Comparison of Central Corneal Thickness and Peripheral Corneal Thickness using Sheimpflug system,
Optical Coherence Tomography and Ultrasound Pachymetry in Normal, Keratoconus and Post-Refractive
Surgery Eyes at Different Corneal Thickness.
Ricardo N. Sepulveda, MDClaudia Maria Prospero Ponce, MS
Karolinne Maia Rocha, MD PhDWilliam J. Dupps, MD PhD
Ronald R. Krueger, MDCole Eye Institute
Cleveland, OH*Authors have no financial interest.
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Purpose: To compare central corneal thickness (CCT) and peripheral corneal thickness (PCT) with scheimpflug system (Pentacam), high-speed optical coherence tomography (Visante) and ultrasound pachymetry (US) in normal eyes, keratoconus suspect and post-laser in situ keratomileusis (LASIK).
Setting: Department of Refractive Surgery, Cole Eye Institute, The Cleveland Clinic. Cleveland, Ohio, USA.
Study Type: Retrospective Analysis
Background
Introduction
• Ultrasound pachymetry is currently the gold standard in measuring CCT.
• Measurements taken with Pentacam and Visante-OCT have demonstrated to be comparable to US.
• To our knowledge, this is the first study comparing central pachymetry between the 3 systems in pre- and post-LASIK eyes, and keratoconus suspects.
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Patients and Methods
• The CCT and PCT were measured with Pentacam (Oculus Inc, Lynnwood, WA, USA) , US (Sonogage, Corneo-Gage Plus, Sonogage Inc., USA) and Visante OCT(Carl Zeiss Meditec Inc., Dublin, CA, USA,) in 163 eyes of 83 patients.
• 3 groups were retrospectively analyzed: Keratoconus suspects, Post-LASIK and Normal patients (without Corneal pathology).
• Keratoconus suspects were identified by the Rabinowitz-Macdonald criteria and using the PathFinder II Corneal Analysis Software for the ATLAS™ Corneal Topography System (Model 9000).
• Influence of age and corneal thickness was evaluated in all groups, categorizing eyes with thin (≤500µm), normal (501-550µm) or thick (≥551µm) corneas using US values.
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Patients and Methods
• Data was collected at 0 mm and 6 mm from Pentacam, 0-2mm and 5-7mm from OCT, and a single value was obtained from US.
• Multivariate generalized estimating equations were used to analyze the correlations between the 3 measurements obtained from the patients’ both eyes.
• Mean CCT and mean PCT difference between devices were obtained for each group using multivariate linear regression.
• Analyzed factors included age, keratoconus suspects and previous refractive surgery (LASIK); subsequently, influence of absolute corneal thickness in pachymetry measurements was determined.
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Results
• 83 patients (163 eyes)
• Mean age: 39 years (range 22-69 yrs.)
• 53 female, 30 male
• 40 eyes were keratoconus suspects, 17 post LASIK and 103 normal eyes.
• Mean spherical equivalent (SE) and Keratometry (Km) are shown in Table 1.
• Keratometry readings ranged from 36.2 D to 59.5 D.
• Mean CCT for each group is shown in Table 2.
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Results
• CCT measurements were higher in US compared with Pentacam (6.49 ±1.84μ; p<0.0005) and Visante OCT (7.48+1.38μ; p<0.0005) for keratoconus suspects, post LASIK and normal eyes, regardless of age and corneal thickness.
• The greatest difference in mean CCT measurements was observed in the post-LASIK group (Table 3), where Pentacam measured thinner CCT than US and OCT.
• Peripheral corneal thickness measurements were superior in Pentacam than in OCT(603.26± 38.83μ vs. 570.61±40.39μ; p<0.0005).
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Results
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Mean Normal Keratoconus
Post LASIK
CCT (value + SD)US
Pentacam OCT
523±28.04
516.28±31.6
515.41±29.16
523.02±41.61
513.57±43.96
512.67±42.31
526.06±66.73
501.66±73.74
516.35±66.14
PCT (value + SD)Pentacam
OCT597.37±3
4.20564.26±3
4.8
575.05±45.28
608.97±42.79
628.82±46.91
600±49.79Mean Difference US vs.
PentacamP value* US vs OCT P
value*
Pentacam vs OCT P value*
CCT (value + SD)Constant
KeratoconusPost LASIK
6.49±1.844.07± 3.3417.75±4.86
0.00050.224
0.0005
7.48±1.383.28±2.512.04± 3.63
0.00050.1900.575
0.89±1.390.21±2.58
15.61±3.66
0.5210.935
0.0005
All Normal Keratoconus
Post Lasik
SE -3.78 ± 3.36
-4.17 ± 3.03
-4.09 ± 3.75
-0.78 ± 2.29
Km 44.77 ± 2.14
44.63± 1.31
46.04 ± 2.6
42.2 ± 2.4
Table 1. Spherical equivalent and Km for all the eyes and for each individual group
Table 2. Mean central(CCT) and peripheral(PCT) corneal thickness with standard deviation in the 3 groups.
Table 3. Mean central corneal thickness difference between Ultrasound, Pentacam and Visante OCT pachymetries in a paired analysis. Standard deviation is also shown.
*P value <0.05 was statistically significant
Results
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Mean Difference
US vs Pentaca
m
P value*
US vs OCT
PValue*
Pentacam vs OCT
P Value
*
CCT (value + SD)
Thick*
Thin**
Constant
6.26±3.67
1.93±3.22
7.66±2.07
0.0890.550
0.0005
3.6±2.64-
2.13±2.32
8.23±1.47
0.1730.359
0.0005
-2.08±3.0
2-
4.44±2.67
0.68±1.64
0.491
0.096
0.679
Table 5. US gives thicker measurements than OCT and Pentacam(p<0.0005). The difference between US and Pentacam were similar at any absolute corneal
thickness.*>551µm*≤500µm
Mean Difference
Pentacam vs OCT
P value*
PCT (value + SD)
ConstantKeratoconus
Post LASIK
1.2±5.054.49±7.1433.16±2.7
1
0.0005
0.8120.53
Table 4. Peripheral corneal thickness was higher in Pentacam than in Visante OCT (constant). No influence was observed in Keratoconus or Post LASIK patients.
*P value was statistically significant if <0.05
Results
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Figure 1. Linear regression plots showing the correlation between US/Pentacam, US/Visante OCT, and Pentacam/Visante OCT respectively in Keratoconus Suspect eyes. A linear correlation can be illustrated with the 3 methods.
Figure 3. Linear regression plots showing the correlation between US/Pentacam, US/Visante OCT, and Pentacam/Visante OCT in normal eyes.
Results
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Figure 4. Peripheral corneal thickness correlation between Pentacam and Visante OCT. Pentacam values were constantly higher than OCT.
Figure 5. Peripheral pachymetries in post LASIK eyes. Correlation between Pentacam and Visante OCT.
Figure 6. Correlation between peripheral pachymetries in eyes unoperated and without disease.
Conclusion
• These recently new devices do not replace US pachymetry but rather complement each other in the preoperative evaluation of refractive surgery candidates, aid in the diagnosis and treatment of keratoconus suspects, help evaluate the lens, screen for glaucoma and allow room for more research in all ophthalmologic fields.
• Ophthalmologists should be familiar with the difference in CCT between US, Pentacam and Visante OCT.
• Pentacam and Visante OCT can be used interchangeably for central pachymetry, however, in post LASIK patients, Visante OCT might perform better pachymetry maps than Pentacam.
• Further studies are suggested in order to establish the influence of age in peripheral pachymetry.
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