Applications of Corneal Topography Beyond Corneal Shape
Corneal topography is a versatile tool that can be used for much more than corneal shape measurement.
By Randy Kojima, FAAO; Patrick Caroline, FAAO; Beth Kinoshita, OD, FAAO; Matthew Lampa, OD, FAAO; Mark André, FAAO; & Chad Rosen, OD
Randy Kojima is a research scientist and clinical instructor at the Pacific University College of Optometry and a consultant to Precision Technology Services and Medmont Instruments, Australia.
Patrick Caroline is an associate professor of optometry at Pacific University. He is also a consultant to Contamac.
Dr. Kinoshita is currently an assistant professor and the director of the Pacific Eye Care Clinic at Pacific University College of Optometry.
Dr. Lampa is an assistant professor at Pacific University College of Optometry and is in private practice in Silverton, Ore. He has been a consultant to SpecialEyes, Alcon, Vistakon, and B+L.
Mark André is an associate professor of optometry at Pacific University. He is also a consultant for CooperVision.
Dr. Rosen is the Clinical Research Fellow in the Vision Research Institute at the Michigan College of Optometry.
For many practitioners, the corneal topographer has become an indispensable tool in today’s eyecare practice. In such offices, it has replaced the keratometer as the principle instrument for measuring corneal shape (Wilson, 1991; Mandell, 1996; Mountford et al, 2004; Cairns and McGhee, 2005; and others. Full list available at www.clspectrum.com/references.asp). These modern video keratoscopy systems offer a more efficient means of capture, databasing of images, and a multitude of analysis options that a keratometer simply cannot perform (Guillon et al, 1986; Naufal et al, 1997; Erdélyi et al, 2005; Ambrosio et al, 2013; and others).
Much has been published on the many analysis, disease detection, and contact lens design options that are available in corneal topographers (Boyd et al, 2013; Szczotka, 1997; Bogan et al, 1990; Maeda et al 1994; and others), but the question arises: For what other applications could the instrument be employed? This article will explore some of the less commonly utilized but valuable clinical functions of corneal topographers.
Tear Film Assessment
In eyecare practice today, the most prevalent video keratoscopy design is the placido-based instrument (Mountford et al, 2004; Wang, 2012; Guarnieri and Guarnieri, 2002). These systems reflect circular rings off of the cornea and measure the shape and elevation of the eye’s surface (Figure 1). In fact, reflection systems actually image the tear film rather than the epithelium (Wilson, 1991; Mandell, 1996; Mountford et al, 2004; Anderson and Kojima, 2007; and others). Therefore, placidobased corneal topographers describe the anterior surface fluid shape, which is used to predict the actual underlying corneal shape.
It can be regarded as a drawback that placido topographers do not actually measure the cornea directly (Mountford et al, 2004; Yanoff and Duker, 2008; Wang, 2012; Naufal et al, 1997; and others). However, it’s advantageous that they measure the active tear film and can assess the tear film quality and breakup time in a non-invasive manner (Alonso-Caneiro et al, 2011; Szczesna et al, 2010; Szczesna et al, 2012). Figure 2 shows the start of tear film breakup, which is visible from the warping of the central or first ring and the “splitting” or “breaking” of the second ring.
Dynamic assessment of the placido reflection can be an effective means of assessing tear film stability. For example, measuring the duration from stable tear film (Figure 1) to the slightest sign of instability (Figure 2) provides a fast and efficient tool to measure tear breakup time (TBUT). This diagnostic evaluation can be performed over a small, acute surface area or can be equally effective in observing larger areas. Figure 3 shows two significant areas of tear film instability that are evident in the breaking, warping, and distorting of the reflected rings within the yellow circles.
The live video assessment of the placido reflection defines when tear instability is present, but it does not measure or quantify the degree of tear film instability. For this reason, numerous topographer manufacturers have begun to implement algorithms to gauge tear film quality (Best et al, 2012; Medmont E300 Corneal Topographer manual). This can help us to better understand the severity of dry eye present in specific patients.
Performing Topography Over Contact Lenses
The intended purpose of a corneal topographer is to measure the anterior surface of the eye. However, it can be used just as effectively over contact lenses for a range of applications.
For example, the same placido reflection used to assess tear film quality and TBUT on the cornea can be employed to understand the wettability and stability of the surface of contact lenses on-eye. This can provide insight for researchers, lens developers, and practitioners alike on the surface wettability characteristics of lenses worn in situ.
Figure 1. The placido reflection displays parallel and defined rings across much of the surface. This indicates even, consistent tear film stability and an absence of tear film breakup. Only the outer-most rings appear to distort as they reach the flatter peripheral corneal surface.
Figure 2. The yellow ring highlights an acute area of tear film breakup near the visual axis. The first and second rings appear to have warped or distorted, indicating tear film instability or breakup.
Figure 3. The yellow circles highlight large areas of placido ring distortion, indicating tear film instability on the corneal surface.
Corneal topographers also provide a valuable analysis option when imaging over soft multifocal contact lenses on-eye. This allows practitioners to observe the distance and near zones of the lenses and the position of the optics in relation to the pupil. Figure 4 shows an example of a corneal topography map imaged over a multifocal soft contact lens. The patient’s visual axis has been marked with the white arrow while the optical center of the multifocal lens is marked with the black arrow. The topographer is able to display the exact position of the lens in relation to the patient’s visual axis. A slit lamp assessment would allow practitioners to appreciate lens decentration, but not measure the actual distance of the optical center from the visual axis. In the Figure 4 example, the optical center of the lens is approximately 1.2mm superior-temporal to the visual axis on this right eye.
When assessing the fit of multifocal contact lenses with the slit lamp, it is also difficult—if not impossible—to determine the exact placement of the distance and near zones over the pupil. In Figure 4, the distance zone of the multifocal lens (central orange) is within the pupil and slightly beyond superiortemporal on this right eye. At the same time, the add zone (red ring) appears in an arcuate pattern as a smile of magnification in the inferior pupil margin only. The location of the optics would indicate that quality simultaneous vision may not be achievable with the present position of the lens. Although the slit lamp is our customary tool for determining contact lens centration clinically, performing topography over soft multifocals may be advisable to critically assess misalignments between the optical and visual axes.
Performing topography over multifocal soft lenses offers the benefit that the various zone powers of the lens can be observed. This allows us to understand the actual magnitude of minus or plus that is presented within a given pupil size. Although many lens manufacturers publish the diameter of the near, intermediate, and distance zones of their lenses, what powers do patients actually benefit from based on their specific pupil size? Figure 5 shows the measurement of the distance and near zones of a soft multifocal lens. The yellow arrow denotes the distance section of the optics of the lens on both the upper topography and the lower graph. The black arrows denote the peak of the add power of the lens. The topographer measured the pupil size as 3.7mm, while the peak of the add power measures a close 3.8mm. If this particular multifocal lens were better centered optically, we could expect the patient to have all of the required powers because the distance and near zones of the lens would fall within the diameter of the measured pupil.
Figure 4. This tangential map was taken over a soft multifocal contact lens with a center-distance, peripheral add design. The white arrow indicates the patient’s visual axis, while the black arrow indicates the approximate center of the optics of this multifocal soft contact lens. The optics of the lens appear to be superior-temporal to the visual axis on this right eye.
Performing topography over contact lenses is not limited to soft lenses only. Images can be taken over GP lenses as well. Figure 6 illustrates an analysis of the anterior surface of a GP multifocal lens on-eye. The map indicates that the lens has minus in the center (blue) and progressive add power toward the periphery (warmer colors). The position of the blue distance zone would indicate a superior-positioning lens, which would be typical of lid-attached GP fits.
The white dotted line across the topography indicates the central optical axis of the lens. The graph immediately below shows the powers across that white dotted line. The topographer measured the pupil size to be 4mm, which prompts us to assess the actual distribution of power across the same diameter of the lens optics. In this case, the flattest central power is 39.25D, while at a 4mm diameter of the lens, the power reads 39.75D. This indicates that the add provided within pupil was +0.50D, although the lens actually creates more magnification as the diameter of measure increases. In this case, the patient had a +2.00D add but the lens was providing only +0.50D at the 4mm pupil in primary gaze.
Figure 5. This topography image was taken over a multifocal soft contact lens. The white arrow corresponds with the graph below and indicates the curvature across the lens surface in cross section. The yellow arrow indicates the center of the distance zone on both the geometric center of the lens and the graph. The black arrows indicate the peak of the add power, which is measured at a 3.8mm diameter (red arrows).
Figure 6. This topography map was taken over a front-surface GP multifocal lens. The white arrow corresponds with the graph below, which displays the curvature of the anterior surface in cross section. The red arrows show the measured pupil diameter of 4mm, while the yellow arrows indicate the range of multifocal power over the measured pupil. This patient would receive a simultaneous add effect of only +0.50D even if the lens were well-centered.
Figure 7. This image was taken over a GP contact lens to check for flexing or warping. Note the blue spherical front optical zone of the lens, which is devoid of an “hourglass-shape” astigmatic appearance.
Figure 8. This image was taken over a spherical scleral GP lens. The simulated keratometry readings along with the astigmatic appearance of the topography would suggest that the lens is flexing or warping on-eye, resulting in residual refractive astigmatism.
To resolve this case, the centration of the lens needs to be improved. Secondly, the manufacturer should be consulted to increase the add power to +2.00D at the patient’s 4mm pupil size. With these modifications, quality simultaneous vision should be achieved. However, it should be noted that some manufacturers prefer slightly superior central positioning for their aspheric multifocals. In such designs, it may be advisable to rotate the white axis line to the vertical to measure the powers presented within the pupil diameter.
Another reason to map over GP contact lenses is to determine the degree of flexing or warpage. For example, if a patient is manifesting residual astigmatism through a GP lens, it could mean that either the lens needs to be exchanged for a toric design, or it is a simple case of lens flexure that can be resolved with an increase in center thickness.
Figure 7 illustrates a map over a GP lens with simulated keratometry readings indicating 0.01D of corneal astigmatism. The image itself shows a single blue color throughout the entire 9mm optical zone of the lens, indicating a spherical anterior surface and therefore an absence of warpage or flexing on-eye.
Figure 9. Normal fixation on a keratoconic cornea.
Figure 10. Superior fixation of eye from Figure 9 to center the placido over the cone.
By contrast, the image in Figure 8 was taken over a spherical scleral lens and shows a high degree of astigmatism on the anterior surface. The simulated keratometry readings indicate a cylinder power of 2.80D, and the image of the anterior surface shows the classic hourglassshaped astigmatism. This patient had an over-refraction of +0.50 -1.75 × 175, which would indicate significant flexing or warping of the contact lens on-eye.
Changing Fixation Points
Each one of the previous cases was based on topography imaging on the visual axis. In other words, the patient was fixating down the center of the instrument, and we appreciate the resultant analysis in relation to the visual axis. However, images can be captured with different fixation positions to better understand specific areas of the cornea (Mountford and Noack, KBA Fitting Guide; Read et al, 2006).
The topography map in Figure 9 was taken with the patient focused on the standard fixation target. To better understand the shape of this keratoconic eye, the patient was then instructed to fixate a number of rings superior to the standard fixation target so that the image could be captured directly over the cone (a method introduced by John Mountford, OD, FAAO). Figure 10 shows the same patient with an image taken over the ectasia to better understand the shape, power, and elevation from this altered position. Imaging in this manner gives us more perspective on the asymmetrical eye to help us better select the correct initial contact lens path.
Capturing images with the patient fixated in different directions can also allow topography software to “stitch” or merge the maps together to increase the surface area of coverage and analysis (Franklin et al, 2006; Caroline and André, 2012). For instance, images can be taken with varied fixation targets that are a number of rings superior, inferior, nasal, and temporal along with a central visual axis capture (Figure 11). With these five or more captures, the topography software will build a global view of the corneal surface, similar to retinal imaging, to present the entire visible iris (Figure 12).
Figure 11. Images were taken on the visual axis and in each of the four principle meridians by altering patient fixation. The centered, nasal, temporal, superior, and inferior topographies can be merged to create a larger view of the corneal surface.
Figure 12. Images from various fixations were used to create a comprehensive view of the corneal surface. The diameter of analysis on this eye is 12.4mm, whereas normal placido imaging of the cornea ranges from 6mm to 11mm depending on the instrument employed.
A Truly Versatile Instrument
We have come to rely on corneal topography for a vast number of applications in practice today, including assessing corneal power, shape, elevation, and wavefront error. It can help us diagnose signs of corneal thinning disorders such as keratoconus and pellucid marginal degeneration, and its contact lens modules assist us in fitting GP lenses by modeling fluorescein patterns.
But the instrument has numerous other applications that we can employ. Topographers and their software continue to evolve, and so should we in our use of equipment in practice. CLS
Images in this article were taken using the Medmont E300 Corneal Topographer, Medmont Pty Ltd., Nanawading, Australia.
To obtain references for this article, please visit http://www.clspectrum.com/references.asp and click on document #216.