Tear Film: Layers, Glands & Break-Up Time
Key Takeaways
- The tear film is a three-layered structure (lipid, aqueous, mucin) approximately 7-9 micrometers thick that lubricates, nourishes, and optically smooths the corneal surface
- Tear film break-up time (TBUT) of 10 seconds or longer is considered stable; values under 10 seconds indicate tear film instability and dry eye
- Meibomian glands in the tarsal plate produce the lipid layer; Meibomian gland dysfunction (MGD) is the leading cause of evaporative dry eye
- The Schirmer test measures aqueous tear production; less than 10 mm of wetting in 5 minutes (with anesthesia) suggests aqueous-deficient dry eye
- For contact lens wearers, the tear film is the interface between the lens and cornea; inadequate tears cause discomfort, deposits, corneal staining, and infection risk
Tear Film: Layers, Glands & Break-Up Time
Quick Answer: The precorneal tear film is a three-layered structure (lipid, aqueous, mucin) approximately 7-9 micrometers thick that lubricates, nourishes, and optically smooths the corneal surface. Tear film break-up time (TBUT) of 10 seconds or longer is considered stable; values under 10 seconds indicate instability and dry eye. For contact lens wearers, the tear film is the interface between the lens and cornea, and its quality determines comfort, vision, and safety.
The Three Tear Film Layers
The tear film is classically described as a trilaminar structure. Although modern research suggests it behaves more like a gradient, the three-layer model remains the NCLE-relevant framework for exam questions.
| Layer | Source | Thickness | Primary Function |
|---|---|---|---|
| Lipid (outer) | Meibomian glands (tarsal plate) | ~0.1 micrometers | Retards evaporation, smooths optical surface |
| Aqueous (middle) | Main & accessory lacrimal glands | ~7 micrometers | Supplies oxygen, nutrients, antibacterial proteins |
| Mucin (inner) | Goblet cells of conjunctiva | ~1-2 micrometers | Anchors tear film to corneal epithelium |
Lipid Layer
The lipid layer is the outermost film, secreted by the Meibomian glands (also called tarsal glands) embedded in the tarsal plate of the upper and lower lids. There are roughly 30-40 Meibomian glands in the upper lid and 20-30 in the lower lid. The lipid layer has polar and non-polar phases that together reduce aqueous evaporation by up to 90 percent. Meibomian gland dysfunction (MGD) is the leading cause of evaporative dry eye and is highly relevant in CL candidates.
Aqueous Layer
The aqueous layer is the thickest component (~7 micrometers) and is produced by the main lacrimal gland (orbital and palpebral lobes) plus the accessory lacrimal glands of Krause and Wolfring. It carries electrolytes, glucose, oxygen, lysozyme, lactoferrin, IgA, and growth factors. The aqueous layer is what the Schirmer test quantifies.
Mucin Layer
The mucin layer is secreted by goblet cells (densely packed in the inferonasal conjunctiva) and by corneal and conjunctival epithelial cells. Mucins are glycoproteins that convert the hydrophobic corneal surface into a wettable, hydrophilic one. Without mucin, the tear film beads up and breaks apart regardless of aqueous volume.
Tear Film Geometry: Meniscus and Lacrimal Lake
At the lid margins, the tear film forms the tear meniscus (lacrimal lake), a small reservoir 0.2-0.5 mm in height along the lower lid margin. A reduced meniscus height (under 0.2 mm) is a clinical sign of aqueous deficiency. Tears drain through the puncta at the medial canthus into the canaliculi, lacrimal sac, and nasolacrimal duct.
Tear Film Break-Up Time (TBUT)
TBUT is the time, in seconds, between a complete blink and the first appearance of a dry spot (dark area) on the cornea under fluorescein and cobalt blue light.
- Normal: 10 seconds or longer
- Suspect instability: under 10 seconds
- Frank dry eye: under 5 seconds
A reduced TBUT indicates that one or more layers are failing, most commonly the lipid layer (evaporative) or mucin layer (wettability). For CL candidates, a low pre-fitting TBUT predicts lens intolerance, deposit formation, and corneal staining.
Schirmer Test
The Schirmer test uses a 5 x 35 mm filter paper strip hooked over the lower lid margin (typically the lateral third, away from the punctum). After 5 minutes, the wetting length is measured.
| Schirmer I (with anesthesia) | Interpretation |
|---|---|
| Greater than 10 mm | Normal aqueous production |
| 5-10 mm | Borderline / mild deficiency |
| Under 5 mm | Aqueous-deficient dry eye |
Schirmer I without anesthesia measures both basal and reflex tearing; with anesthesia it isolates basal secretion. Schirmer II (nasal stimulation) tests reflex tearing.
Contact Lens Relevance
The tear film is the single interface between a contact lens and the cornea. A soft lens sits on the post-lens tear film; a rigid lens rides on the tear film that acts as the true optical element.
Clinical consequences of inadequate tears in CL wearers:
- Comfort: dryness, burning, foreign body sensation, end-of-day intolerance
- Vision: fluctuating vision, especially with rigid gas permeable (RGP) lenses
- Deposits: protein and lipid deposits accelerate when tears stagnate
- Corneal staining: punctate epithelial erosions, especially inferiorly or 3-9 o'clock staining with RGPs
- Infection risk: reduced lysozyme and lactoferrin delivery raises microbial risk
A contact lens itself thins the lipid layer and disrupts mucin distribution at the lens edge. Even a normal tear film may be marginally adequate once a lens is added. Pre-fitting TBUT and Schirmer values are therefore part of the NCLE-recommended fitting workup.
Which gland is the primary source of the tear film's outer lipid layer?
A patient has a fluorescein TBUT of 7 seconds. Which interpretation is most accurate?