Dr Agarwals’ Textbook on Corneal Topography (Including Pentacam and Anterior Segment OCT) Amar Agarwal, Athiya Agarwal, Soosan Jacob
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1Introduction to Corneal Topography and Orbscan
  • Fundamentals on Corneal Topography
  • Topographic Machines
  • Corneal Topography and the Orbscan
  • The Orbscan IIZ Diagnostic System and Zywave Wavefront Analysis
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FUNDAMENTALS ON CORNEAL TOPOGRAPHYChapter One

Guillermo L Simón-Castellvi
Spain
Sarabel Simón-Castellvi
Spain
José Maria Simón-Castellvi
Spain
Cristina Simón-Castellvi
Spain
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INTRODUCTION: HUMAN OPTICS AND THE NORMAL CORNEA
The cornea is the highest diopter of human eye, accounting alone for about 43–44 diopters at corneal apex (about two thirds of the total dioptric power of the eye). It has an average radius of curvature of 7.8 mm. A healthy cornea is not absolutely transparent: It scatters almost 10% of the incident light, primarily due to the scattering at the stroma.
The corneal geography can be divided into four geographical zones from apex to limbus, which can be easily differentiated in color corneal videokeratoscopy:
  1. The central zone (4 central millimeters): It overlies the pupil and is responsible for the high definition vision. The central part is almost spherical and called apex.
  2. The paracentral zone: Where the cornea begins to flatten
  3. The peripheral zone
  4. The limbal zone
Refractive surgery refers to a surgical or laser procedure performed on the cornea, to alter its refractive power. The major refractive component of the cornea being its front surface, it is not difficult to understand that most refractive techniques have involved this frontal surface (PRK, radial keratotomies, …). Nevertheless, posterior surface of the cornea also accounts, and that is the reason why a “posterior surface corneal topographer” like the Orbscan™ - Bauschand Lomb© was developed by Orbtek©, in the race for a more precise refractive surgery.
The cornea of an eagle is almost as transparent as glass: There is almost no scattering of incident light. That alone explains the resolution of an eagle eye being much better than ours. As we are never satisfied, we are now developing new tools and extremely promising laser surgical techniques that have proven to increase human being visual acuity by reducing corneal aberrations: we reduce diopters and also improve visual acuity. The new dream is “supervision”. Topographic and aberrometer-linked LASIK are on the way to achieve this goal of better-than-normal vision. Bausch and Lomb® ̀s Zywave™ combines topography and wavefront measurements to achieve customized computer controlled flying spot excimer laser ablation, which appears to be fundamental in treating irregular astigmatisms or retreating unsatisfied LASIK patients to regularize the corneal shape. Regularizing the corneal shape has the theoretical advantage of improving the quality of vision by means of reduction of halos, glare and any other optical aberrations. We are on the way to achieve an aberration-free visual system, though the influence of all other dioptric surfaces (vitreous, lens, …) and interfaces still has to be ascertained (Table 1.1).
In this chapter we will try to introduce the novice to this interesting new world of instruments recently developed due to the advent of refractive corneal surgery. We have tried to show different maps from different systems, trying to make an interesting basic atlas of corneal topography.
Table 1.1   Indications and uses of corneal topographers
The use of computerized corneal topography is indicated in the following conditions:
  1. Preoperative and postoperative assessment of the refractive patient
  2. Preoperative and postoperative assessment of penetrating keratoplasty
  3. Irregular astigmatism
  4. Corneal dystrophies, bullous keratopathy
  5. Keratoconus (diagnostic and follow-up)
  6. Follow-up of corneal ulceration or abscess (Figure 1.1)
  7. Post-traumatic corneal scarring
  8. Contact lens fitting
  9. Evaluation of tear film quality
  10. Reference instrument for IOL-implants to see the corneal difference before and after surgery
  11. To study unexplained low visual acuity after any surgical procedure (trabeculectomy, extracapsular lens extraction, …).
  12. Preoperative and postoperative assessment of Intacs™ corneal rings (intrastromal corneal rings)
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Corneal maps of rare cases and complications can be found in the different chapters of this book. Please refer to them for better knowledge. There is no perfect system to assess true corneal surface shape, but we still have to rely on the instruments we have, waiting for new instruments and methods being developed for better accuracy. With that goal in mind BioShape AG® has developed the EyeShape™ system, based on a principle called fringe projection. Patterns of parallel lines are first imaged onto a reference and then onto the surface to be measured. Detection of the lines with a digital camera under a tilted angle yields distorted line patterns. The deviation of the detected lines from the original lines together with the tilt make it possible to calculate the absolute height at any point on the surface of the cornea (or not).
 
INSTRUMENTS TO MEASURE THE CORNEAL SURFACE
The normal corneal surface is smooth: A healthy tear film neutralizes corneal irregularities. The cornea, acting as a convex “almost transparent” mirror, reflects part of the incident light. Different instruments have been developed to assess and measure this corneal reflex. These non contact instruments use a light target (lamp, mires, Placido disks, etc.) and a microscope or another optic system to measure corneal reflex of these light targets (Table 1.2 and 1.3).
Table 1.2   Advantages and disadvantages of projection-based systems over reflection-based ones
Advantages
  • Measurement of direct corneal height
  • Ability to measure:
    • irregular corneal surfaces
    • non-reflective surfaces
  • Higher resolution (theoretical)
  • Uniform accuracy across the whole cornea
  • Less operator dependent
  • Do not suffer from spherical bias
Disadvantages
  • Not standard instruments (most are still prototypes):
    • complex to use
    • need clinical experience validation
    • nonstandard presentation maps (more difficult to learn)
  • Longer examination time:
    • longer image acquisition time
    • longer image analysis
  • Fluorescein instillation needed (in some, like the Euclid Systems Corporation® ET-800™)
Table 1.3   Different methods of measuring corneal surface used by modern corneal topographers
  • Placido systems (small cone or large disk) are the most popular
  • Placido cone with arc-step mapping (Keratron™ from Optikon 2000®)
  • Placido's disk with arc-step mapping (Zeiss Humphrey® Atlas™)
  • Slit-lamp topo-pachymetry (Orbscan™ - Bausch and Lomb®)
  • Fourier profilometry (Euclid Systems Corporation® ET-800™)
  • Fringe projection or Moiré interference fringes (EyeShape® from BioShape AG™)
  • Triangulation ellipsoid topometry (Technomed™ color ellipsoid topometer)
  • Laser interferometry (experimental method, it records the interference pattern generated on the corneal surface by the interference of two lasers or coherent wave fronts)
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Keratometry
A keratometer quantitatively measures the radius of curvature of different corneal zones of 3 mm (diameter). The present day keratometer allows the operator to precisely measure the size of the reflected image, converting the image size to corneal radius using a mathematical relation r = 2 a Y/y, where,
  • r : anterior corneal radius
  • a : distance from mire to cornea (75 mm in keratometer)
  • Y : image size
  • y : mire size (64 mm in keratometer)
The keratometer can convert from corneal radius r (measured in meters) into refracting power RP (in Diopters) using the relationship:
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Modern—automated or not-keratometers also known as ophthalmometers directly convert from radius to diopters and inversely. They are mainly used to calculate the power of intraocular lenses through different formulas (Hoffer, SRK-T, SRK-II, Holladay, Enrique del Rio and S Simón, …). Although the theory of measuring corneal reflex may appear to be simple, it is not, since eye movement, decentration or any tear film deficiency may make it difficult to measure accurately, thus creating errors. Modern video methods (topographers) can freeze the reflected cornea image, and perform the measurements once the image is captured on the video or computer screen, allowing greater precision. Notice that most traditional keratometers perform measurements of the central 3 mm, while computerized topographers can cover almost the whole corneal surface.
 
Keratoscopy or Photokeratoscopy
It is a method to evaluate qualitatively the reflected light on the corneal surface. The projected light may be a simple flash lamp or a Placido's disc target, which is a series of concentric rings (10 or 12 rings) or a tube (cone) with illuminated rings lining the inside surface. When we look at the keratoscope, an elliptical distortion of mires suggest astigmatism, and small, narrow and closely spaced mires suggest corneas that have high power (steep regions or short radius of curvature) (Figure 1.1).
The use of keratoscopes is being abandoned in favor of computerized modern topographers which allow qualitative and quantitative measurements of the corneal surface, with higher definition and accuracy (more than 20 rings), and more sensitivity in the peripheral cornea.
Some of the known deficiencies of the Placido's method are:
  • It requires assumptions about the corneal shape
  • It misses data on the central cornea (not all topographers)
  • It is only able to acquire limited data points
  • It measures slope not height
Some more subjective complaints include:
  • It is difficult to focus and align
  • In most topographers, the patient is exposed to high light
Large Placido's disk systems work far away from the eye, while small Placido cones get much closer to the eye. While Placido's disk systems easily create shadows caused by the nose and brow blocking the light of the rings, small cone systems fit under the brow and beside the nose, avoiding shadows, but can get in contact with large noses and make the patient blink and be afraid. Most small cones have a reputation for difficult focusing: Some manufacturers-like Optikon 2000®-have worked out worthwhile automatic capture devices for improved accuracy, precision, and repeatability of measurements.
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Figure 1.1: The “ring verification display” in modern videokeratoscopes is a static picture of what the explorer viewed at the keratoscope. Looking at the keratoscope, the explorer is able to evaluate qualitatively the corneal surface. In this case, notice the huge distortion of the mires on the temporal side of a right eye of a patient who underwent a keratoplasty for a keratoconus, and is wearing a soft plano-T therapeutic contact lens. The distortion of the mires is due to an irregularity at contact lens surface: air is in between the cornea and the lens
 
Computerized Videokeratoscopy: Modern Corneal Topographers
Corneal topography has gained wide acceptance as a clinical examination procedure with the advent of modern laser refractive surgery. It has many advantages over traditional keratometers or keratoscopes: They measure a grater area of the cornea with a much higher number of points and produce permanent records that can be used for follow-up.
Basically, a projection corneal topographer consists of a Placido's disk or cone (large or small) that illuminates the cornea by sending a mire of concentric rings, a video camera that captures the corneal reflex from the tear layer and a computer and software that perform the analysis of the data trough different computer algorithms. The computer evaluates the distance between a series of concentric rings of light and darkness in a variable number of points. The shorter the distance, the higher the corneal power, and inversely. Final results can be printed in colors or black-and-white.
The Placido's disk (Figure 1.2) consists of a series of concentric dark and light rings in the configuration of a disk or a cone, of different sizes, depending on the number of rings and the manufacturer. Usually, it is better to have a large number of rings, since more corneal radius values can be measured. The mires of most systemsexclude the very central cornea and the paralimbal area.
The reproducibility of videokeratography measurements is mainly dependent on the accuracy of manual adjustment in the focal plane. Videokeratoscopes having small Placido cones show a considerable amount of error when the required working distance between cornea and keratoscope is not maintained. The advantages of small cones (optimal illumination and the reduction of anatomically caused shadows) are in no proportion to the disadvantage—poor depth of focus, resulting in poor reproducibility. Which one should you choose, a small Placido cone or large Placido's disk?
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Figure 1.2: The Placido cone consists of a series of concentric dark and light rings in the configuration of a cone of different sizes depending on the number of rings and the manufacturer. Usually, it is better to have a large number of rings, since more corneal radius values can be measured: Notice that while describing the technical characteristics of videokeratographers some manufactures count both clear and dark rings, while others only count light ones. The mires of most systems exclude the very central cornea (where the video camera or CCD is located) and the paralimbal area. Picture shows a large cone of the Haag-Streit® Keratograph CTK 922™ with 22 rings (dark and light rings).(Published with permission from Haag-Streit® AG International)
Not easy to answer: Each family of topographers has advantages and disadvantages. Being no ideal instrument, topographer potential buyers will have to decide upon other important factors, like software ability to exactly reproduce real corneal height, number of rings, price, ….
There are two main groups of corneal topographers: Those which use the principle of reflection (most), and those which use the principle of projection.
Let's notice that the image captured by most topographers is produced by the thin tear layer covering the cornea that almost reproduces the shape or contour of the corneal surface. Most instruments perform indirect measurements of the corneal surface (reflection technique) and extrapolate to know the height of each point of the cornea. Reflection techniques amplify the corneal topographic distortions (Figure 1.3).
Euclid Systems Corporation® ET-800 uses a completely different method of topography called Fourier profilometry using filtered blue light that induces fluorescence of a liquid that has been applied to the tear film before the examination. This projection technique visualizes the surface directly while a reflection technique amplifies the corneal topographic distortions.
 
Causes of Artefacts of the Corneal Topography Map
  1. Shadows on the cornea from large eye-lashes or trichiasis(Figure 1.4)
  2. Ptosis or non-sufficient eye opening (Figure 1.5)
  3. Irregularities of the tear film layer (dry eye, mucinous film, greasy film) (Figure 1.6)
  4. too short working distance of the small Placido disk cone
  5. Incomplete or distorted image (corneal pathology) (Figure 1.7 and Figure 1.8)
 
Understanding and Reading Corneal Topography
The meaningful interpretation of topographic maps requires the examiner to have detailed knowledge and clinical experience on the patterns detailed in them. At first, one must understand how to read the color scales. The untrained eye may find some confusion and sometimes misinterpretation in evaluating corneal maps. Modern topographers (videokeratographers) use the Louisiana State University Color-Coded Map to display corneal superficial powers.
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Figure 1.3: There are different methods of following the clinical course of a corneal ulceration or corneal abscess. While daily slit-lamp examination and daily photographs are invaluable, corneal topographic maps, being less “explorer dependant”, can also be very useful in the follow-up.(Courtesy: Dr Agarwal`s Eye Hospital)
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Figure 1.4: With large Placido' disk topographers, large eyelashes project shadows on the superior cornea: The topographer will be unable to accurately perform the map of that zone. Danger is that extrapolation performed by some systems distorts the true map of the paracentral cornea. Trichiatic cilia projects a shadow that may interfere with the mapping. This situation should be addressed prior to corneal topography
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Figure 1.5: Ptosis or non-sufficient eye opening because of induced photophobia or patient anxiety limits and distorts the mapping of the cornea. Notice that the map is not round but oval
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Figure 1.6: An advanced corneal herpetic keratopathy produces an irregular completely distorted corneal map in which no regular pattern can be identified. Notice that the low-vision patient is unable fixate the fixation light
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Figures 1.7 and 1.8: These two maps may look different but are the same axial diopter map of the left eye of the same patient (keratoconus) measured in different scales, absolute on the left and relative on the right. Notice very high diopter values under corneal vertex, where corneal surface is most elevated
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The power values (measured in diopters) are preferred by clinicians over the radius values (measured in millimeters), although all topographers can map the corneas using both values.
Projection-based topography systems, adopted a similar color scale to represent their height maps. High areas are depicted by warm colors, while low areas are depicted by cool colors.
Facing a corneal topography, care has to be taken to interpret colored maps, since scales (and sometimes color coding) can be modified in most topographers´ software. For patient examination manufacturer sets default values which are operator adjustable (diopter interval, radius interval). When operator adjusts the values to new parameters, color scales are modified.
Rare are the topographers that directly measure the corneal elevation: Most act by extrapolation from corneal curvature and power at each measured point. The Optikon 2000® Keratron™ is one of those systems that accurately maps aspheric surfacesby means of its own method of arc-step mapping.
The range of powers found in the normal cornea range from 39 D found at peripheral cornea, close to the limbus, to 48 D found at corneal apex.
The colors do not always represent an elevation map, they correspond to curvature values. Therefore, the cornea is most curved towards the center (green) and flattens out towards the periphery (blue). The nasal side becomes blue more quickly, indicating that the nasal cornea is flatter than the temporal. Some advanced instruments like the Optikon 2000® Keratron™, are able to directly represent a colored elevation map.
Apart from color maps, most topographers also display values of simulated keratometry, that should be equivalent to those obtained by a keratometer. Simulated keratometry values are obtained form the radius values at the corneal position (3 central millimeters) where the reflection from the keratometer mires would take place.
 
Topographic Scales
Two basic scales are commonly used: Absolute and relative.
 
Absolute, Standardized or International Standard Scale
Same scale for every map produced. Good for direct comparisons between different maps, for screening and for gross pathologies. It was designed to make only clinically relevant information obvious, by setting the interval between the contours of the power plot (i.e. in practice, the contours of colors) at 1.5 diopters (which means it has low resolution).
 
Relative, Normalized or Adaptative Color Scale
Different scale for each map. The computer determines maximum and minimum curvatures for the map and automatically distributes the range of colors. The computer contractsor expands its color range according to 13the range of colors present in a given cornea. It is best suited for looking at variations for a particular cornea. It has the advantage of offering great topographic detail since incremental steps are smaller (around 0.8 diopters) giving high resolution, but suffers from some inconveniences: the meanings of colors are lost (explorer and clinician have to carefully check the meaning of the colors, according to the new scale), a normal cornea may look abnormal while abnormal corneas may appear closer to normal. With this scale, subtle features are made apparent, being good for detail.
 
Computer Displays: Presentation of Topographic Information
When confronted to a topography display, either a printed report or on screen, one should study it in a structured way to avoid mistakes in interpretation, and get the most of it.
Proceed as follows:
  • Check the name of the patient, date of exam and examined eye.
  • Check the scale:
    • Type of measurement (height in microns, curvature in mm, power in diopters)
    • Step interval
  • Study the map (type of map, form of abnormalities, …)
  • Evaluate statistical information (cursor box, statistical indices when given …)
  • Compare with topography of the other eye (always perform bilateral exams, when possible)
  • Compare with the previous maps first verifying they are in the same scale)
  • Apply statistical analysis or other needed software application (contact lens fit, surgical modules, 3-D color maps, neural networks, …)
  • Explain the exam´s results to the patient
To present a corneal topography, each software application (i.e. each instrument) has a large number of computer displays. Most are produced form data of a single application, and are software dependent. Most instruments are able to show: A ring verification, a numerical display, a large number of corneal maps, a simulated keratometry, a meridional plot, and some can display a 3-D reconstruction of the corneal surface.
  1. Ring verification (keratoscopic raw image) (Figure 1.1): Displays a keratoscopic image of the Placido rings reflex on the examined cornea. It is a raw image, that allows qualitative evaluation of the image taken (irregularity of tear film layer, lids aperture, …), helping the examiner to either accept or reject the taken image. It is very useful when there is a question regarding the accuracy of the displayed data.
  2. Numeric display: Of a number of corneal power values along several meridians shown in a radial display. Helpful to make the data amenable to statistical methods (Figure 1.9 and Figure 1.10).
    1. Corneal maps: Details of the most common (axial, tangential, 3-D, …) will be discussed later in this chapter. Each topographer offers different maps or ways of presenting the results. Please refer to your topographer´s manual for more details.
    2. Simulated keratometry readings (SimK): obtained form the radius values at the corneal position (3 millimeters central zone) where, the reflection from the keratometer mires would take place. The major axis is that with the greatest power, and the minor axis is at 90° to it (perpendicular axis). The cylinder is the difference between the major and minor axis. The meridian with the lowest mean power can also be displayed.
    3. Meridional plot: shows the minimum and the maximum corneal power values, displaying a cross-sectional profile of the cornea along the chosen meridian. It is used to show the general shape of the cornea to the patient, and assessing the toricity for contact lens adaptation. The helps identifying the ablation zone limits following LASIK or PRK.
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Figures 1.9 and 1.10: The numeric display shows a number of corneal power values along several meridians in a radial display. It is a very helpful presentation to make the data amenable to statistical methods. Notice that picture on the left (Axial Diopter) displays corneal powers in diopters and that on the right (Axial Radius) shows the same values in millimeters (corneal radius). Most topographers allow you to choose the way you want the results to be shown
 
Common Corneal Maps
  1. Axial map: It is the original and most commonly used map. It provides measurements based on the keratometer formula. It is helpful is evaluating the overall characteristics of the cornea and classify the corneal map (normal or abnormal). It can differentiate between spherical, astigmatic or irregular corneas. It is the most stable type of map, but may confuse the explorer when evaluating the peripheral cornea (Figure 1.11).
  2. Height map: True height data (in microns) is immediately available from systems using the principle of projection, although a reflection system like the Optikon 2000® Keratron™ does a good job with its own arc-step method of representing corneal height. Very useful in numeric or cross-sectional format to quantify the elevation or the depth of a corneal defect (ulceration, laser ablation zone, keratoconus, …). Some topographers display the spherical height map relative to a reference spherical surface, by comparing to a best fit calculated reference sphere.
  3. Tangential map (Figure 1.12 and Figure 1.13): This very useful display provides a measurement of corneal power over a large portion of the cornea, based on a mathematical radius formula. It is more accurate than axial map in the corneal periphery, but is subject to greater variation when comparing several exams that are repeated. It may help detecting mild corneal changes that might not be detected by standard axial map. It is used for locating corneal distances on the map, and to locate a cone or peak position in keratoconus, as well as to locate the ablation diameter and position after laser refractive surgical ablation.
  4. Refractive map: It is a map based on an axial map, using Snell´s law to calculate the refractive power of the cornea. It is mainly used in pre and post corneal surgery.
  5. Elliptical elevation map: It represents the height of the cornea in microns, at different corneal positions, relative to a reference elliptical surface. It is useful to visualize corneal shape. In contrast to the spherical height map-which uses a simple spherical reference- the elliptical elevation map matches better to the inherently elliptical shape of the healthy cornea.
  6. 3-D reconstruction map is used to visualize the overall shape of the cornea in a more realistic way. Understandable for the patient, it can be rotated and tilted as desired. Some instruments like Oculus® Keratograph and Haag-Streit® Keratograph CTK 922 offer excellent comprehensive kinetic three-dimensional (3-D) analysis of corneal topography for simple explanation to the patient (Figure 1.14 and Figure 1.15).
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    Figure 1.11: Axial diopter displays are showed for both right and left eyes. The patient suffered from regular astigmatism (with-the-rule), that gives an oval corneal map, being the most common deviation from optically perfect spherical (round) cornea. The long axis is near the vertical meridian. The shape and colors of the bow tie are influenced by the rate of peripheral corneal flattening: notice the nasal peripheral flattening in left eye (purple color). This binocular report form Dicon´s CT-200 topographer shows pupil size and simulated keratometry of both eyes. RE size pupil is 4.03 mm, and astigmatism 3.12 D at 8°. Notice that the two eyes present a mirror image of each other: This phenomenon is called enantiomorphism
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    Figure 1.12: A “multiple exams” of both eyes of the same patient, a 38-year-old man who underwent LASIK in both eyes at a time for high myopia. Corneal map is overlaid upon the keratoscope eye image to aid interpretation. The overlay shows the spatial relationship between the pupil, the ablation zone and the cornea. Notice that immediately after surgery (the day after), ablation zones differ from each other: it is due to the fact that a different excimer laser was used for each eye. Schwind® Keratom™ was used on right eye, while left eye was operated using the Bausch and Lomb® -Chiron Technolas 217™. Although ablation zone seems more perfect and regular on right eye (tangential diopter map), this does not mean that visual result is better. The meridional plots shown under tangential diopter maps help the surgeon to evaluate the effectiveness and ablation pattern of the excimer laser he or she uses
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    Figure 1.13: Shows a “multiple exams view” of left eye of the same patient, a 58-year-old woman who underwent (a couple of years before consultation) complicated phacoemulsification converted to extracapsular surgery. In the hurry, surgeon sutured the cornea too loose, thus creating a peripheral superior corneal wound defect. High against-the-rule astigmatism is well represented by the axial diopter display (superior right), and well measured by the keratometer display (5.25 D at 87°). But only tangential diopter map (down-right) accurately represented the corneal wound suture defect: notice the red superior area where the sutures used to be
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    Figures 1.14 and 1.15: 3-D reconstruction of orbscan map(Courtesy: Dr Agarwal's Eye Hospital)
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  7. Irregularity map: It calculates a best sphere/cylinder correction for the cornea, subtracting the correction from either axial or tangential data and presents the remaining irregularities. Used after refractive surgery to detectirregularities that may explain a low visual acuity. It reports an index that measures eccentricity (a measure of asphericity) and the amount of astigmatism that has been subtracted form the original corneal data.
Table 1.4   Common overlays that can be added to a topography map to help interpretation
Pupil margin: Displays the visually important region. Helps evaluating photopic pupillary size, and the centration or refractive surgery.
Grids square: Helps defining size and location of abnormalities.
Circular: helps defining size and location of abnormalities.
Polar: Helps defining axis of abnormalities and the assessment of radial keratotomies.
Optical zone: Useful in refractive surgery for planning procedures or assessing results.
Angular scale: Useful in refractive surgery of astigmatism for planning procedures or assessing results. It is similar in use to polar grid (Figure 1.16).
Eye image: More realistic than a simple map, it eases patient' interpretation of the map.
Keratoconus: A peak or keratoconus overlay can be applied by Dicon´s CT-200. It is called Bull´s Eye target: If one peak area exists with an index of 10 or greater, the system automatically marks it with a target, indicating the location of this elevation to some but not all maps (see Figure 1.17 and Figure 1.18).
Keratometer mires: It is a graphic reference showing a 3 mm circle with both major and minor meridians, representing the calculated keratometry readings, 90 degrees apart (perpendicular). It also shows a 5 mm with the steepest and flattest meridians (see Figure 1.16).
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Figure 1.16: Shows a “multiple exams view” of both eyes of the same patient, a young man referred for refractive surgery who -to our surprise- was never diagnosed astigmatic. Axial diopter maps are displayed, in normalized (right eye) and absolute scales (left eye). Elliptical elevation with keratometer overlay maps help better assess true corneal shape and direction or axis of astigmatism. Radius of the reference ellipse are displayed and can be modified by operator: BaseR refers to central radius value, and BaseR (2.5 mm) refers to the radius value at 2.5 mm
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Figure 1.17: Shows an “axial irregularity map” in diopters of the right eye of a 55-year-old man suffering form a paracentral progressive corneal ectasia (central keratoconus). Notice the Q index with a value of -1,25 (measuring eccentricity) and an astigmatism of 4.5 D, resulting form the subtraction of the original corneal data and the best sphere/cylinder for that cornea. An overlay option adds an irregularity index to the map for increasing circles of 1 mm radius, best visualized thanks to the overlay circular grid option. Normal values would be 0.2 or 0.4, but this exceptional case shows 3.5 and 4.0 zonal indices
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Figure 1.18: Different overlays can be added to a topography to help interpretation. The Figure shows a quadruple view of an almost normal cornea of a young contact lens user with mild corneal warpage only diagnosed by means of the tangential maps (c and d). Notice that (b) is displayed in radius (mm) while the rest of maps are displayed in diopters (see the color scale). Map (a) displays a center overlay (small red cross) that indicates where the true center of the cornea is, and a pupil outline overlay that reproduces pupil margin, the visually important region. Map (b) shows a “verify rings” overlay, to better asses the quality of the taken image. Red and green concentric rings should alternate and not cross. The red rings should be located on the outer edge of the white rings, and the green rings should be located on the outer edge of the black rings. Map (c) shows an angular scale that helps to locate the axis of astigmatism. Map (d) shows “eye image” overlay, the image of the patient´s eye is displayed to ease patient´s interpretation of the map. Notice that a paracentral target marks an elevation zone that has to be carefully inspected. Angular scale is also displayed in map (Table 1.4)
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Special Software Applications and Displays
Each available instrument is sold with standard software package and most offer optional packages at additional price. The most common are:
Multiple display option: A customisable multiple display allows simultaneous screen display for rapid analysis and ease of use. Depending on the software of your topographer, you can simultaneously view either one, two or four maps. Extremely practice in daily use to ease work and interpretation.
Surgical applications: Used to predict the results of refractive surgery, and for postoperative evaluation. Some but not all allow refractive surgery simulations and topography linked laser refractive surgery with special excimer laser brand names (Figure 1.19A and B).
Contact lens fitting application: They are used for contact lens fitting, and help choosing the best suggested lens for each case, by simulating the fluorescein pattern and contact lens position of rigid contacts. Not all topographers offer this feature: In some cases this software module is sold as an option. For instance, Dicon´s CT-200™ offers as standard the Mandell Contact Lens Module “Easy-Fit™”, and as an option the Mandell Contact Lens Module “Advanced-Fit™” for toric, bi-toric, keratoconic fitting and postsurgical fitting with Labtalk™. Contact your dealer for more precise information.
The simulated fluorescein feature is intended to reduce fitting time by viewing the effect of changing lens parameters on a personalized basis, depending on the patient´s corneal exam. Let´s notice that the true “in vivo” result of any computerized fluorescein test may vary due to differences caused by lid action on the lens (aperture and weight).
Ask the manufacturer of your topographer for special software applications, and for the possibility to link your topographer and your excimer laser for better results.
 
TOPOGRAPHY MAPS OF THE NORMAL CORNEA
When considering the topography of a normal cornea, we feel the need to remember that there is a wide spectrum of normality. No human cornea demonstrates the kind of regularity found in the calibration spheres of a topographer: The eye is not moulded glass-made. Normal corneal topography can take on many topographic patterns (see Table 1.5 and Figure 1.20):
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Figure 1.19A:
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Figures 1.19 A and B: Dicon´s CT-200™ trend analysis displays a series of exam maps (preoperative exam, first postoperative exam, most recent exam and a choice of a K-trend graph, a pre/postoperative difference map or a post-last difference map. Shown are trend analysis of both eyes of a patient who underwent myopic LASIK with two different excimer lasers. Shown are axial diopter preoperative, tangential diopter immediate postoperative and K-trend graph. Notice that immediately after surgery (the day after), ablation zones differ form each other: It is due to the fact that a different excimer laser was used for each eye. Schwind® Keratom™ was used on right eye, while left eye was operated using the Bausch and Lomb® -Chiron Technolas 217™. K-trend graph shows the major (green) and minor (blue) K values for all exams in the series. The Y axis is power in diopters, and the X axis is the exams´ number spaced out over time. The vertical line marks the date of surgery. Trend analysis eases a rapid overview of healing trend over time
Table 1.5   Normal topographic patterns
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Figure 1.20: Shows a “multiple exams view” of left both eyes of the same patient, a 38-year-old woman prior to LASIK surgery. Corneal topography remains a routine exam for preoperative and postoperative assessment of the refractive patient. This report shows normal, spherical (round), corneas in both eyes (44 D at vertex, and mostly green color in the map). The color zones are approximately circular in shape. Notice that lid aperture is not the same in both eyes, thus making it more difficult to map superior corneal periphery in left eye
Regular astigmatism (with-the-rule) gives an oval axial corneal map, being the most common deviation from optically perfect spherical (round) cornea. If the bow tie is vertical (the long axis is near the vertical meridian) in an axial map, it represents a cornea having with-the-rule-astigmatism.
If the bow tie is horizontal, it represents an “against-the-rule” astigmatism, ninety degrees rotated when compared to a with-the-rule astigmatism.
When the bow tie is diagonal, it represents a cornea having an oblique astigmatism. The shape and colors of the bow tie are influenced by the rate of peripheral corneal flattening, and the appearance is influenced by the scale interval chosen by the explorer. The bow tie may be symmetrical or asymmetrical along the perpendicular meridian: One-half of the bow tie is significantly larger than the other, the corneal apex being located in the direction of the larger bow half, slightly decentered form the visual axis.
In the normal eye, nasal cornea is flatter than temporal. The nasal side of a healthy corneal map becomes blue more quickly, indicating that the nasal cornea is flatter than the temporal. There is a physiological astigmatism of around 0.75 diopter. Physiologically, the axis may not be the same superiorly than inferiorly. In an axial map, the rate of flattening is greater when the color scale interval is larger, and there are many color zones. A focal steepening inferiorly may exist due to the lower tear meniscus.
Generally,the two eyes of the same subject are very similar, and present a mirror image of each other (Figure 1.11 and Figure 1.21). This phenomenon is called enantiomorphism. The knowledge of this fact is useful to decide whether a cornea is normal or not, by comparing to the map of contralateral eye.
Small changes in corneal shape do occur throughout life:
  • In infancy the cornea is fairly spherical
  • In childhood and adolescence, probably due to eyelid pressure on a young tissue, cornea becomes slightly astigmatic with-the-rule
    22
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    Figure 1.21: Enantiomorphism is the phenomenon wherein an individual´s topographies are non-superimposable almost mirror images of each-other. The knowledge of this fact is useful to decide whether a cornea is normal or not, by comparing to the map of contralateral eye. Notice that even pachimetry maps reflect this phenomenon (Corneal thickness was mapped with Bausch and Lomb©Orbscan™ topo-pachimeter)
  • In the middle age, cornea tends to recover its sphericity
  • late in life, against-the-rule astigmatism tends to develop
Short-term fluctuation and diurnal variations are not rare, and usually remain unnoticed by individuals with normal corneas. Some conditions like corneal dystrophies, ocular hypotony, radial keratotomies or contact lens use can make them apparent (Table 1.6).
Table 1.6   Factors that slightly affect the normal curvature of the cornea
Lid closure during sleep time
Tear film quality
Lid pressure on the cornea (weight, exophthalmos)
Intraocular pressure
Menstruation
Pregnancy
 
Comparing Displays
Maps can be compared directly only on the same scale, when taken with the same instrument, and preferably by the same explorer. It is not a good idea to compare maps taken with different instruments: Every instrument uses a different measuring algorithm that may confuse you, specially when comparing subtle details.
Most software applications allow the comparison of different maps (Table 1.7) over time, and even subtract values form two different exams (substraction or difference maps) (Figure 1.22). They are invaluable to the refractive surgeon.
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Figure 1.22: A tangential diopter difference map to the left eye of a 21-year-old patient is shown. The subtraction has been performed between two different eye fixations to determine the existence of any irregularity in the ablation zone. The patient underwent a successful bilateral LASIK surgery to correct a high myopic astigmatism in both eyes a year before
Table 1.7   Uses of substraction or difference maps
Validation of various exams taken in a same session (Figure 1.23)
Ascertain the existence of progressive corneal astigmatism (Figure 1.24)
Comparison of preoperative and postoperative corneal maps (LASIK and PRK)
Follow-up of myopic regression (LASIK and PRK)
Establishing ablation zone centration (LASIK and PRK)
Assessing resolution of corneal warpage in rigid contact lens users
Assessing evolution of a corneal ulcer or abscess
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Figure 1.23: A diopter difference map is useful to assess the validity of the different exams with the same fixation performed in the same session. Low differences due to tear film irregularities, lid aperture and blinking is acceptable. In case of difference between maps taken at the same moment, they need to be repeated, after a few blinks form the patient. If significant difference persists, try instillating a tear substitute in both eyes and wait a few minutes. Should differences persist, repeat the exams in a few days. Image shows a left eye with regular (with-the-rule) high astigmatism: Both axial diopter maps were taken in the same session: Differences exist between the exams. Eye fixation is the same (center): Differences are attributable to different lid aperture and form blinking. Axial diopter difference (down, with a square grid overlay) shows that differences are almost nonsignificant (around 0.25-0.50 diopters), but exist. Such differences are physiological: Difference maps allow validation of various exams taken in a same session
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Figure 1.24: Difference maps ease the astigmatism progression follow-up. Tangential diopter displays show right eye maps of a 22-year-old myopic patient referred for refractive surgery. To our surprise, neither glasses nor contacts had astigmatism. The existence of astigmatism was ascertained with the keratometer, subjective refraction and skiascopy. Corneal topography was performed and helped the demonstration of its existence. Figure shows a difference map between two exams taken with a 3 months delay (see the dates of the exams). Tangential diopter difference is 0 (green), meaning that no changes have occurred in that period of time. The first impression is that the guy never had good refraction, but new topographic exams will be performed 6 months and one year later, before refractive surgery is decided, so as to make sure that no keratoconic formation is on the way
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