Tear Film
Composition · Stability · Dysfunction
Examination question · ~800 words
What is the tear film? Discuss its composition, stability mechanisms, dry eye disease classification, investigations, and evidence-based management.
Try to outline your answer mentally before expanding sections below.
The tear film is a trilaminar fluid interface between the atmosphere and the corneal epithelium. Despite being only a few microns thick (typically around 3 µm over the cornea), it performs functions critical to ocular surface integrity, optical clarity, and defence against pathogens. Disruption of any layer precipitates dry eye disease (DED), the most prevalent ocular surface disorder. Understanding tear film physiology is foundational to corneal, anterior segment, and refractive surgery practice.
The classic three-layer model (Wolff, 1954) — lipid / aqueous / mucin — remains clinically useful, though contemporary evidence (Nichols et al., 2004) supports a two-zone model: an outer lipid layer over a mixed aqueous-mucin gel.
Tear Film Layers — Summary
| Layer | Thickness | Origin | Composition | Function |
|---|---|---|---|---|
| Outer Lipid | 0.1–0.2 µm | Meibomian glands (non-polar); Zeis / Moll (polar) | Non-polar: wax esters, cholesterol esters; Polar: phospholipids | Retards evaporation, spreads tear film, reduces surface tension |
| Middle Aqueous | ~7 µm | Lacrimal gland (reflex + basal); Accessory glands (Krause, Wolfring) | Water (98%), electrolytes (Na+, K+, Cl-); Proteins: lysozyme, lactoferrin, IgA; Growth factors: EGF, TGF-b | Nutrition, O2 supply, antimicrobial, lubrication |
| Inner Mucin | 0.2–0.5 µm | Goblet cells (MUC5AC); Conjunctival epithelium (MUC1, MUC4, MUC16); Corneal epithelium (MUC1, MUC16) | Gel-forming: MUC5AC; Membrane-anchored: MUC1, MUC4, MUC16 | Wettability of hydrophobic epithelium, glycocalyx protection, trapping debris |
Lipid Layer — Meibomian Gland Anatomy
- 25–40 glands in upper lid, 20–30 in lower; each has 10–15 acini
- Meibomian lipids: wax esters and cholesterol esters (non-polar core) + polar phospholipid-containing interface. Note: the precise composition of the polar surface is more complex than a simple phospholipid monolayer — modern lipid biochemistry recognises a mixture of polar lipids at the aqueous-lipid interface.
- Spreads over aqueous on blink; forms lamellar body at lid margin
- TFOS DEWS II (2017): MGD is the leading cause of evaporative DED worldwide
Mucin Layer — Glycocalyx and Wettability
- Gel-forming mucins (MUC5AC) secreted by conjunctival goblet cells form a hydrophilic mesh
- Membrane-anchored mucins (MUC1, MUC16) on corneal epithelial microvilli convert hydrophobic surface to hydrophilic
- MUC16 acts as a barrier against pathogen adhesion
- Goblet cell density highest in nasal conjunctiva; reduced in cicatrising conditions (OCP, SJS)
Stability is assessed by tear film break-up time (TBUT) — the interval between a complete blink and appearance of the first dry spot. TBUT is a classic bedside test. A value <10 s is commonly regarded as abnormal, though thresholds are technique-dependent. Non-invasive TBUT (NITBUT) is preferred when available, as fluorescein instillation itself can destabilise the tear film and produce falsely short readings.
Tear Film Distribution
- Volume: ~6–8 µL on ocular surface; lacrimal lake ~1–2 µL
- Renewal: ~16% per minute; complete turnover ~5–7 min
- Drainage: 75% canalicular (active + passive), 25% evaporation
Blink Dynamics
- Blink rate: ~12–15/min (reduced to 4–6/min during screen use)
- Incomplete blink → failure to replenish inferior tear meniscus → inferior punctate keratopathy
- Upper lid spread rate determines post-blink stabilisation time (~2–3 s)
Tear Film Break-Up Mechanisms (TFOS DEWS II)
| Mechanism | Pathophysiology | Clinical Correlate |
|---|---|---|
| Evaporative | Lipid layer deficiency → increased TEWL | MGD, aqueous-sufficient DED |
| Mucin-deficient | Reduced goblet cells → poor wettability | SJS, OCP, Vitamin A deficiency |
| Aqueous-deficient | Lacrimal gland hyposecretion | Sjogren's, post-radiotherapy |
| Mixed | Combined mechanism (most common) | Prolonged / chronic DED |
Total tear protein ~6–8 mg/mL. Key components and their clinical significance:
| Protein | % of Total | Function | Clinical Relevance |
|---|---|---|---|
| Lysozyme | ~25% | Cleaves bacterial peptidoglycan (muramidase activity) | Reduced in Sjogren's; marker of lacrimal function |
| Lactoferrin | ~25% | Iron chelation → bacteriostasis; anti-inflammatory | Reduced in aqueous DED; available as diagnostic test |
| Secretory IgA | ~15% | Prevents microbial adhesion; neutralises toxins | Produced locally by plasma cells in lacrimal gland |
| Albumin | ~5% | Osmotic balance, drug binding | Increased in inflammation — marker of blood-tear barrier breakdown |
| EGF / TGF-b | Trace | Epithelial proliferation and wound healing | Reduced in neurotrophic keratopathy; basis for cenegermin (rhNGF) therapy |
- Normal: <307 mOsm/L (TearLab Osmolarity System)
- DED threshold: ≥308 mOsm/L (single eye) or inter-eye difference ≥8 mOsm/L
- Hyperosmolarity triggers: IL-1b, TNF-a, MMP-9 → epithelial apoptosis → mucin loss → vicious cycle
Tear osmolarity is an important biomarker of tear-film homeostatic disturbance, but it should be interpreted alongside symptoms and other clinical signs. TFOS DEWS II emphasises overlap between DED and normal values and does not endorse a single standalone diagnostic metric.
InflammaDry: detects MMP-9 >40 ng/mL — marker of ocular surface inflammation; positive result supports DED diagnosis in the right clinical context.
| Type | Subtype | Examples |
|---|---|---|
| Aqueous Deficient (ADDE) | Sjogren's Syndrome DED / Non-Sjogren's DED | Primary / secondary SS; Lacrimal gland disease, post-radiation, Riley-Day syndrome |
| Evaporative (EDE) | Intrinsic / Extrinsic | MGD, low blink rate, incomplete blink / Vitamin A deficiency, contact lens wear, preservatives |
| Mixed | Both mechanisms combined | Chronic DED (most prevalent in clinical practice) |
Schirmer's Test Interpretation
| Test | Method | Normal | Abnormal |
|---|---|---|---|
| Schirmer I (without anaesthesia) | Filter paper at outer 1/3 of lower lid, 5 min | >15 mm | <10 mm suggestive; <5 mm diagnostic |
| Schirmer I (with anaesthesia) | Topical anaesthetic applied first — intended to reduce reflex tearing and approximate basal secretion (does not cleanly isolate basal secretion) | >10 mm | <5 mm diagnostic of aqueous deficiency |
| Schirmer II (Jones test) | Nasal mucosal stimulation after topical anaesthesia | >15 mm reflex | Distinguishes lacrimal gland secretory reserve |
Clinical Tests
- TBUT (fluorescein): a classic bedside test; <10 s is commonly abnormal, though thresholds are method-dependent
- Non-invasive TBUT (NITBUT): Keratograph 5M — Placido ring reflection; preferred when available as it avoids fluorescein artifact and is more reproducible
- Ocular Surface Staining: Fluorescein (corneal epithelial defects), Rose Bengal (cells not protected by mucin), Lissamine green (conjunctival — less toxic than RB)
- Tear Meniscus Height (TMH): Normal ≥0.2 mm; anterior-segment OCT is a useful noninvasive method for TMH assessment (not a universal gold standard, as no single test holds that status in DED diagnostics)
- Meibography: Infrared imaging — quantifies meibomian gland dropout (Meiboscore 0–3)
- Blink analysis: High-speed video — incomplete blink rate
- In vivo confocal microscopy (IVCM): Goblet cell density, sub-basal nerve plexus, inflammatory cell assessment
TFOS DEWS II / AAO Stepwise Management
| Step | Intervention | Key Notes |
|---|---|---|
| Step 1 — Mild | Patient education + environmental modification; Preservative-free lubricant drops (HA, CMC, HP-guar); Lid hygiene + warm compresses (MGD) | Preservative-free critical in chronic use. Sodium hyaluronate preferred for epithelial healing. |
| Step 2 — Moderate | Topical cyclosporine 0.05–0.1% (Restasis, Ikervis); Topical lifitegrast 5% (Xiidra) — LFA-1 antagonist; Omega-3 supplements (limited evidence — DREAM trial); Punctal occlusion (collagen / silicone plugs) | CSA: 3–6 months for full effect; calcineurin inhibitor. Lifitegrast: symptom improvement demonstrated as early as 2 weeks in OPUS-3; blocks T-cell adhesion via LFA-1/ICAM-1. Punctal plugs: consider after assessing ocular-surface inflammation; a selective rather than absolutely contraindicated option. |
| Step 3 — Severe | Autologous serum tears (20–50%); Scleral contact lenses; Oral pilocarpine 5 mg QDS / cevimeline 30 mg TDS; Intense pulsed light (IPL) for MGD | AST: contains EGF, fibronectin, Vit A. IPL: targets abnormal lid vasculature, reduces inflammation. Pilocarpine/cevimeline: muscarinic M3 agonists — Sjogren's. |
| Step 4 — Very Severe | Tarsorrhaphy (temporary or permanent); Amniotic membrane transplant; Salivary gland autotransplantation; Mucous membrane grafting | Reserved for neurotrophic keratopathy, exposure keratopathy, end-stage DED. |
Emerging and Novel Therapies
- Cenegermin (Oxervate / rhNGF): Recombinant nerve growth factor — FDA/EMA approved for neurotrophic keratopathy; promotes epithelial healing and corneal sensitivity
- Perfluorohexyloctane (MIEBO; formerly NOV03): FDA approved May 18, 2023 — first perfluorocarbon drop specifically for evaporative DED; pivotal phase 3 efficacy data from GOBI and MOJAVE trials; forms stable lipid layer substitute
- Thymosin-b4: Phase II trials — promotes wound healing and goblet cell regeneration
- Microwave thermal pulsation (LipiFlow, iLux): Vectored thermal pulsation devices — evidence base growing for MGD treatment
- Lacrimal gland stem cell / organoid therapy: Under investigation for secretory ADDE
| Trial | Year | Drug / Intervention | Key Finding |
|---|---|---|---|
| OPUS-1 / OPUS-2 | 2014–15 | Lifitegrast 5% | Significant improvement in eye dryness score and corneal staining vs placebo |
| OPUS-3 | 2016 | Lifitegrast 5% (symptom-focused) | Primary endpoint (SANDE score) met; led to FDA approval 2016; symptom improvement demonstrated as early as day 14 |
| SONATA | 2017 | Lifitegrast | Confirmed long-term safety and tolerability over 1 year (360-day multicenter randomized safety study) |
| Restasis trials | 2002–03 | Cyclosporine 0.05% (CSA) | Increased Schirmer scores and reduced corneal staining vs vehicle; FDA approved 2003 |
| DREAM study | 2018 | Omega-3 supplementation | Omega-3 NOT superior to olive oil placebo in moderate DED — negative RCT; caution when citing omega-3 as evidence-based therapy (NEJM 2018;378(18):1681–1690) |
| GOBI / MOJAVE (MIEBO) | 2022–23 | Perfluorohexyloctane | Phase 3 trials met primary endpoints for evaporative DED; FDA approved MIEBO May 18, 2023 |
The pivotal negative omega-3 DED trial is the DREAM study (NEJM 2018), NOT a COMET study. COMET is an unrelated trial name. The DREAM study showed omega-3 was not superior to olive oil placebo.
The 1-year lifitegrast safety study is SONATA, not LEVEL. LEVEL is not a recognised lifitegrast landmark trial.
The tear film, though structurally simple, is functionally indispensable. Its trilaminar architecture maintains optical quality, corneal nutrition, and antimicrobial defence. Disruption — whether from lipid, aqueous, or mucin insufficiency — triggers a self-perpetuating inflammatory cascade culminating in dry eye disease. Modern management is guided by the TFOS DEWS II framework, targeting both the underlying aetiology and the inflammatory amplification cycle. Novel agents such as perfluorohexyloctane (MIEBO) and cenegermin, alongside advancing biological therapies, are reshaping the treatment landscape. Understanding tear film physiology remains central to all modern anterior segment practice.
Trap 1 — FALSE
“Rose Bengal stains devitalised (dead) cells”
Rose Bengal stains cells NOT protected by mucin — not simply devitalised cells. Lissamine green is preferred in clinical practice as it is less toxic to the ocular surface.
Trap 2 — FALSE
“Schirmer I WITHOUT anaesthesia tests basal secretion alone”
Schirmer I WITHOUT anaesthesia tests reflex + basal secretion combined. Schirmer I WITH anaesthesia is intended to approximate basal secretion, but even then does not completely eliminate reflex tearing.
Trap 3 — FALSE
“TFOS DEWS II defines DED simply as aqueous deficiency”
TFOS DEWS II defines dry eye as a multifactorial disease of the ocular surface characterized by loss of homeostasis of the tear film, accompanied by ocular symptoms; tear-film instability, hyperosmolarity, inflammation/damage, and neurosensory abnormalities play etiologic roles. It is not defined simply as aqueous deficiency.
Trap 4 — FALSE
“MUC5AC is membrane-anchored on the corneal epithelium”
MUC5AC is a gel-forming mucin secreted by conjunctival goblet cells. MUC1 and MUC16 are membrane-anchored mucins on the corneal epithelium that convert the hydrophobic surface to hydrophilic.
Trap 5 — FALSE
“The COMET study showed omega-3 was not superior to placebo in moderate DED”
The pivotal negative omega-3 DED trial is the DREAM study (NEJM 2018;378(18):1681–1690), not the COMET study. COMET is not a recognised DED landmark trial. The DREAM study showed omega-3 was not superior to olive oil placebo.
Trap 6 — FALSE
“Lifitegrast blocks calcineurin”
Lifitegrast blocks LFA-1 / ICAM-1 interaction (T-cell adhesion), NOT calcineurin. Calcineurin inhibition is the mechanism of cyclosporine.
Q: What is the difference between fluorescein TBUT and NITBUT?
A: Fluorescein instillation itself destabilises the tear film (falsely short TBUT); NITBUT uses Placido ring reflection on the Keratograph 5M — more reproducible and avoids the fluorescein artifact. NITBUT is preferred when available.
Q: Why does Vitamin A deficiency cause dry eye?
A: Vitamin A deficiency causes conjunctival squamous metaplasia → goblet cell loss → MUC5AC deficiency → poor corneal wettability → dry eye and Bitot's spots.
Q: Describe the vicious cycle of DED
A: Tear instability → hyperosmolarity → inflammatory mediators (MMP-9, IL-1b, TNF-a) → epithelial apoptosis → mucin loss → further tear instability — a self-perpetuating cascade.
Q: How does cyclosporine work in DED?
A: Inhibits calcineurin → blocks NFAT nuclear translocation → suppresses T-lymphocyte activation → reduces IL-2, TNF-a, IFN-g → reduces ocular surface inflammation and apoptosis → improves goblet cell density and Schirmer scores over 3–6 months.
A 35-year-old woman with primary Sjogren's syndrome presents with severe bilateral dry eye (Schirmer I: 2 mm OU, TBUT: 2 s, osmolarity 320 mOsm/L) unresponsive to topical CSA for 12 months and punctal occlusion. She is now considering autologous serum tears. Critically evaluate the evidence for autologous serum tears, and outline an evidence-based escalation strategy including systemic options.
Autologous Serum Tears (AST)
Derived from centrifuged autologous blood — contains EGF, fibronectin, Vitamin A, TGF-b, IgA, and lysozyme at near-physiological concentrations. Evidence: multiple RCTs (Tsubota et al., 1999; Kojima et al., 2005) demonstrate superior improvement in corneal staining and symptom scores vs preservative-free artificial tears in severe Sjogren's DED. Concentration: typically 20% for moderate and 50% for severe cases, though protocols vary by centre as preparation is not standardised across centres. Limitations: requires haematology/blood banking, refrigerated storage, 3-monthly preparation, infection risk if contaminated, not universally available on NHS / government formulary.
Escalation Strategy
(1) Upgrade to 50% AST QID; (2) Add topical lifitegrast 5% BD — distinct mechanism from CSA (LFA-1/ICAM-1 block vs calcineurin); (3) Oral pilocarpine 5 mg QDS or cevimeline 30 mg TDS — muscarinic M3 agonists, Level I evidence in primary Sjogren's for stimulating lacrimal and salivary secretion; (4) Scleral contact lenses — maintain a fluid reservoir over the entire cornea, strongest evidence for severe aqueous-deficient DED; (5) Rheumatology co-management — systemic therapy (e.g. hydroxychloroquine) is directed at underlying rheumatological disease; however, evidence for improvement of ocular dryness outcomes specifically is limited and should not be framed as a primary ocular escalation step; (6) Last resort: medial tarsorrhaphy; salivary gland autotransplantation (Jablonski-Stiehler procedure) as definitive secretory substitute.
Monitoring Endpoints
Repeat osmolarity, TBUT, Oxford corneal staining grade, Schirmer score, and MMP-9 (InflammaDry) at 3-month intervals to guide step-up decisions.
References
- Craig JP, et al. TFOS DEWS II Definition and Classification Report. Ocul Surf. 2017;15(3):276–283.
- Wolffsohn JS, et al. TFOS DEWS II Diagnostic Methodology Report. Ocul Surf. 2017;15(3):539–574.
- Jones L, et al. TFOS DEWS II Management and Therapy Report. Ocul Surf. 2017;15(3):575–628.
- Nichols KK, et al. The international workshop on meibomian gland dysfunction: executive summary. Invest Ophthalmol Vis Sci. 2011;52:1922–1929.
- Kanski JJ, Bowling B. Clinical Ophthalmology: A Systematic Approach. 8th ed. Elsevier; 2016.
- Yanoff M, Duker JS. Ophthalmology. 5th ed. Elsevier; 2019.
- Holland EJ, Mannis MJ, Lee WB. Ocular Surface Disease: Cornea, Conjunctiva and Tear Film. Elsevier; 2013.
- Sheppard JD, et al. OPUS-1 study: Lifitegrast ophthalmic solution 5.0% for treatment of dry eye disease. Ophthalmology. 2014;121(2):475–483.
- Tsubota K, et al. Treatment of dry eye by autologous serum application in Sjogren's syndrome. Br J Ophthalmol. 1999;83(4):390–395.
- Wirta D, et al. Perfluorohexyloctane (NOV03) ophthalmic solution for evaporative dry eye disease: results of the GOBI and MOJAVE phase 3 trials. Ophthalmology. 2022.
- Donnenfeld ED, et al. DREAM Study Research Group. n-3 Fatty Acid Supplementation for the Treatment of Dry Eye Disease. N Engl J Med. 2018;378(18):1681–1690.
- Donnenfeld ED, et al. SONATA study: Lifitegrast 5% ophthalmic solution 1-year safety study. Cornea. 2016;35(6):741–748.
- Murri MS, et al. Cenegermin ophthalmic solution for neurotrophic keratitis. Ophthalmol Ther. 2018;7(1):1–12.