.
- Arcuate scotoma: Early glaucoma.
- Bitemporal hemianopia: Lesion at optic chiasm (pituitary tumor).
- Homonymous hemianopia: Lesion posterior to chiasm (optic tract, radiations, cortex).
- Quadrantanopia: Lesion in optic radiations (temporal or parietal lobe).
- Tunnel vision: Advanced glaucoma or retinitis pigmentosa.
Advantages
- Simple, quick, non-invasive.
- No equipment required beyond examiner’s hands or small object.
- Useful in outpatient clinics, emergency, and bedside settings.
- Helps localize lesions along the visual pathway.
Limitations
- Examiner must have normal visual fields.
- Only detects gross defects, not subtle or early changes.
- Subjective and dependent on patient cooperation.
- Not standardized, results vary between examiners.
- Cannot replace automated perimetry for precise mapping.
Clinical Importance
- Glaucoma screening: Detects peripheral field loss.
- Neurological assessment: Detects field defects from optic nerve to cortex.
- Stroke patients: Identifies homonymous hemianopia.
- Pituitary tumors: Early bitemporal hemianopia detection.
- Emergency eye care: Bedside assessment when perimetry not available.
Comparison with Other Field Tests
- Confrontation test: Screening, bedside, gross defects only.
- Manual perimetry (Goldmann): Semi-quantitative, requires skill.
- Automated perimetry (Humphrey): Gold standard for precise mapping and monitoring.
- Amsler grid: Central 10° field assessment.
