Cataracts Flashcards
How does the lens keep transparency?
3 things
Avascular, OFZ, tight/dense packed fibres (regular shaped)
What does the anterior lens surface consist of?
Monolayer of epithelial cells
Where is the germinative zone?
Near the lens equator
What happens at the germinative zone?
Epithelial cells divide & elongate
What happens to the epithelial cell structure at the germinative zone
Cell body flattens & elongates toward the anterior & posterior poles, where they join to form anterior/posterior sutures
What makes up the lens cortex
Epithelial cells that have flattened & elongated
What are sutures
Where the cortex fibres that stop at the anterior & posterior pole, y shaped overlapping branch pattern
What are organelles and where are they located
Membrane bound subunit within epithelial cells, they do energy production (mitochondria), cell repair (DNA in cell nucleus) and other things.
What happens to organelles
During the flattening & elongagtion of epithelial cells in the germinative zone, cells gradually lose organelles resulting in OFZ
What is OFZ
Organelles free zone in the deeper cortex fibres
Why is OFZ important
Within OFZ light scatter is greatly reduced compared to lens cortex.
Organelles have different refractive index compared to cell membranes.
With organelles = get diff RI = higher LS in outer cortical layers.
OFZ = reduced LS
How is organelles driven LS minimalised through centre of iris
Central epi cells near anterior pole have less organelles than perip epi cells.
But even though perip epi cells & outer cortical layer have more organelles = high LS due to diff in RI but are mostly occluded by iris = organelles driven LS therefor minimalised through pupil centre to retina.
Why is the lens being avascular important
Maximum transparency
How does the lens get its nutrients if its avascular
Via aqueous humour surrounding the lens cells
Aqueous humor has different RI to lens fibres, how does this affect light scatter
Difference in RI = high LS as light passes the fibres and AqH.
To avoid this LS, lens fibres have regular hexagon shape structure packed and dense to minimalise extracellular fluid between the fibres.
Lens is flexible for accomodation, so how does this affect the packed dense nature of the lens fibres
The lens fibres have interlocking membrance = interdigitations (tongue & groove type).
This allows the lens movement, minimising extracellular space.
What is oxidative stress
Free radicals: oxygen containing molecules that cause changes to cell structure over time.
Its an imbalance of free radicals & antioxidants in the body.
How do free radicals/oxidative stress affect the lens
Alter protein structure = protein denaturation: make the proteins opaque that normally allow the lens fibre to have structure for transparency.
The denatured proteins = fibres opaque = higher LS
What happens to the lens fibres with protein denaturation
Become opaque = higher LS
How does aging cause high Light scatter
Aging is a form of denaturing = irregularities in protein size, shape, and packed density of the lens fibres = high LS within and between fibres.
Intercellular space increases = increased Aq H in this space = high LS due to difference in RI
Maximum nuclear cataract size
Limited to 6mm equatorially & 4mm sagitally because the nuclear fibres are present at birth.
What can cause a slight reduction in nuclear fibres with age
The compacting effect of cortical fibres that are continually added through life, these surround the lens nucleus
Which of the fibres contributes to increased lens thickness
Cortical fibre production = increased lens thickness on the axial and saggital planes (lens diameter stays constant)
What results with increase in cortical thickness with age
Increased light scatter driven by changes in the deep cortical fibre layers causing uneven increased LS compared to other layers.
2 types of Light scatter
Forward scatter : light scatters towards retina results in cataract related symptoms.
Backwards scattler : light scatters in reverse towards the incident light like when we look at lens through slit lamp. This is why we see the lens better in elderly patients with more opacity and less in young pxs who have no backward light scatter because the lens has more transparency
Cortical fibres lose organelles closer to the nucleus, so we expect these deeper layers to have less light scatter… true of false
False
These deeper layers cause most rapid increase in LS
Key factor for this scatter:
Age related increase in cortical lens compaction. The fibre membranes so closely packed have higher RI than cell contents, meaning the light goes through membrance and cell content (diff RIs) many regions of density resulting in massive increase in LS
What is PSF
Point spread function
Measures the degree the lens spreads that point source (narrow beam e.g pinhole).
What does a greater PSF indicate or result in
The greater the point spread function the greater the detrimental effect of vision
What factors influence PSF
Optical Aberrations: central part of the PSF
Light Scatter: peripheral zones of PSF
An increase in either will negatively effect vision and may induce symptoms
What 2 factors related to PSF can result in reduced vision or related symptoms
Increase in Aberrations and or light scatter.
What 2 types of aberations are there, what are they & how are they corrected
- Low order abs: postive defocus (myopia), negative defocus (hyperopia & regular astigmatism. All grouped into DEFOCUS ABERRATIONS.
Corrected with Rx/CLs - High order abs: spherical aberration, coma, trefoil. All grouped into IRREGULAR ASTIGMATISM
What sx do aberrations cause the patient
Blurred vision
What sxs does irregular astigmatism (higher order aberrations) cause the px
Less like blurry sx. Night vision problems Halos Starburst Monocular diplopia
What type of aberrations, can refractive error help patient with blur symptoms
Lower order: Defocus type
E.g myopic shift in cataract
Astig shift in cortical cataract
But be careful with elderly and adaptation to new rx
What kind of aberrations wont be helped by offering new improved refractive error
Higher order: irregular astigmatism because a new rx unlikely to help with sxs
