AED - Corneal Ectasia - Week 11 Flashcards
Is bowmans membrane a basement membrane like descemets membrane? Explain its structure in this regard.
It is not
It is an acellular layer composed of randomly arranged collagen fibres
How thicc is the corneal stroma typically?
500μm
What is corneal ectasia characterised by? List three things that contribute to this.
Corneal conditions that are characterised by progressive thinning of the corneal stroma
-biochemical weakening
-increased corneal curvature
-irregular astigmatism
List three classifications of corneal ectasia and give an example each.
Degenerative
-keratoconus
-pellucid marginal degeneration
Congenital anatomy
-keratoglobus
Mechanical trauma such as post-surgery
-iatrogenic ectasia
Is keratoconus progressive? Is it symmetrical or asymmetrical? Is it inflammatory?
Progressive, typically asymmetrical, and non-inflammatory
What is keratoconus characterised by? Is it uni- or bilateral?
Progressive thinning of the axial corneal stroma
It is bilateral
What condition is the most common primary corneal ectasia?
Keratoconus
In what regions is the prevalence of keratoconus higher?
Regions where cosanguinity occurs
What is the onset of keratoconus? When does it stabilise? Does its clinical course vary significantly or not?
Typical onset at puberty
Stabilises ~35-40yoa
Clinical course varies significantly
List 6 associations with keratoconus. Are associations common or is it an isolated ocular finding?
Most commonly an isolated ocular finding
Possible associations include:
-atopy (vernal, athsma, eczema)
-leber’s congenital amaurosis
-retinitis pigmentosa
-down’s syndrome
-connective tissue disorders
-mitral valve prolapse
What is the aetiology and pathogenesis of keratoconus?
Uncertain
What is the mode of inheritance for keratoconus? Does it have complete or variable penetrance?
Autosomal dominant with variable penetrance
What does the link between keratoconus and connective tissue disorders suggest?
A possible genetic abnormality in connective tissues
What do family members of a keratoconic individual typically have?
30-50% have subtle topographic abnormalities
Explain two biochemical abnormalities of proteinases in keratoconus.
Increased proteinase activity
Decreased activity of proteinase inhibitors
How does interleukin-1 affect keratocytes (3)?
It is a key modulator of keratocyte proliferation, differentiation, and death
What is interleukin-1 produced by in the eye (2)?
Epithelium and endothelium
How do interleukin-1 receptor levels compare in keratoconic eyes vs normal eyes?
Four-fold more IL-1 receptors in keratoconic eyes
What does epithelial trauma result in the release of and what consequence does this have (2)?
Increased release of IL-1, which results in increased keratocyte loss and stromal thinning
What band of UV light does the cornea absorb most? What does this result in?
Most of the UVB light
Results in the creation of oxygen free radicals, resulting in oxidation damage
List three general biochemical abnormalities in keratoconus.
Proteinases
IL-1
Oxidative damage
List two environmental influences of keratoconus. What consideration may be given for this?
High levels of atopy including hayfever
Vigorous eye rubbing
Consider topical antihistamine or mast cell stabiliser therapy
In what percentage of patients is hayfever an environmental influence for keratoconus?
Approximately half of patients
What is the histopathology of the cornea in keratoconus (biochemical resistance?
50% decrease in biochemical resistance
What is the d cross linking like in keratoconic eyes?
A decrease in d cross links between and within collagen fibres in the anterior stroma
List four primary corneal changes in a keratoconic eye.
Epithelial anomalies
Breaks in bowmans layer
Stromal thinning
Descemets membrane breaks
What happens to the basal epithelial cells in keratoconic corneas (2)? What does this result in (2)?
Basal epithelial cells degenerate
Down-growth of epithelial basal cells into bowmans membrane
Results in a thickened basement membrane-like layer and breaks in bowmans layer occurs
What fills breaks in bowmans layer in a keratoconic eye?
Filled by eruptions of underlying stromal collagen
What are fleischers ring in keratoconic eyes and where can it be found?
Iron deposits in basal cells at the base of the cone
Is lamellae organisation affected in keratoconus? Explain.
Yes, the gross organisation changes
It is more compact and there is loss of arrangement
What happens to fibre bundle thickness and corneal nerve density in keratoconus?
Reduced density of corneal nerves and thicker fibre bundles
Is decemet’s membrane affected early on or late into the disease with keratoconus?
Late into the disease
What results due to breaks in descemets membrane (3)?
Acute corneal hydrops
Aqueaous enters the corneal stroma
Dramatic corneal oedema occurs
Are the symptoms of keratoconus stable or highly variable?
Highly variable
List 7 possible symptoms of keratoconus.
Blurred vision
Increased light sensitivity
Flaring of lights
Difficulty with night vision
Eye strain
Dry/itch/irritated eyes
History of eye rubbing
List 7 signs of keratoconus.
Reduced best corrected visual acuity
Frequent changes in refraction
Scissor reflex with retinoscopy
Charleux oil droplet with ophthalmoscopy
Doubled/distorted mires with keratometry
Corneal topography
Reduced central corneal thickness with pachymetry
What three things can be found with a slit lamp assessment of a keratoconic eye?
Vogt’s striae (corneal folds)
Fleischer’s ring
Corneal stromal thinning
What is corneal nerve visibility like in a keratoconic eye with slit lamp examination?
Increased visibility
Define musnon’s sign for keratoconus.
A V-shaped indentation observed in the lower eyelid when patient gaze is directed downwards
What can sometimes be seen on the apex of a keratoconic eye (2)? What is it called?
Superficial and/or deep opacities (apical scarring)
Describe the four stages of keratoconus severity including the power in D and mm of curvature.
Stage 1 - forme fruste (sub-clinical)
Stage 2 - early - (<45D, >7.5mm)
Stage 3 - moderate (45-52D, 6.5-7.5mm)
Stage 4 - severe (>52D, <6.5mm)
Consider forme fruste keratoconus. What is slit lamp examination and corneal topography like? What about best corrected vision?
Slit lamp exam normal
Minimal or no change to corneal topography over years
Best VA generally 6/6+
What is the management for forme fruste keratoconus (2)?
Spectacle correction to 6/6 or 6/4.8
Soft contact lens
What is early keratoconus primarily diagnosed by (2)?
Corneal topography
-central K value >47.2D
-I/S index >1.4
What is corneal distortion like in early keratoconus (corneal topography)?
Minimal degree of distortion
Is there corneal scarring in early keratoconus? What about striae or fliescher’s ring?
No corneal scarring
Striae and fleischer’s ring may or may not be there
What is the management for early keratoconus (2)?
Spectacle acuity may be normal or mildly reduced
If normal, use spectacle/soft CL correction
If mildly reduced, use toric soft CLs or RGPs
Is subjective refraction repeatable or poorly repeatable with keratoconus? What happens with more significant keratoconus? Explain.
Poorly repeatable
Multifocality of the cornea yields variable endpoint
More significant keratoconus yields higher degrees of refractive variability
When refracting keratoconic patients, what dioptre steps may be necessary for a discernable difference? What about with JCC?
±1.00-3.00D
JCC ±1.00D
List two common features of moderate/severe keratoconus.
Poor vision quality even with a prescription
History of progressive myopic astigmatism
What four findings are common in a slit lamp examination of a moderate/severe keratoconic eye?
Striae
Fleischers ring
Prominent ectasia
Apical scarring
What is the management for moderate/severe keratoconus (4)?
RGPs
Piggyback (RGP + SCL carrier)
Hybrid
Miniscleral/scleral
List and describe the three morphologies of keratoconus and the percentage of cases for each.
Nipple cones - 45%
-near the corneal cenrte or inferionasal
Sagging/oval cones - 50%
-larger, below centre or inferotemporal
Globus cones - 5%
-involves up to 75% of the cornea
Of the three morphologies for keratoconus, categorise them by their size.
Nipple cone - small and steep curvature
Oval cone - larger and ellipsoidal
Globus cone - largest
Which of the three keretoconic morphologies is ideal for smaller diameter RGPs? Explain why.
Nipple cones
The lens will tend to centre towards the cone
Where do oval cones tend to be steep (2)? What kind of RGP is typically required and why?
Inferior or inferotemporal
Small RGP will ride low, larger diameters typically required
Which keratoconic morphology is the most challenging to fit?
Globus cone
Is keratoconus an indication or contraindication for refractive surgery?
Contraindication
What counsel can be given to keratoconic patients? What should management additionally involve?
Counsel to avoid eye rubbing
Manage co-existing or seasonal allergies with topical therapy
What is the gold standard for monitoring keratoconic progression?
Corneal topography
At what age should a child with a family history of keratoconus be assessed for keratoconus and how?
Perform topography early at 6-7 years of age on any child with a history of keratoconus
In what two cases of keratoconus is a referral indicated?
Progressive keratoconus
Consideration for a corneal graft
What is a treatment option for progressive early/moderate keratoconus?
Corneal collagen cross-linking
What are two treatment options for end-stage keratoconus?
Corneal graft
-deep lamellar keratoplasty
-penetrating keratoplasty
In what way does cross-linking occur in the eyes and where (2)?
Naturally with age in the cornea and lens
List two major components of cross-linking and explain how these two interact to result in crosslinking.
UVA (370nm) and riboflavin
Exposure of riboflavin to UVA releases free radicals, which cause cross-linking formation between amino acids on the collagen chains
What effect do cross-links have on collagen fibre diameter?
Diameter increases with cross-linking
What happens to the space between collagen fibrils with cross-linking?
It increases
Can cross-linking be done in individuals with non-progressive keratoconus?
No, it must be progressive and documented
What is the minimum corneal thickness required for cross-linking?
400 microns
Aside from corneal thickness, list 4 requirements for cross-linking.
Absence of significant corneal scarring
No history of herpetic eye disease
Not pregnant/breastfeeding
No autoimmune disorders or impaired wound healing
List four cases a corneal graft would be considered for keratoconus.
Contact lens intolerance
Inability to fit a contact lens by an experienced fitter
Corneal scarring that limits acuity/quality of life
Bilaterally progressive disease with a high risk of visual impairment
What is a serious potential side effect of laser refractive surgery? What is its clinical phenotype like?
Development of rpogressive corneal ectasia following surgery
Clinical phenotype essentially the same as keratoconus
List 2 corneal topographic and 2 pathologic pre-operative risk factors for corneal ectasia following refractive surgery..
Abnormal pre-operative corneal topography
-asymmetric bow-tie (<1D asymmetry)
-inferior steep/skewed radial axis (I/S value <1.4)
Pathologic
-forme fruste keratoconus
-other ectasias
List 8 general risk factors for corneal ectasia following refractive surgery.
Low pre-operative corneal thickness (<450 microns)
High myopia (>8.00D)
Young age
Thin residual stromal bed (<250 microns)
Family history of keratoconus
Eye rubbing
Refractive instability
BCVA <6/6
What is the management for post-refractive surgery corneal ectasia (2)?
As in keratoconus, including options of cross-linking
What is the onset of pellucid merginal degeneration, which gender does it affect more, and what is the mode of inheritance?
20-40s
M»F
No apparent hereditary transmission
Which ethnicities are more affected by pellucid marginal degeneration?
No predilection for ethnicity
What is pellucid merginal degeneration often misdiagnosed as?
Keratoconus due to similarities
What is pellucid marginal degeneration characterised by?
Thinning of the corneal stroma at the peripheral inferior corneal margin
Is pellucid marginal degeneration uni- or bilateral? Is it progressive?
Progressive and bilateral
What characteristic pattern is seen on a corneal topography of pellucid marginal degeneration? What can be said of the central cornea?
Butterfly/crab claw
Central cornea is flatter than the peripheral cornea
What rule of astigmatism does pellucid marginal degeneration produce and what is VA like?
Produces against the rule astigmatism and VA may be reasonably good
List 3 signs of pellucid marginal degeneration.
Stromal thinning in a crescent band from 4-8 o’clock
No vogt striae
No fleischers ring
Can hydrops occur in pellucid marginal degeneration?
Yesd but rare
Is CL fitting more complex for keratoconus or pellucid marginal degeneration?
Pellucid marginal degeneration
What is the management for pellucid marginal degeneration?
Similar to keratoconus, better VA is achievable
What is the success rate of corneal grafting for pellucid marginal degeneration like compared to keratoconus?
Lower
Is keratoglobus uni- or bilateral?
Bilateral
Is keratoglobus inflammatory?
Non-inflammatory
What is the mode of inheritance for keratoglobus?
Non-hereditary
What is keratoglobus characterised by?
Severe thinning of the entire cornea
What does keratoglobus lead to?
Gross corneal protrusion
What four conditions is keratoglobus associated with?
Ehler-danlos type IV
Marfan syndrome
Leber’s congential amaurosis
Blue sclera syndrome
What is a possible cause of keratoglobus given what conditions it is associated with?
Defective collagen synthesis
List 3 signs of keratoglobus.
Myopic irregular astigmatism
Corneal thinning extending to the limbus
Deep anterior chamber due to corneal protrusion
Where does maximal thinning occur with keratoglobus?
Midperiphery
Are contact lenses easy to fit for keratoglobus?
Very difficult
What is the prognosis for keratoglobus like with corneal grafts? Explain.
Poor due to thin host cornea vs donor
