Hypoxia and corneal responses Flashcards
What pathology can occur in the corneal epithelium due to hypoxia?
Microcysts
Vacuoles
Epithelial Plug
What pathology can occur in the corneal stroma due to hypoxia?
Oedema
Neovascularisation
Microbial and sterile keratitis
What pathology can occur in the corneal endothelium due to hypoxia?
Bedewing
Blebs
Polymegathism
How do microcysts appear on observation?
small, spherical inclusions with light reversed inside them
How long does it take microcysts to appear?
1 week to 2 months after starting CL wear
What is within microcysts?
Cellular debris from cells killed by hypoxia
How do microcysts develop?
Form in deepest epithelial layers due to disorganised cell growth and move forward. Stain once on ocular surface
Where will the most microcysts be found?
Where the thickest part of the lens is
What is microcyst rebound?
Number increase initially once CLs not worn due to increased metabolism and growth of epithelium from increased oxygen levels
How do vacuoles appear on observation?
small, spherical inclusions, light not reversed
What is within vacuoles?
Gas and fluid
Where will the most vacuoles be found?
Mid-peripheral cornea in groups of 2-4
Why does corneal oedema form due to hypoxia?
Hypoxia induces anaerobic respiration - this has a by product of lactate/lactic acid which accumulates.
More water enters the cornea too and the endothelial pump is unable to balance the excess fluid.
What are striae in corneal oedema?
White, fine, vertical lines in posterior stroma caused by fluid separation of posterior fibrils
What are folds in corneal oedema?
physical buckling of the posterior stroma
seen as depressed grooves or raised ridges in endothelium
What is haze in corneal oedema?
Hazy, granular appearance to stroma due to gross separation of fibrils
At what point does corneal oedema start to affect VA?
Once it’s over 20% oedema
At what point does corneal oedema start to make the cornea lose transparency?
Over 15% oedema
How long does it take corneal oedema to resolve with no CL wear?
4 hours-7 days
What causes corneal neovascularisation?
Combination of hypoxia, angiogenic suppression, vasostimulation and neural control factors
How should neovascularisation be managed if it’s over grade 1?
Refit to RGP or SiH
Reduce WT
Stop EW
Decrease mechanical stimulation
How should neovascularisation be managed if it’s over grade 2?
STOP LENS WEAR
Px must be managed carefully if wants to return to lens wear
How should neovascularisation be managed if it’s under grade 1?
Minimise toxic/allergic response - change sols
Monitor
How does bedewing appear on observation?
small white inclusions on inferior central cornea (endothelium) which become pigmented, reversed illumination
Why does bedewing occur?
Due to inflammation - cells from iris and ciliary body engulfed into endothelium
What are the sxs of bedewing?
Partial/total CL intolerance
Stinging
Red eyes
Corneal clouding
How long does it take for bedewing to resolve?
Sxs: 3-5 days
Bedewing: 3-5 months
How do blebs appear on observation?
Black, non-reflecting areas of the corneal endothelium
What percentage of CL wearers will get blebs and how quickly do they appear?
100%
Within 10 mins of insertion
What causes blebs?
Reversible localised oedema (due to increased lactic acid and carbonic acid) causes cells to bulge posteriorly
What are guttata? What could they be mistaken for?
Permanent, progressive accumulation of collagen on DM
Mistaken for blebs
What can guttata be a precursor to?
Fuch’s endothelial dystrophy
How do guttata appear on obervation?
Dark spot in central cornea, larger than blebs
What is Fuch’s endothelial dystrophy?
An autosomal dominant inherited condition which causes reduced VA due to corneal oedema and pain due to exposed corneal nerve endings.
What is polymegathism?
Endothelial cell density and regularity irreversibly reduces, due to cell death, leaving remaining cells to fill gaps. No new cells created.
Why does polymegathism occur?
Natural aging process but accelerated by CL wear due to acidic shift
Due to loss of endothelial pump function
