Tear film Flashcards
what does the epithelium do?
-protects the cornea by providing a barrier against water, large molecules, and toxins
what is Bowman’s membrane?
-consists of randomly arranged collagen fibrils
what layer contributes to 90% of corneal thickness?
Stroma
how are the collagen fibrils of the stroma arranged?
- fibrils within each lamellae run parallel
- each layer runs across at right angles
what is the shape of the endothelium? what is its function?
- hexogonal shape
- regulates fluid balance of the cornea and maintain stroma hydration
what percent hydration does the strom need to be at?
78%
where is Dua’s layer and what is it composed of?
- between stroma and descemets membrane
- type 1 collagen
what is the refractive index of tear film?
1.336
does tear film have a shape?
- nope
- it takes on the shape of the cornea
about how thick is tear film?
like 3 micrometers
what are the two main functions of tear film lipid layer?
- provide a smooth optical surface for the cornea
2. slow water evaporation from the surface
how is tear film stability measured?
-by its lack of stability
what does tear film stability refer to? what is considered a stable tear film?
- evaporation rate
- one in which a minimum amount of tears evaporate
why does instability of the tear film occur?
-due to the depletion of aqueous component of tears
about how long is tear break up time? how long until you normally need to blink again?
- 4 sec
- 7 sec
why do irregularities in the anterior surface of the tears have the most potential to degrade the optical quality of the eye?
- refractive difference between air and the tear film is greater than at any other optical interference in the eye
- its the first surface it hits
what are the two techniques used to analyze tear film integrity?
- retroillumination of pupil with a slit lamp
- fluorescein
what kind of disease is dry eye? how is dry eye defined?
- multifactorial disease
- symptoms of discomfort
- visual disturbance
- tear film instability
what is 2.5 times greater in dry eyes than in health eyes?
higher order aberrations like spherical and coma
what are the two main contributions to dry eye?
- blink suppression
- tear film instability
what are some subjective complaints of dry eye?
- dryness
- sandy feeling
- burning/stinging
- red eye
- blurry vision
irregularities in the tear film can cause significant changes in what?
-retinal image quality
where does the contact lens sit?
-IN the tear film
the contact lens is how much thicker than the aqueous layer?
10 times thicker
contact lens interaction with the tear film is dependent on what two things?
- lens material
2. individual’s tear chemistry
what are the two common effects on tear film from contact lenses?
- lens can deplete the levels of components in the tear film by sorption
- lens can stimulate biochemical generation of additional components
corneal epithelium finger like projections extend into the tear film by how much?
-about 1/2 micron
why is the amount that the epithelium projections extend into the tear film significant?
- it is comparable to the wavelength of green light
- so light will scatter if the refractive indices are not comparable
when is a surface considered optically rough?
-when the refractive indices are not comparable
what are the two clinical examples of degradation of tear film integrity?
- irregular reflection from a scared cornea (tear film cant remove uneven contours of the scar)
- Stevens-Johnson syndrome(dysfunction of the epithelia cells)
what happens to tear film with age?
-tear film stability decreases with age
which sex has a longer tear break up time?
-males have longer TBUT than females
which race has a longer TBUT?
white is higher TBUT than asians
what are some environmental components that affect tear film stability?
- CL use
- refractive surgery
- temperature
- smoking and alcohol
- computer time
what does extended time on the computer do to the tear film?
- reduced blink rate
- incomplete blinking patters
