Competency 3.1.7 Flashcards
Layers of the Tear Film
- Deep mucous layer
- Watery aqueous layer
- Superficial lipid layer
Composition of the Tear Film by Layer
- Aqueous layer previously though to constitute 95% of tear film
- Recent research points towards the mucous layer making up more than was initially thought
Wettability and the Tear Film
- Poor wettability means that a liquid is unable to spread out on a given surface
- Cornea is a hydrophobic surface with poor wettability
- Mucous layer overcomes poor wettability
- The stability of the tear film is maintained by soluble lipids which dissolve into the aqueous, lowering surface tension and allowing the spread of tears across the ocular surface.
Lipid Layer in Tear Film
- The lipid layer is secreted by meibomian glands, forming a very thin layer at the top of the tears to prevent evaporation. Meibum has a melting point (normally) at 35 degrees which allows it to be constantly in a liquid state on the tears.
TBUT in Relation to the Layers of the Tear Film
- TBUT is time is taken for the aqueous layer to evaporate and the lipid layer to come into contact with the epithelium
Tear Film Changes Due to Blink
- Upon blinking the aqueous layer is cleared out, with the mucus layer remaining (albeit thinned).
The Functions of the Tear Film
Optical
- Provides smooth refractive surface
- Vision can be affected if this is inconsistent
Immune System
- Bactericidal, chemical and cellular components
- Poor tear film can lead to higher infection susceptibility
Lubrication
- Allows globe and lids to move freely
- Prevents cornea and conjunctiva from drying out
- Causes dryness/discomfort if tear film affected
Nutritional
- Source of oxygen
Surface Maintenance
- Remove debris from environment
Sections of a Slit Lamp Tear Film Assessment
- Initial Assessment
- Lids and Lashes
- Conjunctiva
- Cornea
- Tear Quantity
- Tear Quality
- Other Tests
Initial Tear Film Assessment
-Should be assessed at this early stage as bright light can induce reflex tearing and later actions may interfere and result in an invasive tear film assessment e.g flipping lids
- Assess tear film for debris etc
- Assess tear meniscus height
- Are tears frothy? (MGD)
Lids and lashes Assessment
- examine eyes closed and open
- look for debris and anterior blepharitis
- press bottom lids to check for any blocked glands and document if there are any blocked glands
- Any lid malposition
- Assess punctum (open or closed/current punctal plug)
Conjunctiva Assessment
- Look with all areas of bulbar conjunctiva, both with white light and with dye instilled
- Lissamine green is the best stain for conjunctival staining, though fluorescein is sufficient
- Assess both inferior and superior palpebral conjunctiva in both white light and with dye instilled
Cornea Assessment
- Optical section with white light
- Diffuse viewing with cobalt blue filter
- If staining present then assess further with corneal section
Tear Quantity Assessment
- Use an illuminated horizontal slit in alignment with the lower lid and reduce the width of the beam until it matches the height of the tear height. This is a non-invasive test
- Use Schirmer test. Hook the test strip over the lid margin and measure how far the tear wetting travels along the strip in a five minute interval.
- Phenol red thread test is similar in principle to the Schirmer test but is though to be less invasive and therefore give a better representation of resting tear volume. The thread is pH sensitive and turns from yellow to red when the tears travel along each section of the thread
Tear Quality Assessment
- Non-invasively we can assess TBUT using a Bausch and Lomb Keratometer but keeping eye open until the mires start to distort
- Invasively we can instill fluorescein and measure the first break down in teat film under cobalt blue illumination
Other Tests to Assess Tear Film
- LIPCOF stands for lid parallel conjunctival folds. These are folds which appear nasally and temporally to the limbus along the lid margin, disappear on lifting of lower lid and result in symptoms of dryness/irritation. The are graded grade 0 for 0 folds, grade 1 for 1 fold etc…
- Lid Wiper eptheliopathy is visualised by use of lissamine green along the inside lid margin of the upper eyelid. A thin line can be viewed as normal and 85% of regular CL wearers have some form of this, but the thicker the line the greater the indication of dryness as the tear film is no longer able to provide sufficient lubrication for the lid to move across the ocular surface.
- Tear osmolarity is the amount of salt concentrated in an individuals tears. Hyperosmolarity has been found in those with dry eyes. Osmolarity under 308mOsm/L is considered normal with progressively severe dry eye being associated with higher values above this point
