Early Diagnosis of Corneal Ectatic Disease

With new treatments, such as collagen cross-linking on the horizon, the role of topography is increasingly important.

By Paul M. Karpecki, OD, FAAO

Keratoconus, pellucid marginal degeneration (PMD) and other ectatic diseases of the cornea are among the most frustrating for doctors and patients. Their progressive nature requires frequent updating of contact lens and spectacle prescriptions to maintain useful vision. While RGP or other specially designed contact lenses or intracorneal ring segments may slow disease progression, a high percentage of patients eventually need a corneal transplant. This last-resort option is far from ideal because ectatic conditions affect not only the central cornea but also the far periphery. Donor grafts must be large, which increases the potential for induced astigmatism, long healing times and the activation of inflammatory cells near the limbus. Furthermore, after a transplant, few patients will see as well as they once did.

Given the current lack of a truly successful therapy for corneal ectatic diseases, the eyecare community is following with great interest research involving a potential new treatment � collagen cross-linking (CXL) with riboflavin and ultraviolet-A light. CXL is not yet FDA-approved, but several clinical trials are in progress. Based on the experience of international physicians and studies conducted around the world, CXL is the first treatment that appears to be capable of halting the progression of ectatic conditions, including post-LASIK ectasia.^1-3 CXL has been shown to improve the biomechanical strength of the cornea. It has also been shown to stabilize corneal curvature, in some studies for several years.⁴

The CXL procedure begins with the removal of the corneal epithelium. The cornea is then saturated with riboflavin drops and exposed to UVA light, usually for 30 minutes. The interaction of the light and the riboflavin stimulates production of free radicals that cause the formation of chemical bonds between and within the corneal collagen fibers. Because the riboflavin absorbs the UVA light, it also protects the corneal endothelium and other intraocular structures from photo-damage during the procedure.

New Corneal Analyzer Combines Advanced Technology with Ease of Use Like the iTrace Combo Visual Function Analyzer described in the accompanying article, the recently launched CA-200 Corneal Analyzer (Topcon Medical Systems, Inc.) is equipped with a keratoconus index to help clinicians diagnose keratoconus and other ectatic conditions of the cornea. The CA-200 is a placido-based topography system consisting of 24 rings on a 43D cone. It has corneal coverage from 0.3-10.5 mm and a diopter range from 1-20D. The instrument can operate as a stand-alone unit or in combination with an external PC with communication driven by WiFi data acquisition. It's controlled by an 8-inch touch screen display that makes acquisition, operation and evaluation of the cornea fast and easy. The CA-200 Corneal Analyzer is designed to operate intuitively. It automatically selects the bestfocused image and automatically determines pupil diameter during every eye measurement. Value-added optional software modules, for contact lens fitting, Zernike analysis and network viewing, are also available for the CA-200.

Early Detection Key to Preserving Vision

Early detection and management of corneal ectatic conditions are of the utmost importance for keeping progression in check and safeguarding vision.⁵ The earlier we can diagnose and treat, the more likely we are to prevent debilitating consequences for our patients. Corneal topography is extremely valuable � essential really � for early detection. Patients with any of the following characteristics should be closely monitored with topography:

■ family history of corneal ectatic disease
■ steep K values above 40
■ forme fruste keratoconus
■ against-the-rule astigmatism
■ astigmatism variance of greater than 1D between eyes
■ superior to inferior keratometry variance, 1.5D or more of steepening inferiorly compared to superiorly (Some patients with healthy corneas do exhibit asymmetric astigmatism. If it is with-the-rule, it usually does not indicate ectatic disease. However, if with-the-rule astigmatism changes to against-the-rule, it is cause for concern.)
■ frequent changes in refraction (Note that, in and of itself, a high level of myopia is not an indicator of ectatic disease.)

Making Use of Efficient and Effective Tools

Many quality corneal topography instruments are available to optometrists today. In our practice we find the iTrace Combo Visual Function Analyzer (Topcon Medical Systems, Inc.) improves our ability to diagnose ectatic corneal conditions early. With the iTrace Combo, we can perform not only corneal topography but also wavefront aberrometry, autorefraction, keratometry and pupillometry. Using ray tracing technology, this instrument differentiates the internal from the external optics of the eye. This means we can easily determine if the cornea is the primary source of an eye's aberrations, rather than, for example, the lens.

We can also efficiently assess wavefront error, which is directly applicable to the diagnosis of corneal ectatic disease. We can detect aberrations in the periphery, which are an early indicator of keratoconus. We can detect aberrations such as vertical coma, trefoil and quadrafoil that are typically elevated in patients with keratoconus or PMD. We can separate lower-order and higherorder aberrations and capture RMS values, which represent the total amount of distortion in the eye. High RMS values can be an early indicator of keratoconus.

As with other corneal topographers, the iTrace Combo provides simulated K values and standard axial maps, but it also calculates Z elevation maps, which help us to assess precise cone elevation and location for early diagnosis and monitoring of ectatic disease progression.

The iTrace Combo also allows for multi-zone refraction, which in my experience, makes it a more sensitive device fo r early detection of ectatic problems. The farther out from the corneal center we measure, the more likely we are to detect irregularities in the periphery, which is where the first signs of this group of diseases emerge. This is even more important when we consider that keratoconus tends to be diagnosed in younger patients who typically have larger pupils.

In addition, the iTrace Combo is equipped to utilize the Rabinowitz Keratoconus index, which helps to identify keratoconus suspects based on inferior-superior axial power indices and central corneal power.

iTrace Visual Function Analyzer at a Glance The iTrace Combo Visual Function Analyzer (Topcon Medical Systems, Inc.) provides a complete vision assessment, combining corneal topography, wavefront aberrometry, autorefraction, keratometry and pupillometry. Key features and capabilities of the iTrace Combo: ■ 256-point ray tracing technology for highly accurate wavefront and refractive analysis ■ complete evaluation of the corneal surface (maps include axial, Local ROC (Tangential), Refractive and Z-Elevation ■ measures lower- and higher-order aberrations and obtains complete aberration profile of the eye ■ distinguishes between lenticular and corneal optical aberrations ■ binocular and monocular open field fixation to objectively measure accommodation and overcome instrument myopia ■ over spectacle refraction and wave-front measurement ■ selectable multi-zone refractions ■ Retinal Spot Diagram provides graphical image of patient's total refraction, aberrations and Point Spread Function ■ keratoconus screening based on the Rabinowitz Keratoconus test ■ compare two corneal maps or two wavefront maps from different points in time and review total subtractive "difference" maps ■ portability

Positioned for the Future

As further research into CXL arms us with more information about its potential side effects and long-term results, it seems likely that our patients will be able to take advantage of this exciting new treatment at some point in the future. Optometrists who are experienced and comfortable using corneal topography and adjunctive testing to diagnose and monitor these conditions will be uniquely positioned to play a major role in the treatment of patients with corneal ectatic diseases.

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Dr. Karpecki focuses on corneal disease, ocular surface disease and clinical research at Koffler Vision Group in Lexington, Ky. He can be reached at Paul@Karpecki.com or 859-402-2814.

References

1. Hafezi F, Kanellopoulos J, Wiltfang R, Seiler T. Corneal collagen crosslinking with riboflavin and ultraviolet A to treat induced keratectasia after laser in situ keratomileusis. J Cataract Refract Surg 2007;33:2035-2040.
2. Wittig-Silva C, Whiting M, Lamoureux E, Lindsay RG, Sullivan LJ, Snibson GR. A randomized controlled trial of corneal collagen cross-linking in progressive keratoconus: preliminary results. J Refract Surg 2008;24:S720-S725.
3. Wollensak G. Crosslinking treatment of progressive keratoconus: new hope. Curr Opin Ophthalmol 2006;17:356-360.
4. Raiskup-Wolf F, Hoyer A, Spoerl E, Pillunat LE. Collagen crosslinking with riboflavin and ultraviolet-A light in keratoconus: long-term results. J Cataract Refract Surg 2008;34:796-801.
5. Bromley JG, Randleman JB. Treatment strategies for corneal ectasia. Curr Opin Ophthalmol May 19, 2010: e-pub ahead of print.