Tear film

Question 1

what does the epithelium do?

Answer 1

-protects the cornea by providing a barrier against water, large molecules, and toxins

Question 2

what is Bowman’s membrane?

Answer 2

-consists of randomly arranged collagen fibrils

Question 3

what layer contributes to 90% of corneal thickness?

Answer 3

Stroma

Question 4

how are the collagen fibrils of the stroma arranged?

Answer 4

• fibrils within each lamellae run parallel

- each layer runs across at right angles

Question 5

what is the shape of the endothelium? what is its function?

Answer 5

• hexogonal shape

- regulates fluid balance of the cornea and maintain stroma hydration

Question 6

what percent hydration does the strom need to be at?

Answer 6

78%

Question 7

where is Dua’s layer and what is it composed of?

Answer 7

• between stroma and descemets membrane

- type 1 collagen

Question 8

what is the refractive index of tear film?

Answer 8

1.336

Question 9

does tear film have a shape?

Answer 9

• nope

- it takes on the shape of the cornea

Question 10

about how thick is tear film?

Answer 10

like 3 micrometers

Question 11

what are the two main functions of tear film lipid layer?

Answer 11

1. provide a smooth optical surface for the cornea

2. slow water evaporation from the surface

Question 12

how is tear film stability measured?

Answer 12

-by its lack of stability

Question 13

what does tear film stability refer to? what is considered a stable tear film?

Answer 13

• evaporation rate

- one in which a minimum amount of tears evaporate

Question 14

why does instability of the tear film occur?

Answer 14

-due to the depletion of aqueous component of tears

Question 15

about how long is tear break up time? how long until you normally need to blink again?

Answer 15

• 4 sec

- 7 sec

Question 16

why do irregularities in the anterior surface of the tears have the most potential to degrade the optical quality of the eye?

Answer 16

• 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

Question 17

what are the two techniques used to analyze tear film integrity?

Answer 17

• retroillumination of pupil with a slit lamp

- fluorescein

Question 18

what kind of disease is dry eye? how is dry eye defined?

Answer 18

• multifactorial disease
• symptoms of discomfort
• visual disturbance
• tear film instability

Question 19

what is 2.5 times greater in dry eyes than in health eyes?

Answer 19

higher order aberrations like spherical and coma

Question 20

what are the two main contributions to dry eye?

Answer 20

• blink suppression

- tear film instability

Question 21

what are some subjective complaints of dry eye?

Answer 21

• dryness
• sandy feeling
• burning/stinging
• red eye
• blurry vision

Question 22

irregularities in the tear film can cause significant changes in what?

Answer 22

-retinal image quality

Question 23

where does the contact lens sit?

Answer 23

-IN the tear film

Question 24

the contact lens is how much thicker than the aqueous layer?

Answer 24

10 times thicker

Question 25

contact lens interaction with the tear film is dependent on what two things?

Answer 25

1. lens material

2. individual’s tear chemistry

Question 26

what are the two common effects on tear film from contact lenses?

Answer 26

1. lens can deplete the levels of components in the tear film by sorption
2. lens can stimulate biochemical generation of additional components

Question 27

corneal epithelium finger like projections extend into the tear film by how much?

Answer 27

-about 1/2 micron

Question 28

why is the amount that the epithelium projections extend into the tear film significant?

Answer 28

• it is comparable to the wavelength of green light

- so light will scatter if the refractive indices are not comparable

Question 29

when is a surface considered optically rough?

Answer 29

-when the refractive indices are not comparable

Question 30

what are the two clinical examples of degradation of tear film integrity?

Answer 30

1. irregular reflection from a scared cornea (tear film cant remove uneven contours of the scar)
2. Stevens-Johnson syndrome(dysfunction of the epithelia cells)

Question 31

what happens to tear film with age?

Answer 31

-tear film stability decreases with age

Question 32

which sex has a longer tear break up time?

Answer 32

-males have longer TBUT than females

Question 33

which race has a longer TBUT?

Answer 33

white is higher TBUT than asians

Question 34

what are some environmental components that affect tear film stability?

Answer 34

• CL use
• refractive surgery
• temperature
• smoking and alcohol
• computer time

Question 35

what does extended time on the computer do to the tear film?

Answer 35

• reduced blink rate

- incomplete blinking patters