L9 Corneal Edema

When does edema occur?

occurs when cornea water content is > 78%

What percent of increase in hydration can result in transparency decrease?

>5% increase in hydration

What regions are typically affected by corneal edema?

stromal edema

epithelial edema

Which curvature of the cornea is affected by corneal edema?

posterior curvature is affected due to interweaving of anterior lamellae resisting expansion better than the posterior 2/3

How do hypoxic conditions lead to edema?

hypoxic conditions lead to and increase in anaerobic metabolism

build up of lactate induces swelling by osmotic draw of water

reduction in pH

reduction in pump function

What are the two reasons for corneal edema during sleep?

hypotonic tears

hypoxic metabolism of corneal tissues

Bullous Keratopathy

permanent corneal edema

failure in endothelial pump function or damage to endothelial cells

caused by: endothelial dystrophy, injury, surgery, hypoxia

Fuch's Corneal Dystrophy

gutatta formation

reduced endothelial cell density/expansion of guttate: mild edema

severity of edema increases

opacification of cornea/pain/vascularization of cornea

What are the risk factors for Fuch's corneal dystrophy?

sex: FCD more common in women vs men

genetics: family history increases risk

age: affects typically observed in 30s-40s

Posterior Non-banded Layer

made of collagen type IV

increases in size from birth

Banded layer

made of collagen type VIII

formed during embryonic/fetal development

does not increase with age

What are the forces that regulate corneal hydration?

1. stromal swelling pressure

2. intraocular pressure

3. imbibing pressure: barrier function of the epithelium and endothelium

4. endothelium pump

5. tear evaporation

IP = IOP - SP

What is the standard imbibition pressure under normal physiological conditions?

-40 mmHg

What is the standard intraocular pressure under normal physiological conditions?

15 mmHg

What is the standard swelling pressure under normal physiological conditions?

55 mmHg

In what direction is the force of imbibition pressure?

under normal physiological conditions the direction of the pressure is toward the inside of the stroma

In what direction is the force of intraocular pressure?

this force acts toward the endothelium from the aqueous humor

In what direction is the force of the swelling pressure?

this force pushes outward from the corneal stroma

How is imbibtion pressure created?

negative charge draws in Na+ from aqueous humor

negative imbibtion pressure

draws fluid into cornea

positive imbibtion pressure

water leaving stroma

How does imbibition change with an increase in water content?

imbibition pressure gets closer to 0 with increased water content

How is intraocular pressure regulated?

regulated by equilibrium of aqueous production and drainage

How does swelling pressure change with increased corneal thickness?

decreases with increased corneal thickness

How does swelling pressure change with water content?

decreases with increased water content

How is swelling pressure generated?

the negative charge of GAGs repel one another producing this force, as the cornea swells the GAGs are further apart reducing the repulsive force

What is the cause of Bullous keratopathy?

involves endothelial barrier function and reduced pump function

What are the pressures affecting edema in Bullous keratopathy?

imbibition pressure: -30 mmHg

IOP: 15 mmHg

swelling pressure: 45 mmHg

What occurs to corneal pressures and swelling during acute IOP?

if IOP becomes greater than the swelling pressure, then the imbibition pressure becomes positive because the pump cannot keep up with excess water being pushed in, water is then hydraulically pressed out of stroma by the IOP

the water needs somewhere to go so infiltrates the intracullular speces of the corneal epithelium

How do pressures change in Phthisis bulbi?

there is no IOP

pump function nis disrupted without elevated IOP

the IP and SP are the same value and water is drawn in