KERATOCONUS
Objectives of this lecture:
1. To understand the etiology, diagnosis, and
progression of keratoconus
2. To understand the RGP design alternatives in the
management of this condition
DEFINITION: Keratoconus is a
noninflammatory, self-limiting ectasia of the axial cornea. It is characterized
by thinning and steepening of the central cornea.
I. INCIDENCE
* 0.15% - 0.6%
* Most often in adolescents (10-20 y/o)
* Females > Males
* Bilateral in 90%; one eye is more progressive
* Average of about 7 year progression (varies!!!)
II. ETIOLOGY ???
A. Corneal Tissue Change -
Epithelium or Stroma?
1.
Epithelium - caused by the influence of basal epithelial cells on fibrogenesis
and the anterior stroma
2.
Stroma - stromal scarring and lamellae changes have been documented; donor
collagen is replaced for defective collagen. Increased collagenase activity has
also been reported.
B. Atopic
1.
Primary factor associated with keratoconus
2. Patients often have history of dermatitis, hay fever
3. Positive history of atopy in 35% of keratoconics
4. Raised serum levels of Immunoglobulin E in 47-52%
5. Eye Rubbing is very common:
* 70 % of keratoconic patients
* Trauma of eye rubbing may trigger
the mechanism
* Often associated with atopy
C. Systemic
1.
Connective Tissue Disorders - Ehler-Danlos Syndrome (abnormal crosslinking of
collagen), Rieger's Syndrome and Marfan's Syndrome
2. Down's Syndrome
D. Heredity
1.
Only a small (low %) correlation
2. Five sets of twins reported to have keratoconus
3. Hammerstein: about 20% had 2 family members;
8% overall
4. Predominance of older mothers
E. Contact Lens Wear ???
1.
Associated with long-term PMMA contact lens wear?
2. Possibly associated with low ocular rigidity combined with chronic hypoxia
3. Some cases are actually corneal warpage syndrome:
* Corneal distortion and irregular
distortion and possibly slight steepening
* Gradual progression and
biomicroscopic changes are absent
4. Gasset: 162 keratoconic patients - 26.5% were PMMA wearers; of 1248 hydrogel
"controls" - only one had keratoconus
III. DIAGNOSIS
A. Case History
*
Ghost Images/Monocular Diplopia
* Distorted letters/words
* Asthenopia
B. Vision
*
Gradual Decrease
* Typically this represents the first clinical signof keratoconus
* One eye decreases more than the other
* Scissors motion observed with the retinoscope
C. Ophthalmoscopy
1.
Shadow appears like a cataract centrally
2. Photodiagnosis (the cone is viewed against the fundus reflex)
* Dilated Pupil
* Examine at a 2 foot distance
* Cone viewed against red fundus
reflex
D. Corneal Topography
1.
Progressive steepening
2. Keratometer - may need to attach a +1.25D trial lens to the front (i.e.,
where patient views) of the keratometer to extend the range approximately 8
diopters. For example, if you read the following: 44.00 @ 147; 49.50 @ 049
The actual values are: 52.00/57.50
3. Corneascope - photokeratograph: changes initially occur inferiorally
4. Corneal Modeling System
E. Biomicroscope
1.
Best for detecting subtle changes
2. Thinning via optic section or pachometry (for example, .38mm vs. .50mm)
3. Increased nerve visibility
4. Vogt's straie
5. Fleischer's Ring - yellow-brown from deposition of hemosiderin in stroma
which encircles the base of the cone
6. Apical scarring
IV. PROGRESSION
A. Stage One
1.
Fully correctable with spectacles
2. Slight increase in refractive astigmatism
3. Slight or no keratometric mire distortion
4. Normal keratometry readings
5. Difficulat to diagnose
B. Stage Two (45 - 50D)
1.
Definite corneal distortion
2. Further increase in refractive astigmatism
3. 1 - 4 diopters of corneal steepening
C. Stage Three (50 - 55D)
1.
Decreased spectacle acuity
2. Keratometry readings difficult due to mire distortion
3. 5 - 10 diopter corneal steepening
4. 2 - 8 diopter irregular astigmatism
5. Retinoscopy and ophthalmoscopy show shadows
6. Corneal thinning
7. Straie
8. Pronounced cone shape
9. Photokeratoscopy - circles wide apart in superior-nasal region; close
together in inferior-temporal
D. Stage Four ( > 55D)
1.
Intensification of above signs
2. Apical scarring
3. Munson's Sign is present
V. MANAGEMENT
A. Contact Lenses
1. Spherical Designs
LARGE, FLAT DESIGN
* Fit several diopters flatter than
"K" to halt progression of cone
* Probably encourages apical
scarring
SMALL, STEEP DESIGN
* Apical clearance to minimize
scarring
* Korb: 7 patients diagnosed as
keratoconic with no apical scarring
Design One: 9.4/8.0 fit 1.5D flatter than "K"
Design Two: 8.0/5.8 fit "On
steep K"
Scarring resulted in 4/7 flat fits;
0/7 steep fits
THREE-POINT TOUCH
* Attempt to obtain a mild "feather" touch centrally and
mid-peripherally
* Often fitted slightly (1/3 - 1/4
the difference) steeper than "K"
* Weight distribution minimizes
damage and enhances vision
* Most often recommended
SPECIFIC DESIGNS
* Soper bicurve (peripheral curve = 45D)
* Caroline-McGuire: three types of
cones -
Nipple/small = 8.1/5.5mm OAD/OZD
Oval/medium = 8.6/6.0mm OAD/OZD
Globus/large = 9.1/6.5mm OAD/OZD
4 peripheral curves
* NiCone: Recently reported to be
very successful, it is a "tri-base curve" lens design
RECOMMENDED
SPHERICAL LENS DESIGN
Stage One: Normal lens design
Stage Two: 9.0 OAD/OZD = BCR;
tetracurve
Stage Three: 8.6-8.8 OAD/OZD = BCR;
Tetra or Pentacurve design with 12.0 PCR
Stage Four: 8.0 OAD/OZD = BCR;
Pentacurve design with 12.0 PCR, BCR = 1/3 to 1/4 the difference in K's
PROBLEM - SOLVING
* Adhesion - reduce OZD; flatten PCR
* Central Bubble - flatten BCR
* Excessive edge lift - steepen BCR
2. Aspheric Design
* Better alignment with cornea
* Enhanced centration
* Low eccentricity
* Ellipseecon/VFL-II
CLINICAL CASES
CASE ONE
Keratometry: 45.00 @ 170; 47.25 @ 080 Mild Distortion
Refraction: -4.00 - 2.00 x 170 20/20
-2
SLE: Clear
Lens Design ???
What if this lens decenters inferiorly? (two options)
CASE TWO
Keratometry: 51.00 @ 068; 55.50 @ 151 Moderate Distortion
Refraction: -9.00 - 4.00 x 075 20/40
+1
SLE: Vogt's straie, thinning,
Fleischer's Ring
Lens Design ???
What if this lens adheres?
CASE THREE
Keratometry: 54.25 @ 048; 62.00 @ 138
Refraction: -15.25 - 5.50 x 045
20/50
SLE: Straie, thinning, Munson's
Sign, mild apical scarring
Lens Design ???
Should a PK be considered?