Response to CL wear Flashcards
Etiology of corneal swelling due to hypoxia
Corneal hypoxia Anaerobic metabolism Stromal lactic acid build-up Increased osmotic gradient Excess fluid in stroma
What is the perfect lattice model?
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.
What percent water (state of deturgescence) is required to maintain corneal transparency?
78%
How much O2 is required to prevent corneal swelling in CL wearers and non-lens wearers?
9.9% in non lens wearers
18% in lens wearers
What are some causes of corneal swelling?
Hypoxia
Temperature
Tear osmolarity
Mechanical causes
How does hypertonic vs hypotonic tear film affect corneal thickness?
Hyper = thinner cornea Hypo = thicker cornea (tears flow into cornea)
How does lens wear affect exfoliation rate of corneal epithelium?
Exfoliation rate decreases as wearing time of lenses increases (for all lens types)
How does O2 affect binding of pseudomonas?
Pseudomonas bind more to cells exposed to lower levels of O2 (conventional hydrogels)
Describe limbal hyperemia
Vasodilation of blood vessels
Rapid response to hypoxia
Driven by nitric oxide (vasodilator)
How is limbal hyperaemia affected by lens material?
Less hyperaemia in SiHy wear than in conventional hydrogels (Acuvue 2)
How does lens material affect neovasc?
Hydrogel lens extended wear develop greater neovasc than eyes wearing SiHys
Signs of microcysts
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
Where do we see an increase in micro cysts?
In EW conventional hydrogels, after 6 mos
What is the “rebound effect”?
Switching to higher Dk/t lens temporarily increases microcysts (they’re trying to clean out the cornea…)
Describe Mucin balls
Mini impressions on the cornea
Round, opaque, NOT reverse refractile!
Appear on superficial corneal layers
Fl pools, not staining
What are striae?
Fine, vertical white lines deep in corneal stroma or Descemet’s membrane
How much swelling corresponds to striae?
1 striae = 5% swelling
5 striae = 11% swelling
What are folds?
Descemet’s membrane buckling
How do folds correspond to swelling?
1 fold = 8% swelling
What level of corneal edema is pathological?
16-20%
What level of corneal edema is dangerous?
8-15%
What are blebs?
Acute response to hypoxia (20-30 mins)
Small, dark non-reflective areas in endothelium
Disappear after lens removal
How do blebs form?
pH change in the stroma during hypoxia
What percentage of O2 is required to prevent bleb formation?
16.6%
What is the chronic endothelial consequence of hypoxia?
Polymegathism and pleomorphism of cells
What are some visual symptoms of corneal hypoxia?
Glare and/or halos (esp at night!)
Decreased CS and VA
What is myopic creep?
Increase in myopia of less than or equal to 0.50D
Seen w/ low Dk/t conventional hydrogel (HEMA) lenses
What causes myopic creep (aetiology)?
WE DON'T KNOW! Hypoxia Ref index change in cornea Differential swelling response Mechanical influences
