Corneal Topography of Early Keratoconus

AMERICAN

JOURNAL

OPHTHALMOLOGY®

OF

AUGUST, 1989

NUMBER 2

VOLUME 108

Corneal Topography of Early Keratoconus Leo

J. Maguire, M.D., and William M. Bourne, M.D.

We used a corneal topography analysis system to evaluate nine eyes of seven patients in whom the diagnosis of keratoconus was suspected. There was no slit-lamp evidence of the condition. In seven of nine eyes a cone was identified. Large amounts of corneal distortion were seen in selected patients even though they had excellent spectacle-corrected visual acuity and little or no distortion of the keratometer mires. These findings suggest that corneal topography analysis systems are useful in the detection and description of corneal irregularity in the early stages of keratoconus. The radial keratotomy surgeon should be aware that normal results on slit-lamp examination and normal keratometry and refractive data do not rule out the presence of early keratoconus. is a noninflammatory corneal thinning disorder characterized in its most advanced form by a localized conical protrusion of the cornea associated with an area of corneal stromal thinning most marked at the apex of the cone.V When the characteristic slit-lamp findings of advanced keratoconus are seen, the diagnosis can be made readily. The earliest stages of keratoconus usually develop between puberty and 30 years of age." The astute clinician suspects the condition in adolescents and KERATOCONUS

young adults who complain of progressive myopic astigmatism and subtle spectacle blur even when classic slit-lamp findings are not present. The finding of a focal round or oval area of internal reflection in the central or inferior cornea while inspecting the red reflex with the direct ophthalmoscope helps confirm the diagnosis.V As early as 1946 Amsler' recognized the utility of photokeratoscopy in the detection of the early stages of keratoconus in which spectaclecorrected visual acuity may still be excellent and slit-lamp findings of the condition minimal to nonexistent. Rowsey, Reynolds, and Brown" described similar findings. Recent advances in computer-based analysis of keratoscope images 6-8 now offer the opportunity to evaluate in exquisite detail the patterns of power distribution seen in the earliest stages of keratoconus and offer the opportunity for earlier diagnosis and a better understanding of the degree of corneal irregularity compatible with a given level of visual function. We used a highly sensitive computer-based corneal topography analysis system to try to detect the presence of keratoconus in patients without slit-lamp evidence of the disease who had normal keratometry readings and excellent spectaclecorrected Snellen visual acuity.

Material and Methods Accepted for publication May 2, 1989, From the Department of Ophthalmology, Mayo Clinic, Rochester, Minnesota, This study was supported in part by National Institutes of Health grant EY 02037 (Dr. Bourne), Research to Prevent Blindness, Inc., and the Mayo Foundation (Dr. Maguire). Reprint requests to Leo]. Maguire, M.D., Department of Ophthalmology, Mayo Clinic, 200 First St. S.W., Rochester, MN 55905.

©AMERICAN JOURNAL OF OPHTHALMOLOGY

108:107-112,

From May 1988 through January 1989, we performed topographic analysis on a group of eyes in which no slit-lamp evidence of keratoconus (stromal thinning, Fleischer's ring, Vogt's striae, anterior stromal scar) was found but in whom the diagnosis was suspected. Of the seven patients in the study, five had defiAUGUST,

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nite slit-lamp evidence of keratoconus in the fellow eye. In the remaining two patients no slit-lamp evidence of keratoconus was found in either eye, but the diagnosis was entertained because of patient complaints of mild spectacle blur in at least one eye despite best-corrected visual acuity of 20/30 or better. None of the eyes with suspected keratoconus showed slit-lamp evidence of the condition. In all cases bestcorrected visual acuity with spectacles was 201 30 or better, with less than 2.25 diopters of cylinder in the spectacle refraction. No patient had more than 3 diopters of keratometric astigmatism (Table). Patients 1 and 7 had worn a soft contact lens in the eye studied. We obtained keratoscope images of the study eyes with the Corneal Modeling System (Computed Anatomy, New York, New York), a computer-based corneal topography analysis systern." Four keratoscope video images were captured for each eye. The technician did not accept to computer memory any image for which the aiming laser images were not exactly superimposed, any image in which patient fixation was in doubt, or any image showing evidence of tear film artifact. All video images were processed using the revised Corneal Modeling System analysis software updated in May 1988. Notation was made of any errors in the autodigitizing process. The video images were ranked in order of overall quality, taking into consideration the amount of surface area covered by the keratoscope mires and the quality of the autodigitizing process. The image with the highest rank was used later to generate the color-coded topography maps. None of the maps were generated until after collection of all visual acuity data. In those cases in which a cone was identified on the topography maps, we determined corneal power at the cone apex and the location of the apex of the cone relative to the visual axis point.

Results Of the nine eyes studied, seven showed definite evidence of early keratoconus. The cone apex was located 1.3 to 2.5 mm from the visual axis in all patients. All cones were located inferior to the visual axis between the 245- and the 301-degree hemimeridians. The degree of irregularity observed varied widely between patients.

The most subtle cone detected was in the left eye of Patient 7 (Fig. 1). In this case each color in the contour map represents a small (0.3diopter) range in surface power. Colors in the blue spectrum represent lower powers and colors closer to the red spectrum represent higher powers. Power in this case ranges from a low of 41.5 to greater than 44.5 diopters. The area of the cone apex, identified by the red color interval, is surrounded by concentric bands of increasingly lower corneal power. Lowest corneal power is seen in the superior half of the cornea. The most advanced cones observed were in the left eye of Patient 2 (Fig. 2) and the left eye of Patient 3 (Fig. 3). In Figure 2, each color represents a 1.1-diopter range of power and overall power ranges from 39.9 to greater than 50.9 diopters. In Figure 3, each color represents a 1.2-diopter range and the overall range is between 38.9 and greater than 50.9 diopters. Both of these patients had excellent spectaclecorrected visual acuity and less than 2 diopters of keratometric astigmatism despite the presence of this degree of irregularity (Table). The power distribution in the other patients with identifiable cones showed similar patterns to those shown in Figures 1 through 3, with degrees of irregularity greater than those shown in Figure 1 and less than those seen in Figures 2 and 3. Patient 1 was the only patient with definite keratoconus in the fellow eye who did not show an early cone in the study eye (Table). Patient 2 showed topographic evidence of keratoconus in the left eye (Fig. 2) but no evidence of early involvement in the right eye (Fig. 4).

Discussion Our results suggest that very early stages of keratoconus can be detected by using a sensitive corneal topography analysis system. The analysis system was able to detect a pattern of power distribution consistent with keratoconus in seven of nine patients in whom the diagnosis was suspected. Those who are interested in detecting pilots at risk for the development of visually disabling keratoconus before investing in their training should find the results of this study encouraging." Investigators interested in better understanding the earliest stages of the natural course of cone development and those interested in determining the true incidence of unilateral keratoconus and the incidence of

Fig. 1 (Maguire and Bourne). Computer-generated color contour map of the left eye of Patient 7. Each color represents a 0.3-diopter range of surface power. The range extends from a low of 41.5 to greater than 44.5 diopters. The area of the cone apex, identified by the red color interval, is surrounded by concentric bands of increasingly lower corneal power. Lowest corneal power is seen in the superior half of the cornea. The cursor (cross) has been placed at the cone apex.

Fig. 2 (Maguire and Bourne). Contour map of the left eye of Patient 2. This cornea is more irregular. Each color interval represents a 1.1-diopter range of power. The overall range extends from 39.9 to greater than 50.5 diopters. The location of the cone apex is readily apparent. Visual acuity was 20/20, with a spectacle correction of -2.50.

Fig. 3 (Maguire and Bourne). Contour map of the left eye of Patient 3. Each color represents a 1.2-diopter range and the overall range is 38.9 to greater than 50.9 diopters.

Fig. 4 (Maguire and Bourne). Contour map of the right eye of Patient 2. Each color represents a 0.3-diopter range and the overall range is 41.9 to greater than 44.8 diopters. Unlike the patient's opposite eye (Fig. 2), no topographic evidence of early keratoconus is seen.

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Corneal Topography of Early Keratoconus

Vol. 108, No. 2

TABLE SUMMARY OF CORNEAL MODELING SYSTEM FINDINGS TOPOGRAPHIC APEXOF CONE

VISUAL PATIENT NO., ACUITYWITH AGE (YRS), SEX EYE SPECTACLES

1,62, M

RE.

20/20-

2,27, M

L.E. RE. RE. L.E.

20/20 20/20 20/20

4,29, F

RE.

20/25

5,29,M

L.E.

20/20

6,30, F

RE.

7, 19, F

L.E.

3,27, M

20/30

MANIFEST REFRACTION

-1.00 +0.75 x 14 -2.50 +9.75' -3.00 -2.25 +1.25 x 12 -2.00

DISTANCE FROM VISUAL HEM1MAXIMUM AXIS MERIDIAN POWER (MM) (0) (OEG)

KERATOMETER MIRE DISTORTION

KERATOMETRY READING

No evidence of keratoconus

43.87 x 44.12 x 98

Mild

2.3 263 52.80 No evidence of keratoconus 45.2 2.5 255 2.0 280 52.2

43.12 43.00 43.25 43.75

Mild None None Moderate

x x x x

43.87 45.75 44.00 45.50

x x x x

33 112 05 05

1.6

271

50.7

45.25 x 47.00 x 85

Mild

-1.25 +2.00 x20

2.5

245

47.6

42.25 x 45.00 x 40

None

20/20

-2.00 +0.75 x 166

1.3

301

45.8

43.75 x 43.87 x 163

None

20/20

Plano

2.1

259

44.5

43.25 x 44.00 x 90

None

SLIT-LAMP FINDINGS IN FELLOW EYE

Advanced keratoconus

Severe keratoconus Penetrating keratoplasty for keratoconus in 1978 Penetrating keratoplasty for keratoconus in 1985 Moderate

'Cataract secondary to nonpenetrattng BB injury; cataract extraction age 10 years.

early keratoconus in the normal population appear to have a new tool." Most importantly, topographic analysis, when used as a screening test, may help ophthalmologists interested in refractive corneal surgery avoid operating on an eye with unsuspected abnormalities in corneal curvature. Many variables have been suggested to contribute to the lack of refractive accuracy of radial keratotomy" and other refractive surgical procedures. 12 Heterogeneity of the topography of the preoperative surface has only recently been suggested as one of them. 13 It has been assumed that if the refraction, visual acuity, and keratometry readings are normal, the corneal surface is normal. The results of our study suggest otherwise. All of the patients in whom early keratoconus was identified had excellent visual acuity with spectacle correction and no slit-lamp evidence of the condition. Four of the seven patients showed no distortion of the keratometer mires. Three others showed only subtle mire distortion. These keratometric findings are not sur-

prising because the degree of keratoconus in these eyes was not severe and the cones were not located near the two points on the corneal surface measured by the keratometer. The refractive surgeon may well ask whether some patients with poor results after radial keratotomy have had similar preoperative topographic findings. This pilot study suggests that computerbased corneal topography analysis systems similar in design to the Corneal Modeling System are useful in the detection of early stages of keratoconus. If our findings are confirmed by other investigators, such systems should be considered for use in population-based studies to determine the true incidence of early keratoconus and to determine the degree of topographic heterogeneity in the normal population. If significant corneal irregularity is found in a relatively large percentage of normal subjects, computer-based topography analysis may become a useful preoperative screening procedure for patients interested in refractive corneal surgery.

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References 1. Maguire, L. J.: Ectatic corneal degenerations. In Kaufman, H. E., Barron, B., McDonald, M. B., and Waltman, S. R (eds.): The Cornea. New York, Churchill Livingston, 1988, pp. 485-490. 2. Krachmer, J. H., Feder, R., and Belin, M. W.: Keratoconus and related noninflammatory corneal thinning disorders. Surv. Ophthalmol. 28:293, 1984. 3. Amsler, M.: Quelques donnees du problerne du keratocone. Bull. Soc. Belge Ophthalmol. 129:33, 1962. 4. - - : Keratocone classique et keratocone fruste. Arguments unitaires. Ophthalmologica 111:96, 1946. 5. Rowsey, J. J., Reynolds, A. E., and Brown, R: Corneal topography. Corneascope. Arch. Ophthalmol. 99:1093, 1981. 6. Klyce, S. D.: Computer-assisted corneal topography. High resolution graphic presentation and analysis of keratoscopy. Invest. Ophthalmol. Vis. Sci. 12:1426, 1984.

August, 1989

7. Maguire, L. J., Singer, D. E., and Klyce, S. D.: Graphic presentation of computer analyzed keratoscope photographs. Arch. Ophthalmol. 105:223, 1987. 8. Gormley, D. J., Gersten, M., Koplin, R. S., and Lubkin, V.: Corneal modeling. Cornea 7:30,1988. 9. Barry, W. E., and Tredici, T. J.: Keratoconus in USAF personnel. Aerospace Med. 43:1027, 1972. 10. Kennedy, R H., Bourne, W. M., and Dyer, J. A.: A 48-year clinical and epidemiologic study of keratoconus. Am. J. Ophthalmol. 101:267, 1986. 11. Binder, P. S.: Radial keratotomy in the United States. Where are we six years later? Arch. Ophthalmol. 105:37, 1987. 12. Maguire, L. J., Klyce, S. D., Singer, D. E., McDonald, M. B., and Kaufman, H. E.: Corneal topography in myopic patients undergoing epikeratophakia. Am. J. Ophthalmol. 103:404, 1987. 13. Dingeldein, S. A., and Klyce, S. D.: Computer assisted corneal topography of normal corneas. Arch. Ophthalmol. 107:512, 1989.

OPHTHALMIC MINIATURE

Being now prepared to make the section of the cornea, the surgeon, holding the knife as he would a pen, should rest the ring and middle finger of the right hand upon the temple of the patient, near the outer canthus of the eye; and place the flat part of the blade of the knife upon the surface of the cornea, and try whether he can carry the point of the instrument to the nose, without shifting the position of the fingers on the cheek. By touching the cornea, the patient is warned of the commencement of the operation .... Frederick Tyrrell, A Practical Work on the Diseases of the Eye and Their Treatment, Medically, Topically, and by Operation, Vol. II. London, John Churchill, 1840, p. 396