12.3 Tear Film Assessment: Schirmer, Tear Break-Up Time, Vital Dyes & Corneal Sensitivity
Key Takeaways
- Schirmer I without anaesthetic measures reflex plus basal tearing; with anaesthetic it approximates basal secretion.
- Less than 10 mm of wetting in 5 minutes is generally abnormal and less than 5 mm indicates severe aqueous deficiency.
- Tear break-up time under 10 seconds indicates an unstable tear film and under 5 seconds is clearly abnormal.
- Lissamine green and rose bengal stain devitalised cells and mucin-deficient areas; fluorescein stains epithelial defects.
- The Cochet-Bonnet aesthesiometer quantifies corneal sensation by the length of an extended nylon filament.
Order of testing matters
Tear tests interfere with one another. Perform them least invasive first:
- History and symptom questionnaire
- Osmolarity (if available) — must precede any drop
- Tear meniscus height at the slit lamp
- Tear break-up time (requires fluorescein)
- Corneal and conjunctival staining (fluorescein, then lissamine green)
- Schirmer test
- Corneal sensitivity
Performing a Schirmer first stimulates reflex tearing and corrupts everything after it. Instilling anaesthetic first invalidates the Schirmer I and corneal sensitivity.
Schirmer tests
A standardised 5 mm × 35 mm strip of Whatman No. 41 filter paper is folded at the notch and hooked over the lower lid at the junction of the lateral and middle thirds, away from the cornea. The patient closes the eyes gently (or looks up, per protocol) for 5 minutes, and the wetted length is read from the fold.
| Test | Anaesthetic | What it measures |
|---|---|---|
| Schirmer I | No | Basal plus reflex secretion |
| Basal secretion test (Jones) | Yes — topical anaesthetic, then blot the fornix | Approximately basal secretion |
| Schirmer II | Yes, plus nasal mucosa stimulation with a cotton swab | Reflex secretion from the nasolacrimal reflex arc |
Interpretation (Schirmer I, 5 minutes):
| Wetting | Interpretation |
|---|---|
| Over 15 mm | Normal |
| 10–15 mm | Borderline |
| Under 10 mm | Abnormal, consistent with aqueous deficiency |
| Under 5 mm | Severe aqueous deficiency; consider Sjögren syndrome |
Sources of error: the strip touching the cornea (reflex tearing, falsely high), a strip placed centrally rather than laterally, a patient who keeps their eyes open under a fan or bright light (evaporation, falsely low), squeezing, and inconsistent timing. Record which variant was performed — a Schirmer result without its anaesthetic status is uninterpretable.
Tear break-up time
Tear break-up time (TBUT) measures tear film stability, which is the dominant problem in evaporative dry eye — the commonest form.
- Instil a minimal amount of fluorescein: moisten a strip with non-preserved saline, shake off excess, touch the inferior fornix. Too much fluorescein artificially lengthens the measured time.
- Ask the patient to blink several times, then hold the eyes open without blinking.
- View with the cobalt blue filter and a broad beam at low magnification.
- Start timing at the last complete blink; stop at the first dark spot, streak or dry area in the tear film.
- Repeat three times and average.
| TBUT | Interpretation |
|---|---|
| Over 10 seconds | Normal |
| 5–10 seconds | Borderline / marginal instability |
| Under 5 seconds | Clearly abnormal |
Non-invasive break-up time (NIBUT), measured by watching a projected grid or placido ring pattern distort, avoids the confounding effect of the dye and is increasingly standard on modern topographers and surface analysers.
A recurring break-up spot in the same location points to a local surface abnormality — a scar, a Salzmann nodule, an epithelial basement membrane irregularity or a raised suture.
Vital dyes
Three dyes, three different meanings. Confusing them is a standard examination trap.
| Dye | Stains | Viewed with | Notes |
|---|---|---|---|
| Sodium fluorescein | Epithelial defects — areas where cells are missing and the dye pools in the intercellular spaces or on bare stroma | Cobalt blue filter, enhanced by a Wratten yellow #12 barrier filter | Does not stain intact epithelium; the standard dye for abrasions, ulcers, dendrites and the Seidel test |
| Rose bengal | Devitalised and dead cells, and epithelium unprotected by mucin | White or red-free light | Stings considerably; largely displaced by lissamine green |
| Lissamine green | Same targets as rose bengal — devitalised cells and mucin-deficient areas | White or red-free light; best viewed 1 to 4 minutes after instillation | Well tolerated, the modern standard for conjunctival staining |
Staining patterns worth recognising:
| Pattern | Suggests |
|---|---|
| Inferior interpalpebral punctate staining | Aqueous-deficient dry eye, exposure |
| Interpalpebral band (3 and 9 o'clock) conjunctival staining | Evaporative dry eye, exposure, rigid lens 3-and-9 staining |
| Superior punctate staining | Superior limbic keratoconjunctivitis, foreign body under the upper lid — evert the lid |
| Linear vertical abrasions | Foreign body under the upper lid |
| Dendrite with terminal end bulbs | Herpes simplex epithelial keratitis |
| Positive Seidel — a stream of diluted green fluorescein from a wound under cobalt blue | Aqueous leak; full-thickness wound — surgical emergency |
The Seidel test is a named technique worth spelling out: apply concentrated fluorescein over the suspect site, view under cobalt blue, and watch for a dark rivulet of aqueous diluting the dye as it flows. A positive Seidel means an open wound.
Corneal sensitivity
Corneal sensation is carried by the long ciliary nerves from the ophthalmic division of the trigeminal nerve (V1). Testing it is a COMT-level Supplemental Testing task.
Qualitative bedside test. Twist a wisp of cotton wool to a fine point and touch the cornea from the side, out of the patient's line of sight so the response is not anticipatory. Compare the two eyes and compare central with peripheral cornea. Record as normal, reduced or absent, and compare sides. Anaesthetic must not have been instilled.
Quantitative test — the Cochet-Bonnet aesthesiometer. A nylon monofilament of fixed diameter is extended from a holder. A longer filament bends more easily and therefore applies less pressure; a shorter filament is stiffer and applies more pressure.
- Start at the full 60 mm length.
- Touch the cornea perpendicularly until the filament just begins to bend.
- If there is no response, shorten in 5 mm steps and repeat.
- Record the longest filament length that produces a consistent response.
A normal cornea responds at or near 60 mm. Progressively shorter thresholds indicate progressively reduced sensation.
Causes of reduced corneal sensation:
- Herpes simplex and herpes zoster keratitis — a key diagnostic clue, and the reason a herpetic ulcer may be surprisingly painless
- Diabetes mellitus
- Long-term contact lens wear
- After LASIK, PRK or penetrating keratoplasty (corneal nerves are transected)
- Trigeminal nerve lesions, acoustic neuroma, brainstem disease
- Chronic topical anaesthetic abuse
- Chemical injury
Why it matters clinically: a cornea without sensation loses its protective blink and its trophic nerve supply, producing neurotrophic keratopathy — persistent epithelial defects that do not hurt and can progress to melting and perforation before the patient complains. Reduced sensation therefore changes both the diagnosis and the follow-up interval.
A Schirmer test performed without anaesthetic shows 4 mm of wetting in 5 minutes. What does this indicate?
Which dye stains devitalised cells and areas unprotected by mucin, and is better tolerated than rose bengal?
Using a Cochet-Bonnet aesthesiometer, a patient responds only when the filament is shortened to 25 mm. What does this mean?
In what order should tear film tests be performed?
A stream of diluted fluorescein is seen flowing from a corneal wound under cobalt blue illumination. What does this signify?