Rotating Scheimpflug Topographic Parameters Important in

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ROTATING SCHEIMPFLUG TOPOGRAPHIC PARAMETERS
IMPORTANT IN DISTINGUISHING KERATOCONUS
SUSPECT FROM NORMAL EYES
Clayton Falknor, MD, Orkun Muftuoglu, MD, R. Wayne Bowman, MD,
Steven Verity, MD, James P. McCulley, MD
Some of the authors have received consultant reimbursement from Alcon Labs, Inc.
None of the authors have financial interest in the subject matter of this poster.
Iatrogenic corneal ectasia

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Complication of corneal refractive surgery
Progressive corneal steepening, myopia, astigmatism, loss of BCVA
Pre-operative risk factors
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Keratoconus is characterized by:
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Thin residual stromal bed after LASIK
Forme fruste keratoconus or keratoconus
Non-inflammatory, progressive corneal thinning and bulging
Irregular astigmatism and myopia
Potentially severe corneal scarring
Keratoconus is identified by:


Examination findings (Fleisher ring, Vogt striae, subepithelial fibrosis,
stromal thinning, scissoring of retinoscopic reflex)
Central or paracentral steepening on topography
Subclinical keratoconus

Traditional method to identify subclinical keratoconus
is Placido disk-based corneal topography
 Reflection-based
system
 Measures slopes of anterior corneal surface only
 Axial curvature method subject to misalignment of
corneal apex and corneal sighting point

Contribution of posterior corneal surface important
 Projection-based
 Orbscan
systems
(Bausch and Lomb, Salt Lake City, Utah, USA)
 Pentacam (Oculus Optikgeraete GmbH, Germany)
Keratoconus suspects



Topographic designation
No evidence of keratoconus on examination
Multiple methods to define
 Modified
Rabinowitz/McDonnell method (central K
steeper than 47.2D, I-S >1.4D)
 Maeda/Klyce KPI index
 Smolek/Klyce KSI index (based on >10 indices and
neural network)
 KISA% (Rabinowitz)
Pentacam Comprehensive Eye Scanner


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Rotating Scheimpflug camera
Monochromatic slit light source rotates with camera
25-50 slit images per acquisition
Eye movement monitoring by 2nd camera



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Less than 0.6mm decentration
Rotates 180º in 2 seconds
All images include central cornea
Corneal elevation data independent of visual axis and corneal
apex
http://www.oculus.de/chi/downloads/dyn/sonstige/sonstige/pent
acam_aao_2006.pdf
Purpose


Evaluate Pentacam parameters important in
distinguishing keratoconus suspects from normal
Pentacam parameters to detect keratoconus
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Pachymetry
Progression index of corneal
thinning
Corneal volume within fixed
diameter
Keratometry readings and axis
AC volume, depth and angle
Posterior elevation over best-fit
sphere (option of toric ellipsoid)
Zernike HOA of anterior and
posterior surfaces

Corneal variance indices
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ISV (index of surface variance)
IVA (index of vertical asymmetry)
IHA (index of height asymmetry)
IHD (index of height
decentration)
Rmin (radius minimum)
KI (keratoconus index)
CKI (center keratoconus index)
Pentacam keratoconus identification
Patients

Controls (normals presenting for keratorefractive surgery)
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Keratoconus suspects
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72 eyes of 41 patients
Inclusion: underwent pre-operative screening for keratorefractive Sx, normal
corneal exam, available topography maps
Exclusion: prior ocular surgery or trauma, ocular disease likely to affect corneal
HOA’s
15 eyes from 10 patients
Selected from normals with keratoconus suspect indication by Smolek/Klyce KSI on
topography.
Keratoconus (diagnosed clinically with topography support)



108 eyes of 54 patients (34 men, 20 women)
Inclusion: distorted keratometry mires, abnormal retinoscopic reflex, Vogt’s striae,
Fleischer’s ring, corneal scarring, available topography maps
Exclusion: prior corneal surgery, extensive corneal scarring
Posterior corneal elevation

Mean posterior
elevation
Keratoconus
98.7 ± 46.3 µm
 Keratoconus
suspects
16.9 ± 6.1 µm
 Controls
8.6 ± 3.8 µm
Table 1. Sensitivity and specificity of posterior elevation cut-off points to detect
keratoconus and keratoconus suspects

Keratoconus
Keratoconus suspects
Cut-off point
Sensitivity
Specificity
Sensitivity
Specificity
10
100
0
93
61
15
100
12
67
91
20
100
53
20
100
25
100
66
20
100
30
96
84
0
100
35
93
94
0
100
40
89
98
0
100
45
86
100
0
100
(µm)
Pentacam keratoconus parameters
Table 2. Comparison of Pentacam parameters between keratoconus suspect and normal eyes
Keratoconus Suspect
Control
Mean ± SD
Mean ± SD
P*
Corneal Vol (7mm diam)
23.7 ± 1.3
24.3 ± 1.8
0.264
K Thinning Prog Min
0.7 ± 0.5
0.6 ± 0.2
0.749
K Thinning Prog Avg
1.1 ± 0.6
0.9 ± 0.3
0.048
K Thinning Prog Max
1.6 ± 0.8
1.1 ± 0.5
0.011
K1
45.0 ± 2.5
42.7 ± 1.4
<0.001
K2
46. 3± 2.2
43.6 ± 1.6
<0.001
KAxis
94.2 ± 69.3
78.5 ± 65.1
0.332
Pachy Pupil (µm)
513 ± 40
538 ± 41
0.032
Pachy Thinnest (µm)
508 ± 40
536 ± 41
0.015
AC Vol (mm3)
192 ± 25
189 ± 49
0.394
AC Depth (mm)
3.39 ± 0.42
3.09 ± 0.28
0.004
AC Angle
43.6 ± 0.4
40.2 ± 7.0
0.122
Corneal variance indices
Table 3. Comparison of Pentacam parameters between keratoconus suspect and normal eyes
Keratoconus Suspect
Control
Mean ± SD
Mean ± SD
P*
Sensitivity
Specificity
ISV
25 ± 12
19 ± 6
0.062
7
98
IVA
0.23 ± 0.17
0.18 ± 0.08
0.466
33
89
KI
1.04 ± 0.04
1.01 ± 0.03
0.007
20
98
CKI
1.00 ± 0.02
1.00 ± 0.01
0.537
7
100
IHA
9.4 ± 10.9
5.2 ± 3.8
0.360
13
100
IHD
0.017 ± 0.013
0.01 ± 0.005
0.008
53
79
Rmin
7.06 ± 0.24
7.58 ± 0.29
<0.001
7
100
ABR
1.4 ± 0.2
1.3 ± 0.5
0.727
100
9
Summary of results
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Maximum posterior corneal elevation

For keratoconus
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For keratoconus suspect
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Cut-off of 35 µm, sensitivity 93% and specificity 95%
Cut-off 15 µm, sensitivity 67% and specificity 91%
Cut-off 10 µm, sensitivity 93% and specificity 61%
Progression index average and maximum all significantly
different in keratoconus suspects vs. controls
Other significant parameters:
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Corneal variance parameters : KI, IHD, Rmin
Pachymetry at pupil center and thinnest
Keratometry (flat and steep)
AC depth
Zernike analysis
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Both anterior and posterior elevation data decomposed into Zernike higher-order aberration polynomials
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Real differences between keratoconus and controls within the third through sixth orders
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Anterior surface: vertical coma, trefoil
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Posterior surface: vertical coma, spherical aberration, and fifth-order vertical coma
Pentacam for keratoconus
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Pentacam is useful for identifying keratoconus suspects
Corneal variance parameters are set too high to
capture keratoconus suspects, but specificity is excellent
for confirmation purposes
Best Posterior elevation cut-off between 10 and 15 µm.
Important parameters include pachymetry, progression
of corneal thinning, keratometry, corneal variance
indices, and Zernike HOA’s (especially vertical coma)

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