The Lens and Cataracts (also do lens flashcards from OP4102)
what is the anterior radius of curvature of the lens? what about the posterior radius of curvature?
-8-14mm
-4.5-7.5mm
what is the unaccommodated axis length of the lens and what speed does it grow at each year?
-4mm
-0.023mm/year
what is the equatorial diameter of the adult lens?
10mm
what keeps the lens in position in the eye?
suspensory ligaments called zonules of zinn
what are the zonules of zinn made of? what is their diameter?
fibrillin, 1-2 micrometers
what happens to the lens in accommodation?
- ciliary muscles contract
- relieves the tension of the radiating fibres of the zonule so the lens assumes a more globular shape - thicker section for light to travel through so increased dioptric power
- sphincter pupillae muscle contracts to pupil becomes smaller and light through the thickest central part of lens reaches retina
what does protein content do in the lens?
it causes its refractive power to be increased e.g. fish lenses have a much higher protein concentration as refractive index of water is much higher than in air (1.333 compared to 1.00) so their lens needs a greater refractive power
why does the lens have a refractive index gradient?
A lens with a refractive index gradient means you can minimise chromatic aberrations and get a sharper image.
why do some animals have slit pupils in the light?
because with a circular pupil, you’re only using two refractive zones whereas with a slit, you’re exposing more refractive index gradients - different refractive index zones will refract their corresponding wavelengths of light so having more refractive index zones available, you can minimise chromatic aberrations
what are the three parts that make up the lens?
-an elastic capsule
-lens epithelium (on anterior surface of the lens found under the lens capsule)
-lens fibres
how are adult lens fibres produced?
- Cuboidal Epithelial cells
elongate, forming columnar cells - Apical surface of the cell grows
and pushes anteriorly - Basal surface grows and
pushes posteriorally - Both processes continue to
grow, meeting opposite fibres at
irregular sutures
how big is each lens fibre?
Each fibre 4 by 7micrometers , hexagonal in cross-section up to 12mm in length
what forms the nucleus and cortex of the lens?
older lens fibres form the nucleus and the younger nucleated fibres form the cortex
what is the difference in structure between cortical and nuclear fibres?
cortical fibres have a nucleus and organelles whereas nuclear fibres have no nucleus or organelles
how does the lens capsule change with age?
it increases in thickness
what percentage of the lens is protein?
90%
what are the two types of proteins in the lens?
- insoluble - membrane and cytoskeletal
proteins - soluble – crystallins - responsible for
special properties of the lens.
what are the three main groups of crystallins in the lens?
- Alpha crystallin
- Beta crystallin
- Gamma crystallin
what is alpha crystallin?
small heat shock protein (sHSp) - a globular protein that forms aggregates and occurs in all the major body tissues
what are 2 properties of alpha crystallin?
-it is polydisperse so forms aggregates of different sizes
-it cannot crystallise
what does alpha crystallin do?
Chaperones/protects target proteins in
response to stress eg increased
temperatures, oxidation etc. (protects other proteins from thermal denaturing)
how is the state of alpha, beta and gamma crystallins different?
alpha and beta are oligomers (a polymer with relatively few repeating units) whereas gamma is a monomer
why is the lens transparent?
it has structural regularity at the atomic and the molecular level so
-regular arrangement of lens fibres in a lamellar conformation so minimal intercellular space (cataracts disrupt this regular and precise organisation)
-the relative dehydrated state of the lens
-avascularity of the lens
how can precise packing of crystallins in the lens be disturbed?
-increased water accumulation
-vacuole formation within the lens fibres
-formation of high molecular wight protein amorphous aggregates (when high weight Amorphous aggregates reach a size comparable with wavelength of light - Light scattering)
when in the embryonic lens do crystallin appear?
around week 6
how can UV light cause lens opacification?
- UV absorption causes changes in crystallin structure and aggregation
- energy of the photons in UV break chemical bonds and lead to change in molecular structure
- causes formation of large amorphous aggregates
what wavelengths of light does the lens absorb?
300 and 400 nm
why is protein denaturation a problem in the lens?
due to the chaperone role of alpha crystallin
give 5 symptoms of catarct
- Blurred or hazy vision
- Problems with NV first
- Problems driving at night
- Glare
- Like looking through mist
give 3 general signs of cataract
- Opacity of natural lens of the eye
- Can be classified according to position
of the opacity - Most common is nuclear sclerosis
which is age related
what are the first wavelengths of light to get lost in a cataract?
the short wavelengths so you stop being able to see warmer tones
what is the most common cataract type? what does it look like?
cortical cataract so looks like spikes coming into the center and in retro illumination you can see glare associated with this in the patient’s vision
what is a posterior sub capsular cataract?
where the back of the lens becomes opacified - common in people with disease like diabetes or uveitis and patients even in their 20s can develop this hence not age related
give 5 rare types of catarcat
-christmas tree or polychromatic
-anterior capsular
-traumatic
-posterior pole
-diabetic snowflake cataract
what is a Christmas tree cataract?
also polychromatic cataract - due to changes in the lens, can be due to lipid changes as well as protein changes and its more rare
is anterior or posterior rupture to lens worse? why?
Anterior rupture of capsule is much worse than posterior rupture as the aqueous has more liquid than the vitreous
what is significant about management for posterior pole cataracts?
need a specific surgical technique as they are very common to rupture but most patients with this usually have good va so not worth risk of surgery
why does diabetic cataract look the way it does?
snowflake cataract where multiple layers of the lens have different opacities
why does diabetes cause cataract?
due to rapid
osmotic changes in the
lens due to poor diabetic
control
what are the 2 causes of anterior capsular cataract?
-trauma
-idiopathic
what are traumatic cataracts also known as?
stellate or rosette
what can cause posterior pole cataract
congenital with AD inheritance link
why grade cataracts and how does it work?
Grading cataracts are used for research purposes to categorise the cataract and explain to people what they may look like - not used clinically to make surgical decisions - n grading, c grading and p grading where the higher the level of cataract, the higher the grading.
why give cataract patients a questionnaire? How does it work?
because va is not always the best way to measure how cataract impacts daily life due to other affects like contrast.
Negative score means you’re good, positive score = worse cataract that’s affecting your life and people with symptomatic cataract have a score of 0. Patients post surgery should increase their score by a mean of ~3.5. You want to know their best corrected symptoms
what is the cataract questionnaire called?
CATProm-5
what ocular assessment would you do for someone with cataract? what is an extra thing ophthalmologists would do?
- Visual acuity
- Contrast sensitivity?
- IOP
- Anterior eye check – look for signs of blepharitis – increased risk of
endophthalmitis – lid hygiene recommended - Lens check
- Posterior eye
Biometry
give 8 examples of ocular conditions that would affect cataract surgery and how they would affect it
- Corneal scarring – poor view
- Anterior chamber depth - difficult surgery
- Previous trauma – causes zonules to be weakened or damaged
- Intra-ocular injections – zonular damage, risk of posterior lens touch
- Pseudo- exfoliation – weak zonules
- Diabetes – poor dilation, risk of bleed if NVI
- Fuchs endothelial dystrophy – risk of post op corneal decompensation
- Deep sockets – difficult access for surgery
what systemic checks would you need to do before cataract surgery?
- Blood pressure – needs to be controlled prior to surgery
- Blood sugar if patient is diabetic
- Review of medication – some will affect how the eye behaves in surgery
- Allergies
- Ability to mobilise and lie flat for surgery
- Other health checks only needed if a general anaesthetic is being carried out
give 6 medical problems that may affect cataract surgery with examples
- Poor mobility – unable to get on and off the bed easily
- Spinal problems – unable to lie flat for surgery
- Dystonia – unable to keep still for surgery
- Thromobcytopenia – may require medication prior to surgery
- Dementia or learning difficulties – not co-operative
- Use of alpha-blockers such as Tamsulosin / Doxazocin– will cause intra-operative floppy iris syndrome (IFIS) (where the iris shrinks during surgery)
what is biometry?
collecting and analysing data using stats methods where you sit at the machine and it can calculate multiple lenses that will be good for the power of the eye. Cl wearers should not wear their Cls 2 weeks prior to this appointment
give 3 reasons that youd have to repeat biometry
- Axial length below 21 mm and above 26mm
- If there is more than 0.3mm difference
between the two eyes - if the cataract is more than 4 years old
what kind of IOL is supplied under the NHS?
a monofocal one (px can be fitted with one distance and one reading if patient has had
experience of this with contact lens
wear)
when do NICE recommend you use a toric IOL?
when corneal astigmatism is >2.5D
give 2 more complex IOLs you can use in cataract surgery
-EDOF which mean your need to use reading glasses would be less
-Multifocal lenses
what is an EDOF IOL?
extended depth of focus lens so has a single extended focal point which allows greater range if focus and reduces dependence on near vision correction
what are the negatives of using a multifocal IOL
- Prone to ghosting and halos
- Difficult to adapt
- Need to manage patient expectations
what is a multifocal IOL?
a lens with multiple points of focus produced either by diffractive rings or variable refractive materials
what are the topical anaesthesia is uses in cataract surgery?
Local anaesthetic drops and intracameral lidocaine
what happens in sub-tenons block and why can’t it be done on both eyes at once?
Small incision made in the inferior medical
canthus and a blunt canular takes
anaesthetic to posterior eye
because it will block extra ocular muscles, lid movement and optic nerve function,
therefore cannot block both eyes
what oral sedation can you use for any local anaesthetic procedure?
diazepam or temazepam
give reasons why GA is avoided in cataracts
Elderly patients with small vessel dementia can be progressed with general anaesthetic - avoid unless really needed
what should not be a reason to give GA for cataract surgery?
anxiety
when might you use GA with cataract surgery?
-learning difficulties
-dimentia
-movement disorders
what do the surgeons feet do in cataract surgery?
one foot controls the phaco machine and the other controls the microscope (binocular microscope)
what are the 10 steps in cataract surgery?
- Incision
- Viscoeleastic
- Caspularhexis
- Hydrodissection
- Sculpt
- Segment (quadrant) removal
- Irrigation aspiration
- Lens insertion
- Removal of visco elastic
- Antibiotic and wound hydration
learn the steps of cataract surgery slides 32 - 36 on cataract surgery pptx in block 8 under lecture slides
okay
what are the most common intra-operative complications in cataract surgery?
- Extension of the rhexis
- Zonular dehisence
- Posterior capsular rupture
- Loss of vitreous
- Dropped nucleus
what are the most post operative complications in cataract surgery?
- Prolonged inflammatory response
including macular oedema - Corneal decompensation
- Refractive surprise
what can increase risk of intraoperative complications?
- White cataract
- Multiple Intravitreal injections
- Ocular co-morbidity
- High refractive error – both myope
and hypermetrope - Poor co-operation
how common are intraoperative cataract complications?
uncommon - approx 1% of cases
how do ypu deal with prolonged inflammatory response in post op cataract surgeries?
All patients will have antibiotic and steroid post operatively for us to 4 weeks
when is prolonged inflammatory response post cataract op even more common?
- where surgery has been prolonged or complication has occurred
- Common where ocular co-morbidity especially co-existed ocular inflammatory disease
how do you deal with post op uveitis?
-flare in AC and CMO in oct 4 weeks post op is normal
-asymptomatic Px dont need treatment
-symptomatic can have NSAID 3 times a day and steroid 2x a day for a further 4 weeks but if this persists then refer to secondary care
how do you deal with corneal decompensation (oedema post op)
-be concerned if va is reduced
-treat with mild topical steroid, can consider 5% (hypertonic) saline for further 4 weeks
-refer to secondary care if does not want to resolve
when may you not do phaco? what other surgeries are there?
if the nucleus is very dense with Small incision manual cataract surgery
(SIMCS)
ECCE and ICCE (extra and intra capsular cataract extraction) is where you flap the cornea open and just remove the cataract - this was the old way of doing it before phaco.
how is a capsular rupture dealt with in cataract surgery?
- Manage vitreous loss
- If nuclus is dropped will
require vitrectomy to
remove - Lens placed in sulcus,
anterior IOL or scleral suture
what can cause higher risk of corneal decompensation post cataract op?
-if prolonged surgery or endothelial injury
during surgery
-if there’s pre-exisiting corneal pathology – Fuchs endothelial dystrophy
what defines refractive suprise post cataract surgery?
+/- 1.00D deviation from expected
combined spherical error (SE)
what can cause refractive surprise post cataract op?
-using the wrong biometric formula
-if viscoelastic trapped behind the IOP causing capsular distension
how can you correct refractive surprise post cataract op?
- Glasses
- Contact lenses
- YAG capsulotomy
- IOL exchange
