Download Ocular Surface – Tear Film

Tear Film Structure and
Function
Rachel Redfern, OD, PhD, FAAO
September 29, 2016
Tear Film
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What is the tear film?
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What are its characteristics?
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What are its
components/structure?
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Why is it important?
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What are some methods for
clinical examination?
What is the tear film?
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Fluid secreted to protect the
ocular surface from stress:
• Chemical
• Physical
• Microbial
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Serves as the first refractive
surface of the eye
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Lubrication
Normal Tear Film
Modified from Pflugfelder, Dry Eye and Ocular Surface Disorders, Elaine Kurie Illustrations
Tear Film: Why is it important?
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Essential for maintaining:
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health of the cornea and conjunctiva
optical quality of the cornea
Disruption leads to:
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compromised corneal and conjunctival
physiology
reduced visual performance
What is the tear film?
Normal Tear Film
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Exceedingly complex
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Composition is dynamic to
maintain homeostasis
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Components interact to create
a hydrated gel
Modified from Pflugfelder, Dry Eye and Ocular Surface Disorders, Elaine Kurie Illustrations
Tear Film Structure
• The anterior lipid layer provides
stability by interacting with the
mucin-aqueous phase
• Secretory mucins mix with the
aqueous layer
• Membrane-associated mucins on
the microplicae of the epithelium
form the glycocalyx
From Gipson IK, Argueso
Tear Film Structure: Lipid Component
Meibomian Glands
visible through
conjunctiva
ducts on lid margin
Tear Film Structure: Lipid Component
Length
• Follows the tarsus (4-5mm)
Number
• Upper lid: 30-40
• Lower lid: 20-30
Volume
• Higher in upper lid (26µl vs. 13µl)
Relative functional
contribution (upper vs.
lower) to the tear film lipid
layer is unknown…
Tear Film: Lipid Component Summary
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Outmost, superficial layer
of the tear film
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Function (s):
1) Reduces evaporation
2) Stabilize the tear film
3) Barrier to pathogens
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Complex mixture
Tear Film: Aqueous Component
• Functions:
Protection (e.g., immune defense)
Wound healing
Stability promoting
Enzymes (catalyze other reactions)
Metabolic
Structural/mechanical
Cell signaling
Cell adhesion
Transport
Aqueous!
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Tear Film Summary
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Fluid secreted to protect the ocular
surface from stress
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Chemical, microbial, physical,
hypoxia
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Lipid layer = meibomian glands
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Aqueous = lacrimal gland
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Mucin = goblet cells
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Mostly parasympathetic control
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Hormonal influences
Ocular Surface Lab
Tear Osmolarity
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Tear lab
Tear ferning
Tear Secretion
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Schirmer’s Test
Phenol Red Thread
Tear Film Stability (Keratograph)
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NITBUT
Lipid Layer Evaluation
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Meibography (Keratograph)
Interferometry (Lipiview)
Tear Ferning
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Ions in the Tear Film
+ Sodium and Potassium
Chloride and Bicarbonate
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Useful in the detection of
hyperosmolarity
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The shift in the salt-tomacromolecule
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↓ Proteins & Mucins
↑ Ions
TearLab
TearLabTM
TearLab
DEFINITION: number of
osmoles (Osm) of solute
per litre (L) of solution
NaCl dissociates Na+ and
Cl− ions
1mole of NaCl becomes
two osmoles in solution
Tear Production
Tear Secretion
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Schirmer Strip
Phenol Red Thread
Tear Stability
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TBUT
(SLE optional)
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NITBUT
Keratograph
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Meibography (Keratograph)
• IR light at 8001400nm
• Increased contrast
between the
glands and
surrounding tissue
LipiView
Lipid Layer Evaluation
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interferometry to
measure the lipid
layer’s thickness
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color assessment of
the tear film by
specular reflection
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interferometric color
units (ICU, nm)
Questions?