Article - Corneal topography in Ehlers-Danlos Syndrome

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Corneal topography in Ehlers-Danlos Syndrome

by:

Mark L. McDermott, MD, Jack Holladay, MD, Liu, PhD, E. Puklin,

MD, Dong H. Shin, MD, PhD, W. Cowden, MD

Abstract

Purpose:

To assess the use of corneal topography in conjunction with slitlamp

biomicroscopy and retinoscopy to diagnose keratoconus in a large group of

patients with Ehlers-Danlos Syndrome (EDS).

Setting:

Kresge Eye Institute, Wayne State University, Detroit, Michigan, USA.

Methods:

Thirty-six patients (72 eyes) with genetically typed EDS had slitlamp

biomicroscopy, retinoscopy, and videokeratography with the EyeSys

instrument. The presence or absence of slitlamp keratoconus findings was

correlated to a presumptive diagnosis based on corneal topography using

derived topographic indexes associated with keratoconus. These topographic

indexes included central corneal power (CCP), difference in CCP,

inferosuperior asymmetry (I-S) value, and asphericity (Q). Axial and profile

difference maps were generated and analyzed for findings suggestive of

keratoconus.

Results:

In 72 eyes, no keratoconus was found using slitlamp biomicroscopy. No eye

had an I-S value greater than 1.60 diopters (D), 2 eyes had a CCP greater

than 46.50 D, and 2 eyes had a Q value less than - 1.00. Eight of 36 pairs

of eyes had an intereye CCP greater than 0.92 D. In both eyes of the patient

with Q values less than - 1.00, the profile difference maps were mildly

abnormal.

Conclusions:

Slitlamp biomicroscopy of the cornea was unremarkable in all patients. Only

1 patient had Q values and profile difference maps that were mildly

suggestive of keratoconus. Even after adding topography to the examination,

it appears that keratoconus in a known population of patients with EDS

remains rare.

J Cataract Refract Surg 1998; 24:1212-1215

Previous studies characterizing the corneal abnormalities associated with

Ehlers-Danlos Syndrome (EDS) have relied on slitlamp biomicroscopy,

keratometry, and pachymetry. Most corneal abnormalities described were

related to an alteration in corneal curvature, reduction in corneal

thickness, or both (1,2). These corneal curvature alterations include cornea

plane, keratoconus, and keratoglobus. Most corneal changes were found in

patients with EDS type VI. In patients who do not have EDS, the reported

incidence of keratoconus (50 to 230 per 100,000 persons)(2,3) is variable

and may be underreported because conventional keratometry may fail to detect

small, inferiorly displaced cones that lie outside the central area sampled

by the instrument (1,4).

Based on clinical experience, many corneal surgeons maintain that corneal

ectasia, including keratoconus, is more common with EDS than in the general

population (2). Since these observations are, in part, anecdotal or based on

studies published before digital videokeratoscopy, we questioned whether

using this technique would uncover additional cases of subclinical

keratoconus not detected by slitlamp findings in a genetically defined

population of EDS patients.

Other investigators (2) have performed topographic analysis of the cornea

using digital videokeratoscopy to identify preclinical keratoconus in the

general population. Using the data derived from digital processing of the

video image, Rabinowitz and McDonnel (5-7) published topographic indexes

they believe to be sensitive indicators of subclinical keratoconus. Our

study's use of these indexes is extrapolated from Rabinowitz and McDonnell's

studies, which concentrated on identifying subclinical keratoconus in family

members of patients with keratoconus.The indexes used consisted of central

corneal power (CCP), computed by placing the cursor at the center of the

innermost ring; the intereye difference in CCP; the inferosuperior asymmetry

(I-S) value. The I-S value is the difference between the average inferior

corneal power and average superior corneal power computed at equal distances

from the center of the cornea. The average inferior corneal power is the

mean of corneal power readings at 5 points (210, 240, 270, 300, and 330

degrees) 3.0 mm from the center of the cornea. The average superior corneal

power is the mean of corneal power readings at 5 corresponding points (30,

60, 90, 120, and 150 degrees) 3.0 mm from the center of the cornea. In 1997,

Holladay (8) reported that an asphericity index (Q) is also a sensitive

indicator of keratoconus. To assess their ability to predict keratoconus,

the indexes and color-coded axial and profile difference maps for patients

with EDS were compared with slitlamp findings.

Patients and Methods

Thirty-six patients (72 eyes) with EDS were evaluated: type 1 (7 patients),

type II (7 patients) type III (17 patients), type IV (4 patients), and type

VI (1 patient). All patients had slitlamp biomicroscopy performed by a

fellowship-trained corneal specialist, central corneal pachymetry,

retinoscopy, and digital videokeratoscopy. Slitlamp examination was used to

detect keratoconus findings; no specific attempt was made to detect

keratoglobus.

Corneal analysis was done using the EyeSys instrument (EyeSys Technologies).

Version 3-1 software for the Holladay Diagnostic Summary was used to

generate topographic maps. Axial and profile difference maps for all eyes

were examined for corneal ectasia suggestive of keratoconus. Holladay's

profile difference map (8) was used as it is helpful in diagnosing corneal

diseases such as keratoconus, keratoglobus, and pellucid marginal

degeneration in which the cornea changes its overall shape. This map

compares the patient's actual cornea to the normal aspheric cornea. The

difference between the two is plotted on a color-coded map. If the patient's

cornea is steeper than the normal aspheric cornea, the difference is plus

and it is toward the red. In a keratoconic cornea, this map can easily

delineate the extent and location of a cone. The CCP, intereye difference in

CCP, and I-S value were computed using a custom software program developed

by one of the authors (D.L.)(4-6). Asphericity was calculated using software

included in the Holladay Diagnostic Summary, version 3-1.

Results

All slitlamp findings of the cornea were unremarkable. No eye showed

evidence of prominent corneal nerves, Fleischer ring, Vogt's striae, breaks

in Bowman's or Descemet's membrane, heightened endothelial reflex, apical

thinning, or inferior cone formation. No scissoring of the reflex was

observed on retinoscopy. Central corneal pachymetry showed no values less

than 0.4 mm. Table 1 (not included here) shows computer-assisted topographic

indexes for all EDS types. To analyze the data, published values 2 standard

deviations from normal were used as follows: CCP greater than 46.50 diopters

(D) (5); intereye difference in CCP 0.92 (5); IS-value greater than -1.60 D

(5) Q less than -1.00 (8).

These values were used to analyze the data in Table 1 (72 eyes). No eye had

an I-S value greater than 1.60 D. In 2 eyes, the CCP exceeded 46.50 D

(patients 23 and 26, left eyes). Of 26 pairs of eyes, 8 had an intereye

difference in CCP greater than 0.98 D (patients 10, 11, 14, 23, 15, 26, 27,

and 32). In 2 eyes (patient 24 both eyes), the Q value was less than -1.00.

In that same patient, the profile difference maps were mildly suggestive of

keratoconus (Figure 1 - note included here).

Discussion

In this study, an isolated inter-eye difference in CCP exceeding 0.92 D

appeared relatively non-specific since 8 of 36 patients appeared to be

keratoconus suspects. It is unlikely these patients have keratoconus since

the slitlamp examinations, I-S values, and Q values were normal and the

profile difference maps were unremarkable. When the I-S values were

computed, no eye had a value greater than the 1.60 D threshold. Although I-S

values were calculated at 3.0 mm with the EyeSys system, they may represent

slightly different values than those reported by Rabinowitz and McDonnell

(5) using the CMS device with a different algorithm. In 1 case with a normal

I-S value (patient 24, both eyes), both the profile difference map and Q

values were mildly suggestive of keratoconus, despite negative slitlamp

findings of keratoconus.

Our findings are somewhat surprising given the initial hypothesis; that is,

in a group having a higher incidence of keratoconus than the general

population, no definitive cases were uncovered, even with the addition of

topography. It is possible that if a larger group of patients were collected

or a prospective study of the present group undertaken, some patients may be

found to have keratoconus. However, in this large cohort of 72 eyes, no

definitive cases of keratoconus were seen.

References

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Cornea 1993; 12:54-49

2. Maguire LJ, Meyer RF. Ectatic corneal degenerations. In: Kaufman HE,

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Churchill-Livingstone, Inc, 1988; 485-510

3. Kennedy RH, Bourne WM, Dyer JA. A 48-year clinical and epidemiologic

study of keratoconus. Am J Ophthalmol 1986; 101:267-273

4. Maguire LJ, Bourne WM. Corneal topography of early keratoconus. Am J

Opthalmol 1989; 108:107-112

5. Rabinowitz YS, McDonnell PJ. Computer-assisted corneal topography in

keratoconus. Refract Corneal Surg 1989; 5:400-408

6. Rabinowitz YS, Garbus J, McDonnell PJ. Computer-assisted corneal

topography in family members of patients with keratoconus. Arch Ophthalmol

1990; 108:365-371

7. V, McDonnell PJ. Computer-assisted corneal topography in parents

of patients with keratoconus. Arch Ophthalmol 1992; 110:1412-1414

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Cataract Refract Surg 1997; 23:209-221

Presented in part at the 66th annual meeting of the Association for Research

in Vision and Opthalmology, Sarasota, Florida, USA, May 1993

Drs. McDermott, Puklin, and Shin have no proprietary interest in any product

mentioned. Dr. Holladay is a software contributor to EyeSys Technologies,

and Dr. Liu is employed by EyeSys Technologies.