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APCO > CL Sequencing Physiology > Flashcards

CL Sequencing Physiology Flashcards

1
Q

Why are central K readings misleading indicators of overall corneal topography?

A

Because cornea should be spherical which is not the case.

It tells you central is 4mm only, nothing about spherical

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2
Q
  1. Is the cornea symmetrical?

2. The curvature flattens towards which part of the cornea?

A
  1. Not symmetrical

2. Periphery

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3
Q

What is the normal HVID?

A

~12mm

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4
Q

What are the 4 topographic zones?

A
  1. Central (4mm spherical)
  2. Paracentral (~ diameter of scotopic pupil)
  3. Peripheral (flattest and aspheric)
  4. Limbal
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5
Q

Cornea provides how much refractive power of the eye?

A

2/3

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6
Q

Why is cornea curved, transparent with clear optics?

A

It all thanks to cellular organisation and the air TF interface

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7
Q

Slight irregularities can be compensated by?

A

Overlying TF

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8
Q

What allows cornea to withstand shear forces of blinking, EM, microbial infection and FB?

A
  • cell specialisation
  • cell turnover
  • tight junctions
  • surface glycocalyx
  • TF
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9
Q

How many layers does cornea have? What are they?

A

5 layers!

  1. Epithelium
  2. BOwman’s layer
  3. Stroma
  4. Descemet’s membrane
  5. Endothelium
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10
Q

How many cells does epithelium have? What are they?

A

3 cell types!

  1. Basal cells
  2. Wing cells
  3. Squamous cells
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11
Q

Microplicae and microvilli on the epithelium surface play a role in TF by doing what?

A

Anchoring the mucus layer (glycocalyx)

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12
Q

How long is cell shedding and turnover (basal to the surface) without CL wear?

A

~7 days

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13
Q

Does epithelium negatively or positively impacted by CL wear?

A

NEGATIVE!

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14
Q

Epithelium is a permeability barrier to what?

Adjacent cells are connected by what?

A
  1. water
  2. ions
  3. most hydrophilic molecules (eg. fluorescein)
  4. infective pathogens through tight junctions (desmosomes)
  5. Aquaporins (water channels)
  6. Gap junctions (ion transport)
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15
Q

In the stroma, how do intermediate filaments in the cytoskeleton are linked through?

A

Linked through the hemidesmosomes via anchoring fibrils to anchoring plaques

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16
Q

CL wear can reduce the epithelial barrier function related to what?

A
  1. Hypoxia dose

2. Overnight wear

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17
Q

How thick is the stroma and it is how many % of the cornea?

A

Thickness: 0.50mm thick/ 500 microns

It is 90% of the cornea

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18
Q

What does stroma contain?

A

2-3% keratocytes (fibroblasts) and 1% ground substance (GAGs) interspersed between lamellae

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19
Q

What are keratocytes in the stoma?

A

They are:

  • thin
  • flat cells
  • tight junctions
  • 10 microns in diameter with long processes
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20
Q

What are GAGs in the stroma?

A

They are:

  • proteoglycans
  • poly-ionic
  • induce water retention
  • influence fibril arrangement
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21
Q

What are stromal Lamellae?

A

A regular arrangement of collagen fibrils within each lamellae provides optical clarity

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22
Q

What happens when there’s infection and/or irritation in stromal lamellae?

A

Leads to epithelial cells will release cytokines and chemokines to begin immune defence

23
Q

What kinds of WBC will go to the infected site in stromal lamellae?

A
  1. Limbal vasodilation
  2. Keratocyte migrate
  3. Interferon
  4. Neutrophils
  5. Natural Killer cells
  6. Macrophages
24
Q

What happens when there’s corneal insult in stromal lamellae?

A

Leads to:

  1. Disorganised spacing
  2. Oedema (Striae)
  3. Tissue loss (ulcer)
  4. Clarity loss (infiltrate + scarring)
25
Q

What is the corneal endothelium?

Do endothelial cells lost with age?

A

A mono-layer of squamous cells 500,000 cells at birth (4,500 cells/mm2)

Yes, it loses with age

26
Q

What does corneal endothelium have/do?

A
  1. Inter-locking irregular edges, tight junctions, lots of organelles, metabolic activity
  2. allows passage of ions and small molecules
  3. Maintains osmotic gradient by metabolically driven pumping ions out of the stroma into the aqueous
27
Q

Endothelium function

A
  1. Pumping function: maintains 78% corneal hydration for optimal transparency & normal thickness
  2. Minimal endo cell count: 1000 cells/mm2 - needed to maintain this state
28
Q

In endothelium:

Age, pathology, trauma, oxygen deficiency (hypoxia) can lead to?

A
  • loss of pumping ability
  • corneal oedema
  • corneal striae
  • polymegethism (size)
  • polymorphism (shape)
    of the cells
29
Q

What does innervation do?

A
  1. There’s 50-80 myelinated and unmyelinated precorneal nerve trunks -> enters mid stroma -> lose myelin at the cornea and branch anterior centrally
  2. Dense subepithelial plexus, unsheathed nerve endings enter the epithelium and branch out among basal cells
  3. Richly innervation, initiates blink reflex, wound healing (trophic role), and tear secretion
30
Q

What is the transition zone between cornea and conjunctiva/sclera?

A

1.5mm Horizontal
2mm Vertical

  • Surgical reference point and significant change between steeper corneal curvature and flatter sclera
31
Q

Why do you need to respect the limbus?

A

Because it has a:

  • minor role in corneal nutrition
  • Important role for defence
  • location of limbal stem cells

So… need to avoid CL touch, interaction or pressure on this area

32
Q

Terminal arteries of limbus have distinctive looping, so what do you have to look out for?

A
  • Engorgement
  • NVZ
    with CL wear
33
Q

What does the cornea need to maintain tissue health and control hydration?

These nutrients are supplied by what?

A
  1. Oxygen
  2. Glucose
  3. Vitamins
  4. Amino acids
  5. Other metabolites

These nutrients are supplied by (oxygen supply):

  1. Peri-limbal vasculature - 55mmHg O2 tension
  2. TF - 155mmHg O2 tension
  3. Aqueous humour - 30-40mmHg O2 tension
  4. Palpebral vasculature - 55mmHg O2 tension
34
Q

How much is corneal influx on average?

A

6microlitres/cm2/hr

35
Q

If you wear CL, it reduces or increases oxygen availability to the cornea?

A

REDUCES

36
Q

What permits corneal flux through the CL?

A

CL oxygen permeability (Dk)

37
Q

What is Dk?

What is Dk/t?

A

Dk: measures oxygen permeability of CL
- means: how much oxygen is passing through the material under certain given conditions

Dk/t: measures oxygen transmissibility of CL
- Dk/t calculated using Dk of CL material and thickness. Usually refers to a CL with certain power

** Higher these values, better Dk/t of the material. Higher oxygen flow to the cornea and can improve comfort and longer-term tolerance

  • Dk/t is dependent on CL thickness
  • Local Dk/t is important. There’s a significant impact on central and peripheral CL thickness, especially for higher powers
38
Q

What happens to the cornea when we sleep?

A

5% overnight oedema (increase in corneal thickness) in NORMAL eyes is observed WITHOUT CL wear

39
Q

What happens when there’s a 5% increase in corneal hydration?

A

Leads to light scatter
- increase water uptake of GAGs between fibrils causes fibre aggregation

–> seen as striae after overnight eye closure even w/o CL wear

But return to normal/ baseline within 1 hour of waking

40
Q

Think about the impact of CL EW, or even long-day wear of low Dk/t CL on the diffusion of CO2 and O2

We all know eye closure + CL wear will reduce oxygen tension.
So, low O2 tension leads to?

A

Build up of:

  1. Lactic Acidosis
  2. Increased osmotic load
  3. increase water intake into cornea
    - –> OEDEMA!

** Long term –> tissue change

41
Q

What are the signs you can see/ complications from CL?

What are they and what causes them?

A
  1. Corneal NVZ
    What are they?

Causes:

  1. Epithelial Microcysts
    What are they?

Causes:

  1. Corneal Striae What are they?

Causes:

  1. Endothelial polymegethism
    What are they?

Causes:

  1. Endothelial Blebs
    What are they?

Causes:

42
Q

What has a significant impact on TF flow and CL fitting, movement, insertion, and removal?

A
  1. Lid action
  2. Aperture Size
  3. Gravity
43
Q

What is a TF?

A

Tears:

  • a complex, structured, surface film secreted by lacrimal and accessory glands
  • which protects and nourishes the OS and provide goof optical front surface
44
Q

What are the roles of the TF?

A
  1. Optical:
    - TF maintains an optically uniform interface between the air and anterior surface of CL
  2. Mechanical:
    - TF acts as a vehicle for the continual blink-mediated removal of intrinsic and extrinsic debris and particulate matter from the front and underneath the CL
  3. Lubricant:
    - TF ensures a smooth movement of eyelids over the front surface of the CL, and CL over the globe, during blinking
  4. Bactericidal:
    - TF contains defence mechanisms in the form of proteins, antibodies, phagocytotic cells, and other immunodefence mechanisms that prevent CL-induced infections.
  5. Nutritional:
    - TF supplies epithelium with necessary supplies of oxygen, glucose, AA, and vitamins underneath CL via lid-lens tear pump
  6. Waste Removal:
    - TF acts as intermediate reservoir for removal of corneal metabolic byproducts (CO2, lactate) from underneath the CL via lid-lens tear pump.
45
Q

TF vs CL thickness

And what are the things you should consider?

A

5 microns vs 100 microns

*CL are highly disruptive to the TF structure

Consider:

  • Impact of lens edge design and movement
  • Pre-lens TF - especially the lipid layer
  • Post-lens TF - tear exchange, aqueous, and mucins
46
Q

CL-associated keratitis can result from many mechanisms like what?

A
  1. Infection
  2. Toxic solutions
  3. Bacterial endotoxicity
  4. Immunological rxn
  5. Trauma
  6. Hypoxia
  7. Metabolic Disturbance

Other aetiological factors:

  1. Breakdown of trapped post-lens TF debris
  2. Lens deposits
  3. Poor px hygiene
47
Q

What does the chemical messages in the immune cascade lead to?

A
  1. Redness
    2, Recruit inflammatory cells (WBC)
    into the cornea
48
Q

Why do we avoid CL edge touch with the limbal area?

A

Don’t want lens edge to interact with area bcoz can induce NVZ
(The vessels can grow into the cornea and can leak to the stroma)

49
Q

What are the 4 clinical signs associated with corneal hypoxia?

How is corneal thickness impacted by CL wear?

A
  1. Epithelial blebs
  2. Stroma oedema
  3. Polymegethism
  4. Corneal striae
  5. NVZ ingrowth BV in cornea
50
Q

What are the key influences of the eyelid on CL fit?

A

Rotational force on CL

Push CL down and back

51
Q

What impact does CL wear have on the corneal TF?

A

Highly disruptive TF, consider pre and post-lens TF, the impact of lens edge design and movement?

52
Q

What are the functions and composition of the TF?

A

Lipid, aqueous and mucin

- Each of them can affect CL position, deposition differently

53
Q

What is the immune response to corneal insult?

A

Breakdown of the post-lens TF debris, deposits in lens and poor px hygiene –> can cause redness, inflammatory cells into cornea

APCO (13 decks)

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