Flashcards in Corneal Function Deck (81):
What are the functions of the cornea?
What are some characteristics the cornea needs to accomplish it's functions?
How could the cornea fail?
-Couldn't protect against microbes
-Change in curvature
What are some factors that affect refraction of light through the cornea?
-curvature of ANTERIOR SURFACE
-change in REFRACTIVE INDEX from AIR TO CORNEA (tear film)
-corneal THICKNESS (light continues to refract through the cornea; changes power of eye)
-curvature of the POSTERIOR SURFACE
-change in REFRACTIVE INDEX from CORNEA TO AQUEOUS
What's the total power of the eye?
What's the power of the cornea?
(2/3 to 3/4 the power of the eye)
What kind of focus does the cornea have?
(Once the lens stops changing shape, you look at where you look, and that's it)
When you take K-readings, what part of the cornea are you measuring?
Front surface (about the same power as the front surface)
What are 3 criteria for transmitting light through the cornea?
Keys to maximize light transmission through the cornea?
-SMOOTH OPTICAL SURFACE (tear film - does it have a good TBUT?)
-regularity of EPITHELIAL CELLS
-absence of BLOOD VESSELS
-arrangement of correct spatial arrangement of COLLAGEN FIBRILS in STROMA
-less than 1% LIGHT SCATTER (majority in epithelium and endothelium) *so it's very effective
How does the cornea protect you long-term vision?
-minimize risk of infection (tear film -> bling microbes away)
-prevent damage to retina (UV light is blocked by cornea)
-avoid penetrating injury
-swift healing mechanism
What is one of the most sensitive tissues in the human body?
Where is the innervation of the cornea the greatest?
In the center
How much greater is the cornea innervated than in the skin?
300-600 times greater than the skin
How much greater is the cornea innervated than in the roots in teeth?
20-40 times greater than the roots in teeth
The cornea is supplied by which division of which nerve?
Ophthalmic division of trigeminal nerve (60-80 long ciliary nerves)
Does the cornea have myelinated or unmyelinated nerve endings, and why?
Unmyelinated b/c it helps with the transparency of the cornea
What are 3 things the cornea is sensitive to?
What does touch to the cornea cause in order to protect it?
Involuntary lid closure (in on 5, out on 7)
Normal endothelium has a coefficient of variation (CV) of what?
What happens to the density and the CV (coefficient of variation) of cell size when you have a patient who has work contact lenses for 20+ years?
Cell density decreases
CV increases (0.66 when it should be about 0.25)
Once there is damage to the endothelium, can it be fixed?
No; it doesn't come back
The stroma makes up how much of the cornea?
What is the regularity size of collagen fibrils and spacing between them for maintaining corneal transparency?
Size = 300A (angstrom = 10^-8)
Spacing = 550A
What charged molecules located around each collagen fibril maintain the precise arrangement by their bonds?
-Negatively charged molecules
-Bonds with water molecules
Corneal transparency is optimal when the stroma is how much water?
75% to 80%
What does the cornea lack in order to maintain it's transparency?
-blood and Lymph vessels
The cornea receives it's strength primarily from what 2 layers?
-Dua's Layer (6th layer) = new layer
The tough layer of collagen and laminitis that protects the underlying layers in the cornea
How thick is Bowman's layer? Is it cellular or acellular?
8-14 microns thick
What is the Bowman's layer resistant to?
Foreign bodies (will scar)
How thick is Dua's Layer (the newest 6th layer)?
15 microns thick
Where is Dua's Layer located?
Anterior to Descemet's membrane
How does Dua's Layer help the cornea maintain it's strength?
It can withstand high pressures (200kPa)
What are the layers of the cornea, in order from most external to most internal?
The body and cornea are how much water?
How does the epithelium maintain ideal corneal hydration?
With tight junctions
How does the stroma maintain ideal corneal hydration?
Anionic proteoglycans pull water in
How does the endothelium maintain ideal corneal hydration?
Slowly leaks fluids into the cornea/stroma
What does corneal deturgescence mean?
It's the mechanism by which the cornea remains hydrated (78% water content)
What are the 3 main things that corneal hydration is dependent on?
1) barrier function (of epithelium and endothelium)
2) anionic characteristics (of molecules w/in the stromal matrix that account for the tendency of the stroma to imbibe water)
3) water and ion transport (through the epithelial and endothelial cell membranes, including ion channels, ion cotransporters, and energy-utilizing ion pumps)
In the cornea, how do ion flow and osmotic gradients move water?
Out of the cornea from the stroma through the endothelium and into the aqueous
From the stroma through the epithelium into tears
What are the major driving forces for water transport across the epithelium and endothelium (down its concentration gradient)?
Cl- extrusion and Na+ absorption
A patient has some corneal edema 5 minutes after having cataract surgery. Is this an issue?
No; it is normal for a patient to have some edema because you just did a lot to their eye
A patient has some corneal edema 1-7 days after having cataract surgery. Is this an issue?
Yes; the edema should have pretty much healed by now, and this needs to be checked further
What is a characteristic of phthisis bulbi?
Very very low pressure in the eye; there is no vision coming out of this eye anymore
(It looks like it has a white scab over the cornea)
Small integral membrane proteins residing in the plasma membrane; some are water-selective and others also transport glycerol
Why kind of channels do aquaporins form across the plasma membrane?
Bidirectional osmotic water transport channels
Aquaporins function not only as channels, but have some role in what?
Cellular processes (particularly in cell migration)
If a patient has a closed angle attack, what can you do to relieve the pressure if their cornea is clear?
You can do a PI (peripheral iridotomy) with a laser
What classifies as VERY HIGH IOP?
50 mm Hg
VERY HIGH IOP on the order of 50 mm Hg or higher can move excessive water where and cause what?
Into the corneal stroma from the anterior chamber and overwhelm the endothelial transport system
What does a minor abrasion of the corneal epithelium causing a loss of the zonular occludens barrier result in?
Localized area of edema and haziness
What can epithelial edema cause?
Decreased visual acuity when it separates cells, causing surface irregularities; it's uncomfortable and can be painful
What can more extensive epithelial abrasions to the cornea cause?
Fluid entrance into the stroma
What is one technique used commonly to treat corneal epithelial damage?
Put a bandage CL on top to help the epithelium to grow back
Corneal healing is unique because why?
The tissue is entirely avascular
Where does most oxygen to the cornea come from?
Air (you don't get much when you sleep)
Where do corneal nutrients come from?
Diffusion through the tear film and active transport through aqueous
What does corneal healing rely on?
Peptide growth factors
What is the fastest healing tissue in the entire body?
How long does it normally take corneal abrasions to heal?
(Sometime you may not even see anything on the pt's cornea if they scratched their eye a day or 2 before they come to see you)
How long does it take the corneal epithelium to be completely replaced?
Why do PRK patients have to keep the bandage CL on their eye for about 2 weeks?
Because that's about how long it takes the corneal epithelium to be completely replaced
What is recurrent corneal erosion?
Pretty much tearing the surface of the cornea off (like opening your eyes upon awakening)
What parts of the cornea consume most of the oxygen?
Endothelium and epithelium
The endothelium and epithelium consume how much more oxygen than the stroma?
25-30 times that of the stroma (the strom is not as active as the other 2 layers)
What nutrient can the epithelium store?
What are the 3 nutrient requirements of the cornea?
What is a corneal ulcer?
Inflammatory of infective condition that comprises the integrity of the epithelium; it can cause vessel growth
What is the permeability of the epithelium and the endothelium of the cornea?
They are both hydrophobic (non-polar compounds)
What is the permeability of the corneal stroma?
It has a high water content (polar compounds)
Molecules in the cornea should be soluble in what?
Both lipids and water
What is the epithelium impermeable to?
What can the removal of the corneal epithelium facilitate?
Uptake of water soluble drugs
_____-soluble materials can generally penetrate the cornea more easily
To reach the anterior chamber, some drugs require what?
What is ectasia?
-weakness of the cornea (sagging) (Loss of stability and structure)
What does keratoconus look like?
Sag/weakening of the cornea
(Loss of stability and structure)
With high or low pressures inside the eye, you can have a loss of stability and structure. If you have a thinning of the cornea, what can this be associated with?