Fig. 2.1
The nipple shape of a cone on the tangential curvature map
Oval Cone
It is larger (5–6 mm), ellipsoid, and commonly displaced inferotemporally (Fig. 2.2). Contact-lens fitting is more difficult in this pattern.
Fig. 2.2
The oval shape of a cone on the tangential curvature map
Globus Cone
It is the largest (>6 mm) and may involve over 75 % of the cornea (Fig. 2.3). Contact-lens fitting is a difficult challenge.
Fig. 2.3
The globus shape of a cone on the tangential curvature map
N.B: In mild cases, cone morphology may be indeterminate.
Tomographic Patterns
Tomographically, patterns can be curvature-based, elevation-based, or curvature-based.
Curvature-Based Patterns
Identifying and defining the patterns on the curvature map is subjective, and it is prone to some degree of error as a result of observer variability [8].
Errors may rise as a result of different interpretation by different observers (inter-observer variability) and as a result of inconsistent interpretation of the same map on repeat observations by the same observer (intra-observer variability) [9, 10].
In addition, using different color scales in the settings of the map display plays a big role in the intra- and inter-observer variability [11].
Color Scale Settings
There are two types of color scale, the normalized and absolute scales. In the normalized scale, the computer provides color contour maps based on the average dioptric value of each cornea. The disadvantage is that the color of two maps cannot be compared directly and have to be interpreted based on the keratometric values from their different color scales. In the absolute (standardized), the computer displays all corneas on the same scale, making comparison between corneas possible. Additionally, the color increments can be chosen to in 0.5, 1.0, or 1.5 diopter (D) steps. It has been shown that using the 1.5 D steps is better to avoid false positives resulting from overestimation of corneal irregularities [12, 13].
In the Klyce/Wilson Scale, the dioptric range extends from 28.0 D to 65.5 D in 1.5 D steps [12], while in the Universal Standard Scale it covers from 30.0 D to 67.5 D in 1.5 D steps [13], both providing the widest range of powers without scarifying the sensitivity to detect clinically significant features.
However, some settings are recommended for the color scale in specific devices. Belin and Ambrosio suggested the following settings for the users of Oculus Pentacam (OCULUS Optikgeräte GmbH, Wetzlar, Germany) [14]:
Belin Intuitive Color Bar for both elevations
Ambrosio 2 for pachymetry
Belin Intuitive or Ambrosio 2 for curvature
Absolute Normal scale for pachymetry and curvature
Relative minimum (2.5 μm) (this is ± 75 μm scale) for elevation
61 colors for all the maps
Arce suggested the following settings for screening for the users of Galilei (Ziemer Ophthalmic Systems AG, Switzerland) [15]:
ANSI style with CGA 5 μm for both elevations
German style with GCA 20 μm for pachymetry
Default style with CGA 1.0 D and CGA 1.5 D for anterior axial (sagittal) and anterior instantaneous (tangential) curvature maps, respectively
Default style with CGA 0.25 D for posterior axial (sagittal) and instantaneous (tangential) curvature maps
The Normal Cornea
The normal cornea flattens progressively from center to periphery by 2–4 D, with the nasal area flattening more than the temporal area, and this is shown on the curvature map as the nasal side becomes blue (flat) more quickly. The topographic patterns of the two corneas of an individual often show mirror-image symmetry, known as enantiomorphism, and small variations in patterns are unique for the individual. The approximate distribution of keratographic patterns described in normal eyes includes the following: round (23 %), oval (21 %), symmetric bowtie (SB) (18 %), asymmetric bowtie (AB) (32 %, of which 20 % are steep), and irregular (7 %) [16–18].
In regular astigmatism, the cornea shows a SB, which consists of two symmetric segments, “a” and “b”. The SB may be aligned vertically, horizontally, or diagonally, representing with-the-rule (Fig. 2.4), against-the-rule (Fig. 2.5), or oblique astigmatism (Fig. 2.6), respectively. Figure 2.7 represents enantiomorphism.
Fig. 2.4
In regular astigmatism, the normal pattern is the symmetric bowtie (SB) consisting of two segments (a) and (b). In case of with-the-rule astigmatism, the SB is vertically oriented
Fig. 2.5
Horizontal symmetric bowtie indicating against-the-rule astigmatism
Fig. 2.6
Oblique symmetric bowtie indicating oblique astigmatism
Fig. 2.7
Enantiomorphism. The right eye has a mirror shape of the left eye
Topographic Shape Patterns Characterizing Irregularity [16, 18–20]
There are several patterns of corneal curvature, and according to some factors, these patterns can be considered within the norm, suspicious or abnormal ranges.
Figure 2.8 illustrates major curvature patterns.
Fig. 2.8
Patterns of the anterior curvature map. SS superior steep, IS inferior steep, SB symmetric bowtie, SB/SRAX symmetric bowtie with skewed steepest radial axis index, AB/IS asymmetric bowtie/inferior steep, AB/SS asymmetric bowtie/superior steep, AB/SRAX asymmetric bowtie with skewed steepest radial axis index
Round. The steepest part of the cornea (hot spot) is round and often decentered.
Oval. The steepest part of the cornea (hot spot) is oval and may be centered or decentered.
Superior steep (SS). The steepest part of the cornea is localized in the upper part of the cornea.
Inferior steep (IS). The steepest part of the cornea is localized inferior to the apex of the cornea.
Irregular. Corneal surface has no particular shape, that is, in this pattern steep areas are mixed with flat areas.
Symmetric bowtie (SB). This pattern may be an indicative of normal astigmatism or occasionally symmetric pattern of KC.
Symmetric bowtie with skewed steepest radial axis index (SB/SRAX). It is a SB with angulation (skew) between the axes of segments “b” and “a”. In this case, corneal astigmatism is called “non-orthogonal astigmatism,” or the “lazy 8” pattern. Angulation is considered clinically significant when it is >22°.
Asymmetric bowtie/inferior steep (AB/IS). The curvature power of segment “a” is higher than that of segment “b”. If the difference is more than 1.4 D on the 3 mm central circle, it is considered significant.
Asymmetric bowtie/superior steep (AB/SS). The curvature power of segment “b” is higher than that of segment “a”. If the difference is more than 2.5 D on the 3 mm central circle, it is considered significant.
Asymmetric bowtie with skewed steepest radial axis index (AB/SRAX). It is an AB with angulation between the axes of the two segments. Angulation is considered clinically significant when it is >22°.
Vertical D. It is a circular shape, where the two segments are connected laterally.
Vortex pattern. Flat and steep areas are distributed in a vortex pattern, indicating corneal distortion (Fig. 2.10).
Fig. 2.10
Vortex pattern
Elevation-Based Patterns
The Normal Cornea
An elevation map describes the height details of the measured corneal surface by matching it with a reference surface (RS). Points above the RS are considered as elevations and expressed in plus values, while those below the RS are considered as depressions and expressed in minus values, as shown in Fig. 2.11. In corneal astigmatism, one meridian is steeper than the other and is located under the RS taking minus values, contrary to the flatter meridian which takes plus values (Fig. 2.12).
Fig. 2.11
General principle of the elevation map
Fig. 2.12
The principle of display of corneal astigmatism on the elevation map
There are three main shapes of the RS that were introduced by Belin in 1990. They are best-fit sphere (BFS), best-fit ellipse, and the best-fit toric ellipsoid (BFTE) [21, 22].
The BFS describes (qualifies) the shape of the measured surface [23], while the BFTE estimates (quantifies) the values of that surface [3].
The shape of the normal cornea is an aspheric ellipse or toric ellipsoid [3, 24]. The best RS for screening is BFS [25]. In the BFS float mode, corneal surface takes the shape of a symmetric hourglass in case of regular astigmatism. The hourglass is vertically, horizontally, or obliquely oriented in with-the-rule, against-the-rule, and oblique astigmatism, respectively. Figure 2.13 shows a vertical hourglass representing with-the-rule astigmatism [26].
Fig. 2.13
Vertical symmetric hourglass, representing with-the-rule astigmatism, as shown by the elevation map using a best-fit sphere reference surface
For routine use in refractive surgery screening, the ideal diameter of the RS is 8 mm and the ideal mode is the float mode. When viewing the maps limited to the central 9-mm area, a valid map would have no or very limited extrapolated data. Extrapolated data are marked by either black dots or white areas. In case of significant extrapolated data, the exam should be repeated [23].
The Abnormal Cornea
Shape (BFS Float Mode)
As mentioned above, the normal shape of a corneal surface with regular astigmatism is the symmetric hourglass.
Abnormal shapes include the following [26]:
- 1.
- 2.
Irregular patterns (Fig. 2.16). They are seen in abnormal distorted corneas.
Fig. 2.16
Irregular elevation map patterns
- 3.
Isolated island (Fig. 2.17, white arrow). It is encountered in normal corneas with minor astigmatism as well as abnormal distorted corneas with central or paracentral protrusion.
Fig. 2.17
Isolated island pattern
Parameters (BFTE Float Mode)
Roughly speaking, values within the 5-mm central circle > +12 μm and > +15 μm on the anterior and posterior elevation maps, respectively, are considered as abnormal [26].
According to Holladay, the correspondence of a hot spot on the tangential map with the lowest value in the relative pachymetry map and the > +15 μm on the posterior elevation map diagnoses forme fruste keratoconus (FFKC) [3].
Parameters (BFS Float Mode)
Another method for quantification is to look at the values corresponding to the thinnest location (TL) using the BFS float mode. Table 2.1 shows the normative data in myopic and hyperopic populations according to this method. Usually 3 SD parameters are used for quantification [23].
Table 2.1
Normal values of elevations corresponding to the thinnest location
Population | Elevation + SD | Anterior (μm) | Posterior (μm) |
|---|---|---|---|
Myopic | 1 SD | 3.7 | 8.3 |
2 SD | 5.7 | 13 | |
3 SD | 7.7 | 17.7 | |
Hyperopic | 1 SD | 2.1 | 16.3 |
2 SD | 4.3 | 22.1 | |
3 SD | 6.5 | 27.8 |
Cone Location (BFS Mode)
In ectatic corneal disorders, the cone can be localized by the BFS float mode [25], and can be quantified by the BFTE float mode [3]. On the BFS, the cone can be central, paracentral, or peripheral when its apex is located within the central 3-mm zone, between 3-mm and 5-mm, or outside the central 5-mm zone, respectively (Fig. 2.18) [26]. When the cone is peripheral, the elevation map shows “kissing-birds” sign as seen in Fig. 2.18, bottom right [26].
Fig. 2.18
Cone localization using the elevation map with best-fit sphere float reference surface. Upper, localization zones: the 3-mm zone and the 5-mm zone; bottom left, central cone; bottom middle, paracentral cone; bottom right, peripheral cone showing the “kissing-birds” sign
Identifying cone location is important for the treatment plan, especially when intracorneal ring segment implantation is indicated.
Pachymetry-Based Patterns
- (a)
The normal cornea: the normal pachymetry map has a concentric shape as shown in Fig. 2.19.
Fig. 2.19
The normal concentric shape of the pachymetry map
- (b)
Abnormal shapes include [20] the following:
- 1.
Horizontal displacement of the TL (Fig. 2.20).
Fig. 2.20
Horizontal displacement of the thinnest location on the pachymetry map
- 2.
Dome shape. The TL is vertically displaced (Fig. 2.21).
Fig. 2.21
Dome shape on the pachymetry map. The red arrow points at the vertically displaced thinnest location
- 3.
Bell shape. There is a thin band in the inferior part of the cornea (Fig. 2.22). It is a hallmark for PMD.
Fig. 2.22
Bell shape on the pachymetry map. This indicator of inferior band of thinning is a hallmark of PMD
- 4.
Globus. A generalized thinning reaching the limbus (Fig. 2.23).
Fig. 2.23
Globus shape on the pachymetry map. The thinning extends from limbus to limbus. This is a hallmark for keratoglobus
- 1.
Take-Home Message
Morphologically, the cone is classified based on size and shape into nipple, oval, and globus.
Identification of morphologic patterns is important, basically for contact-lens fitting.
Identification of curvature-based patterns contributes to categorizing the cornea into normal, suspect, and abnormal.
The absolute color scale should be used in the curvature map for standardization.
The normal pattern in regular astigmatism is SB; otherwise, there is a wide range of other patterns that can be seen in both normal and abnormal corneas.
The most important curvature pattern characterizing abnormality is AB/SRAX.
In elevation maps, the BFS is the best to qualify, while both BFS and BFTE can quantify the measured corneal surface.
By BFS, the cone can be localized as central, paracentral, and peripheral. This is important for some management modalities.
Based on the pachymetry map, the normal cornea has a concentric shape; otherwise, it may take a pattern of horizontal displacement, dome, bell, or globus shapes.
Classifications in Ectatic Corneal Diseases
Based on the thinning location and pattern, ectatic disorders are categorized into keratoconus (KC), pellucid marginal degeneration (PMD), and keratoglobus (KG) [1, 2].
Keratoconus
Definitions
KC has usually been defined as a chronic, bilateral, degenerative, non-inflammatory thinning disease of the cornea, characterized by progressive steepening, thinning, and apical scarring [4, 27, 28].
Recently, an additional tomographic characterization has been added to the definition of KC to be an ectatic corneal disease, characterized by abnormal posterior elevation, abnormal corneal thickness distribution, and clinical non-inflammatory corneal thinning [29].
Classifications
As of now, there is no clinically adequate classification system for KC [29].
Amsler-Krumeich Classification
The first KC classification based on disease evolution was proposed by Amsler [32, 33]. After that, Krumeich et al. [34] made some modifications to Amsler’s classification and came up with the Amsler-Krumeich grading system for KC (Table 2.2). This grading system depends on the simulated keratometric readings (Sim-K), central corneal thickness, and clinical examination in order to grade the severity of KC from 0 (the least) to 4 (the most severe stage). However, this over-20-year-old analysis is limited [29] because it does not reflect much of the more modern diagnostic measurements [35].
Table 2.2
Amsler-Krumeich classification of keratoconus
Severity | Mean central K (D) | Thickness (μm) | Spherical equivalent (D) | Cornea |
|---|---|---|---|---|
4 | >55 | <200 | Not measurable | Central scars |
3 | 53–55 | 300–400 | > −8 Myopia, induced astigmatism, or both | No central scars |
2 | <53 | 401–500 | [−5, −8] Myopia, induced astigmatism, or both | No central scars |
1 | <48 Eccentric steepening | >500 | < −5 Myopia, induced astigmatism, or both | No central scars |
Alio-Shabayek Modification
Alio and Shabayek added another modification to Amsler-Krumeich grading system, that is, corneal high-order aberrations, especially coma-like aberrations (Table 2.3) [36].
Table 2.3
Alio-Shabayek classification of keratoconus
Stage | Mean central K (D) | Thickness (μm) | Spherical equivalent (D) | RMS of coma-like aberration (μm) | Cornea |
|---|---|---|---|---|---|
4 | >55 | <200 | Not measurable | >4.5 | Central scars |
3 | >53 to ≤55 | 300–400 | > −8 Myopia, induced astigmatism, or both | >3.5 to ≤4.5 | No central scars |
2 | >48 to ≤53 | 401–500 | [−5, −8] Myopia, induced astigmatism, or both | >2.5 to ≤3.5 | No central scars |
1 | ≤48 Eccentric steepening | >500 | < −5 Myopia, induced astigmatism, or both | 1.5–2.5 | No central scars |
Ishii et al. Modification
Ishii et al. [37] described a new classification based on Amsler-Krumeich grading system. In their classification, they integrated visual acuity, minimum radius of curvature of the anterior corneal surface, and six indices, namely, ISV, index of surface variance, which describes corneal surface irregularity; IVA, index of vertical asymmetry, which describes curvature symmetry; KI, keratoconus index, which also describes curvature symmetry; CKI, center keratoconus index, which describes the severity of central KC; IHA, index of height asymmetry, which is similar to IVA but based on corneal elevation, and is thus more sensitive; and IHD, index of height decentration, which describes the decentration in elevation data in the vertical direction. Table 2.4 represents the classification and is adapted from the Wavelight® Allegro Oculyzer™ User Manual [38].
Table 2.4
Classification stages of keratoconus adapted from the classical Amsler-Krumeich standards
CDVA
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