OP: Corneal Edema and Metabolism

What is the water content of the corneal stroma? How does corneal edema occur? What is the threshold for impact on vision?

• 78%

• More leak and less pump

• >5% increase = light scattering + reduced refractive function

What are the 2 regions typically affected by corneal edema? What is the most common way they occur? Can they occur independently? Furthermore, which curvature is affected the most and why?

• Stromal + Epithelial Edema

• Simultaneously affects both regions (majority)

• Each can occur independently

• Typically, posterior curvature but not anterior curvature is affected

  • Anterior weaving of lamellae resists expansion

What is Bullous Keratopathy? What is the functional cause of it, what is the presentation, and what are the 4 etiologies?

• Permanent corneal edema

• Failure in endothelial pump fxn or dmg to endothelial cells

• Stromal and epithelial edema

• Several causes

  • Endothelial dystrophy’s (Fuch’s)

  • Injury

  • Surgery

  • Hypoxia

What are the 5 forces that contribute to the 78% water content of the corneal stroma?

• Imbibition pressure (IP)

• Stromal swelling pressure (SP)

• Intraocular pressure (IOP)

• Endothelium pump

• Tear evaporation/miosis (minor)

What generates imbibition pressure, what does + or - pressure indicate, what does it draw and from where, how are Na+ ions associated with negative charges in a non-edemic stroma, and what does imbibition pressure allow for? Lastly, what is the normally expected value in mmHg?

• Negative imbibition pressure = draw fluid into cornea or can take on more water

• Positive = water leaving cornea or cannot take on more water

• Generated by (-) charge on GAGs

  • Draws in Na+ and H2O from AH

  • In non-edemic stroma only a portion of negative ions are associated with Na+

  • Allows for further Na+ infiltration and water movement

• -40 mmHg

What is IP a part of, what is it counteracted by, what does it depend on, how does it change with increasing/decreasing H2O content, what does a negative value indicate, and what other force is it affected by?

• Force is part of the leak

• Counteracted by endothelial pump

• IP value dependent on H2O content:

  • IP increases with increasing H2O content

  • IP decreases with decreasing H2O content

  • Negative value indicates more potential imbibition

• IP value is affected by IOP

What direction is swelling pressure directed towards, what is its relationship with water content, and what is the normally expected range? Lastly, how is this force produced and how does corneal swelling affect it?

• Outward directed force (repulsive)

• Inverse relationship with water content

  • Decreases with increased water content

  • Increases with decreased water content

• 55 mmHg

• 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 IOP regulated by, where does it generate a force, how does it affect water, and what is it distinct from?

• Regulated by equilibrium of aqueous prodxn and drainage

• Generates a force from anterior chamber toward epithelium

• Water is forced into the cornea (hydraulic pressure)

• Distinct from the negative imbibition force which draws in water through GAGs

What does IP = ?

IP = IOP - SP (15 - 55)

How does compromised/decreased endothelial pump function affect corneal hydration/IP/SP? What about tear evaporation?

• Endothelial pump

  • When compromised/decreased:

    • More water content in cornea

    • IP: increases

    • SP: decreases

    • Tear evaporation (minor)

    • More evaporation draws water from cornea

    • Less evaporation can cause edema

Where is edema found in bullous keratopathy? What does their IOP look like and how does the equation of IOP - SP = IP change? Lastly, what causes the edema found in the presentation and where does the excess water go?

• Bullous Keratopathy

  • IOP normal

  • Edema found in stroma + epithelium

  • Normal = 15 (IOP) - 55 (SP) = -40 (IP)

  • BK = 15 (IOP) - 45 (SP) = -30 (IP)

  • Stromal edema occurs d/t leak exceeding pump fxn

  • Epithelial edema occurs d/t IOP exceeding pump fxn

    • Excess water is forced btwn epithelial cells

How does ocular hypertension affect the amount of water coming into the epithelium? Why? Lastly, what occurs when IOP > SP?

• Elevated IOP pushes more than normal amount of water in through leaky barrier from AH

• Pump fxn cannot keep up with infiltration of water

• If high enough, it pushes water out of stroma

  • Occurs when IOP exceeds SP

  • IP becomes positive

  • Epithelial edema without stromal edema

What does hypotony do to pump fxn? How does this affect IOP? What kinda of edema will result and why? What does the equation of IP = IOP - SP look like in a normal person vs a person with phthisis bulbi? What will the equation change to over time and why?

• Pump fxn disrupted

• IOP = 0

• Stromal edema only

  • Water being drawn into stroma via IP

  • Epithelium does not have IP, no water drawn into epithelium

• Normal = -40 = 15 - 55

• Phthisis bulbi = -45 = 0 - 45

• Since H2O quickly infiltrates stroma, equation will rapidly drop to 0 = 0 - 0

  • Water drawn in = IP decreases

  • Water drawn in = SP reduced

  • IP = SP b/c IOP = 0

How does hypoxia affect anaerobic metabolism? What will this cause a build up in and how will this build up affect water movement? Lastly, how does hypoxia affect pH and how does this affect pump function/ion transport efficiency?

• Hypoxia

  • Increase in anaerobic metabolism

    • Build up lactate

    • Osmotic draw of water

  • pH reduction

    • Inhibition pump function

    • Ion transport efficiency reduced

Where does glycolysis occur and does it require O2? What about TCA/Ox. phos? Lastly, what is the reaction for glycolysis and TCA/Ox. phos?

• Glycolysis occurs in the cytoplasm and doesn’t require O2

• TCA/Ox. phos occurs in mitochondria and requires O2

• 1 glucose → 2 pyruvate + 2 ATP + 2 NADH

• 2 NADH + 6 O2 → 2 NAD+ + 6 CO2/H2O

What is pyruvate converted to following anaerobic glycolysis? What affects the rate of this overall reaction? Furthermore, what catalyzes the reaction? Lastly, give the 2 step reaction of anaerobic glycolysis.

• Pyruvate converted to lactate under physiological conditions following anaerobic glycolysis

• Rate of pyruvate to lactate conversion increases with hypoxia

• Lactate dehydrogenase catalyzes the reaction

• 1 glucose → 2 pyruvate + 2 ATP + 2 NADH

• 2 pyruvate + 2 NADH + 2 NAD+ → 2 Lactate

What are the 2 causes of corneal edema whilst sleeping?

• Hypotonic tears

• Hypoxic metabolism of corneal tissues

What is an example of Descemet’s membrane related edema? What is gradually lost in this disease, what forms, how does this affect endothelial cell density, how does this affect the hydration of the endothelium, and what 3 presentations can this cause in the cornea?

• Fuch’s corneal/endothelial corneal dystrophy (FCD/FECD)

• Gradual loss of endothelial cells

  • Guttata formation

  • Reduced endothelial cell density/expansion of guttata: mild edema

  • Severity of edema increases

  • Opacification of cornea + pain/vasc of cornea

What are the predominant risk groups in regards to sex, genetics, and age for FCD/FECD?

• Sex: Women > Men

• Genetics: Family Hx

• Age: 30-40s with some early onset occasionally

How do guttata affect the endothelial cell layer? Furthermore, what is descemet’s, what 3 functions does it have, and what does damage of this layer result in with regard to hydration?

• Guttata negatively affect endothelial cell layer

  • Descemet’s is a special BM that allows support of endothelial cells

  • Facilitates adhesion to stroma

  • Supports pump function

  • Supports cell viability

  • Damage to this layer can cause permanent edema or bullous keratopathy

What are the 2 zones of Descemet’s membrane, how does each change with age, what are each made from, and when is each formed?

• banded vs non-banded zones

• Posterior non-banded layer

  • Increases from 2 to 10 microns w/ ~ 1 micron/year

  • Made from COL-IV

  • Formed after birth

• Banded layer

  • Does not increase with age

  • Primarily made of COL-VIII

  • Formed during embryonic/fetal development

What are the 3 structures that COL-VIII can exist as?

• COL-VIII has subunits that can form helices or helical tetramers

What causes FCD/FECD? How does this affect Descemet’s and each of its respective layers?

• Can be caused by a congenital mutation in COL8A2 which encodes for COL-VIII

  • Thicker Descemet’s

  • Abnormal accumulation of COL-VIII in posterior

  • Disturbed assembly of COL-VIII in banded layer

What do the energy requirements of the corneal endothelium look like? What does the corneal stroma/epithelium need energy for? Lastly, what is counterintuitive about the corneas design in regards to energy production?

• Endothelium: energy required to maintain pump fxn (corneal tissue with highest energy req per tissue volume)

• Cornea stroma: maintenance/prodxn of GAGs and collagen fibrils

• Corneal epithelium: mitosis and migration of epithelial cells

• Energy needs to be delivered while being an avascular tissue

What is the main source of glucose for the entire cornea? How does glucose move throughout the cornea? What is cellular uptake of glucose mediated by? Furthermore, what contributes a minor amount of glucose to corneal tissue? What about a negligible amount? Lastly, how does the concentration of glucose change from corneal epithelium to endothelium?

• The AH provides the main source of glucose for the entire cornea

• Glucose diffuses through the cornea

• Cellular uptake is mediated by glucose transport proteins

• A minor amount of glucose comes from glycogen stores in the epithelium

• A negligible amount comes from tears/limbus

• Increases

What is the major transporter of glucose into corneal cells, is it expressed in epithelial or endothelial cells, and what does it allow for?

• GLUT = major transporter of glucose into corneal cells

• Expressed in epithelial and endothelial cells

• Allows bi-directional transport via a diffusion gradient

What are the 3 main ways glucose is utilized in the cornea? Furthermore, what are the metabolic pathways that use glucose dictated by (3 things)?

• 3 main ways

  • Anaerobic glycolysis: 85% of usage

  • Aerobic glycolysis: Krebs/CAC/TCA/Ox. Phos.

  • Hexose-Monophosphate shunt

• Metabolic pathways that use glucose are dictated by:

  • Oxygen concentrations

  • Mitochondrial number

  • Active enzymes available

What is the equation following glucose to the HMS or glycolysis?

Glucose → G6P → HMS or Glycolysis

Where does the oxygen supply of the cornea come from, how does it travel through the cornea, what depends on its partial pressure, and what organelle does metabolism depend on? Lastly, how does oxygen concentration change from the corneal epithelium to the endothelium?

• Oxygen supply comes from tear film and diffuses posteriorly in an open eye

• Glucose metabolism is dependent on partial pressure of oxygen

• Metabolism is affected by mitochondrial density

• High to low

How is O2 drawn into the cornea? What 2 factors generate this force?

• Oxygen is drawn into the cornea d/t diffusion gradient

• Gradient generated by:

  • Consumption of oxygen by the component layers of the cornea

  • Difference in partial pressure of O2 in atmosphere vs AH

Which metabolic pathway predominates in the cornea? Furthermore, how does the density of mitochondria in the epithelium compare to the endothelium? Lastly, which engages in more TCA/Ox. phos. and which engages in more anaerobic metabolism? What about glycolysis?

• Anaerobic metabolism

• Epithelium < Endothelium

• Endothelium = TCA/Ox. phos.

• Epithelium = Anaerobic

• Glycolysis = all tissues (precursor to anaerobic/TCA+Ox. phos.)

What is the ATP yield of glycolysis, TCA/krebs, and ox. phos.? What about anaerobic?

• Glycolysis = 2 ATP = end of anaerobic but start of aerobic

• TCA/Krebs = 2 ATP

• Ox. Phos. = 32 ATP

  • Total = 36 ATP

What are ROS and what do they contain? What are they missing and when do they form? Why are they problematic?

• ROS are chemically reactive molecules containing oxygen

• They are missing an e-

• Forms during mitochondrial metabolism (aerobic metabolism)

• Highly reactive with other biomolecules

What two molecules reduce ROS and how? How are they replenished and what is required for their replenishment?

• Ascorbic acid and ALDH3A1

  • Strong e- donor to ROS

• Regain e- via glutathione pathway requiring NADPH

How does the pO2 of stromal cells compare to that of tears and AH?

• The available O2 of tissues between the epithelia (stromal cells) will be a value between the pO2 of tears and AH

What is the hexose monophosphate shunt utilized by? What is another name for it? What are the 2 main functions it has and why?

• Utilized by corneal epithelium

• Also known as Pentose Phosphate Shunt

• 2 main fxns:

  • Produce NADH

    • Used in synthesis of lipids

    • Limits ROS generated by aerobic glycolysis (gluthathione pathway)

  • Produces ribose-phosphate

    • needed for RNA/DNA synthesis

What is the precursor that feeds into the HMS? What enzyme is required for this and what can go wrong with this enzyme? What does this result in?

• Pathway begins with G6P → HMS

• G6P-dehydrogenase

  • Essential for HMS

  • Deficiency present in 400m worldwide

  • Deficiency = risk factor for pterygium and cataracts (d/t reduced ROS protection)

What is favism?

• Hemolytic anemia from eating fava beans or inhaling their pollen

What enzyme catalyzes the conversion of pyruvate to lactate?

• Lactate dehydrogenase

What does the concentration of lactate in the corneal stroma look like in comparison to the AH? What is this facilitated by and how is excess lactate removed from the corneal tissue?

• Higher in cornea stroma vs AH

• Facilitated by MCTs

• Cannot pass thru epithelial barrier into tear film thus any excess diffuses posterior and is transporter out of the cell

How does keratocyte metabolism affect GAG prodxn in the stroma and what is it dependent on? Make sure to note how it affects both the posterior and anterior stroma, as well as which GAG is favored and why.

• Affects type of GAG prodxn

• Dependent on NAD+/NADH ratio

• Posterior stroma

  • Less O2 = more anaerobic metabolism = more lactate = shifts NAD+/NADH ratio to NAD+

  • Keratan prodxn favored as NAD+ inhibits dermatan’s enzyme

• Anterior stroma

  • More O2 = more aerobic metabolism = less lactate = shifts NAD+/NADH ratio to NADH

  • Dermatan prodxn favored

What produces CO2, how does it diffuse in the corneal tissue, what does the pCO2 look like in AH vs tears, what does increasing CO2 do, and how does the concentration of CO2 change from corneal epithelium to endothelium?

• Produced by aerobic metabolism

• CO2 diffuses from AH to tears

• pCO2 AH > tears

• Increasing CO2 = lower pH = increase HCO3- (b/c of CO2 + H2O → HCO3- + H+)

• Low to high

How does anaerobic metabolism affect pH and why? How does the amount of CO2 change with anaerobic metabolism and how does this affect the pH?

• Anaerobic metabolism produces lactate/lactic acid and because of the physiological pH it is almost entirely found as lactate (-) + H+

• Increases in anaerobic metabolism which will decrease pH via the carbonate buffer system and carbonic anhydrase activity

What does the rate of CO2 expelled vs O2 absorbed look like?

• CO2 expelled > O2 absorbed

What is the structure of glycogen, what is it comprised of, when is it generated and where, where is it packed into, what is required for its synthesis, how is the synthesis affected in diabetic patients, and lastly how is it converted in times of low energy?

• Polymerized (branched), modified glucose residues

• Generated in the presence of excessive glucose within the corneal epithelium

  • Converted to glycogen and packed into cytoplasmic granules for future use

  • Requires energy input for synthesis

  • Increase occurs in diabetic patients

• When energy is needed, glycogen is converted to G6P which enters glycolysis