Wavefront analysis detects pellucid marginal degeneration before LASIK

Bausch & Lomb Zywave used on patient with decreased vision and monocular diplopia in his left eye.

Issue: October 1, 2004

ByEugenio M. Candal, MD

ByLaura T. Muller, MD

ByDianna L. Seldomridge, MD, MBA

ByRandy J. Epstein, MD

Case report

An 18-year-old white man with monocular astigmatism was referred by an ophthalmologist for a corneal consultation before possible LASIK surgery. The patient complained of decreased vision and monocular diplopia in his left eye only for 6 months prior to presentation.

The patient’s past medical and ocular histories were unremarkable. On examination, the patient’s uncorrected visual acuity was 20/20 in the right eye and 20/30 in the left eye. Manifest refraction was –0.50 sphere=20/20 in the right eye and –0.75 +1.75 × 010=20/25 in the left eye. Slit lamp and dilated fundus examinations were normal in both eyes.

Corneal topography was performed with the TMS-1 unit (CBD Ophthalmic, Tomey Technology). This revealed against-the-rule astigmatism and a “crab’s claw” pattern in the left eye (Figures 1 and 2). This pattern was felt to be consistent with the topographical findings characteristically associated with pellucid marginal degeneration. With the TMS-1 program for keratoconus screening, the left eye was positive utilizing the Rabinowitz method only. The Klyce-Maeda indices for keratoconus were negative in both eyes (Figures 3 and 4). Orbscan II testing in the right eye revealed a posterior float of 0.018 mm. The left eye had a relatively increased posterior float of 0.04 mm (Figures 5 and 6).

Utilizing the Bausch & Lomb Zywave aberrometer, we performed wavefront analysis on this patient (Figures 7 and 8). The right eye was normal, with total higher-order aberrations of 0.36 µm. The Zernike RMS at 6 mm was 0.31 µm. There were markedly increased total higher-order aberrations in the left eye of 1.79 µm. Vertical and horizontal coma and spherical aberrations were particularly increased. The Zernike RMS at 6 mm was 1.6 µm. The patient was told he was not a candidate for refractive corneal surgery and was referred for contact lens fitting.

Topography in the right eye was normal.	Topography in the left eye showed against-the-rule astigmatism in a “crab’s claw” pattern.
Keratoconus screening programs of the right eye were both negative.	Keratoconus screening with the Rabinowitz method was positive in the left eye. Testing using the Klyce-Maeda method was negative.
Orbscan in the right eye was normal. The posterior float measured 0.018 mm.	Orbscan in the left eye showed a relatively increased posterior float of 0.04 mm.
Wavescan in the right eye was normal, with higher-order aberration of 0.36 µm. The Zernike RMS at 6 mm was 0.31 µm.	Wavescan in the left eye showed markedly increased total higher-order aberrations of 1.79 µm. The Zernike RMS at 6 mm was 1.6 µm.
Images: Epstein RJ, et al

Discussion

Pellucid marginal degeneration was first described by Krachmer as a rare, non-ulcerative peripheral thinning disorder involving the inferior cornea. Diagnosis is typically made between the ages of 20 and 40 years. Incidence is approximately equal in men and women. This ectatic disorder results in high against-the-rule astigmatism. In contrast to keratoconus, where the greatest corneal protrusion is in the area of maximal thinning, the greatest corneal protrusion is noted to be above the area of maximal thinning in pellucid marginal degeneration.

Maguire et al originally described the topography of pellucid marginal degeneration. It typically displays decreased corneal powers in the vertical axis associated with increasing dioptric powers toward the inferior peripheral limbus. This gives the characteristic inverted horseshoe- or moustache-shaped steepening observed on topography.

Maeda et al recently compared the ocular wavefront aberrations of 35 keratoconic eyes and 38 normal eyes. Their results revealed a statistically significant difference in higher-order aberrations, coma-like aberrations and spherical-like aberrations between normal and keratoconic eyes under photopic and scotopic conditions. Of note was a dominance of coma-like aberrations over spherical-like aberrations in keratoconic eyes.

Schwiegerling et al have investigated the corneal wavefront aberrations in patients with keratoconus utilizing elevation data obtained by corneal topographical analysis. Their study suggested that keratoconic corneas could be differentiated from normal corneas with the Z3 term of Zernike polynomials in corneal topography.

Wavefront analysis of a patient with suspected pellucid marginal degeneration is an invaluable confirmatory test. Our studies have demonstrated elevated higher-order aberrations in other eyes with pellucid marginal degeneration. This is consistent with Maeda’s findings in patients with keratoconus. Wavefront analysis represents a new, highly sensitive and important diagnostic tool that can detect ectatic degenerations and exclude these patients from consideration for laser refractive surgery, given present technology. Large studies evaluating wavefront aberrations in patients with pellucid marginal degeneration have yet to be performed but will help to further characterize this condition.

It is important that clinicians familiarize themselves with the topographic characteristics of this condition, as these patients frequently seek the services of refractive surgeons. Recent reports indicate that they have a high risk of developing iatrogenic ectasia after LASIK surgery, and many of these patients are “missed” by routine keratoconus detection programs. If there is any doubt as to the presence of this condition, the combination of Orbscan testing, which usually reveals an increased posterior float as demonstrated by Rao et al, confirmed by Zywave wavefront analysis can be helpful in identifying these patients before surgery.

For Your Information:

Randy J. Epstein, MD, can be reached at Chicago Cornea Consultants, 806 Central Ave., Highland Park, IL 60035; 847-432-6010; fax: 847-432-8241; e-mail: repstein@chicagocornea.com. Dr. Epstein is a paid consultant for Bausch & Lomb.

References:

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

Maguire LJ, Klyce SD, et al. Corneal topography of pellucid marginal degeneration. Ophthalmology. 1987;94(5):519-524.

Maeda N, Fujikado T, et al. Wavefront aberrations measured with Hartmann-Shack sensor in patients with keratoconus. Ophthalmology. 2003;109(11):1998-2003.

Schwiegerling J, Greivenkamp JE. Keratoconus detection based on videokeratographic height data. Optom Vis Sci. 1996; 73:721-728.

Fogla R, Rao SK, Padmanabhan P. Keratectasia in 2 cases with pellucid marginal corneal degeneration after laser in situ keratomileusis. J Cat Refract Surg. 2003;29:788-791.

Rabinowitz Y. Videokeratographic indices to aid in screening for keratoconus. J Refract Surg. 1995;11:371-379.

Klein SR, Muller LT, et al. Poor visual outcomes following refractive surgery in patients with clinically unsuspected pellucid marginal degeneration. Ophthalmology. 2003 AAO supplement;250.

Rao S, Raviv T, Majmudar PA, Epstein RJ. Role of Orbscan II in screening keratoconus suspects before refractive corneal surgery. Ophthalmology. 2002;109:1642-1646.