Characteristics of Ocular Higher-Order Aberrations in Patients with Pellucid Marginal Corneal Degeneration

1. Characteristics of Ocular Higher-Order Aberrations in Patients with Pellucid Marginal Corneal Degeneration Yoshinori Oie, MD,1 Naoyuki Maeda, MD,1 Ryo Kosaki, MD,1 Kohji Nishida, MD,2 Takashi Fujikado, MD,3 and Yasuo Tano, MD1 1Department of Ophthalmology, Osaka University Medical School, Suita, Japan 2Department of Ophthalmology and Visual Science, Tohoku University Graduate School of Medicine, Sendai, Japan 3Department of Applied Visual Science, Osaka University Medical School, Suita, Japan Financial interest Dr. Maeda received research grant from Topcon Corporation.

3. Methods The medical records of 20 eyes with pellucid marginal corneal degeneration (PMCD group), 76 eyes with keratoconus (KC group), and 105 normal eyes (control group) were retrospectively reviewed. Patients attended the outpatient clinic of the Department of Ophthalmology at Osaka University Medical School between February 2002 and May 2006. The higher-order aberrations (HOAs) for the central 4 mm diameter to estimate photopic vision were obtained up to forth order with the Hartmann-Shack wavefront analyzer (KR-9000PW, Topcon, Corporation, Tokyo, Japan) All data from the KR-9000PW database were extracted by using a prototype program of calculating Zernike vector analysis. The magnitudes and orientations of trefoil, coma, tetrafoil, secondary astigmatism in addition to the magnitude of the spherical aberration and total HOAs (third order component (S3), fourth order component (S4); S3 + S4) were compared among the three groups. The simulated retinal image of Landolt ring of pellucid marginal corneal degeneration, keratoconus, control eyes was also investigated.

4. Zernike vector terms

5. Diagnosis of pellucid marginal corneal degeneration and keratoconus Pellucid marginal corneal degeneration Inferior corneal thinning with ectasia of the cornea above the area of thinning, with no evidence of scarring, vascularization or lipid deposition by slit-lamp examination Keratoconus Central thinning of the stroma with Fleischer ring and/or Vogt’s striae by slit-lamp examination. Pellucid marginal corneal degeneration suspect, keratoconus suspect Typical topographic patterns of pellucid marginal degeneration or keratoconus was observed in the axial power videokeratographic map for visual inspection without any abnormal findings in the slit-lamp and visual acuity examinations. Eyes diagnosed with pellucid marginal corneal degeneration suspect and keratoconus suspect were excluded in this study.

6. Results

8. Each of the third- and fourth-order Zernike vector terms on polar coordinates

9. Simple averages of magnitude for total HOAs and each Zernike vector term

10. The percentages of eyes with comet-like image with two tails in simulated retinal images of Landolt rings

11. Discussion We suggest that the inferior slow pattern of coma in PMCD and KC groups is caused by the inferosuperior asymmetric pattern in power distribution. The difference of the sign of spherical aberration might be also helpful to differentiate pellucid marginal corneal degeneration and keratoconus. We speculated that there might be the association between steep axis (infero-nasal and infero-temporal) and the direction of two tails in the retinal image in pellucid marginal corneal degeneration. The HOAs patterns of PMCD and KC groups are characterized by the following three patterns of trefoil, coma, and spherical aberration.

12. Conclusion Although pellucid marginal corneal degeneration and keratoconus are categorized as noninflammatory corneal thinning disorders, the patterns of higher-order aberrations in both groups have different characteristics possibly due to the difference in the position of the corneal apex relative to the entrance pupil. The eyes with pellucid marginal corneal degeneration and keratoconus may possess distinctively different quality of vision.