Transcript Rotating Scheimpflug Topographic Parameters Important in

ROTATING SCHEIMPFLUG TOPOGRAPHIC PARAMETERS
IMPORTANT IN DISTINGUISHING NORMAL FROM
KERATOCONIC CORNEAL MORPHOLOGICAL FEATURES
Clayton Falknor, MD, Orkun Muftuoglu, 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.
Keratoconus
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Essential to identify keratoconus prior to keratorefractive surgery
Keratoconus is characterized by:
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Keratoconus is identified by:
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Non-inflammatory, progressive corneal disease
Stromal thinning and anterior bulging of cornea
Irregular astigmatism and myopia
Potentially severe corneal scarring
Fleisher ring (corneal iron line at base of cone)
Vogt striae (stromal stress lines within cone)
Scissoring of retinoscopic reflex
Apical scarring and/or subepithelial fibrosis
Central or paracentral steepening on topography
Subclinical (forme fruste) keratoconus is difficult to identify
Identification of keratoconus
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Traditional method to identify subclinical keratoconus is
corneal topography
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Placido disk-based
Measures slopes of anterior corneal surface only
Axial curvature method subject to misalignment of corneal apex
and corneal sighting point
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May lead to misleading maps, eg normal eyes may show asymmetric
bow-tie or inferior steepening
Contribution of posterior corneal surface important
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Orbscan (Bausch and Lomb, Salt Lake City, Utah, USA)
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Slit-scanning beam combined with Placido ring technology
Posterior surface recreated with triangulation algorithms
Not all images include central cornea
Pentacam (Oculus Optikgeraete GmbH, Germany)
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
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Evaluate parameters obtained by Pentacam
important in distinguishing keratoconus from normal
Pentacam parameters to detect keratoconus
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Pachymetry
Progression index of corneal
thinning
Corneal volume within fixed
diameter
Simulated keratometry
AC volume, depth and angle
Posterior elevation over best-fit
sphere (option of toric ellipsoid)
Zernike HOA of anterior and
posterior surfaces
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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)
Patients
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Keratoconus (Diagnosed clinically with topography support)
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Controls (normals presenting for keratorefractive surgery)
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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
72 eyes of 36 patients (16 men, 20 women)
Inclusion: underwent pre-operative screening for keratorefractive Sx,
normal corneal exam, available topography maps
Exclusion: prior ocular surgery or trauma, suspicion for keratoconus or
pellucid marginal degeneration by topography
Age-matched
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Keratoconus group 36.4 ± 11
Control group 43 ± 14
Posterior corneal elevation
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Mean posterior elevation
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Keratoconus 98.7 ± 46.3 µm
Control 11.8 ± 6.12 µm
Difference significant
(p<0.001)
With Orbscan IIz, posterior
elevation optimal cutoff point to
discriminate keratoconus and
keratoconus suspect versus normal
corneas was 40 µm
(Rao et al & Fam et al)
Table 1. The sensitivity and specificity of different
posterior corneal elevation levels to distinguish
between keratoconic eyes and controls.
Keratoconus
Controls
Cut-off point
(µm)
Sensitivity
Specificity
10
100
0
15
100
12.3
20
100
53.4
25
100
66.2
30
96.4
84.5
35
93.2
94.9
40
89.0
98.9.
45
86.2
100
Pentacam keratoconus parameters
Table 2. Comparison of Pentacam parameters between Keratoconic eyes and normal controls
PROGMIN
PROGAVG
PROGMAX
CVolume7mm (mm3)
K1
K2
Kmean
Kaxis
PACHYPupil (µm)
PACHYThinnest (µm)
ACVol (mm3)
ACDepth (mm)
ACAngle (deg)
Keratoconus
Mean ± SD
Controls
Mean ± SD
P*
1.79 ± 2.50
2.75 ± 2.71
3.85 ± 3.62
22.92 ± 1.85
46.68 ± 6.03
50.81 ± 6.45
50.2 ± 6.1
86.93 ± 55.56
481.00 ± 60.64
450.72 ± 70.67
193.8 ± 35.8
3.4 ± 0.4
39.8 ± 6.9
0.71 ± 0.41
0.97 ± 0.48
1.26 ± 0.62
24.23 ± 1.65
42.46 ± 2.36
43.49 ± 2.25
43.5 ± 2.3
79.51 ± 64.13
533.19 ± 44.19
530.18 ± 43.17
183.2 ± 56.1
3.1 ± 0.6
39.8 ± 8.8
<0.001
<0.001
<0.001
<0.001
<0.001
<0.001
<0.001
0.422
<0.001
<0.001
0.081
<0.001
0.886
Corneal variance indices
Table 3. Comparison of corneal variance indices between keratoconic
eyes and normal controls
ISV
IVA
KI
CKI
IHA
IHD
RMIN
ABR
Keratoconus
Mean ± SD
Controls
Mean ± SD
P*
Sensitivity
Specificity
94.34 ± 51.90
0.96 ± 0.57
1.23 ± 0.18
1.03 ± 0.11
22.11 ± 19.72
0.09 ± 0.07
6.14 ± 0.85
2.51 ± 1.10
26.41 ± 29.46
0.24 ± 0.39
1.01 ± 0.07
1.00 ± 0.02
5.39 ± 3.90
0.01 ± 0.02
7.46 ± 0.39
1.46 ± 0.57
<0.01
<0.01
<0.01
<0.01
<0.01
<0.01
<0.01
<0.01
98
95
92
66
46
92
86
100
65
86
88
89
78
63
46
0
Summary of results
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Mean posterior corneal elevation significantly higher in keratoconus
compared to controls
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For cut-off of 35 µm, sensitivity 93% & specificity 95%, comparable to
Orbscan
Progression index minimum, average, and maximum all significantly
different in keratoconus vs. controls
Other significant parameters:
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All corneal variance parameters (all based on anterior surface)
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Most sensitive: ISV, Abr, IVA, KI, IHD
Most specific: CKI, KI, IVA
Pachymetry at pupil center and thinnest, flat and steep keratometry, AC
depth, corneal volume of central 7mm diameter
Not significantly different:
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Keratometry axis, AC volume, AC angle
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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Trefoil, coma, fourth-order astigmatism, spherical aberration all differ both anteriorly and posteriorly
For both anterior and posterior surfaces, vertical coma most important HOA
Conclusions
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Pentacam is useful for identifying keratoconus
Both anterior and posterior corneal surface
parameters are important
Advantages over other instruments
 Measures
central corneal zone
 Elevation data independent of reference axis
 Zernike analysis of anterior and posterior corneal HOA