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
Aberrations reduce/effect visual acuity, true or false
True
A lot of Light scatter causes VA reduction, true ir false
False
Hiw can a patient maintain good VA but have significant light scatter from media opacity
Opacity causes the light scatter which is randomly distributed across entire retina resulting in veiling luminance
What is veiling luminance & its cause
When light scatters across the retina from a media opacity, resulting in a diffuse ‘blanket’ of background luminance which is superimposed onto the eyes retinal image. This raised background luminance reduces the image contrast because it has reduced the luminance difference between dark and light image regions.
What symptoms are associated with veiling luminance
Glare symptoms:
Stars, halos around lights,
Pain/discomfort fixating near light source
Vision reduced as you get closer to light source
What is glare
Glare is caused by light scatter and varies with intensity and location of light source.
E.g sun, headlights, streetlights etc
The light scatter on the retina causes a diffuse increase in background luminance causing a reduction in image contrast = veiling luminance
When does light scatter cause a reduction in VA for the patient
When a light source is close to fixation providinf maximal incident illumination to lens opacities close to the pupil (centre of lens), therefore maximal light scatter and veiling luminance.
Then VA will be poor (not due to image blur from aberrations) but because retinal contrast will be so low that all the image detail is lost
A patient complaining of symptoms of reduced vision, esp when driving at night, glare type, but achieves good VA while testing. What can we do?
Possibly light scatter, patient complains of poor vision but the VA chart has high contrast with minimal glare source hence good VA.
Case history questions important here,
Contrast sensitivity pelli robson
Glare testing: measure VA with light source near fixation to help with a more real world measure for this patient.
How to best test and check for cataract
Retinoscopy: for PSC & cortical because it retroilluminates the opacity so seen in silhouette. No dilation needed because dark room so semi-dilated pupil.
Nuclear cataract seen by diffuse dim ret reflex but not well defined (but not even with slit lamp).
What is mixed cataract
When cataracts co-occur within the same lens.
Opacities are part nuclear &/or PSC &/or cortical
What is nuclear cataract / nuclear sclerotic cataract
Age related sclerosis of lens nucleus
Gradual hardening of nucleus
Opacities
What causes the grey white opacity in nuclear cataract
Protein related changes
Protein denaturation from aging, changes the structure of the proteins resulting in non transparency because the dense packness of cells is no longer.
Where can you find chromophores
Lens nucleus
What are chromophores
Responsible for visible light: Absorb it resulting in reduced ret illumination.
This can impact visual function.
In the lens nucleus, they Absorb mainly blue light: resulting in yellowish brown appearance
What causes the patient to see washed out blue colours with nuclear sclerosis
Chromophores in lens nucleus, absorbs blue light so causes px to see blue as washed out
Ehy can nuclear sclerosis cause myopic shift
Can lead to increase in refractive index resulting in myopic shift
What is the average myopic shift in nuclear cataract
- 0.4D
Sxs patient may experirnxe with nuclear sclerosis
Blurred vision ( may be from aberrations due to myopic shift)
Glare sxs
Altered colour vision
What causes the cortical volume to increase
Age
New lens fibres grow from epithelial cells with age
In cortical cataract, where do the opacities begin
Periphery sections of the lens cortex
Close to the lens equator
What happens to the fibre in cortical cataract to result in protein changes ( reduced transparency)
Fibres become over hydrated
Shape of cortical cataract
C shaped crescents & progress to spoke shaped wedges (cuneiform)
What causes the cortical cataract to become a large region
Shade type opacities at the same depth as spoke shaped, they coalesce to form regions of diffuse opacities causing large regions (sections/quadrants) of opacities
Prevalence of cortical cataract
30% in >45yrs
Why is most cortical cataract asymptomatic
When its in the extreme periphery and small in size, hidden behind the itis so doesnt effect pxs visual function
In cortical cataract: when will it be symptomatic
Once there is pupillary involvment
Cortical cataract sxs
Blurred vision (astig blur due to cyl changes in cortex fibre RI, average 0.50-1DC) Monocular diplopa (driven by uncorrected regular or irregular astig blur) Glare sxs from LS
Severity of visual sxs depend on (with a cortical cataract)
- how much the spoke opacities encroach the pupilliary zone (depends on shape, size, pupil dilation e.g night vision pupil more dilated so more sxs than during day when pupil miosis and opacity behind iris)
- pupil dilation (reduced depth of focus)
- angle between light source & visual axis ( e.g headlights)
Other than the sxs of blur, glare from LS, monoc dip, what does pupil dilation also result in when cortical cataract present
Result in reduced depth of focus causing greater aberrations = greater retinal blur
Most common location on lens for cortical cataract
Infero nasal quadrant
Due to UV exposure
What location on the lens does cortical cataract likely to begin
Equatorial region
What can also influence the cortical fibres & causes friction in the lens fibres
Accomodation: increases lens power ( 90% of this thickness occurs in lens nucleus )
But as nucleus hardens with age, friction between nucleus fibres ( now less flex) & cortical fibres ( now more flex) = causes damage to cortical fibres assoc with opacity development
What happens to epithelial cells to cause PSC
Lens fibres are made via lens epi cells migrating along lens ant surface entering transition zone where they elongate to straddle the equator.
Some of these epi cells take a different pathological path: migrate without elongation twrds posterior pole where they become vacuole shaped.
These cells then form thin opaque layer
Where does PSC form
Btwn posterior capsule & lens fibre terminating at posterior sutures
What does the metabolic activity of the epi cell do in PSC
Disrupts the regularity of the underlying lens fibres at posterior sutures.
The effect of these abnormal epi cells & changes to underlying fibres = LS & increased opacity in this region
Describe what PSC looks like
The opacity itself is typically well circumferenced, circular region at posterior pole.
Shape and density can vary
Why is PSC easily missed on Slit lamp with optic section
Without dilation, difficult to get shape/size beyond pupil margin
If undilated: Angle on SL needs to be very narrow resulting in greater corneal reflections
Best way to asses PSC
SL: retro illumination
Dark room = semi-dilated pupil & opacity seen via silhouette
Cortical & PSC Cataract sxs vary depending on what factors
Opacity size
Density
Shape
Proximity to visual axis
PSC sxs
Blurred vision ( from a combo of opacity induced aberrations and light scatter) Glare (from opacity induced light scatter)
PSC sxs are influenced by
- pupil size : ambient light levels critical (brighter environment = more miosis = Light entering eye cant bypass PSC opacity)
- Amount of converge-induced miosis : more severe sxs at near than distance, due to miosis at near preventing light to bypass psc opacity
- Vision at night often better: pupil largest so vision better than daytime
THIS IS OPPOSITE TO CORTICAL CATARACT - proximity to light sources to visual axis:
With dense opacity near visual axis = high LS = max veiling luminance
Both these mean disproportionate effect on CS esp early PSC where reduced CS causes severity of sxs for px.
Why is it important to explore px sxs (glare sx rather than blurry) in PSC
Very difficult to detect early (small) PSC, opacities near visual axis
Early PSC NR VISUAL AXis CAUSES SEVERE SXS FOR PX which vary with weather/ light lvls
What is congenital cataract
Cataract of any opacity present at birth
Most common cause of congenital cataract
Idiopathic in bilateral and unilateral
Some rare causes of congenital cataract
Hereditary Metabolic disorders (diabetes) Abnormalities with autosomal dominant inheritance (marfans disease) Chromosomal abnormalities (Down’s syndrome)
Why is early Dx and tx critical in congential cataract
To avoid amblyopia
What can congenital cataract cause
If dense enough then Leukocoria: greyish white pupil
What does subluxated mean
Anomoly of lens position (not on pupil centre)
What is a subluxated lens called
Ectopia lentis (dislocation of natural lens)
Causes if ectopia lentis
Traumatic displacement Genetic disorder ( marfans )
Consequence of ectopia lentis and treatment
Unusual refractice error (perip of lens on visual axis)
Rx or surgery
Meaning of aphakic px
Born without lens or no lens
If patient born with lens but is aphakic, what could be the reason
Cataract surgery: but px not suitable for IOL implant ( other condition meaning that implant wont help vision e.g. advance amd, diabetic retinopathy with limits patients vision)
