corneal topography

Six Signs You Need a Corneal Topographer …and how to choose one.

Dianne Anderson, O.D., F.A.A.O. AURORA, Ill.

Corneal topography can be a significant asset in optometric practice, whether you are diagnosing and managing corneal disorders or fitting challenging patients. Using topography to its full potential, particularly in a contact lens practice, can not only save chair time but also advance your fitting skills and success with specialty contact lens designs.

You may be on the fence about whether you should invest in a corneal topographer. After all, any capital expenditure raises numerous questions: Will the instrument enhance the care you provide to patients? Will it help you grow your practice? Will it save time? What sort of return on investment can you expect? Can you afford it? If you recognize any of the following scenarios, it may be time to invest in a topographer.

1 You refer contact lens patients to other practices for topography.

If you find yourself ordering and dispensing numerous diagnostic contact lenses or if questionable astigmatic refractions and irregular keratometry mires send you directly to the referral pad, consider how much more convenient having your own topographer would be for you and your patients. Patients will be impressed with the technology, and you can collect the service fee and immediately schedule a specialty lens fitting. (See “Table 1. Diagnostic and Procedural Codes for Corneal Topography,” page 41.) Also, keep in mind that referring patients for topography may give them the impression that you're not fully capable of meeting their needs, and they may choose not to return to your practice.

2 You enjoy working with GP lenses for keratoconus.

The prevalence statistics for keratoconus have been on the rise, possibly because more clinicians are using corneal topography to aid in their diagnoses. Not all patients who have irregular astigmatism have keratoconus, however. Some may have pellucid marginal degeneration (PMD), and the only way to differentiate the two conditions is by corneal topography (See figure 1, page 39). This is an important distinction when prescribing a contact lens.

Figure 1. Topography maps clearly show the differences in curvature between keratoconus (A) and pellucid marginal degeneration (B).

Keratoconus patients do well with a variety of different lens designs, ranging from small diameter GPs to hybrids to semi-scleral lenses. Patients who have PMD fare best with large-diameter lenses, such as hybrids and semi-scleral lenses. In either case, the initial diagnostic lens choice is most accurately calculated from the evaluation of corneal topography. The dynamics of the lens will be more physiologically compliant with the underlying cornea when you consider the peripheral contours of the cornea. In addition, many new GP designs allow unique manipulations of the optic zone, peripheral curves and diameter to customize the fit for improved visual performance and comfort. Thus, corneal topography plays an important role not only in diagnosis but also in lens design.

3 You manage difficult astigmatic CL fits.

Approximately 30% of children and adults have some astigmatism.^1,2 In some cases of regular astigmatism, patients have great vision with spectacles, but unsatisfactory visual results with standard contact lenses. The reason for this may be due to the magnitude and location of corneal astigmatism.³ Keratometers measure the curvature of the cornea's steepest and flattest meridians over the central 3mm, while corneal topographers provide a more accurate representation of the cornea's true shape over 10mm. With topography, you can determine whether a patient's astigmatism is apical or limbal-to-limbal (see figure 2, page 40), which will influence your lens choice. Apical astigmatism may be adequately corrected with spherical GP lens- es or soft toric designs, while limbal astigmatism may require bitoric GPs or custom soft toric lens designs for optimal centration, comfort and visual acuity.

Figure 2. Apical astigmatism (left) is condensed within the central area of the cornea. Limbal astigmatism (right) extends to the entire corneal diameter.

4 You rehabilitate patients who have contact lens-induced corneal warpage.

Patients who overwear or overextend the life of their contact lenses may develop corneal edema and warpage, resulting in transient visual impairment. This condition usually presents as a significant change in refractive astigmatism that correlates to a significant change in corneal astigmatism. You may find irregular keratometry mires, making it difficult to differentiate corneal warpage from early keratoconus. Not only will corneal topography help you confirm a diagnosis of corneal warpage, it will also help you educate your patients. Discontinuing lens wear and repeating topography in three-to-four weeks may show a significant difference map (see figure 3, page 40) as the warpage resolves. Using these maps as visual aids, you can explain the problem to patients and, hopefully, prevent a recurrence.

Figure 3. Axial difference map of lens-induced corneal warpage. This patient had been wearing a highly aspheric back-surface multifocal RGP (top left). Notice the normalization of corneal appearance after discontinuing CL wear for four months (bottom left).

5 Your patients are inquiring about LASIK.

In any successful contact lens practice, patients are likely to inquire about their candidacy for LASIK, and you must be prepared to answer their questions and refer them for further evaluation, should they look like a candidate for the procedure. If you have a corneal topographer, you can begin the LASIK evaluation right there, explaining any unusual patterns that could disqualify the patient from having surgery. And, even if the map shows normal topography, this is a good opportunity for you to discuss corneal reshaping as an alternative to LASIK.

6 You are interested in corneal reshaping.

Access to corneal topography can open up the world of corneal reshaping, which, in my experience, scores high marks with patients and offers an excellent return on investment. And because the FDA has placed no age restriction on corneal reshaping, it is a great alternative to eyeglasses or daytime contact lenses for adolescents. The future of corneal reshaping also looks promising, as new lens designs, such as hyperopic/presbyopic, toric and possibly hydrogel lenses, come to market. In addition, ongoing research shows the potential for myopia control with corneal reshaping.⁴

A corneal topographer is a must-have instrument in any practice offering corneal reshaping. Axial or sagittal maps help monitor refractive changes, and tangential maps help determine the orientation of those changes on the cornea. Difference maps can help determine whether changes in lens parameters are required, and elevation maps show whether the corneal toricity is great enough to require a toric lens. (See figure 4, page 41).

Figure 4: Elevation map of a cornea with a high amount of limbal with-the-rule astigmatism. This illustration shows how a spherical lens will be tight along the horizontal meridian and loose along the vertical meridian. CRT spherical lens on this toric cornea will decenter, (left image) where CRT Dual Axis(right image) achieves proper centration for a successful treatment outcome.

Choosing a topographer

If you can relate to any of the above scenarios, then you should consider adding a corneal topographer to your practice. Numerous manufacturers offer various models of topographers. A comprehensive review of each model is beyond the scope of this article. I can, however, provide a basic overview to help you get started in your search.

The two main types of corneal topographers are placido disc and slit-scanning. Placido-disc systems project concentric rings onto the cornea and directly measure the anterior curvature, which is helpful for diagnosing diseases of the cornea and for fitting contact lenses. Some placido disc systems: Atlas Model 9000 (Carl Zeiss Meditec); CTS (Reichert); Easygraph and Keratograph (Oculus); Magellan Mapper (Nidek); Medmont E300 (Precision Technology Services); Keratron Scout and Piccolo (EyeQuip); TMS-4 (Tomey USA); and CA-200, KR-8000PA (Topcon). The average cost of these systems is $16,000.

Slit-scanning devices use light rays to scan the entire cornea. They directly measure anterior and posterior surface elevation and generate corneal thickness data, which is helpful when evaluating a patient for refractive surgery candidacy. Some slit-scanning devices: Orbscan (Bausch + Lomb); Pentacam (Oculus); and Galilei G2 (Ziemer). The average cost of these instruments is $50,000.

Some manufacturers also offer instruments that combine corneal topography with full optical wavefront aberrometry. This combination enables you to generate wavefront-guided spectacle lens prescriptions, which leads to high-end spectacle lens sales. Some topographer/wavefront analyzers: OPD Scan II (Marco); i.Profiler plus (Carl Zeiss Meditec); KR-1W (Topcon); and iTrace Combo (Tracey). The average cost of these instruments is $25,000.

It's important to understand the applications and limitations of each model. Basic features include axial and refractive maps, tangential maps, difference or subtractive maps and elevation maps. More advanced features include pathology detection/corneal analysis, contact lens fitting software and Zernike analysis/wavefront aberrometry. Some models combine autorefraction, keratometry and wavefront aberrometry with corneal mapping. A combination instrument may be a good choice if you're just getting started with topography and don't own an autorefractor, autokeratometer or wavefront aberrometer.

The quality and detail of the images produced by a corneal topographer will differ depending on the resolution of the image, and prices vary accordingly. Models capable of producing high-resolution images with a large number of data points generate more detailed maps.

When considering what you will need to spend for a corneal topographer, note that some manufacturers price their topographers with or without the required laptop or desktop computer. Also, you may need additional software to perform certain functions, such as pathology detection and custom GP lens designing. Further, most manufacturers require licensing of review software for networking purposes. These upgrades are usually sold separately.

A practice-builder

Adding a corneal topographer to your practice will help you provide patients with comprehensive eye care. Using this instrument will also expand your knowledge and fitting skills with specialty lens designs and help you efficiently fit or design the appropriate lenses for each patient. The right combination of instruments and self-motivation will have you well on your way to creating a profitable and rewarding specialty contact lens practice and gaining referrals from corneal specialists as well as your optometric colleagues. OM

1. Kleinstein RN, Jones LA, Hullett S, et al; Collaborative Longitudinal Evaluation of Ethnicity and Refractive Error Study Group. Refractive error and ethnicity in children. Arch Ophthalmol. 2003 Aug;121(8):1141-7.
2. Bourne RR, Dineen BP, Ali SM, et al. Prevalence of refractive error in Bangladeshi adults: results of the National Blindness and Low Vision Survey of Bangladesh. Ophthalmology. 2004 Jun;111(6):1150-60.
3. Cho P, Cheung SW, Edwards M. The longitudinal orthokeratology research in children (LORIC) in Hong Kong: a pilot study on refractive changes and myopic control. Curr Eye Res. 2005 Jan;30(1):71-80
4. Kakita T, Hiraoka T, Oshika T. Influence of overnight orthokeratology on axial elongation in childhood myopia. Invest Ophthalmol Vis Sci. 2011 Apr 6;52(5):2170-2174.

Dr. Anderson specializes in orthokeratology, keratoconus, post-surgical lens fits and anterior segment disease. E-mail her at dianne.anderson@ comcast.net, or send comments to opto metricmanagement@gmail.com.