5.3.3- fitting complex cl and advise px with complex correction Flashcards
What is paediatric aphakia and why does it happen?
Absence or loss of the crystalline lens due to:
Congenital cataracts (3-4/10000 live births, 40-50% unilateral)
Trauma
Lens subluxation (Marfan’s syndrome)
What is the average corneal radius of curvature in new born and how does this change with time?
6.9mm and it rapidly flattens in the first 6 months
How does the axial length of the eye change for a newborn?
17mm to 21 mm in first 6 months of life
What is an aphakic child rx going to be in first few years of life?
Aphakic spec rx approx +25DS to +15DS in first few years of life but prescription will require frequent changes
What are the dis/advantages of paediatric aphakic spectacle wear?
advantages: no risk of infection, can be well tolerated
disadvantages: more challenging in unilateral cases, expensive and break easily, cosmetic issue, heavy, cause peripheral distortion
What are the dis/advantages paediatric aphakic cl’s?
advantages: no weight issues, easier for parents once insertes, good cosmesis
disadvantages: risk of infection if cleaning regime not followed, initially more of a challenge for parents
What is the initial cl choice for a paediatric aphakic cl fitting and why?
SiHy as increased modulus, better stabilisation, reduced risk of corneal hypoxia
3 monthly replacement lenses
some available in a wide range of parameters (up to +50DS)
What measurements are needed for the initial cl choice for a paediatric aphakic cl?
case history, retinoscopy, keratometry and corneal diameter that are usually done in theatre
What are the normative values for initial contact lens choice in aphakic paediatric px?
How to do an aftercare for a paediatric aphakic cl wearer?
H&S: Any problems reported by parents? (red/sticky eyes, handling issues, frequent lens loss, lens decentering) Compliance with cleaning regime, wear time
Assess lens fit- centration and movement
Over-refraction
What do you need to consider when prescribing an rx for an aphakic child?
They can’t accommodate and infants like to look at closer objects so overcorrect by +2 DS until mobile and toddling, toddlers overcorrect by +1 DS.
When start pre school give bifocal with +3 addition for near work or distance cl’s with reading glasses (some aphakes manage without a bifocal for near due to magnification effect so assess on an individual basis)
What contact lens spec would you use to fit a 6 month old aphake?
Ultra Vision Paediatric lens
SiHy lathe cut 74% water content
What glasses would you dispense and aphakic 12-18 month old and how? (power of lenses, frame material, shaoe e.t.c)
-pd should be measured canthus to canthus or pupil centres
-power would be approx +20
-bvd is difficult to measaure and the frame will be fixed anyway
-these lenses are available in this power:
-34mm lenticular 1.5
-40mm lenticular 1.67/1.74
-42mm lenticular 1.5
-47mm lenticular 1.53
-50mm lenticular 1.67
frame:
-nylon/TR90/Grilamid (equivalent frames)- suitable materials
-round frame so px doesn’t look over top
-adjustable bridge headband and unbreakable hinges
-many brands available and a small frame supplement can be claimed for such frames
What is the current understanding of scleral shape?
Peripheral cornea is approximately a straight line and continuous with the scleral conjunctiva
Central cornea is curved
How are modern scleral lenses shaped to fit?
tangents/series of curves in the centre that approach a straight line at the periphery
What are reverse geometry lenses and why are they good?
Flatter in the centre and steeper in the periphery are corneal lenses for post graft px and orthokeratology
How were scleral lenses made in the past and now?
Before moulds were needed but now modern lathes cut lenses reproducibly and within microns of tolerance
How does a scleral lens fit/function?
full corneal clearance leaving a tear/fluid reservoir- this has poor oxygen transmission so the material dK needs to be maximised
What are the modern GP materials and their dK’s?
Boston EO- 58 Boston XO- 100 Optimum Extra- 100 Boston XO2- 141 Menicon Z- 161 Optimum Infinite- 180
What are the indications for scleral lenses and why?
Irregular corneas as they are most comfortable
keratoconus where a low cone (even rgp corneal lens would decentre down and not achieve a good fit)
Why would you fit a regular eye with scleral lenses?
corneal cyls: px where soft toric lenses show unstable rotation but RGPs are uncomfortable (true for moderate to high cyls)
presbyopes: particularly astigmatic presbyopes, the tear lens neutralises astigmatism without the need for stabilisation- can use simultaneous vision optics or modified monovision
RGP wearers losing tolerance (18hrs a day and now dropping wear time)-high myopes that hate spec distortion but losing tolerance to rgp (maybe 3 and 9 o clock staining)
mild to moderate dry eye and need protection of fluid reservoir and barrier between lid and cornea
What are the 4 sub categories of scleral lenses (name/diameter/fitting relationship)?
corneo scleral- 12.9-13.5mm- corneal bearing and scleral touch
semi scleral- 13.6-14.9mm- corneal and scleral bearing
mini scleral- 15.0-18.0mm- scleral bearing and minimal corneal clearance
full scleral- 18.1 -24+mm- scleral bearing and maximum corneal clearance
How are scleral RGP lenses fitted in terms of ocular measurements and how is the first lens selected?
Primarily by sag or vault (in microns)- curvature is a less effective way to define the fit.
Most commonly lenses are fitted from fitting sets and guides based on the corneal condition
Calculated from OCT or topography: use the depth at a 10mm chord + average calculated for the landing point of the lens (variation in depth of the cornea is most pronounced in the central 10mm and the peripheral cornea varies little)
How are scleral lenses inserted?
Lens bowl has to be filled with saline
Use scleral lens inserter in practice but px should know how to use tripod fingers so that they don’t rely on easily misplaced device or don’t clean properly
Head down and forward (perpendicular to floor), px pulls lower lid down, practitioner pulls to lid and lift cl up until it suckers onto eye
Once scleral lenses have been put onto the eye what needs to be checked and why?
Check for bubbles, use either a blue pen light or cobalt blue light on slit lamp-
Air bubbles in the scleral lens cause visual problems if it goes on pupil because of difference in refractive indices and persistent corneal dehydration with corneal desiccation- self limiting problem
px would notice vision problem or it would get uncomfortable; px will remove and refill
What are the 3 key principles to assess the fit of a scleral lens?
3 key principles:
- central clearance
- limbal clearance
- scleral landing
How to assess the central clearance of a scleral lens and change the fit accordingly?
Lens will fully clear the central cornea, target clearance varies (usually between 200-300 microns on insertion, rule of thumb is smaller lens clears less and a larger lens clears more but follow fitting guide), scleral lenses settle back so initial target clearance should be greater than settling clearance, wide/diffuse beam and a blue light to see if any contact with cornea (if notice any touch then replace with deeper lens straight away) but this makes it difficult to see the pupil and iris- when you know there’s no dark areas of corneal touch you need to accurately assess clearance so use a narrow beam and white light to assess the clearance (optic section will show grey hazy cornea, bright NaFl band, space, reflection from the front surface)- tear reservoir is ideally the same thickness as the lens.
Anterior OCT you can measure and compare real thickness
Deeper or shallower lens will need to be inserted until correct- once correct depth then leave to settle for minimum 30 mins
Why is settling important with scleral lenses and what impact does it have?
The conjunctiva is soft so the lens will settle back into it- it varies from px to px so allow for 100 microns to 150 microns- that’s why initial clearance is greater than the target settled clearance
All other aspects of fit should be assessed post settling (limbal clearance and scleral landing)
How to assess the limbal clearance and change fit accordingly?
The lens should vault the limbus and land out onto the scleral conjunctiva- use white light (blue light can’t see limbus clearly)
Fluorescein should extend out beyond the limbus
360 degree clearance is aim otherwise inadequate clearance causes staining and discomfort
Different designs have difffering approaches to amending limbal fit e.g. change in angle, change in base curve (flatten peripheral corneal fit) to lift or lower at limbus, specify lift or drop in microns
How to assess scleral landing?
Assess in white light with high mag- post settling
Check conjunctival vessels to see if blood can flow without impingement from the lens edge (blanching inside the lens edge and a build up of blood outside where it is blocked)
How to remove scleral lens?
RGP technique of using lids either side and pushing lens out
or use an RGP lens sucker by placing towards the top of the kens and lever away from the eye rather than pulling from the middle (as too much surface tension will pull on eye)
What are some scleral lens complications and how to solve?
Conjunctival prolapse- conjunctiva gets ‘sucked’ up under the lens to cover the limbus in any meridian- doesn’t cause issues and is temporary so it resolves on removal, to prevent it avoid overvaulting the limbus
Deposits- front or back surface deposits. Back surface deposits- the inside of especially high depth lenses are difficult to clean well manually- consider px using Progent which will chemically deep clean protein away. Front surface deposits can affect vision and comfort during the day- this can lead to a poorly wetting lens. Use a John Mountford Squeegee Technique which is us a large DMV sucker with a drop of cleaning/conditioning solution and swipe over the lens in situ for approximately 15 seconds to clear the surface.
Excessive settling back- choose a larger lens with a larger landing zone to spread weight more evenly
Fogging- fluid reservoir goes cloudy so px has to remove and refill (occurs in 20-30% of scleral lens wearers), worse in the first month or so of wear- it doesn’t resolve the px adapts. To solve don’t over vault the limbus and peripheral cornea, can experiment by filling the lens with non preserved artificial tears- begin with 50/50 mix of saline and artificial tears and experiment with the mix
What’s the difference between topology and tomology?
Topology is to measure the surface of a structure (tear film/surface topolofy of cornea)
Tomology looks at structure (slices throuch anterior segment to see depth of cornea, front surface, back surface)
How do surface topogrophers work?
Placido rings are projected onto cornea and the comparison of the size of the image on cornea and actual size, distance between- canwork out power of cornea.
Computer software can work out power of cornea- if rings are closer=steeper, if rings separated=flatter, calculates power of the cornea in different areas, global view of cornea 9-10mm diameter
What assumptions do surface topographers make?
Assume power is 0 at the centre of cornea with a regular surface (so an intact tear film)
Use mm radius of curvature in calculators rather than conversion to D to reduce errors- IOL powers are calculated using mm
How to analyse corneal topography?
- Look at raw data: name and laterality of test, check miers projected onto cornea- make sure rings are complete (missing image means computer will extrapolate data)
- Alignment: is it centred on the pupil
- Scale: check how steeness is scaled and if ths can compare to other topographers
- Asphericity: check if it matches normally topography, should be asphere (flatter in periphery and steeper in centre)
- Toricity: check for astigmatism- ir/regular
- Symmetry: symmetrical in an axis (vertical/horizontal), symmetry between eyes
What are the different methods of corneal topography/different ways of measuring power on the surface of cornea?
Axial map: smooth map, overview of cornea, measure the power at any one point from fixed centre of curvature so the power is more broadly applied on the cornea (can see general pattern)
Tangential: look at power at each individual point along cornea by using radius of curvature at a perpendicular point tangential to that point on the cornea, so a busy map for surgeons but not so good for cl fittings
How is asphericity of a cornea measured?
Using Q factor: measure how oblate (+0.5 or above (e.g. px whose had radial keratotomy)) or prolate a cornea is. A normal cornea should be slightly prolate (around -0.4), a hyperprolate is something like -1 would suggest disease like keratoconus
How does Q factor affect IOL selection?
To minimise spherical aberration in an eye the Q factor of the cornea is added to the IOL Q factor
How to analyse tomology?
- Look at raw data: cornea, iris, lens- make sure cornea is gained from limbus to limbus, make sure full depth of AC is there, px should be well aligned (not slanted), appex of cornea should be centred (rather than line of sight like with topology). 3D image on anterior segment is given so can measure the thickness/volume of cornea, depth of AC, heatmap of cornea
- Refractive four maps(?): look at laterality and image. One map is an axial map, anterior elevation map (normal is between 12-15), posterior elevation map (normal is no greater than 18), corneal thickness map from limbus to limbus (using difference between both elevations)
How is tomography and topology used to diagnose keratoconus?
Picks up early keratoconus: steepening on posterior surface of cornea which then becomes on the anterior surface, appex moves inferiorly.
In usual tomography early signs are masked by best fit sphere where elevation is as related to an average of curvature of cornea so subtle changes are not detected as it is lost in average- better softwares take out central/inferior area (where keratoconus commonly occurs) and fits best fit sphere everywhere else and then puts the cut out section back on to highlight subtle areas
What are clinical signs of keratoconus?
Steep cone that is inferior/central with bulging and scarring (Munson’s sign): large astigmatism, irregular mires, broken Bowmans membrane, Vogt’s striae leading to a broken Bowman’s membrane leading to scarring (degredation of vision), oil drop reflex, scissor reflex with ret, progressively steepening K readings
What conditions are associated with keratoconus and who have it mostly?
vernal keratoconjunctivitis (allergic eyes)
blue sclera (collagen problem, weak collagens when cornea is made of collagen)
enlarged corneal nerves
retinitis pigmentosa
young males (mid teens to early twenties), high astigmatism, FH, allergic, eye rubbing, Down’s syndrome
If variable astigmatism is shown in corneal topology, what conditions may it suggest?
Dry eye keratoconus radial keratotomy pterygium epithelial basement membrane dystrophy Salzmann's dystrophy (tear film abnormality) Lid issues that press on cornea Contact lenses worn recently- out of soft contact lenses for 2 weeks, out of hard contact lenses for 4 weeks to give measurement stability
How to manage advanced keratoconus, how to manage keratoconus at other stages?
contact lenses, glasses, full thickness corneal transplant is rarely done, more often DALK (deep anterior lamellar keratoplasty)
collagen cross linking (cornea of minimum stromal thickness of 400 microns, can sometimes thicken cornea with a hypotonic solution)- prevents progression of disease by stabilising cornea but doesn’t correct vision
Intracorneal rings- two crescent shaped plastic polymers inserted- to treat irregular astigmatism and flatten central cornea- good for mild to mod keratoconus- not for advanced or central scarring
excimer laser- to reshape cornea (only by 50 microns) as long as stabilise with collagen cross linking
implantable cl or secondary phakic lens to deal with residual ametropia
What are common signs of keratoconus on topography?
steep keratometry readings
inferior steepening
asymmetry, thinning
thinnest point inferior
How are intracorneal rings implanted and what are some complications?
Femtosecond laser is used to create channel in corneal for plastic implants
Scarring at the wound
Ring segment can erode forward
Ring segment can break (but no issues normally)
Ring segment can erode into anterior chamber
White opacity at edge of ring- only if red and injected would that suggest infection and be worried
vascularisation around ring
How can intracorneal rings be combined with other treatments?
iris clip on IOL
implantable cl behind iris
to correct myopia and astigmatism once cornea stable
How is CXL (collagen cross linking) performed?
It is within the cornea’s molecular matrix and cross linking is photochemically triggered using a combination of vitamin B2 and UV light
- Epithelium is removed (9mm)
- Riboflavin 0.1% is applied- vitamin B2 drop
- Ensure adequate stromal thickness minimum 400 microns
- Apply ultraviolet light wavelength 370nm, irradiance 3mw/cm2
- Allow diffusion 30 mins (accelerated CXL has 3 min duration, with a higher power of UV exposure)
What are the complications of corneal cross linking?
risk of infection, scarring, haze in cornea
What are some sx of keratoconus that px complain of?
Progressively worsening vision, irregular astigmatism, glare
How does keratoconus occur/develop?
The central/paracentral corneal stroma undergoes progressive thinning and loss of structural integrity, leadign to cone-like deformation and adverse impact on vision.
Typically effects both eyes and px present with irregular astigmatic changes from the biomechanical warping of the cornea
When is penetrating keratoplasty (PK- corneal transplant) good to do and why is it successful?
When corneal thinning is excessive or very eccentric
The cornea does not have a direct blood supply so risk of rejection is reduced
How is DALK performed and why is it good?
deep anterior lamellar keratoplasty is when just the epithelium and stroma is replaced (host endothelium and Descemet’s membrane is intact so risk of rejection is reduced- but limited evidence)
What surgery can be performed alongside CXL and how/why?
transepithelial photorefractive keratectomy (trans-PRK).
A specifically targeted excimer laser reshapes the cornea to creat a more regular shape, so cl fitting is easier and an improvement in refraction to allow px to use glasses instead of cl’s (usually indicated in cl intolerant px)
