keratoconus & corneal ectasia

What is the definition of keratoconus

progressive bilateral, asymmetric disease characterized by steepening and distortion, apical thinning and corneal ectasia

What are the key characteristics of KCN

• asymmetric

• onset: early teens to early 20’s

• progression: stops by 4th decade, faster during significant hormonal changes (ie. pregnancy, puberty)

What is the prevalence of KCN

• 1 in 375 people

• 1 in 223 children

What is the pathophysiology of KCN

• Abnormality in biomechanics of anterior 1/3 of stroma weakens the structural integrity of the cornea —> protrusion forward of posterior surface of cornea

where in the cornea is it typically located

• central or paracentral cornea

• inferior temporal

What increases (inflammatory wise) in KCN

• increase in MMP9, TNF-alpha, IL6

KCN is a disease of the _______ stroma

• anterior cornea is affected in early stages

• in later stages, descemet;s membrane and posterior limiting lamina may be affected

anterior stroma

What is the classical triad of the histologic changes in KCN

• thinning of the corneal stroma

• breaks in bowmans layer

• iron deposition in the basal layers of the corneal epithelium

when theres degeneration of epithelial cells what happens next?

fragmentation in bowmans layer

• abnormal enzymes in epithelium > excess collagenase and reduced protease inhibitors in stroma > death of keratocytes in the stroma

What is helpful in differential diagnosis of subclinical early/ KCN

epithelial thinning over apex of cornea

Accumulation of ferritin particles within and between basal cells is known as what?

• fleischer ring

Breaks in ______ layer are present in > ____% of eyes with KCN

• bowmans

• 70%

What happens in the stroma during KCN

• reduced number in lamallae/ density of collagen

• loss of inerlamellar weaving

• keratocytes turn into fibroblasts and myofibroblasts

• induce fibrosis and contribute to formation of scars

What happens in descemet’s membrane during KCN

• irregularities, thinning, breaks/ deformities in 20% of severe KCN cases

• breakdown of corneal stroma results in descemet’s being more susceptible to hydrostatic pressure of IOP

What are hydrops?

breaks that occur from forward -movement of posterior cornea —> allowing influx of aqueous into the stroma and epithelium

What are some common associations of KCN

• metabolic changes in the corneal tissue

• atopy

• connective tissue disorders

• eye rubbing

• inheritance

___% of keratoconic patients have atopy

50%

• excezma

• allergies

• atopic dermatitis

primary symptom of atopy is

itching

• thus many KCN pts are aggressive eye rubbers

• SO advise pts to not rub their eyes

________% of pts with keratoconus have a family history

6-16%

What are the associated genetic syndromes of KCN

• down syndrome

• woodhouse sakati syndrome

• marfan syndrome

• ehlers-danlos

What is the goal when treating KCN

• stop progression of disease

• preserve the pts visual acuity

To diagnose KCN the following must be present:

• abnormal posterior ectasia

• abnormal corneal thickness distribution

• clinical non-inflammatory corneal thinning

symptoms of KCN

• blurred/ distorted vision

• ghost images

• H/O several pairs of glasses that have not worked

• asthenopia

• halos around lights

Signs of KCN

• gradual decrease in VA at distance and near

• reduced contrast sensitivity

• scissor reflex during retinoscopy

• increasing myopia and irregular astigmatism (ATR/ oblique axis typically)

• difficult refraction

How does KCN look on keratometry

• distorted mires

• oblique axis of corneal astigmatism

• steep curvature

• difficulty to diagnose early KCN with keratometry

what helps to locate the apex of cone and monitor progression of condition

topography

How is an axial map helpful in KCN

• provides more global curvature

• useful for GP lens fitting

How is tangential map useful in topography

• can detect subtle differences in curvature

• early diagnosis

• monitoring progression

What are the 4 topography patterns that denote KCN

1. nipple

2. oval

3. globus

4. marginal

Nipple shape

smaller more centralized moutnain

oval shape

inferior, moderate mountain

globus shape

large diameter, encompassing most of cornea

marginal shape

nonround/ nonoval cone in the corneal periphery

How can you detect KCN via topography with:

1. central corneal power:

2. I-S value

1. >47.2D

2. >1.4D

What is the bottom line of KCN

asymmetry

How does TOMOGRAPHY help with EARLY detection of keratoconus

• changes in POSTERIOR corneal surface will occur FIRST

• ectatic corneas exhibit more rapid thinning from the periphery of the cornea to the apex

What is one of the best tomography maps for KCN monitoring

belin-ambrosio enhanced ectasia display

• Anterior elevation at thinnest point

• Posterior elevation at thinnest point

• Change in anterior elevation

• Change in posterior elevation

• Corneal thickness at thinnest point

• Location of thinnest point

• Pachymetric progression

• Ambrosio relational thickness

• Kmax

What are signs of KCN on tomography maps

• areas of higher elevation on anterior and posterior maps

• corneal thinning on pachymetric map

• color map shows increased power in isolated area of cone

Name three slit lamp signs of KCN

1. fleischers ring

2. vogts striae

3. corneal scarring

Fleisher ring

vogt striae

corneal scarring

Prominent corneal nerves

What causes corneal hydrops

• rupture in descemets memrbane

• aqueous humor flows through damaged endothelium > leading to corneal edema + eventual scarring

Name two external signs of KCN

1. munsons sign - altered shape of eyelid on downgaze due to protrusion of the cone

2. Rizzuti’s sign - point of light formed on iris by illuminating cornea with penlight from the side

More diagrams of hallmark signs

What are aberrations

optical imperfections that prevent light from achieving a tight focus on the retina, even with conventional optical correction

• lower order: spherical and cylindrical refractive error

• higher order: coma, trefoil

How can aberrations be measured

wavefront aberrometers

• those with corneal ectasias experience MORE higher order aberrations

Most common aberration seen in KCN

• coma

• trefoil

• spherical aberration

s/x: light distortion, comet-like tails on objects/lights, difficulty with night vision, starburst patterns, double vision

What are two common corneal warpages seen CL wear

(imitators of KCN)

• inferior steepening on topography

• scissor reflex on retinoscopy

  • ie. thick soft lenses worn for extended periods, high riding GP lenses

How is this different than KCN

• CL warpage reverses completely within a few weeks of d/c CL wear

• steepening below 50D

• KCN have clinical signs and is progressive

• no abnormal posterior elevation

how to manage CL warpage

• discontinue wear

• refit to higher DK lens material with better fitting relationship

what is pellucid marginal degeneration

progressive, bilateral corneal disorder with a peripheral band of thinning of the inferior cornea

What are common characteristics of pellucid

• 4-8’oclock location

• 1-2mm unaffected area between area of thinning and the limbus

• may be a disease on the same spectrum as KCN

• later age of onset

• better vision for longer

What is the infamous topography signs of pellucid

• large amount of ATR astigmatism

• “kissing dove” “crab claw” appearance

areas of greatest thinning and steepest corneal curvature are close to _______

the limbus

more images of pellucid

KCN on topography maps

PMD on topography maps

what is keratoglobus

entire cornea THINS, mostly near the limbus

• bilateral

• present from birth

• non-progressive

_____% of refractive surgery candidates have subclinical KCN

5-7%

post-op ectasia occurs in ___% to ____% of refractive surgery cases

0.04% to 0.06%

• vast majority are LASIK cases

• why surgeons like to evaluate corneal thickness and biomechanics pre-operatively to avoid this

What does the Amseler-Krumeich system look at to classify ectasias

• based on K’s, CCT, refraction and degree of scarring

How did CLEK study grading of KCN characterize:

• Mild

• Moderate

• Severe

• mild: <45D

• Moderate: 45-52D

• Severe: >52D

What does the Belin ABCD system look at for KCN classification

• anterior-surface curvature

• posterior-surface curvature

• corneal pachymetry at thinnest location

• corneal scarring

Subclinical KCN

• An eye with positive or suspicious topographic findings

• NO slit lamp findings, but with KCN in the fellow eye

Forme fruste KCN

• an eye with no topographic findings

• NO slit lamp findings and KCN in the fellow eye

What is defined as progression:

At LEAST TWO of the following

• steepening of ANTERIOR corneal surface

• steepening of POSTERIOR corneal surface

• THINNING or INCREASE in rate of corneal thickness change from periphery > thinnest point

T or F: it is possible to predict rate of progression for each patient

FALSE - impossible

• typically progresses over 3-8 years

What does CLEK study signify as progression

• 0.20D increase in flat K per year

• ¼ patients steepened by 3D in 7 years

• 12% needed keratoplasty over 8 years

• do NOT fit patients with KCN in flat GPs

___ % of KCN pts indicated the diagnosis had some impact on their lives

__% indicated a moderate or severe impact on life

• 90%

• 40%

  • why pt education is so important (ie. prognosis, loss of vision or need of transplant)

Overall. pts with KCN have more _______ coping mechanisms

• dysfunctional

  • may impact their relationship with healthcare providers

  • influences clinical perception that they are less respectful, less conforming, and less cooperative than other patients

Why is spectacle correction a less common form of treatment for KCN pts

refractive error can change rapidly

• do not correct irregular astigmatism and HOA

• anisometropia may occur due to asymmetry of disease

• only best used if specs are in early disease, before CL use (back up)

List 5 CL that may be useful in correction of KCN pts

1. corneal GP lenses

2. scleral lenses

3. soft lenses

4. piggyback lenses

5. hybrid lenses

In majority of KCN cases, lenses are considered _______ _________

medically necessary

what is used as a last resort option for those with KCN and poor visual acuity and difficulty with fitting CL

surgery

• 10-22% of pts with KCN require a corneal transplant

• IF impossible to achieve stable CL fit, IF excessive scarring + thinning

What was FDA approved in the US in 2016

(hint: age 14-65 years with progressive KCN or corneal ectasia after refractive surgery)

corneal cross-linking

How does corneal crosslinking work?

addition of molecular bonds to INCREASE the mechanical strength of the corneal stroma

• increase rigidity of collagen lamellae

• increase collagen fiber diameter

• stabilize the shape of cornea

What is the exact mechanism that corneal cross-linking uses

riboflavin (vit B2) + UVA radiation

• photosensitizer - absorbs UV radiation causing a cleavage of oxygen, which splits off and causes cross-linking in the tissue

• more crosslinks = STRONGER tissue

Mechanism of surgical procedure

What is the post-op care for corneal cross-linking

• Bandage CL worn for 3-5 days until epithelium is healed

• Topical antibiotic x 1 week

• Topical corticosteroid x 2-3 weeks

• Remain out of CLs for ~1 month after procedure

What may happen after the corneal cross-linking procedure

• cornea may change shape after procedure

• CL may need to be refit

• epithelium may not fully heal for up to 1 year

What is the purpose for corneal crosslinking

stabilize progression of KCN

• reduced myopic refractive error and flattens K readings by 2-3 D

• may improve subjective vision function (glare, halos, starbursts)

• resulted in 25% reduction rate of corneal transplants

Epi-off procedure requires a min. corneal thickness of _____ um

400 um

• can be artificially thickened by using HYPOTONIC topical drops during procedure

what are the complications of corneal cross linking

• corneal haze - 90% of pts

• eye pain

• abrasion

• MK

• sterile infiltrates

• corneal edema

• corneal opacity

• scarring

Which treatment of cross-linking are currently “off-label”

• epi -on (no epithelial debridement, less discomfort, takes longer to get riboflavin to stroma)

• accelerated CXL - increased intensity of irradiation to reduce treatment time

• combined CXL with other procedures

  • CXL + refractive surgery ‘

  • CXL + intrastromal corneal ring segments

What is Penetrating keratoplasty (PKP)

full-thickness corneal transplant (all layers)

• endothelial loss begins immediately (drops to 800/ mm2)

• impacts tolerance to reduced O2 states

what is anterior lamellar keratoplasty (DALK)

• leaves the endothelium

• less risk of rejection

KCN may recur with with corneal transplant procedures

• PKP

• DALK

life expectancy of a graft is ______ years

15-25 years

What is a problem with intrastromal corneal ring segments (IRCS)

• very LOW Dk - may observe neovascularization

• implant may migrate - extrusion possible, inflammation + staining possible

• difficult to fit CL over

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