CORNEA & SCLERA (PART 2) Flashcards
What are the factors that contribute to the prevention of stromal swelling?
barrier function of endothelium.
the pump function of endothelium.
water maintains 78%
When the endothelium barrier or pump is disrupted, what is the amount of water flow amount that the cornea swells?
pump= cornea swells 33 uL/hr barrier= cornea swells 127 uL/hr
Normal leakage of fluid is VITAL to cornea. Why?
It is VITAL because this fluid brings nutrients to cornea (glucose and amino acids).
If Temperature of cornea is cooled what happens?
cornea swells. But at normal temp. it is at normal thickness.
The endothelium of the cornea has a Na+K+ATPase pump located at the basolateral membrane.
True or False.
True
What is Oaubain? What does it do?
It is a inhibitor to the corneal endothelial Na+K+ ATPase pump. It cause the cornea to swell, eliminate transendothelial potential difference, and inhibits temperature reversal.
Cornea with Gutattas present, is the pump increase or decrease in activity?
Increase.Corneas with guttata: ⇧ pump site density, greater capactiy for the pump to counteract the leak
Cornea with inflamed edematous cornea condition, is the pump site increased or decreased?
Decrease. Inflamed, Edematous cornea: ⇩ pump site density despite the increased permeability
what is sodium potassium ATPase do?
control hydration of cornea, production of aqueous humor. enzyme found in plasma membrane of cell.
which part of the cornea readily takes up water?
Stroma. However, water has to be pumped out to maintain corneal detergescence.
Potassium ATPase is pumped out into channels in between endothelial cells. True or False.
True.
Cornea stroma readily take up water from back. However excessive water has to be pumped out. This is done by Na+ K- ATPase pump. It pulls water and sodium out of cornea back into aqueous humor. There is removal and production of aqueous. The amount is equal.
True or False
True.
Cornea stroma readily take up water from back. However excessive water has to be pumped out. This is done by Na+ K- ATPase pump. It pulls water and sodium out of cornea back into aqueous humor. There is removal and production of aqueous. The amount is equal.
True or False
True
Na+ ions being pushed generates osmotic pressure (Descemte’s Membrane). Where Na+ goes water follows. This can cause edema. True or False.
True.
What happens when the endothelium gets injured.
ENDOTHELIUM CANNOT REGENERATE.
Migration of endothelial cells over the wound edge to the periphery.
Development of tight junctions to establish the endothelial barrier.
What is a keratoplasty?
cornea replacement.
You only replace the central button of the cornea, not the whole thing.
For proper corneal function, endothelial cells must be same size and shape. True or False.
True.
Polymorphism means….
change in shape
polymegathism
change in size
Polymorphism and polymegathism occur with age. True or False.
true.
What is normal corneal variation (CV)?
approx. 0.25
anything greater than 0.25 is not normal. means cell size is variable= polymegathism
Healthy cornea has 70-80% hexagonal cells
A ⇩in hexagons with an ⇧in cells with < 6 sides shows endothelial stress = pleomorphism
True or False
True.
endothelial cells are what shape and have how many sides?
hexagonal shape. 6 sides.
Does LASIK surgery have an effect on the amount of endothelial cells?
No. It doesn’t change. LASIK has no effect on endothelium. Doesn’t harm it.
After LASIK, there must be residual 200 µm of stroma above the corneal endothelium to adequately maintain and protect the corneal endothelial structure and barrier function.
True or False.
True
LASIK can improve CV dramatically. Yes or No. How long does it take after surgery is done?
Yes. CV of cell size improves 3 yrs after LASIK
Change in CV likely 2o to patient wearing contact lenses prior to surgery (CL removal)
With PK corneal replacement surgery can you lose endothelial cells? If so, at what rate?
Yes.
Endothelial cell loss rate: 7.8% per year (3-5 yrs after PK), this is 13x the rate in normal corneas.
With Cataract surgery can you lose endothelial cells? If so, How much?
Yes.
Cataract extraction with intraocular lens implantation can cause endothelial cell loss
Mean overall endothelial cell loss = 8.5% after 12 months
Describe what happens in those with KCN or Type 2 Diabetes endothelium?
With KCN and Type 2 Diabetes:
Endothelial morphology changes
NO decrease in cell density
⇧CV of cell size and ⇩% of hexagonal cells (50%)
KCN: Stress on cornea causes endothelial remodeling
With other diseases of long duration (type I DM) there may also be a decrease in cell density
Describe what happens to the endothelium cells in glaucoma patients? does it increase or decrease.
Decrease.
After 40 yrs of age there is a ⇩ in endothelial cell density (compared to non-glaucomatous suspect population)
Special care taken when doing cataract surgery on those with glaucoma or diabetes
Can a change in endothelium cells happen with contact lens wearing? What kind polymegathism or polymorphism?
Yes. Polymegathism.
what’s hypoxia?
cut off oxygen to cornea.
polymethylmethelate lenses are…..
hard contact lenses
The cornea is richly supplied with sensory nerves. It is innervated by the ophthalmic division of the trigeminal nerve, via the anterior ciliary nerves.
It is clear that Descemet’s membrane and the endothelium are not innervated in humans.
The cornea is one of the most sensitive tissues in the body. True or False.
True.
Pain receptor of the cornea is….
nociceptor
Nociceptors. These receptors in cornea have lowest threshold for stimulation. This is what makes corneal ulcers, abrsasions, etc. so painful. True or False.
True
what are the two effects of Sensory Denervation?
Sensory Denervation causes:
High incidence of epithelial erosions and neurotrophic ulcers
The loss in foreign body sensations cause mechanical cornea damage
What conditions or diseases can cause Sensory Denervation?
Herpes Simplex, Diabetes, Stroke, Neuropathy.
LASIK does not cause a decrease in conjunctiva and corneal sensitivity. True or False.
False.
With CL wear there comes a decrease in corneal sensitivity. True or False.
True.
The amount of oxygen found in aqueous humor is low or high compared to tears?
low
Corneal metabolism receives oxygen from what 3 sources?
atmosphere, limbal vascalature, aqueous humor
Corneal epithelium consumes 10 times more oxygen than the stroma. True or False.
True.
how much oxygen is delivered to eye with open and closed?
open= 21% closed= 8%
where does the eye receive its oxygen to the cornea when sleeping or with closed eye conditions.?
the vascularized superior palpebral conjunctiva.
how much oxygen is found in the aqueous humor vs, the tears?
40 mmHg = aqueous humor
150 mmHg = tears
After cataract removal (ie aphakia), oxygen tension may increase in the aqueous humor as a result of decreased oxygen metabolism by the crystalline lens. Thus,in the aphakic eye, oxygen in the aqueous humor may supplement oxygen dissolved in the tears, better meeting the corneal oxygen demands and allowing greater tolerance to hypoxic stress such as contact lens use.
True or False.
True.
what is aphakia?
post cataract surgery….removed cataract eye.
Metabolic requirement to the cornea are supplied by what?
aqueous humor
Glucose can be derived from what part of the cornea?
corneal epithelium
Glucose derived from the aqueous humor or glycogen stores is converted to pyruvate by the Embden-Meyerhof pathway
True or False
True.
CL lens wear causes a increase in oxygen to cornea.
T or F
False. decrease oxygen. Resulting in edema and 20% increase in corneal thickness.
what are symptoms of epithelial edema?
Halo and rainbow formation
Increased glare sensitivity
Decreased contrast sensitivity
what are symptoms of stromal edema?
Buckling of the stroma and Descemet’s membrane cause vertical striae
What can happen when the Na+ K+ ATPase pump gets inhibited?
edema or problems in the endothelium
Polymegathism can happen in what tyrpe of patients?
Diabetic and long time contact lens wearers.
What is the byproducts of long term hypoxia and stress that has the ability to inhibit the Na+K+ATPase of the endothelial metabolic pump?
12HETE (12-Hydroxyeicosatetraenoic acid)
EW lenses have been associated with what metabolic symptoms in epithelium of cornea?
Decreased rate of mitosis
Reduced oxygen uptake and glucose utilization
Smaller numbers of intercellular desmosomes
Reduced metabolic activity of the epithelium can compromise the barrier of cornea and make it vulnerable how?
Increased likelihood of ulcerative microbial keratitis
lack of vitamin A causes what condition?
Xerophthalmia = dryness of eye in cornea and conjunctiva due to lack of vitamin A.
Keratinization of the epithelium.
Vitamin A is required for mucin production
Xerophthalmia occurs late in the progression of vitamin A deficiency
what is the volume of adult tear film, what is max amount held in cal-de-sac, what is average drop of topical med?
Volume of adult
tear film: 7-9 µL
Max amount of cul-de-sac fluid: 20-30 µL
Average drop of topical meds: 50 µL
Loss of corneal epithelium greatly enhances penetration of hydrophilic water-soluble pharmacologic agents.
True or False
Loss of corneal epithelium greatly enhances penetration of hydrophilic water-soluble pharmacologic agents
Stroma hydrophilic nature provides a barrier to lipid based drugs. T or F
True.
In the corneal endothelium drug penetration is not mostly determined by molecular size. T or F
False. it is determined by molecular size.
how can you increase likelihood of drug penetration?
increasing duration of the contact of the drug with the ocular surface.
1) Press on the lacrimal sac
2) Use viscous drops, suspensions, ointments
Slow-release delivery systems
3) Contact lenses (drug embedded in lens)
4) Porcine collagen corneal shields
why are drug preservatives used?
to increase shelf life of drug and protect eye from infection. prevent bacterial infection
what common preservative is used?
BAK or benzalkonium chloride
what is a side effect of BAK?
epithelium and endothelium susceptible to damage. inhibits epithelial wound healing.
One drop of 0.01% BAK causes increase in permeability of the cornea to fluorescein
T or F
True
The preservatives BAK and chlorhexidine cause endothelial cell degeneration and corneal edema in vitro.
true or false
true
what is used to stabilize BAK?
EDTA Ethylenediaminetetraacetic acid
Used to stabilize BAK-containing formulations.
what is the downside of EDTA? what type of patients shouldn’t have drug with EDTA in it?
break down tight junctions (chelates calcium which is required for the maintaince of tight junctions).
dry eye patients shouldn’t use EDTA.
Describe the sclera?
Has fibroblasts, collagen, and proteoglycans. Made of type 1 collagen.
What percentage of sclera is water and what percentage is dry weight?
water= 70%
dry weight= 30%
how much water does the cornea have?
78%
Sclera is not avascular. T or F
False. Avascular
Episclera overlies the sclera. T or F
True
The Sclera is avascular with what two exceptions from what two parts?
superficial vessels of the episclera and the intrascleral vascular plexus
The posterior sclera contains the sclera canal and the lamina cribrosa for passage of the optic nerve and contains the sites of perforation by the long and short po
sterior ciliary arteries, the short ciliary veins, the ciliary nerves, and the vortex veins. The tendons of the rectus muscles insert into the superficial sclera collagen.
true or false
true.
what does the lamina cribosa do?
fenestrated allows cells to pass by. part of sclera.
sclera is 95% of the globe. T or F
true.
Sclera thickest where? thinnest where?
near optic nerve thickest.
decreases towards the equator.
what is the difference between the cornea and sclera collagen fiber and arrangement?
Scleral fiber diameter ranges from 25-230 nm
Arranged in bundles
VS
Corneal fiber diameter is uniform at 25 nm
Arranged parallel
what is associated with myopia and the sclera?
axial elongation of the globe, change in rate of proteoglycan synthesis, accumulation in sclera.
Name some aging effects on the Sclera.
With increasing age:
1) Progressive degeneration of collagen and elastic fibers
2) Loss of GAGs (Glycose Amina Glycans)
3) Scleral dehydration
4) Accumulation of lipids and calcium salts, responsible of the yellowing of the sclera.
Which cause:
Increase in tissue density
Sclera thinning and yellowing
Decreased sclera elasticity.
Describe innervation of sclera.
Stroma of the sclera is devoid of innervation, but does allow the passage of nerves
Sympathetic innervation present in monkeys
Scleral spur axons of parasympathetic origin are present in humans
Cholinergic innervations rare or absent in both monkeys and humans
How are drugs delivered through sclera?
diffusion through aqueous pathway
what factors can control the diffusion rate of sclera?
Tissue hydration
Tissue thickness
Size and volume fraction of proteoglycans present
The sclera swells least near a pH of 4
Does age alter the permeability of sclera? What alters the permeability?
No. Age doesn’t effect permeability. What alters it is when you cut the sclera, surgically. Sclera is the thinnest where the muscle inserts itself.
Does cryotherapy and laser treatment alter sclera?
No
Change of IOP- 15 to 60 mm Hg can DECREASE sclera permeability to small molecules by half.
True or False
True
With increased molecular weight and radius, scleral permeability increase or decrease?
Permeability is a STRONG or weak function of MW (molecular weight)
decreases.
strong function of MW.
When the sclera is exposed to various prostaglandins in organ culture there is INCREASED permeability
This causes an INCREASED expression of matrix metalloproteinases
True or False
True
what are metalloproteinases?
detect growth of the eyeball in sclera.
Large molecule transcleral delivery can be improved significantly by doing what 3 things?
For large molecules, transscleral deliver could be improved significantly by:
1) Taking advantage of thinner regions of tissue
2) sclera hydration
3) Transient modification of sclera ECM.
