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Lens Disorders > Lena Structure And Function > Flashcards

Lena Structure And Function Flashcards

1
Q

What makes the lens unique compared to other structures in the eye?

A

It can alter its refractive power over short periods via accommodation and also over long periods via age related changes

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2
Q

What is the function of the lens?

A

To transmit light with minimum defocus and light transmission and defocus a dependant on the degree of light scatter

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3
Q

Finish the sentence: the greater the degree of scatter…

A

The more the vision will be affected because the greater the opacity in the structure

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4
Q

What symptoms does light scatter lead to?

A

Glare and haloes or maybe even blur

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5
Q

What cells sit at the anterior surface?

A

A monolayer of epithelial cells

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6
Q

Where does the germinative zone sit?

A

Near the equator

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7
Q

Describe the function of the germinative zone:

A

Epithelial cells divide here and start to alter their structure

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8
Q

Explain 1/3 ways the germinative zone alters the structures of the epithelial cells?

A

The cell body flattens and elongates towards the anterior and posterior poles where they join to form the anterior and posterior sutures

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9
Q

What makes up the lens cortex?

A

The epithelial cells

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10
Q

Describe the appearance of lens cortex fibres:

A

The fibres aren’t continuous and they terminate at the anterior and posterior sutures

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11
Q

What is the shape of the sutures?

A

Y shaped

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12
Q

Explain 2/3 ways the germinative zone alters the structure of the epithelial cells:

A

During flattening and elongation the epithelial cells lose their organelles

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13
Q

What is the function of organelles?

A

Energy production/repair

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14
Q

What leads to an organelle free zone?

A

A loss of organelles

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15
Q

In the organelle free zone, how is light scatter?

A

Light scatter is reduced compared to the cortex

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16
Q

Why is light scatter reduced in a place where there’s less organelles?

A

Because if the difference in refractive indicies compared to the cell membrane

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17
Q

What drives light scatter?

A

Light passing through different structures and the differences between the refractive index

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18
Q

Where is the greatest amount of scatter?

A

In the outer cortical layers (place where there’s most organelles) and the peripheral epithelial cells

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19
Q

Why isn’t light scatter very effective in the peripheral epithelium/outer cortical layers?

A

Because it’s occluded by the iris so light scatter is minimal across the pupil

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20
Q

Explain 3/3 ways that the germinative zone alters the structure of the epithelial cells?

A

The lens needs to have no blokd or nerve supply in order to maintain its transparency

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21
Q

How are lens nutrients transmitted to the lens?

A

Via diffusion from the aqueous which surrounds the lens.

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22
Q

What is the difference between organelle free zone and aqueous?

A

They both have different refractive indicies so although there’s less light scatter at the OFZ, there’s an increase of light scatter as the light passes through the layers of the aqueous

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23
Q

How is scatter avoided?

A

By minimising the extracellular space between lens fibres and the amount of extracellular fluid. This minimisation Is aided by having fibres that are hexangonal shaped to allow them to be packed alongside each other

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24
Q

What is the difference in structure that allows the lens fibres to change shape to move with accommodative changes?

A

The fibres need to have the flexibility to allow them to move with each other and they need to maintain their closeness by having interlocking membranes which contain tounge and groove joints which allows form movement but minimal space

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25
Q

Which structure has the least amount of organelles?

A

The nucleus

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26
Q

How does a lens maintain its transparency?

A

1-by having tightly packed cells
2-having no organelles
3-having no blood supply

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27
Q

Describe the lenses biochemical composition:

A

Loss of lens fibre elasticity causes a loss in accommodation and there’s increased abberations and scatter which reduces vision

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28
Q

What do all cells go through?

A

Oxidative stress

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29
Q

Explain oxidative stress:

A

When free radicals and oxidants cause changes to the cell structure and there’s an imbalance of free radicals and anti oxidants

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30
Q

Which Protein structure does oxidative stress alter?

A

The proteins that allow lens fibres to have the structure needed for transparency

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31
Q

What does alterations of lens fibres (due to oxidative stress) cause?

A

It causes protein denaturing and the denatured proteins are opaque which therefore denatured proteins increase opacities due to the way they increase light scatter

32
Q

Name 1/2 age related changes to the protein structure:

A

Changes to the size and shape which changes how close they are packed together

33
Q

What does irregularity in size, shape and packing cause?

A

An increase in light scatter

34
Q

What is the 2/2 way in which age related changes increase light scatter?

A

Intracellular spacing can become larger causing an increase of aqueous between fibres

35
Q

What are the oldest fibres in the lens?

A

Nuclear lens fibres which are present at birth

36
Q

What is the maximum size a nuclear cataract can be?

A

It’s limited to the nuclear fibres and it’s about 6x-4x

37
Q

Why does the nucleus region reduce in size with age?

A

Because of cortical fibre production surrounding the nucleus

38
Q

What does cortical fibre production do to the lens?

A

It increases its thickness

39
Q

What does an increase of cortical thickness with age correlated with?

A

An increase in light scatter

40
Q

With cortical thickness there’s an increase of light scatter, what changes drive this?

A

The changes in the deeper cortical layers (closest to the nucleus) which produce a different amount of scatter compared to the other layers

41
Q

What is a point source?

A

Narrowest beam of light we can present

42
Q

Describe the point spread function:

A

We can measure the degree in which the ocular media spreads the point source

43
Q

If the spread is greater what happens to the effect on vision?

A

Greater the spread the worse the vision

44
Q

What is the central part of the PSF associated with?

A

Abberations

45
Q

What are the peripheral zones in the psf associated with?

A

Light scatter

46
Q

An increase in light scatter or abberations causes what?

A

Reduced vision and induces symptoms

47
Q

What does the nature of symptoms due to abberations and light scatter depend on?

A

Whether they’re abberations or scatter

48
Q

What increases the severity of the symptoms?

A

Increases with age

49
Q

Describe defocus abberations:

A

These are lower order abberations which include myopia, hyperopia and regular astigmatism

50
Q

How are defocus abberations corrected?

A

With sph or cyls

51
Q

What are high order abberations?

A

Spherical abberations, coma, trefoil

52
Q

What are high order abberations referred to as?

A

Irregular astigmatism

53
Q

What sx will arise from abberations?

A

Blur

54
Q

What sx will appear in irregular astigmatic px’s

A

Issues with night vision , haloes and mon diplopia

55
Q

What does the height on a PSF mean and the width mean?

A

Light intensity and the width is the spread of light across the retina

56
Q

The greater the spread of the px’s eye what happens to vision?

A

The worse the vision gets

57
Q

How can we predict the abberations influence on va?

A

We can do it via the width of the point spread but the higher the abberations, the lower the acuity and the more the px is significantly undercorrected which will cause blurred vision

58
Q

If abberations are of the defocus type, can we restore vision and how?

A

If they’re defocus they’ll be a refractive error change therefore a change in the correction will improve va

59
Q

What is the problem with prescribing specs?

A

Whether or not the px will be able to adapt to the new correction

60
Q

If the abberations are irregular astigmatism, will specs still sort the problem?

A

No it’s unlikely to provide improvement

61
Q

Can you predict va with light scatter?

A

No, because the media can induce lots of scatter but the px can still maintain good va

62
Q

What does light scatter influence?

A

Retinal illumination

63
Q

When light is scattered, what happens to it?

A

It’s randomly distributed across the whole retina to create veiling luminance

64
Q

What happens as we raise the background contrast?

A

Contrast is reduced due to the reduction of luminance j

65
Q

What is light scatter associated with?

A

Glare

66
Q

How did glare vary?

A

With location of the light source and the intensity of the light source

67
Q

Name some examples of glare symptoms?

A
  • Stars/haloes
  • Pain/discomfort when fixation is close to a light source
  • vision that deteriorates with increasing distance between fixation and a light source
68
Q

Why do sx of glare appear when a light source is close to fixation?

A

Because the closer the light, the max illumination of any opacity close to the pupil which maximises scatter and veiling luminance

69
Q

If fixation is close to a light source in a px with glare/scatter issues, how will the va be?

A

VA will be poor but not because of image blur but because the contrast will be so low that image detail will be lost

70
Q

What does scatter help explain?

A

Explains why some px’s will complain about poor vision but have good va in the testing room where there’s min glare and a high contrast chart

71
Q

When px’s who suffer from glare generally but have good va in the testing room, what will make their va worse in outdoor conditions?

A
  • very bright sun
  • sun being low in the sky
  • reflected sun off a surface
72
Q

What should we do if we suspect light scatter?

A

We need to specialise our case hx for that and ask specific questions relating to the light scatter and can also consider testing contrast sensitivity with Pelli robson or glare testing (measuring va with a light source close by)

73
Q

What is a complication with finding a cataract?

A

You won’t be able to see opacities unless you have a dilated pupil

74
Q

What maybe the best way to assess a psc/cortical cataract?

A

With a ret

75
Q

Why is using a ret to find opacities with psc/cortical cataracts best?

A

Because the opacities get retroilluminated by the ret and are seen in sillohette. The darkness of the room provides a semi dilated pupil and can reveal more peripheral sections of the lens

76
Q

Other than finding psc/cortical, what else can a ret be used to check? (Not refraction)

A

It can check the density of nuclear cataracts by the degree of diffusion and dimming of the ret reflex

Lens Disorders (13 decks)

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