KEY CONCEPTS
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Placido-based corneal topography and Scheimpflug anterior segment optical tomography are long-established valuable technologies widely used in clinical practice and refractive surgery screening protocols.
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With the combination and integration of Placido topography and dual Scheimpflug tomography, the Galilei and the CGA color scales (by Carlos G. Arce, 2003) have facilitated the interpretation of data on maps independent of the parameter studied and without forcing the verification of numeric values.
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This chapter demonstrates the use of the Galilei system to detail the four primary signs that identify keratoconus: ectasia, steepening, asymmetry, and thinning. They are not synonyms. There are not keratoconus without ectasia however, it may be steepening, asymmetry or thinning, isolated or in combination, without ectasia.
Introduction
Keratoconus (KC) is a chronic time-dependent deformation of the cornea caused by the biomechanical failure of the corneal tissue induced or promoted by a congenital predisposition, and may be triggered by iatrogenic, surgical, traumatic, inflammatory, environmental, or unknown factors.
Our current understanding of keratectasia has been molded by the available technology. New signs in incipient cases have been discovered since the corneal profiles of Mandell and the computer-assisted topography of the anterior corneal surface by Klyce. We recognize Placido-based corneal topography (by Antonio Placido, 1880) as a long-established valuable technology widely used in clinical practice and refractive surgery screening protocols, although it may be inadequate for early diagnosis. Modern equipment, such as slit-scan and rotational Scheimpflug tomography, have increased our knowledge of concepts such as elevation topography, wavefront, and optical pachymetry. Furthermore, despite their lesser ability to detect surface curvature differences, anterior segment imaging systems based in Scheimpflug photography have introduced diagnostic strategies not possible with the assessment of the anterior corneal surface alone.
With the combination and integration of Placido topography and dual Scheimpflug tomography, the Galilei (Ziemer Ophthalmic Systems AG, Port, Switzerland) and the CGA color scales (Carlos G. Arce, 2003) have facilitated the interpretation of data on maps independent of the parameter studied and without forcing the verification of numeric values. The CGA scales are, basically, comprehensive absolute scales with the yellow step(s) fixed in accepted normal limit values codifying the meaning of all colors. The suggested settings for all maps are summarized in Table 16.1 . New Galilei reports and combinations of maps using posterior curvature, best fit toric aspheric (BFTA) elevation, total corneal power, and wavefront maps go beyond the traditional and insufficient four-map presentation with anterior curvature, anterior and posterior best-fit-sphere (BFS), and pachymetry ( Fig. 16.1 A). , Special windows, such as the refractive (see Fig. 16.1 B) and asymmetry (see Fig. 16.1 C) displays, , the cone location and magnitude index X (CLMI.X; Fig. 16.2 ), the percentage tissue altered (PTA; see Fig. 16.2 ), and the thickness progression ( Fig. 16.3 ) report, have been developed, improving our diagnostic capability with suspicious or borderline cases prior to refractive surgery.
The characteristics of the three types of primary keratectasia—KC, pellucid marginal degeneration (PMD), and keratoglobus—have been described elsewhere using the Galilei. More recently, we identified the differences between secondary acquired keratectasia after the cornea, predisposed or not, was weakened by incisions, scars, or tissue ablation. , We also reported the detection of at least six types of corneas with KC signs that may appear before ectasia. They have been usually confused with corneas with KC despite seem to be in a pre-KC stage because do not have ectasia. In this chapter, we will demonstrate the use of the Galilei system to detail the four primary signs of KC: ectasia, steepening, asymmetry, and thinning.
Tomographic Characteristics of Ectasia
KC progresses slowly from an apparent normal but predisposed susceptible eye. Ectasia is an abnormal forward protrusion of either of the two surfaces of the cornea and is the most recognized topo-tomographic sign of the biomechanical failure in KC. Nevertheless, ectasia, steepening, and KC are not synonymous.
Using the Galilei system, ectasia is typically identified as a central or paracentral well-delimited zone with the steepest anterior keratometry reading (K max ) greater than 48 diopters (D) and/or posterior K max steeper than –7.0 D, and with the anterior most-elevated best-fit-sphere point (BFS max ) higher than 12 µm and/or a posterior BFS max of 17 µm. In the anterior tangential curvature maps, the zone of ectasia is observed as an orange-to-red central area surrounded by a yellow-green ring of variable width with normal values (Brazilian flag sign) usually located between 6- and 9-mm diameter ( Fig. 16.4 ). , In the BFS maps, the protrusion is characteristically shown by a circular or semi-circular pattern with concentric color steps, from the normal green to the more elevated yellow, that we call ‘fried-egg” pattern.
ECTASIA AND STEEPENING OF K MAX
A recent controversial multicenter study did not consider the steepening of surfaces as mandatory for KC diagnosis. , However, although steepening is a classic indicator of KC progression, a keratometry value (K) alone is not the best way to discover early KC, because of its low sensitivity. In suspected cases, a simulated keratometry (SimK) greater than 45.43 D and a posterior mean K steeper than –6.41 D had 46.51% and 67.44% sensitivity and 96.19% and 73.33% specificity, respectively. In cases with established KC, a SimK greater than 45.43 D and a posterior mean K steeper than –6.62 D had 59.3% and 67.4% sensitivity, and 96.2% and 91.14% specificity, respectively.
Although we believe K max is a better parameter than the average K, we conclude that a single steep K value is insufficient for the diagnosis of KC, when ectasia is not demonstrated as shown by symmetric steeper toric, not hyperprolate, corneas ( Figs. 16.1 and 16.5 ), or by asymmetric corneas with a normal range of K max (see Fig. 16.5 ). In these cases, the biomechanical failure and forces molding the cornea were not sufficient to cause central asymmetry and/or focal bulging. We, therefore, classified them at pre-KC stages. ,
ECTASIA, INCREASED ASPHERICITY, AND TOTAL CORNEAL SPHERICAL ABERRATION
The asphericity (Q factor) and the e 2 (squared eccentricity) of surfaces are particularly useful for identifying corneas with ectasia. Basically, both indices represent the relationship between the central and the peripheral curvature. If the center is flatter than the periphery, the surface is aspherical oblate. If the center is steeper, it is aspherical prolate. Q factor and e 2 also represent conical sections. If center and periphery have the same curvature, the surface is truly spherical (e 2 = –Q = 0) or is spherical on average with prolate and oblate hemimeridians that compensate one another. An e 2 = ±1.0 indicates a parabolic shape. If 0 < e 2 < 1.0, or 0 > e 2 > –1.0, the surface is elliptical prolate or oblate, respectively, and if e 2 > 1.0 or e 2 < –1.0, it has a hyperbolic prolate or oblate profile, respectively ( Fig. 16.6 ).
Currently, a corneal surface would be considered abnormal and hyperprolate if it has e 2 ≥ 0.8, especially if associated with a negative total corneal spherical aberration in microns. When either or both faces reach e 2 = 1.0, such a cornea has KC until otherwise proven. However, there are asymmetric and misshapen corneas with sinusoid profiles (such as initial PMD or advanced KC, depressed superiorly and protruding inferiorly) that may have e 2 < 1.0.
In the Galilei system, the e 2 and Q factor are calculated from the central 8-mm diameter aligned by default with the first Purkinje image but can be measured centered to the pupil. They are not necessarily interchangeable with other devices as these values depend on the alignment of data and the size of the measured area, especially in asymmetric corneas. Although the posterior surface is usually more prolate than the anterior, when analyzed separately, the anterior Q factor had slightly better specificity and sensitivity for corneas diagnosed with confirmed KC ( Fig. 16.7 ). An increased e 2 for both surfaces correlated directly with the loss of visual quality in eyes with KC, directly with the steepening of the anterior surface, inversely with the steepening of the posterior surface ( Fig. 16.8 , top), and inversely with the spherical aberration (see Fig.16.8 , middle). ,
The total corneal spherical aberration depicts the wavefront profile of light rays focusing after crossing the cornea. Spherical aberration may be expressed in microns or in diopters, and zero spherical aberration means that the circle of least confusion is so small that all light rays focus on a common point. Positive spherical aberration in microns (negative in diopters) occurs when the cornea has greater power at the periphery, whereas negative spherical aberration in microns (positive in diopters) occurs when there is more power at the center.
When the anterior corneal surface is spherical, the total corneal spherical aberration (measured from 6-mm diameter centralized to the pupil) is between 0.25 and 0.30 µm. In oblate anterior surfaces, such as after radial keratotomy (RK) or myopic LASIK, it tends to be more positive in microns, while it is close to zero when the anterior surface is prolate and ideally aspheric (e 2 = –Q ≈ 0.60). However, most corneas with a hyperprolated anterior surface and e 2 = –Q ≥ 0.80 already have a negative total corneal spherical aberration in microns. , Some corneas with KC and a sinusoid shape, however, may have spherical aberration close to zero or even positive in microns. ,
ECTASIA AND HEIGHT OF CENTRAL PROTRUSION ASSESSED FROM BEST-FIT-SPHERE MAPS
The BFS is the curvature average of each surface calculated within a specific diameter. With this value, the system generates a spherical uniform reference surface that is compared with each corneal face. The values that appear in BFS maps correspond to their difference, point-to-point in microns. Currently, the anterior and posterior BFS shown in most devices is set from a central 8-mm diameter, as it was originally in the Galilei. Using the (American National Standards Institute), ANSI-CGA 5-µm scale, Galilei BFS maps of normal with-the-rule (WTR) corneas have a green horizontal band (±5 µm) with few or nonyellow steps within the 4-mm diameter central region (gray circle on maps). The BFS max is marked by a small dark dot ( Fig. 16.9 A). We use 10 to 12 µm (first yellow) as the cutoff value for the front surface and 15 to 17 µm (second yellow) for the posterior. , , , When the cornea has a lump-like zone of ectasia, the Galilei maps typically have concentric yellow circles or semicircles forming patterns called yellow tongue (see Fig. 16.9 B), fried egg, peninsula (see Fig. 16.9 C), or island ( Fig. 16.10 A, and 16.10 B). ,
