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Topography aids diagnosis, management in all areas of practice
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Originally used to describe the corneal surface in the same language as keratometry, topography has grown into a robust diagnostic tool for refractive surgery, contact lenses fitting, treatment of ocular surface disease and surgical planning. To utilize topography, we must understand the various types of technology now available as well as the language used to describe the cornea in 2010.
The original topographers used placido disc targets to create axial curvature maps. This method uses computer analysis of reflected images from multiple concentric rings projected on the cornea. Unfortunately, the axial map does not take into account spherical aberration, making it a weak descriptor of refractive power. Nor does it accurately depict corneal shape, as measurement grows increasingly less accurate toward the periphery.
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  Images: Swartz T |
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As refractive surgery demanded better accuracy and more detail about corneal shape, complex arc-step algorithms enabled accurate measurement of the corneal elevation from placido images as well as the creation of other curvature maps to describe the human cornea.
Placido systems are widely used. They have a reliable, long track record, tend to be less expensive than other systems and are easy to use and efficient. Elevation data is calculated rather than measured directly, however, and placido disc systems are not able to measure pachymetry or provide information about the posterior cornea.
 Figure 2. The wavefront and topography display of the iTrace unit depict the axial map (lower right) of a patient with a history of radial keratotomy with reduced best corrected vision. The total eye wavefront (upper right) appears irregular with astigmatism. The corneal wavefront (lower left) shows coma consistent with the decentered optical zone of the RK incisions. The internal optics wavefront shows significant irregularity, indicating not all the visual issues are due to the RK, and the patient was educated that a gas-permeable lens might fix some, but not all, of the visual problems. |
Various curvature maps such as axial, instantaneous (also called local radius of curvature and tangential maps), refractive and keratometric maps are now found on various systems. These maps describe curvature in diopters. Elevation maps describe elevation differences in the cornea based on a best fit sphere and use microns as a unit of measurement.
Topography vs. tomography
To improve our understanding of the corneal shape, tomography was introduced. This is the creation of a three-dimensional model from two-dimensional images.
 Figure 3. The Pentacam’s Belin-Ambrosio display incorporates a modified best-fit sphere with analysis of corneal pachymetry centered around the thinnest point relative to the periphery. Image: Ambrosio R |
The first system to accomplish direct measurement of elevation used slit-scanning technology. The Orbscan IIz (Bausch & Lomb, Rochester, N.Y.) corneal topography system captures 50 images using light slits at a 45° angle projected through the cornea, in combination with a placido disc target. Computer software produces elevation maps of the anterior and posterior cornea, calculates the corneal thickness and provides a curvature map.
This technology was the first to allow evaluation of the posterior corneal surface. The system’s “quad map” commonly included four maps: axial or tangential curvature, anterior and posterior elevation, and pachymetry.
Slit-scanning was the only method to evaluate the posterior cornea for several years prior to Scheimpflug imaging. Scheimpflug imaging is based on the Scheimpflug principle of photography.
In a typical camera, the film plane, lens plane and plane of sharp focus are all parallel. In Scheimpflug cameras, an oblique tangent can be drawn from the image, object and lens planes. The point of intersection is the Scheimpflug intersection, where the image is in best focus. The Pentacam (Oculus, Lynnwood, Wash.) and the Galilei (Ziemer, Alton, Ill.) both use this technology. Elevation and pachymetry maps are produced with each system.
The newest technology is the combination of anterior segment optical coherence tomography (OCT) and placido disc topography. The Atlas (Carl Zeiss Meditec, Dublin, Calif.) topographer is combined with the Visante OCT (Carl Zeiss Meditec) and is referred to as the Visante Omni. This system combines placido curvature measurements with OCT to render curvature, elevation and pachymetric information.
Advances in topographic analysis
This variety of technologies advanced the methods we use to analyze the corneal surface. Corneal wavefront, pachymetry, neural networks and contact lens fitting modules are now commonly associated with topography.
 Figure 4. A contact lens fitting module may facilitate fitting irregular corneas by simulating the interaction of the lens with the cornea with computer-created fluorescein patterns. |
Corneal wavefront, or the description of the corneal power in the same language as whole-eye wavefront, is a powerful tool for diagnosis of corneal irregularities, particularly coma in early keratoconus. When corneal wavefront and whole-eye wavefront are compared in systems such as the iTrace system (Tracey Technologies, Houston), diagnosis of complex vision problems may be easily accomplished. This system enables the clinician to locate the aberration on the anterior corneal surface or posteriorly in the lens. Measurement of the corneal wavefront also allows surgeons to tailor aspheric lens implantation with the goal of reducing the spherical aberration and improving visual outcomes.
Corneal pachymetry was also possible using elevation based systems, and pachymetry maps are now commonly associated with topography. These maps may allow earlier detection of ectatic disorders and better monitor corneal edema. The Pentacam offers upgraded software that analyzes the change in pachymetry from the thinnest point on the cornea in addition to modifying the best fit sphere elevation measurements to determine the risk of keratoconus.
It is possible to facilitate the identification of corneal disease using neural networks, software programs that analyze various parameters to indicate the likelihood that a cornea would fall into a diagnostic category such as keratoconus suspect, corneal warpage, post-refractive surgery or simply irregular. This software package is most beneficial to general practitioners.
 Figure 5. Wavefront correction may be incorporated into a contact lens to achieve optimal vision correction in normal eyes and may address irregularities resulting from previous refractive surgery or keratoconus. |
Contact lens fitting modules assist clinicians in ordering lens parameters by simulating lens to cornea relationship. Programs may incorporate known lens designs to enable successful fittings in those with highly irregular corneas. Wavefront may be incorporated into the lens prescriptions as well.
Benefits to clinical practice
While topographers were originally used primarily by corneal specialists and refractive surgeons, they are now used by other specialists, particularly cataract surgeons. With premium IOLs gaining in popularity, identification of irregular corneas that may contraindicate implantation of presbyopia-correcting lenses is critical. Quick references for corneal astigmatism to identify candidates for toric IOLs are found on all systems. Measurement of spherical aberration as a component of the corneal wavefront guides implantation of the aspheric IOLs.
Clinicians fitting contact lenses as well as those treating corneal disease regularly use topography. Lens fitting modules and neural networks facilitate patient care.
Topography can also be used to diagnose and treat ocular surface disease. Dry eye, anterior and posterior blepharitis and tear film instability typically manifest as irregular astigmatism on topography.
In extreme cases, the patient suffers vision loss, but the clinical picture looks normal. An irregular topography map may be the only clue to the irregular ocular surface. Retesting the patient following treatment can also be a useful tool for monitoring improvements as well as encouraging patient compliance.
Reimbursement for topography
While most third parties will not reimburse for a diagnosis of astigmatism (378.21), they will reimburse for topography coded with selected medical diagnoses. A technical and professional component may be billed separately. The physician’s order for the test with medical rationale must be in the chart. A separate interpretation and report should be completed for the test, which should include the testing date, a comment on testing reliability, the printout of the test, diagnosis, impact on treatment and prognosis, and physician’s signature.
  Figures 6a and 6b. Dry eye may manifest as
irregular astigmatism and data loss not clinically noted upon slit lamp
biomicroscopy (left). Treatment of the dry eye using approaches such as
Restasis, Azasite, oral doxycycline, punctum plugs or omega-3 fatty acids, in
conjunction with artificial tears, may dramatically improve the topography and,
subsequently, the best corrected vision. (right, post-treatment).
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Testing for contact lens fittings in the absence of keratoconus will probably not be reimbursed, nor will diagnoses of dry eye or (ironically) irregular astigmatism (367.22). Patients may sign an Advanced Beneficiary Notice (ABN) to ensure payment should the third party not reimburse for medically necessary testing, as desired.
Using topography in clinical practice helps diagnose ocular disease, establish the etiology of vision loss, manage anterior segment disorders and plan for surgery. Advanced applications such as early keratoconus detection, corneal wavefront and contact lens fitting applications will continue to expand its utility.
For more information:
- Tracy Swartz, OD, MS, FAAO, is center director of VisionAmerica of Huntsville, Ala. She can be reached at (256) 533-8801; tswartz@eyehealthpartners.com. Dr. Swartz has no direct financial interest in the products mentioned in this article. She is a paid consultant to Tracey Technologies.
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