KEY CONCEPTS
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This chapter reviews a case of keratoconus in one eye and early keratoonus in the contralateral eye.
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A review of corneal imaging systems and their abmormal parameters to detect keratoconus is performed.
Case Study
A 40-year-old male presented for follow-up for keratoconus and for evaluation for corneal collagen cross-linking in his right eye. His best-corrected visual acuity on the Snellen chart was 20/20 in both eyes with a rigid gas permeable lens in his right eye and spectacle correction in his left eye. Manifest refraction was −8.50 + 5.00 × 180 and −2.25 + 1.75 × 25 for right and left eye, respectively. Central cornea thicknesses were 487 and 575 microns for right and left eye, respectively. Slit-lamp examination showed apical thinning with a Fleischer ring in his right eye and a normal examination in his left eye.
Corneal topography via the Galilei Dual Scheimpflug topographer (Ziemer Ophthalmic Systems, Port, Switzerland) was obtained. His right eye ( Fig. 39.1 ) showed clinical keratoconus with a K max over 60 diopters (D) and over 6 D of irregular astigmatism with significant inferior thinning and steepening. The thinnest pachymetry reading was abnormal at 466 microns, and the I-S index was elevated at 10.26 D. The elevation maps of both the anterior and posterior cornea were abnormal. The keratoconus probability (KP) and keratoconus prediction index (KPI) were both 100%. Collagen cross-linking was planned for the right eye as topography showed evidence of progression.
Corneal topography of the left eye ( Fig. 39.2 ) showed inferior steepening corresponding to the thinnest corneal thickness with a borderline I-S index at 1.51 D and an elevated KP and KPI. The thinnest corneal thickness was 559 microns. The posterior elevation map showed an abnormal elevation of 15 microns in the 3 mm zone. This area of posterior elevation coincided with the area of corneal thinning. The anterior elevation maps were normal. The percent probability of keratoconus (PPK) was abnormal at 42.9%. Given these findings on topography, he was diagnosed with subclinical keratoconus (SCK) in his left eye. The plan for this eye was continued observation.
Introduction
Early keratoconus (keratoconus suspect) can be classified as either SCK or forme fruste keratoconus (FFKC). These terms are frequently interchanged in the literature. However, there is consensus that the specific term FFKC should only be used when the eye in question combines normal topography, normal slit-lamp examination, and keratoconus in the fellow eye. SCK refers to those eyes with normal slit-lamp examination, topographic/tomographic signs of keratoconus or suspicious topography (but not enough to define as keratoconus), and keratoconus in the fellow eye. Without symptoms or definite clinical findings, SCK and FFKC can be challenging to detect and to differentiate from normal eyes.
Clinical keratoconus in the fellow eye represents the primary risk factor for SCK and FFKC. The correct diagnosis is obtained when screening for risk factors for SCK, and ranges from 1% to 6% of eyes being screened for myopic refractive surgery. Along with a deep ablation, SCK is the next strongest risk factor for the development of postrefractive ectasia, and some studies have found it to be present in up to 88% of eyes with postrefractive ectasia. , The diagnosis is purely topographic. In addition to screening for refractive surgery, the diagnosis is important, as there is a higher rate of progression to clinical keratoconus compared with normal eyes. A study of over 100 patients with unilateral clinical keratoconus with a normal contralateral eye showed that approximately 50% developed clinical keratoconus in the “normal” eye, suggesting that some of these eyes may have had SCK that was initially misdiagnosed. In addition to refractive surgery, there is support for acquiring corneal topography for diagnosing keratoconus in patients with at least 2 D of astigmatism, as the rates of clinical keratoconus and SCK in this population are 6.3% and 7.8%, respectively.
Diagnosis
Many tools have been used for diagnosing SCK, including videokeratography, elevation-based topography, corneal biomechanics, wavefront aberrations, and various scoring systems. Over the years, topographic markers have changed, as have advancements in slit scanning and Scheimpflug imaging, posterior corneal data, and anterior segment optical coherence tomography (OCT). Recently, more emphasis has been placed on elevation-based topography rather than curvature-based topography and corneal thickness. It should be emphasized that there is no single parameter or technology that has a 100% sensitivity and specificity (area under the curve of 1.00) for SCK, and many experts suggest the approach of using multiple parameters and modalities.
Curvature-Based Topography
Curvature-based topography includes Placido imaging and videokeratography: a variety of indices are used to diagnose SCK with this modality. Central keratometry >47.2 D or >1 D compared with the contralateral normal eye is suggestive of SCK. An I-S index (difference in mean keratometry of the inferior hemicornea from the superior hemicornea at the 3-mm corneal ring) ranging from 1.4 to 1.9 also supports SCK. Anything greater than 1.9 is associated with clinical keratoconus. Another measurement that may have to be manually calculated depending on the topography device used is the skewed radial axis (SRAX). The SRAX index represents the most acute angle between the steepest semimeridian axis in the superior hemicornea and the inferior hemicornea. Any value greater than 21 degrees rules out keratoconus.
KISA% index is a calculation that incorporates many of the values previously mentioned: central corneal power, I-S index, simulated corneal astigmatism, and SRAX index. The KISA% index is the product of these four variables multiplied by 0.3. Any value greater than 60 and less than 100 supports SCK, whereas a value greater than 100 suggests clinical keratoconus. Additional formulas and tools, such as the Cone and Location Map Index (CLMI), may also assist providers in detecting SCK. The CLMI is an index that refers to the difference of the average corneal curvature in the steepest 2-mm diameter circle from the average corneal curvature of the area outside of the designated 2-mm circle.
Curvature-based topography is effective in diagnosing and confirming clinical keratoconus. However, it can miss SCK. The downfall of curvature-based topography is the failure to characterize the posterior cornea and to provide elevation-based maps. It is now evident that the posterior cornea can show elevation in early SCK before showing any abnormalities on curvature-based topography. Therefore the trend has been toward elevation-based topography and the use of multiple modalities in recent years, especially when screening for laser refractive surgery.
Elevation-Based Topography
The main advantages of elevation-based topography are that it includes a broader image of the cornea (9 mm) and provides posterior corneal data with corneal thickness. Posterior corneal changes that would be missed on curvature-based topography have been shown to be associated with SCK.
Two techniques can provide elevation-based topography: slit scanning and Scheimpflug imaging. In the slit-scanning technique two scans of 20 projections are performed on both sides of the eye. The Scheimpflug image is created by a rotating camera that moves around the optical axis. The Orbscan II (Bausch and Lomb, Inc., Rochester, NY), the Pentacam (OCULUS Inc., Arlington, WA), and the Galilei instruments all employ Scheimpflug imaging.
ORBSCAN
The Orbscan II was one of the first slit-scanning topographies on the market. It provides pachymetry, curvature, and elevation values simultaneously. In 2014, Jafarinasab et al. conducted a study using the Orbscan II on patients with clinical keratoconus, SCK, and normal eyes. Many variables, such as anterior and posterior best-fit-spheres, were statistically significantly different between SCK and normal eyes. For example, mean posterior corneal elevation was 106.80 microns, 36.60 microns, and 22.80 microns in clinical keratoconus, SCK, and normal eyes, respectively. However, it was difficult to find a specific cutoff in either anterior or posterior corneal elevation that had a high sensitivity and specificity in detecting SCK ( Table 39.1 ).
| Parameter | Device(s) | Cutoff Value | Sensitivity | Specificity | AUC |
| Maximum anterior elevation in 5-mm zone | Galilei | 4.5 | 82.6 | 36.1 | 0.615 |
| Maximum posterior elevation in 5-mm zone | Galilei | 12.5 | 91.3 | 38.3 | 0.619 |
| Keratoconus probability | Galilei | 11.6 | 39.1 | 95.5 | 0.669 |
| Maximum anterior elevation in 3-mm zone | Galilei | 3.5 | 87 | 43.6 | 0.678 |
| Keratoconus prediction index | Galilei | 5 | 56.5 | 83.5 | 0.71 |
| Maximum posterior elevation in 3-mm zone | Galilei | 11.5 | 60.9 | 77.4 | 0.718 |
| Radius of anterior best-fit-sphere (mm) | Galilei | 7.645 | 82.6 | 69.7 | 0.828 |
| Mean keratometry | Galilei | 45.49 | 65.2 | 94.7 | 0.833 |
| Flat keratometry | Galilei | 43.8 | 82.6 | 72.9 | 0.84 |
| Radius of posterior best sphere (mm) | Galilei | 6.27 | 87 | 87.1 | 0.892 |
| Three-model structure (elevation + surface indices + pachymetry data) | Galilei | Not available | 93.8 | 80.8 | 0.936 |
| Three-model structure (elevation + surface indices + keratometry data) | Galilei | Not available | 97.7 | 84.6 | 0.952 |
| Posterior corneal elevation | Orbscan | ≥14 | 92.86 | 9.93 | 0.698 |
| Anterior corneal elevation | Orbscan | ≥10 | 75 | 31.21 | 0.698 |
| Posterior corneal elevation | Orbscan | ≥27 | 75 | 57.45 | Not available |
| Anterior corneal elevation | Orbscan | ≥14 | 64.29 | 78.01 | Not available |
| Fifth order vertical coma aberration of front cornea | Pentacam | ≥0.023 | 70.6 | 61.8 | 0.72 |
| Index surface variance | Pentacam | ≥22 | 74.5 | 61.8 | 0.8 |
| Index vertical asymmetry | Pentacam | ≥0.14 | 82.3 | 73.2 | 0.86 |
| BAD_D | Pentacam | ≥1.54 | 81.1 | 73.2 | 0.86 |
| BAD_D + IVA + ISV + fifth order vertical coma aberration of front cornea | Pentacam | Cutoffs are displayed above | 83.6 | 96.9 | 0.96 |
| Inferior superior value | Pentacam | 0.51 | 53.3 | 75 | 0.599 |
| Index height decentration | Pentacam | 0.0075 | 76.7 | 51.7 | 0.604 |
| Index vertical asymmetry | Pentacam | 0.115 | 70 | 66.7 | 0.722 |
| Pachymetry apex | Pentacam | 531 | 66.7 | 75 | 0.732 |
| Parameter | Device(s) | Cutoff Value | Sensitivity | Specificity | AUC |
| ART max | Pentacam | 363.5 | 56.7 | 88.3 | 0.739 |
| BAD_D | Pentacam | 1.01 | 80 | 66.7 | 0.754 |
| IVA + pachymetry apex + IHD + ART max + ISV | Pentacam | Cutoffs are displayed above | 83 | 83 | 0.86 |
| 13 Variables , a | Pentacam+ SD OCT | See reference | 100 | 100 | 1 |
| Epithelium standard deviation | SD OCT | 1.35 | 73.7 | 58.2 | 0.671 |
| Epithelium minimum-maximum | SD OCT | –6.5 | 68.4 | 65.5 | 0.676 |
| 11 Variables , a | SD OCT | See reference | 89 | 89 | 0.96 |
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