CLM - Introduction: History and Physiology of CLs - Week 1

Question 1

List the three main corneal metabolic pathay, and their energy contribution in percentage.

Answer 1

Anaerobic glycolysis - 50%
Krebs Cycle - 15%
Pentose phosphate pathay - 35%

Question 2

WWhich metabolic pathay is more active in corneal endothelium and hy?

Answer 2

TCA cycle due to higher energy requirements

Question 3

List 4 reasons why O2 flux is important for the cornea.

Answer 3

Needed for aerobic glycolysis
Maintains epithelial and endothelial pumps
Corneal hydration
Corneal transparency

Question 4

Why is ATP needed in the cornea (2)?

Answer 4

For moving water from the tears into the stroma

Moving ater and bicarbonate from the stroma into the anterior chamber

Question 5

What happens ith insufficient O2 flux (5)?

Answer 5

Reduced ATP production
Accumulation of lactate in the stroma
Osmotic imbalance
Corneal swelling
Loss of control of separation between corneal collagen

Question 6

What happens with accumulation of lactate in the stroma?

Answer 6

Reduced pH

Stromal acidosis

Question 7

What happens with loss of control of separation between collagen fibres?

Answer 7

Loss of corneal clarity

Question 8

What cellular and biochemical changes happen with corneal hypoxia (5)?

Answer 8

Decreased epithelial mitosis
Reduced number of corneal hemidesmosomes
Reduced density of corneal nerve fibre endings
Epithelial microcysts and vacuoles
Neovascularisation of the cornea
Endothelial abnormalities

Question 9

What happens with reduction in the density of corneal nerve fibre endings (3)?

Answer 9

Reduced corneal sensitivity
Reduced protective mechanism
Reduced wound healing

Question 10

What two factors affect how much oxygen is enough?

Answer 10

Depends on the individual:

• individual variation in O2 demand
• variation in corneal thiccness

Question 11

What normally happens to the cornea overnight and how does this affect corneal oxygen concentration?

Answer 11

It swells, reducing corneal oxygen

Question 12

Define the following:
Dk
Dk/t
EOP

Answer 12

Dk - oxygen permeability
Dk/t - oxygen transmissability
EOP - equivalent oxygen percentage

Question 13

What are the units for Dk?

Answer 13

x10^-11

Question 14

What are the units for Dk/t?

Answer 14

10^-9

Question 15

What is Dk/t directly related to in hydrogel lenses? Explain what this means.

Answer 15

Water content

Improve oxygen performance by increasing water content

Question 16

What happens to contact lens thickness and Dk/t with increasing water content?

Answer 16

Increased water content results in thicker lenses which decreases effective Dk/t, but still a benefit in increased water content

Question 17

Does water content of SiH contact lenses limit their Dk/t? Explain why.

Answer 17

No, the silicone material has an oxygen permeability several times greater than water

Question 18

What does equivalent oxygen percentage measure?

Answer 18

Corneal oxygen consumption after contact lens wear

Question 19

What should the EOP (and Dk/t) of a contact lens be to meet the Holden-Mertz criteria under daily wear conditions? Give ideal and acceptable values.

Answer 19

Ideal - 9.9% or a Dk/t of 24

Acceptable - 15

Question 20

What should the EOP (and Dk/t) of a contact lens worn overnight be to meet the Holden-Mertz criteria? Give ideal and acceptable values.

Answer 20

Ideal - 17.9% or a Dk/t of 87

Acceptable - 34

Question 21

What is the current proposal regarding extended ear critical Dk/t?

Answer 21

It should be revised to at least 125

Question 22

What is the superior contact lens brand for Dk/t?

Answer 22

Biofinity from CooperVision

Question 23

Aside from Dk/t, what other factors should be taken into account for contact lens fitting?

Answer 23

Lens fit - tighter fit, less oxygen
Lens thickness - toric, myopic/hyperoptic
Area covered - rigid smaller than soft
Individual patient differences in O2 requirement