CLs And Oxygen

Question 1

How does cornea receive oxygen

Answer 1

• atmosphere
• aqueous
• perilimbal vasculature

Question 2

What happens if the cornea doesn’t get enough oxygen

Answer 2

• corneal oedema
• neovascularisation
• red and bloodshot
• damage to endothelial layer
• vulnerable to infection

Question 3

What is oxygen transmissibility - Dk/t

Answer 3

• the degree that oxygen passes through material of a given thickness

T=Dk/t

T= transmissibility
D= diffusion coefficient
k= solubility of oxygen in material
t= thickness

Question 4

Corneal neovascularisation mechanism,signs and management

Answer 4

Caused by stromal oedema releasing vasostimulatory elements

• no real symptoms
• new BVs from limbus
• manage by increasing Dk/t
• good prognosis

Question 5

What is polymegathism, signs and management

Answer 5

• caused by long term hypoxia causing structural damage to endothelial cells

Signs
- no symptoms
- possible CL intolerance with reduced VA and Photophobia
- differing cell size in endothelium

Management
- refit with higher O2 lens
- change to dailies
- prognosis not good as endothelium doesn’t regenerate

Question 6

What is myopic creep

Answer 6

• optically stimulated myopic increase
• counter balanced by higher modulus SiHy lenses, which flatten central cornea

Question 7

Corneal oedema - SEAL, signs and management

Answer 7

• superior epithelial arcuate lesions
• caused by mechanical trauma by inflatable lenses
• FB sensation
• arcuate staining 1mm from superior limbus
• manage by removing lens until clear
• if persistent try flatter lens or RGP

Question 8

Corneal oedema - striae

Answer 8

• diabetics, elderly and keratuconus
• caused by hypoxia - lactic acid causing osmotic shift
• asymptomatic
• manage by switching to dailies

Question 9

Corneal oedema - endothelial folds

Answer 9

• prevalent in older hydrogel EW lenses
• caused by hypoxia and corneal oedema
• Px may have blurred vision
• straight dark lines in endothelial mosaic
• switch to dailies and increase Dk/t

Question 10

Benefits of SiHy, and what Px could be offered them

Answer 10

• breathable for healthy wear
• outstanding comfort
• signs of corneal oxygen deficiency
• possibly stopped wearing lenses due to discomfort
• want to wear lenses full time