Response to CL wear

Question 1

Etiology of corneal swelling due to hypoxia

Answer 1

Corneal hypoxia
Anaerobic metabolism
Stromal lactic acid build-up
Increased osmotic gradient
Excess fluid in stroma

Question 2

What is the perfect lattice model?

Answer 2

Normal healthy cornea has regularly spaced fibrils.
With swelling, fibrils become irregular and cornea loses lattice pattern.
See reduced transparency of cornea and more scattering of light.

Question 3

What percent water (state of deturgescence) is required to maintain corneal transparency?

Answer 3

78%

Question 4

How much O2 is required to prevent corneal swelling in CL wearers and non-lens wearers?

Answer 4

9.9% in non lens wearers

18% in lens wearers

Question 5

What are some causes of corneal swelling?

Answer 5

Hypoxia
Temperature
Tear osmolarity
Mechanical causes

Question 6

How does hypertonic vs hypotonic tear film affect corneal thickness?

Answer 6

Hyper = thinner cornea
Hypo = thicker cornea (tears flow into cornea)

Question 7

How does lens wear affect exfoliation rate of corneal epithelium?

Answer 7

Exfoliation rate decreases as wearing time of lenses increases (for all lens types)

Question 8

How does O2 affect binding of pseudomonas?

Answer 8

Pseudomonas bind more to cells exposed to lower levels of O2 (conventional hydrogels)

Question 9

Describe limbal hyperemia

Answer 9

Vasodilation of blood vessels
Rapid response to hypoxia
Driven by nitric oxide (vasodilator)

Question 10

How is limbal hyperaemia affected by lens material?

Answer 10

Less hyperaemia in SiHy wear than in conventional hydrogels (Acuvue 2)

Question 11

How does lens material affect neovasc?

Answer 11

Hydrogel lens extended wear develop greater neovasc than eyes wearing SiHys

Question 12

Signs of microcysts

Answer 12

15-50 um inclusions in epithelium (form in basal layers, cellular debris)
2-3 mos to occur, 4-6 to peak
Reverse refractile in retro illumination
Shiny staining w/ Flu

Question 13

Where do we see an increase in micro cysts?

Answer 13

In EW conventional hydrogels, after 6 mos

Question 14

What is the “rebound effect”?

Answer 14

Switching to higher Dk/t lens temporarily increases microcysts (they’re trying to clean out the cornea…)

Question 15

Describe Mucin balls

Answer 15

Mini impressions on the cornea
Round, opaque, NOT reverse refractile!
Appear on superficial corneal layers
Fl pools, not staining

Question 16

What are striae?

Answer 16

Fine, vertical white lines deep in corneal stroma or Descemet’s membrane

Question 17

How much swelling corresponds to striae?

Answer 17

1 striae = 5% swelling

5 striae = 11% swelling

Question 18

What are folds?

Answer 18

Descemet’s membrane buckling

Question 19

How do folds correspond to swelling?

Answer 19

1 fold = 8% swelling

Question 20

What level of corneal edema is pathological?

Answer 20

16-20%

Question 21

What level of corneal edema is dangerous?

Answer 21

8-15%

Question 22

What are blebs?

Answer 22

Acute response to hypoxia (20-30 mins)
Small, dark non-reflective areas in endothelium
Disappear after lens removal

Question 23

How do blebs form?

Answer 23

pH change in the stroma during hypoxia

Question 24

What percentage of O2 is required to prevent bleb formation?

Answer 24

16.6%

Question 25

What is the chronic endothelial consequence of hypoxia?

Answer 25

Polymegathism and pleomorphism of cells

Question 26

What are some visual symptoms of corneal hypoxia?

Answer 26

Glare and/or halos (esp at night!)

Decreased CS and VA

Question 27

What is myopic creep?

Answer 27

Increase in myopia of less than or equal to 0.50D

Seen w/ low Dk/t conventional hydrogel (HEMA) lenses

Question 28

What causes myopic creep (aetiology)?

Answer 28

WE DON'T KNOW!
Hypoxia
Ref index change in cornea
Differential swelling response
Mechanical influences