CLM - Introduction: History and Physiology of CLs - Week 1 Flashcards
List the three main corneal metabolic pathay, and their energy contribution in percentage.
Anaerobic glycolysis - 50%
Krebs Cycle - 15%
Pentose phosphate pathay - 35%
WWhich metabolic pathay is more active in corneal endothelium and hy?
TCA cycle due to higher energy requirements
List 4 reasons why O2 flux is important for the cornea.
Needed for aerobic glycolysis
Maintains epithelial and endothelial pumps
Corneal hydration
Corneal transparency
Why is ATP needed in the cornea (2)?
For moving water from the tears into the stroma
Moving ater and bicarbonate from the stroma into the anterior chamber
What happens ith insufficient O2 flux (5)?
Reduced ATP production
Accumulation of lactate in the stroma
Osmotic imbalance
Corneal swelling
Loss of control of separation between corneal collagen
What happens with accumulation of lactate in the stroma?
Reduced pH
Stromal acidosis
What happens with loss of control of separation between collagen fibres?
Loss of corneal clarity
What cellular and biochemical changes happen with corneal hypoxia (5)?
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
What happens with reduction in the density of corneal nerve fibre endings (3)?
Reduced corneal sensitivity
Reduced protective mechanism
Reduced wound healing
What two factors affect how much oxygen is enough?
Depends on the individual:
-individual variation in O2 demand
-variation in corneal thiccness
What normally happens to the cornea overnight and how does this affect corneal oxygen concentration?
It swells, reducing corneal oxygen
Define the following:
Dk
Dk/t
EOP
Dk - oxygen permeability
Dk/t - oxygen transmissability
EOP - equivalent oxygen percentage
What are the units for Dk?
x10^-11
What are the units for Dk/t?
10^-9
What is Dk/t directly related to in hydrogel lenses? Explain what this means.
Water content
Improve oxygen performance by increasing water content
What happens to contact lens thickness and Dk/t with increasing water content?
Increased water content results in thicker lenses which decreases effective Dk/t, but still a benefit in increased water content
Does water content of SiH contact lenses limit their Dk/t? Explain why.
No, the silicone material has an oxygen permeability several times greater than water
What does equivalent oxygen percentage measure?
Corneal oxygen consumption after contact lens wear
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.
Ideal - 9.9% or a Dk/t of 24
Acceptable - 15
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.
Ideal - 17.9% or a Dk/t of 87
Acceptable - 34
What is the current proposal regarding extended ear critical Dk/t?
It should be revised to at least 125
What is the superior contact lens brand for Dk/t?
Biofinity from CooperVision
Aside from Dk/t, what other factors should be taken into account for contact lens fitting?
Lens fit - tighter fit, less oxygen
Lens thickness - toric, myopic/hyperoptic
Area covered - rigid smaller than soft
Individual patient differences in O2 requirement
