Ординатура / Офтальмология / Английские материалы / Wavefront Analysis Aberrometers and Corneal Topography_Boyd_2003

Much has been written about the typical videokeratography findings in keratoconus.22-24 The typical topographic pattern of keratoconus is characterized by high central corneal power, a steeper inferior cornea, a large difference between the power of the corneal apex and that of the periphery, and often a disparity in the central power between the two corneas of a given patient (Figure 5 and Figure 6). However, a significant number of keratoconus corneas have a cone that is centered.24 The superior cornea is steeper than the inferior cornea in rare cases of keratoconus. 24

Several computerized algorithms have been devised to aid the clinician in identifying kerato- conus.13-16 For example, the Rabinowitz-McDonnell index is based on the steepest area (SimK) and the difference in curvature between inferior and superior points (I/S). Table 1 summarizes this index.

Table 1

Adapted Rabinowitz-McDonnell Index

Pellucid marginal corneal degeneration is frequently misdiagnosed as keratoconus or merely an irregular corneal pattern (Figure 7).17 This is especially common in early cases. For example, SchmittBernard and coworkers 37 reported a case of severe, progressive, kerectasia after LASIK with enhancement in a patient that was thought to have kerato-

conus. Rabinowitz-McDonnell quantitative topographic indexes (Sim K and I/S) also suggested the diagnosis of keratoconus. However, review of the topographic maps revealed the eye actually had the classic features of pellucid marginal degeneration. A similar case was reported by Lanfond and coworkers.38

The topography in pellucid marginal degeneration is characterized by a very steep contour in the peripheral peri-limbal cornea with high power radiating in towards the center from the inferior oblique meridians (Figure 7). This pattern typically suggests a butterfly or a "lazy C" configuration. Additionally, there is an area of flattening down the center of the cornea. This topographic pattern typically generates a refraction with high against-the-rule astigmatism. In contrast, keratoconus has no central corneal flattening and an unpredictable astigmatism axis on refraction.

CONTACT LENS-INDUCED WARPAGE

Contact lens–induced corneal warpage is a common topographic abnormality detected in refractive surgery candidates (Figure 8).3,39,40 This condition is frequently confused with early keratoconus. Rigid contact lenses are especially likely to alter corneal topography and produce highly variable changes in corneal contour depending on the fit of the lens and the initial corneal shape. Warpage induced by rigid contact lenses typically takes longer to resolve. In most patients wearing rigid lenses the topographic and refractive changes will resolve to a normal, stable pattern within three weeks.40 In some severe cases, however, it may take months for a normal and stable pattern to return after discontinuation of lens wear.40 Our routine is to ask patients who wear rigid gas permeable lenses to discontinue them for a minimum period of three weeks prior to the preoperative examination. Most cases of mild warpage will resolve by then.40 If warpage is still present on corneal topography, the patient is followed at one to two month intervals until there is a return of a normal and stable topographic pattern. Warpage induced by rigid contact lenses will resolve while soft contact lenses are being worn. Many patients who have per-

sistent warpage tolerate discontinuation of rigid lenses if soft lenses are prescribed, even if the quality of vision isn’t as good with the soft lenses.

In contrast, warpage induced by soft contact lens typically resolves within three days of discontinuing lens wear.40 The rare cases soft contact lensinduced warpage lasting more than three days is detected on corneal topography. These cases are monitored at weekly intervals for the return of a normal and stable topographic pattern.

The signs of contact lens-induced warpage include central irregular astigmatism, loss of normal progressive flattening from the center to the periphery of the cornea, and a correlation between the resting position of the contract lens on the cornea and the topographic pattern (Figure 9). For example, in the

case of a superior riding contact lenses, it is common to find flattening over the superior cornea and relative steepening of the inferior cornea.40 This topographic pattern often mimics that found in early keratoconus (Figure 6). It is important to follow such cases without lens wear, monitoring the topography for return to a normal and stable pattern. 41 Manifest refraction and best spectacle-corrected visual acuity are other factors that often change over time following removal of the lenses. Typically, changes in these parameters occur in parallel with those observed in corneal topography. Significant error affecting uncorrected, and even best spectacle-corrected, visual acuity can be introduced by proceeding with LASIK or PRK before the return to a normal and stable topographic pattern.

OTHER CORNEAL DISORDERS

TERRIEN'S MARGINAL

DEGENERATION

Terrien's marginal degeneration is a peripheral thinning disorder of the cornea that is more common in young males. It can, however, be noted in either males or females of almost any age. The peripheral corneal guttering is most commonly detected in the superior cornea, although it may affect the entire circumference of the cornea in advanced cases. The area of thinning is frequently vascularized with an intact epithelium.

The topographic changes produced by Terrien's marginal are prominent flattening of the corneal contour.42 High against-the-rule astigmatism is also commonly present. The against-the-rule astigmatism is attributable to the predilection for involve-

ment of the superior and inferior limbal corneal tissue. These patients are not candidates for corneal refractive surgery.

PTERYGIUM

Pterygium is a fibrovascular connective tissue overgrowth of the cornea. A pterygium can produce marked changes in the refraction and corneal topography.43 Most pterygia are located in the horizontal meridian. This often induces with-the-rule astigmatism caused by localized flattening of the cornea central to the leading apex of the pterygium (Figure 10). 44-48 This astigmatism may be induced by mechanical traction of the pterygium or the pooling of tears adjacent to the head of the pterygium, or both. 43

CONCLUSION

Corneal topography remains an important tool for the diagnosis and treatment of corneal disorders. New diagnostic algorithms increase the utility of corneal topography for detecting ectatic diseases of the cornea. However, the clinician should continue to rely on visual inspection of a color-coded corneal map with an absolute scale. Corneal topography and wavefront analysis should be considered complimentary. Corneal topography can be used to determine which wavefront abnormalities are derived from the cornea and which are derived from other ocular structures.

REFERENCES

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2.Klyce SD. Computer-assisted corneal topography. High-resolution graphic presentation and analysis of keratoscopy. Invest Ophthalmol Vis Sci 1984;25:1426–35.

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8.Klyce, SD. Corneal topography and the new wave. Cornea 2000;19:723-729.

9.American National Standard Ophthalmics— Corneal Topography Systems—Standard Terminology. Requirements. ANSI Z80.23-1999. Optical Laboratories Association, American National Standards Institute, Inc, 1999.

10.Smolek MK, Klyce SD, Hovis JK. The Universal Standard Scale: proposed improvements to the American National Standards Institute (ANSI) scale for corneal topography. Ophthalmology 2002;109:361-9.

11.Wilson SE, Klyce SD. Quantitative descriptors of corneal topography: A prospective clinical study. Arch Ophthalmol 1990;109:349-53.

12.Chastang PJ, Borderie VM, CarvajalGonzalez S, Rostene W, Laroche L. Prediction of specta- cle-corrected visual acuity using videokeratography. J Refract Surg 1999;15:572-9.

13.Maeda N, Klyce SD, Smolek MK, Thompson HW. Automated keratoconus screening with corneal topography analysis. Invest Ophthalmol Vis Sci 1994;35:2749-57.

14.Maeda N, Klyce SD, Smolek MK. Comparison of methods for detecting keratoconus using videokeratography. Arch Ophthalmol 1995;113:870-4.

15.Smolek MK, Klyce SD. Current keratoconus detection methods compared with a neural network approach. Invest Ophthalmol Vis Sci 1997;38:2290-9.

16.Rabinowitz YS, Rasheed K. KISA% index: a quantitative videokeratography algorithm embodying minimal topographic criteria for diagnosing keratoconus. J Cataract Refrac. Surg. 1999;25:1327-35

17.Ambrósio R Jr, Klyce SD, Smolek MK, Wilson SE. Pellucid Marginal Corneal Degeneration. J Refract Surg 2002;19:86-8.

18.Holladay JT. Corneal topography using the Holladay Diagnostic Summary. J Cataract Refract Surg 1997;23:209-21.

19.Dingeldein SA, Klyce SD. The topography of normal corneas. Arch Ophthalmol 1989;107:512-518.

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21.Smolek MK, Klyce SD, Sarver EJ. Inattention to nonsuperimposable midline symmetry causes wavefront analysis error. Arch Ophthalmol 2002;120:439-47.

22.Maguire LJ, Klyce SD, McDonald MB. Corneal topography of pellucid marginal degeneration. Ophthalmology 1987;94:519–24.

23.Maguire LJ, Bourne WM. Corneal topography of early keratoconus. Am J Ophthalmol 1989;108:107–12.

24.Wilson SE, Klyce SD. The topography of keratoconus. Cornea 1991;10:2–8.

25.Rasheed K, Rabinowitz YS. Surgical treatment of advanced pellucid marginal degeneration. Ophthalmology 2000;107:1836-40.

26.Krachmer JH. Pellucid marginal corneal degeneration. Arch Ophthalmol 1978;96:1217¯1221.

27.Krachmer JH, Feder RS, Belin MW. Keratoconus and related noninflammatory corneal thinning disorders. Surv Ophthalmol 1984;28:293¯322.

28.Rabinowitz YS. Keratoconus. Surv Ophthalmol 1998;42:297¯319.

29.Santo RM, Bechara SJ, Kara-José. Corneal topography in asymptomatic family members of a patient with pellucid marginal degeneration. Am J Ophthalmol. 1999;127:205-207.

0.Kim WJ, Rabinowitz YS, Meisler DM, Wilson SE. Keratocyte apoptosis associated with keratoconus. Exp Eye Res 1999;69:475-81.

31.Kaldawy RM, Wagner J, Ching S, Seigel GM. Evidence of apoptotic cell death in keratoconus. Cornea. 2002;21:206-9.

32.Hernandez-Quintela E, Samapunphong S, Khan BF, Gonzalez B, Lu PC, Farah SG, and Azar DT. Posterior corneal surface changes after refractive surgery. Ophthalmology 2001;108:1415-22.

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34.Iskander NG, Anderson Penno E, Peters NT, Gimbel HV, Ferensowicz M. Accuracy of Orbscan pachymetry measurements and DHG ultrasound pachymetry in primary laser in situ keratomileusis and LASIK enhancement procedures. J Cataract Refract Surg 2001; 27:681-5

35.Fakhry MA, Artola A, Belda JI, Ayala MJ, Alio JL. Comparison of corneal pachymetry using ultrasound and Orbscan II. J Cataract Refract Surg 2002;28:248-52.

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1.Lebow KA, Grohe RM. Differentiating contact lens induced warpage from true kerato-conus using corneal topography. CLAO J 1999;25:114-22.

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Section III: Clinical Applications of Topography

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__________________

Renato Ambrósio Jr, MD Department of Ophthalmology,

University of Washington, Seattle, WA Department of Ophthalmology, University of São Paulo, São Paulo, Brazil Clínica e Microcirurgia Oftalmológica Renato Ambrósio, Rio de Janeiro, Brazil

Marcelo V. Netto, MD

Department of Ophthalmology,

University of Washington, Seattle, WA

Steven E. Wilson, MD

Professor and Chair

Department of Ophthalmology,

Grace E. Hill Chair in Vision Research

University of Washington,

Seattle, WA

Corresponding author: Steven E. Wilson, M.D.,

Department of Ophthalmology, Box 356385, Seattle, WA 98195-6485,

Tel: 206-543-5575, Fax: 206-543-4414

Acknowledgements: Supported in part by an unrestricted grant from Research to Prevent Blindness, New York, NY, and US Public Health Service grant EY 10056 from the National Eye Institute, National Institutes of Health, Bethesda, MD.

Proprietary interest statement: The authors have no proprietary or financial interest in relation to this manuscript.

INTRODUCTION

Keratoconus is characterized by non-inflam- matory stromal thinning and anterior protrusion of the cornea. Patients with this disorder are poor candidates for refractive surgery because of the possibility of exacerbating keratectasia. The development of the corneal ectasia is a well recognized complication of the LASIK and attributed to unrecognized preoperative forme fruste Keratoconus. It is known that posterior corneal elevation is an early presenting sign in keratoconus and hence it is imperative to evaluate posterior corneal curvature (PCC) in every LASIK candidate

ORBSCAN CORNEAL

TOPOGRAPHY SYSTEM

The ORBSCAN (BAUSCH & LOMB) corneal topography system mainley uses a scanning optical slit scan that is fundamentally different than the corneal topography that analyses the reflected images from the anterior corneal surface by the Placido rings system. The high-resolution video camera captures 40 light slits at 45 degrees angle projected through the cornea similarly as seen during slit lamp examination. The slits are projected on to the

anterior segment of the eye: the anterior cornea, the posterior cornea, the anterior iris and anterior lens. The data collected from these four surfaces are used to create a topographic map. This technique provides a lot of information about anterior segment of the eye, such as anterior and posterior corneal curvature and corneal thickness. It has an acquisition time of 4 seconds.1 It improves the diagnostic accuracy and it has passive eye-tracker from frame to frame, 43 frames are taken to ensure accuracy. It is easy to interpret and has good repeatability. The diagnosis of keratoconus is a clinical one and early diagnosis can be difficult on clinical examination alone. ORBSCAN has become a useful tool for evaluating the disease, and with the advent of its use, abnormalities in posterior corneal surface topography have been identified with keratoconus. Posterior corneal surface data is problematic because it is not a direct measure and there is little published information on normal values for each age group. In the patient with increased posterior corneal elevation in the absence of other changes, it is unknown whether this finding represents a manifestation of early keratoconus. The decision to proceed with refractive surgery is therefore more difficult. We always use the ORBSCAN system to evaluate our potential LASIK candidates preoperatively to rule out primary posterior corneal elevations.

PRIMARY POSTERIOR CORNEAL ELEVATION

All eyes undergoing LASIK are examined by ORBSCAN. Eyes are screened using quad maps with the normal band (NB) filter turned on. Four maps include (a) anterior corneal elevation: NB = ± 25 µ of best-fit sphere. (b) posterior corneal elvevation: NB = ± 25 µ of best fit sphere. (c) Keratometric mean curvature: NB = 40 to 48 D, K.

(d) Corneal thickness (pachymetry): NB = 500 to 600 µ. Map features within normal band are colored green. This effectively filters out variation falling within normal band. When abnormalities were seen on normal band quad map screening, a standard scale quad map was examined. For those cases with posterior corneal elevation, we also generated threedimensional views of posterior corneal elevation. In all eyes with posterior corneal elevation, the following parameters were generated (a) radii of anterior and posterior curvature of the cornea, (b) posterior best fit sphere, (c) difference between the corneal pachymetry value in 7mm zone and thinnest pachymetry value of the cornea.

Section III: Clinical Applications of Topography

Figures 1-6 show the various topographic features of an eye with primary posterior corneal elevation detected during our pre-LASIK assessment. In figure 1(general quad map) upper left corner map is the anterior float (sphere comparative system) upper right corner map is posterior float, lower left corner is keratometric map while the lower right is the pachymetry map showing a difference of 100 µm between the thickest pachymetry value in 7mm zone of cornea (613 µm) and thinnest pachymetry value (513 µm). In figure 2, normal band scale map of anterior surface shows "with the rule astigmatism" in an otherwise normal anterior surface (shown in green), the posterior float shows significant elevation inferotemporally. In Figure 2 only the abnormal areas are shown in red so it is easy to make out. Figure 3 is three-dimensional representation of the maps in figure 2. Figure 4 shows three-dimensional representation of anterior corneal surface with reference sphere. Figure 5 shows three-dimensional representation of posterior corneal surface showing a significant posterior corneal elevation. Figure 6 shows amount of elevation (color coded) of the posterior corneal surface in microns (50 µm).