Lec 7 Keratoconus Flashcards
Associations with keratoconus
Eye rubbing/atopy
Sleep apnoea
CT disorders like Marfans/Ehlers Danlos
Maori/Pacifica/NZ
Trisomy 21
Properties of keratoconic corneas
Thinner fragile and less sensitive
Prone to corneal moulding and NaFl staining from RGP
Prone to apical scarring - Vortex scarring
What’s special about keratoconus and CL tolerance
They have reduced corneal sensitivity from scarring and increased nerve fibre tortuousity
BUT if RGP intolerant use sclerals/mini scleral
Topographer vs keratometer
Topographer maps out cornea and gives K readings but keratometer just gives K readings
Categorisation of nipple oval and global cone
Nipple is <3mm
Oval is 3-5.5mm
Global is >5.5mm
What does forme fruste mean
Topographer is Good at early cone detection?
So means watch out if you pick up keratoconus one eye as it means the other eye prob has some too
no such thing as monocular keratoconus
What topography indices are good for keratoconus
True elevation maps better than axial or tangential
Tangential maps are good for cone location and normalise scale
Topographer have an inbuilt CL fitting programme and what is this good and bad for
Good at predicting final CL fitting success and is good starting point and cuts chair time instead of using set of trial lens
Manual better for more severe Kc
Advantages of topography
Confirms diagnosis
Finds location/size of cone ape
Monitors progression
Differentiates pellucid/terriens
Demonstrates to patient the problem
Signs of keratoconus on non slit lamp
Pachymetry <480um
Topography indices I-S To show if inferior is diff to superior K reading
Scissoring/swirling ret reflex
Irregular astig/monocular diplopia/poor spec VA
Signs of keratoconus on slit lamp
Vogt striae- vertical lines in epi due to buckling of posterior cornea
Corneal nerve visibility
Fleishers ring
Apical scarring
Hydrops
What are hydrops
Sudden corneal oedema as AQ humour invades posterior cornea through split Decemets membrane
Painful but self resolving though scarring can reduce vision
What’s good about hydrops
Fibrosis occurs and this causes the corneal curvature to flatten over time so makes CL fitting easier ;)
What’s the Fitting options for a keratoconic amount
Mild <45, moderate (45-52), severe >52, post graft
Mild - spec ok, soft toric lenses ok BUT if VA 6/12 or better than no subsidy
Moderate - RGP or hybrids
Severe - RGP/Hybrid/Semi scleral/soft piggyback/grafting
Post graft - may get better spec VA/may use CL still
Difference between a standard RGP and a keratoconic design
Keratoconus cornea steeped in a specific area but then flattens at high rate so:
1) increase edge lift
2) small OZ to vault cone area
Main rule for fitting an RGP on keratoconic
First definite apical clearance fit
As if you get touch/bearing then get apical scarring AND if u get too much vaulting then visions bad
How can a toric periphery help a keratoconic cornea
KC corneas usually WTR so toric peripheries can avoid tightness at 3&9 o’clock as well as reducing inferior standoff
Biochemistry of a normal cornea
Normal stress turns into ROS/RNS but antioxidant enzymes and lipid peroxidation enzymes eliminates them
So minimal mtDNA damage = good cell function
Biochemistry of keratoconic cornea
Abnormal antioxidant enzymes which means ROS/RNS accumulate activating/up regulating ECM degradation enzymes thinning cornea
ROS/RNS also cause MtDNA damage increasing ROS production and decreasing Oxidative phosphorylation = bad cell function
Therapies for Keratoconus
Corneal cross linking
INTACS
Describe corneal cross linking
Riboflavin drops with UVA stiffens cornea by anterior corneal collagen cross linking resulting in increased stiffness and improves resistance to swelling and enzyme degradation
Good for progressing teens
Describe INTACS
Corneal rings that flatten shape for mild Kc improving VA
20% ineffective and not suitable for thinned or scarred corneas so can range from 0-7lines improvement
Does keratoconus slow down with age
KC onset is teens and progresses till 30-40 cuz natural cross linking of collagen fibres
What’s pellucid marginal degeneration and what’s good about it
Steep at bottom of cornea “crab claws” or “kissing swans”
Hard to fit with CL as flat superior and stele inferior
GOOD as has regular astig usually so specs OK
What happens after a graft for keratoconus (keratoplasty)
More regular cornea so better in glasses than CL but still often need fit in CL or semi sclerals
- semi scleral better than RGP when elevation diff across cornea is >350
What’s the key eligibility for grafts in keratoconus
CONTACT LENS INTOLERANCE
What’s important in grafts for keratoconus
Grafts have endothelial cell density ECD reduced and hypoxia can reduce ECD even more which can lead to graft rejection so must avoid limbal pressure
