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The Interpretation of Corneal Topographic Maps
The Interpretation of Corneal Topographic Maps MELANIE C. CORBETT, DAVID P.S. O'BRART, DAVID C. SAUNDERS* and EMANUEL S. ROSEN* Department of Ophthalmology, St Thomas' Hospital, Lambeth Palace Road, London, SE1 7EH, and • Manchester Royal Eye Hospital, Oxford Road, Manchester, M13 9WH, UK
This article is the second of a series which describes the role of corneal topography, and highlights its value in different clinical settings. Using a case of keratoconus, it illustrates the variety of ways in which topographic data from videokeratoscopes can be presented: twodimensional topographic maps using absolute, normalized and adjustable scales; numeric and isometric maps; cross-sections, and three-dimensional wire nets. Each of these representations have their own benefits, limitations and applications. They can be used in combination to provide detailed information about the topography of the corneal surface. Keywords: Corneal topography; Videokeratoscopes; Data presentation; Contour maps; Keratoconus.
The majority of instruments which assess corneal as the scale and the axes. The most appropriate topography do so by analysing the position of mires format for a given case is the one which demonreflected from the ocular surface [1]. Changes in the strates its topography to best effect. When examinshape of spacing of the reflected mires are only ing serial measurements, care should be taken to apparent to an observer when caused by fairly express all the sets of data in the same format, so marked abnormalities of the corneal topography. like can be compared with like. Computer-analysis of the mire positions quantifies Using data from a case of keratoconus, this article the corneal shape and displays the data in formats explains how to interpret a variety of topographic which are much more sensitive to small abnor- maps, and gives guidance on the benefits, limitamalities of the surface [2]. tions and applications of each format. Quantitative The first computerized display of the corneal sur- descriptors of corneal topography will be discussed face was by a three-dimensional wire mesh repre- in later articles. Some of the features described sentation, from which the observer could still only differ slightly between topography systems. The appreciate relatively gross distortion [3, 4]. The EyeSys and TMS (Topographic Modelling System) sensitivity of this technique was slightly improved will mainly be used in this series. by presenting the deviation of the corneal shape from spherical [5]. For a topography system to be of CASE REPORT value in clinical practice, its data should not only be accurate, but presented in a forniat which enables An 18-year-old patient was referred to the ophthalthe clinician to diagnose abnormalities of corneal mologist with left keratoconus and an apparently shape, and easily compare serial measurements. normal right eye. This became possible with the development of colRefraction: Right: -1.00/-1.00 x 60° 6/6 our-coded contour mapping, in which the corneal Left: -2.00/-1.25 X 110° 6/12. surface is represented in two dimensions (x and y ), with the third dimension (z, the curvature or power Corneal topography confirmed the presence of keraof the cornea) being encoded in the colour scheme [6]. toconus on the left, but also detected a subclinical In modern topography systems data can be discone on the right [7, 8]. played in different ways by altering variables such Correspondence to: Miss Melanie C. Corbett, FRCS, Department of Ophthalmology, StThomas' Hospital (Block 8), Lambeth Palace Road, London, SEl 7EH, UK.
TWO-DIMENSIONAL CONTOUR MAPS
Topographic data is presented as corneal contour
0955-3681/94/030153+07 $08.00/0 © 1994 W.B. Saunders Company Limited
Eur J Implant Ref Surg, Vol 6, June 1994
154
M.C. Corbett eta/.
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Fig. 1 Absolute scale map of a keratoconic comea. The scale of diopteric power to the left is the same for all comeas examined, enabling maps to be compared. The white ring outlines the pupillary margin
maps, in which the contours join points of equal slope (or power). The warmer colours (red and yellow) represent the steeper areas, whereas the cooler colours (green and blue) mark the flatter ones; the scale being shown on the maps.
Absolute scale map (Fig. 1)
An absolute scale map is one in which there is a fixed colour coding system, in that the same colours always represent the same diopteric values. This enables easy comparison to be made between the maps of two . individuals, two eyes, or from one occasion to the next. The centre of the cone is in the steep yellow region inferonasally. The cornea flattens off peripherally where it is colour-coded green.
corneal indices at this point (radius of curvature, power, axis and distance from the centre). Normalized/relative scale map (Fig. 3)
A normalized scale map uses a set number of colours which are automatically adjusted to fill the range of diopteric values for that single map. This has the advantage of using narrower steps between the contours to provide more detail, but the clinician must then decide how much of the detail is clinically relevant. It is particularly important when studying normalized or relative scale maps to take note of the scale. The colour distribution for the patient's right eye with a subclinical cone looks remarkably similar to that of the left eye with overt keratoconus, although the diopteric steps between contours are much smaller. These maps are unhelpful, and can even be misleading, when making comparisons of two eyes.
Magnified absolute scale map (Fig. 2)
The zoom function enhances the absolute scale view of the cone and the optical zone. The cursor (marked by a cross) has been moved to the apex of the cone. The box in the bottom right hand corner gives the
Adjustable scale map
The adjustable scale map enables the user to select the diopteric range and step value (down to 0.1 D) of Eur J Implant Ref Surg, Vol6, June 1994
155
The Interpretation of Corneal Topographic Maps 52
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M.C. Corbett eta/.
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Stiles-Crawford effect. The corneal areas contributing least to the retinal image are shaded darker
the contours. This can combine the detail of the normalized scale map, with the ability to compare images as in the absolute scale map.
scale when determining the contour of a certain point, or comparing two maps [9, 10]. CROSS-SECTIONS
Demonstration of Stiles-Crawford effect (Fig. 4). Refraction profile (Fig. 6)
The Stiles-Crawford effect refers to the ability of the eye to minimize the visual impact of aberrations arising from light passing through the peripheral cornea. It is the result of retinal cones being much more sensitive to light which enters the eye paraxially, than to light entering obliquely through the peripheral cornea. On the map demonstrating this effect, the peripheral cornea is shaded, leaving the bright colours at the centre to demonstrate the topography of the optically important part of the cornea. On any of the two-dimensional topography maps the position of the pupil can be demonstrated. This again gives an estimation of the area of cornea through which light passes to contribute to the retinal image.
The refraction profile (EyeSys) shows data from the lower cornea on the left, and upper cornea on the right. The diopteric power of a corneal point (y-axis) is plotted against its distance from the central cornea (x-axis). The lines are coloured red in the steep axes (291 o and 111 °), and blue in the flatter axes (216° and 36°). Below, the difference between the two axes (i.e. the astigmatism) is plotted in green (note that the scale is inverted, with smaller degrees of astigmatism appearing higher up the scale). The upper cornea is relatively spherical, whereas the lower cornea has greater astigmatism due to the presence of the cone. Isometric map (Fig. 7)
Numeric map (Fig. 5)
The numeric maps plots colour-coded numeric values (diopters or mm curvature) along 8 to 16 meridians. This replaces the need to refer to the
On an isometric map (TMS), the rings are straightened into lines from oo to 360°. The diopteric power of each point on a ring is plotted against the axis on which it lies. Isometric maps are displayed both twoEur J Implant Ref Surg, Vol 6, June 1994
The Interpretation of Corneal Topographic Maps
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dimensionally, when the lines appear superimposed as if viewed from the side; and three-dimensionally, when the lines are spread out, as if viewed from a higher point. The straighter the lines, the more spherical the cornea. This is particularly useful for determining whether astigmatism is regular.
resented. It has the disadvantage of only demonstrating relatively gross irregularities of the corneal surface. Current computer software can formulate these representations relatively easily to provide visual impact, but most systems rely upon two dimensional maps and cross-sections to provide clinically relevant information.
THREE-DIMENSIONAL REPRESENTATIONS Three-dimensional wire net {Fig. 8)
The three-dimensional wire net was one of the earliest ways in which corneal topography was repEur J Implant Ref Surg, Vol 6, June 1994
ACKNOWLEDGEMENTS Miss Corbett holds the Williams Research Fellowship for Medical and Scientific Research of the University of London. Mr O'Brart
158
M.C. Corbett eta/.
Fig. 7 Isometric map (TMS). The power of points on each ring are plotted against their axis, as if the rings have been straightened out. Above: two-dimensional view. Below: three-dimensional view
holds a research fellowship sponsored by The Iris Fund for the Prevention of Blindness.
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REFERENCES
6
1 MC Corbett, DPS O'Brart, DC Saunders, ES Rosen. The assessment of corneal topography. Eur. J. Implant Ref Surg., 1994; 6:98-105. 2 SD Klyce, SE Wilson. Methods of analysis of corneal topography. Refract. Corneal Surg., 1989; 5: 368-371. 3 SD Klyce. Computer-assisted topography: High resolution graphic presentation and analysis of keratoscopy. Invest. Ophthalmol. Vis. Sci ., 1984; 25: 1426-1435. 4 JA Young, IM Siegel. Isomorphic corneal topography: A
7 8 9 10
clinical approach to 3-D representation of the corneal surface. Refract. Corneal Surg. , 1993; 9: 74-78. JW Warnicki, PG Rehkopf, DY Curtin, SA Burns, RC Arffa, NC Stuart. Corneal topography using computer analyzed rasterstereographic images. Applied Optics 27: 1135-1140. L.J Maguire, DE Singer, SD Klyce. Graphic presentation of computer analysed keratoscope photographs. Arch. Ophthalmol., 1987; 105: 223-230. V Gonzalez, PJ McDonnell. Computer-assisted corneal topography in parents of patients with keratoconus. Arch. Ophthalmol., 1992; 110: 1412-1414. L.J Maguire, WM Bourne. Corneal topography of early keratoconus.Am. J. Ophthalmol., 1989; 98: 107-112. JJ Rowsey, AE Reynolds, DR Brown. Corneal topography. Corneascope.Arch. Ophthalmol. , 1981; 99: 1093-1100. RC Arffa, SD Klyce, M Busin. Keratometry in epikeratophakia. J . Refract. Surg., 1989; 2: 61-64.
Eur J Implant Ref Surg, Vol6, June 1994
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This article is the second of a series which describes the role of corneal topography, and highlights its value in different clinical settings. Using a case of keratoconus, it illustrates the variety of ways in which topographic data from videokeratoscopes can be presented: twodimensional topographic maps using absolute, normalized and adjustable scales; numeric and isometric maps; cross-sections, and three-dimensional wire nets. Each of these representations have their own benefits, limitations and applications. They can be used in combination to provide detailed information about the topography of the corneal surface. Keywords: Corneal topography; Videokeratoscopes; Data presentation; Contour maps; Keratoconus.
The majority of instruments which assess corneal as the scale and the axes. The most appropriate topography do so by analysing the position of mires format for a given case is the one which demonreflected from the ocular surface [1]. Changes in the strates its topography to best effect. When examinshape of spacing of the reflected mires are only ing serial measurements, care should be taken to apparent to an observer when caused by fairly express all the sets of data in the same format, so marked abnormalities of the corneal topography. like can be compared with like. Computer-analysis of the mire positions quantifies Using data from a case of keratoconus, this article the corneal shape and displays the data in formats explains how to interpret a variety of topographic which are much more sensitive to small abnor- maps, and gives guidance on the benefits, limitamalities of the surface [2]. tions and applications of each format. Quantitative The first computerized display of the corneal sur- descriptors of corneal topography will be discussed face was by a three-dimensional wire mesh repre- in later articles. Some of the features described sentation, from which the observer could still only differ slightly between topography systems. The appreciate relatively gross distortion [3, 4]. The EyeSys and TMS (Topographic Modelling System) sensitivity of this technique was slightly improved will mainly be used in this series. by presenting the deviation of the corneal shape from spherical [5]. For a topography system to be of CASE REPORT value in clinical practice, its data should not only be accurate, but presented in a forniat which enables An 18-year-old patient was referred to the ophthalthe clinician to diagnose abnormalities of corneal mologist with left keratoconus and an apparently shape, and easily compare serial measurements. normal right eye. This became possible with the development of colRefraction: Right: -1.00/-1.00 x 60° 6/6 our-coded contour mapping, in which the corneal Left: -2.00/-1.25 X 110° 6/12. surface is represented in two dimensions (x and y ), with the third dimension (z, the curvature or power Corneal topography confirmed the presence of keraof the cornea) being encoded in the colour scheme [6]. toconus on the left, but also detected a subclinical In modern topography systems data can be discone on the right [7, 8]. played in different ways by altering variables such Correspondence to: Miss Melanie C. Corbett, FRCS, Department of Ophthalmology, StThomas' Hospital (Block 8), Lambeth Palace Road, London, SEl 7EH, UK.
TWO-DIMENSIONAL CONTOUR MAPS
Topographic data is presented as corneal contour
0955-3681/94/030153+07 $08.00/0 © 1994 W.B. Saunders Company Limited
Eur J Implant Ref Surg, Vol 6, June 1994
154
M.C. Corbett eta/.
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Fig. 1 Absolute scale map of a keratoconic comea. The scale of diopteric power to the left is the same for all comeas examined, enabling maps to be compared. The white ring outlines the pupillary margin
maps, in which the contours join points of equal slope (or power). The warmer colours (red and yellow) represent the steeper areas, whereas the cooler colours (green and blue) mark the flatter ones; the scale being shown on the maps.
Absolute scale map (Fig. 1)
An absolute scale map is one in which there is a fixed colour coding system, in that the same colours always represent the same diopteric values. This enables easy comparison to be made between the maps of two . individuals, two eyes, or from one occasion to the next. The centre of the cone is in the steep yellow region inferonasally. The cornea flattens off peripherally where it is colour-coded green.
corneal indices at this point (radius of curvature, power, axis and distance from the centre). Normalized/relative scale map (Fig. 3)
A normalized scale map uses a set number of colours which are automatically adjusted to fill the range of diopteric values for that single map. This has the advantage of using narrower steps between the contours to provide more detail, but the clinician must then decide how much of the detail is clinically relevant. It is particularly important when studying normalized or relative scale maps to take note of the scale. The colour distribution for the patient's right eye with a subclinical cone looks remarkably similar to that of the left eye with overt keratoconus, although the diopteric steps between contours are much smaller. These maps are unhelpful, and can even be misleading, when making comparisons of two eyes.
Magnified absolute scale map (Fig. 2)
The zoom function enhances the absolute scale view of the cone and the optical zone. The cursor (marked by a cross) has been moved to the apex of the cone. The box in the bottom right hand corner gives the
Adjustable scale map
The adjustable scale map enables the user to select the diopteric range and step value (down to 0.1 D) of Eur J Implant Ref Surg, Vol6, June 1994
155
The Interpretation of Corneal Topographic Maps 52
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M.C. Corbett eta/.
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Fig. 4
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the contours. This can combine the detail of the normalized scale map, with the ability to compare images as in the absolute scale map.
scale when determining the contour of a certain point, or comparing two maps [9, 10]. CROSS-SECTIONS
Demonstration of Stiles-Crawford effect (Fig. 4). Refraction profile (Fig. 6)
The Stiles-Crawford effect refers to the ability of the eye to minimize the visual impact of aberrations arising from light passing through the peripheral cornea. It is the result of retinal cones being much more sensitive to light which enters the eye paraxially, than to light entering obliquely through the peripheral cornea. On the map demonstrating this effect, the peripheral cornea is shaded, leaving the bright colours at the centre to demonstrate the topography of the optically important part of the cornea. On any of the two-dimensional topography maps the position of the pupil can be demonstrated. This again gives an estimation of the area of cornea through which light passes to contribute to the retinal image.
The refraction profile (EyeSys) shows data from the lower cornea on the left, and upper cornea on the right. The diopteric power of a corneal point (y-axis) is plotted against its distance from the central cornea (x-axis). The lines are coloured red in the steep axes (291 o and 111 °), and blue in the flatter axes (216° and 36°). Below, the difference between the two axes (i.e. the astigmatism) is plotted in green (note that the scale is inverted, with smaller degrees of astigmatism appearing higher up the scale). The upper cornea is relatively spherical, whereas the lower cornea has greater astigmatism due to the presence of the cone. Isometric map (Fig. 7)
Numeric map (Fig. 5)
The numeric maps plots colour-coded numeric values (diopters or mm curvature) along 8 to 16 meridians. This replaces the need to refer to the
On an isometric map (TMS), the rings are straightened into lines from oo to 360°. The diopteric power of each point on a ring is plotted against the axis on which it lies. Isometric maps are displayed both twoEur J Implant Ref Surg, Vol 6, June 1994
The Interpretation of Corneal Topographic Maps
157
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Fig 6 Refraction profile (EyeSys). Above: variation of diopteric power with zone diameter in the steepest (red) and flattest (blue) axes, with the inferior cornea shown on the left and the superior cornea on the left. Below: astigmatism (green) is the diopteric difference between the steepest and flattest axes (note the inversion of the axis)
dimensionally, when the lines appear superimposed as if viewed from the side; and three-dimensionally, when the lines are spread out, as if viewed from a higher point. The straighter the lines, the more spherical the cornea. This is particularly useful for determining whether astigmatism is regular.
resented. It has the disadvantage of only demonstrating relatively gross irregularities of the corneal surface. Current computer software can formulate these representations relatively easily to provide visual impact, but most systems rely upon two dimensional maps and cross-sections to provide clinically relevant information.
THREE-DIMENSIONAL REPRESENTATIONS Three-dimensional wire net {Fig. 8)
The three-dimensional wire net was one of the earliest ways in which corneal topography was repEur J Implant Ref Surg, Vol 6, June 1994
ACKNOWLEDGEMENTS Miss Corbett holds the Williams Research Fellowship for Medical and Scientific Research of the University of London. Mr O'Brart
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Fig. 7 Isometric map (TMS). The power of points on each ring are plotted against their axis, as if the rings have been straightened out. Above: two-dimensional view. Below: three-dimensional view
holds a research fellowship sponsored by The Iris Fund for the Prevention of Blindness.
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REFERENCES
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1 MC Corbett, DPS O'Brart, DC Saunders, ES Rosen. The assessment of corneal topography. Eur. J. Implant Ref Surg., 1994; 6:98-105. 2 SD Klyce, SE Wilson. Methods of analysis of corneal topography. Refract. Corneal Surg., 1989; 5: 368-371. 3 SD Klyce. Computer-assisted topography: High resolution graphic presentation and analysis of keratoscopy. Invest. Ophthalmol. Vis. Sci ., 1984; 25: 1426-1435. 4 JA Young, IM Siegel. Isomorphic corneal topography: A
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clinical approach to 3-D representation of the corneal surface. Refract. Corneal Surg. , 1993; 9: 74-78. JW Warnicki, PG Rehkopf, DY Curtin, SA Burns, RC Arffa, NC Stuart. Corneal topography using computer analyzed rasterstereographic images. Applied Optics 27: 1135-1140. L.J Maguire, DE Singer, SD Klyce. Graphic presentation of computer analysed keratoscope photographs. Arch. Ophthalmol., 1987; 105: 223-230. V Gonzalez, PJ McDonnell. Computer-assisted corneal topography in parents of patients with keratoconus. Arch. Ophthalmol., 1992; 110: 1412-1414. L.J Maguire, WM Bourne. Corneal topography of early keratoconus.Am. J. Ophthalmol., 1989; 98: 107-112. JJ Rowsey, AE Reynolds, DR Brown. Corneal topography. Corneascope.Arch. Ophthalmol. , 1981; 99: 1093-1100. RC Arffa, SD Klyce, M Busin. Keratometry in epikeratophakia. J . Refract. Surg., 1989; 2: 61-64.
Eur J Implant Ref Surg, Vol6, June 1994
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Fig. 8 3-dimensional wire net (Maastricht Shape Topographer). The MST measures the true shape of the corneal surface in terms of height rather than slope. The reconstruction can be displayed as a three-dimensional wire net, but most clinical information is obtained from the two dimensional and cross-sectional representations, and conversion of height information to power or radius of curvature
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