Elsevier

Ophthalmology

Volume 111, Issue 4, April 2004, Pages 752-757
Ophthalmology

Original article
Corneal regular and irregular astigmatism assessed by Fourier analysis of videokeratography data in normal and pathologic eyes1 ,

https://doi.org/10.1016/j.ophtha.2003.06.019Get rights and content

Abstract

Purpose

To assess corneal regular and irregular astigmatism using Fourier series harmonic analysis of videokeratography data in normal subjects, as well as in subjects with pathologic and postsurgical conditions.

Study design

Retrospective, case-control study.

Participants

Two hundred normal eyes, 58 eyes with keratoconus, 24 eyes with suspect keratoconus, 100 eyes that underwent LASIK, 101 eyes that underwent photorefractive keratectomy (PRK), and 79 eyes that underwent penetrating keratoplasty (PK).

Methods

Videokeratography data were decomposed, using Fourier analysis, into spherical power, regular astigmatism, asymmetry, and higher order irregularity.

Results

The normal range of the Fourier indices was defined as the mean ±2×standard deviation in the normal eyes, which were 40.81–47.13 diopters (D) for spherical power, 0–1.04 D for regular astigmatism, 0.02–0.68 D for asymmetry, and 0.05–0.17 for higher order irregularity. The keratoconus and suspect keratoconus groups showed significantly greater values in all indices than did the normal group (P<0.001 or 0.0001, Mann–Whitney test with Bonferroni correction). Eyes that had undergone LASIK and PRK had significantly smaller spherical power and regular astigmatism (P<0.0001) and significantly larger asymmetry (P<0.0001) than the normal eyes. All indices were significantly greater in the PK group than in the normal group (P<0.0001). Among the eyes tested in this study, eyes with keratoconus had the largest asymmetry, whereas eyes that had undergone PK had the most irregular corneas.

Conclusions

The normal range was defined for the corneal irregular astigmatism index (asymmetry and higher order irregularity) to support future studies in this field. Eyes with ocular pathologic and postsurgical conditions were evaluated using the normal range.

Section snippets

Subjects and methods

Corneal topography was obtained using videokeratography (TMS-2) in 200 eyes of 100 normal subjects, 58 eyes of 45 patients with keratoconus, and 24 eyes of 24 patients with suspected keratoconus, 100 eyes of 53 patients who had undergone LASIK, 101 eyes of 53 patients who had undergone PRK, and 79 eyes of 79 patients who had undergone PK. The measurements were repeated 4 times, and the best-focused, properly aligned image was chosen. The measurements were conducted by a nonmasked, experienced

Results

Color-coded maps representing each Fourier index are shown for a normal eye (Fig 1A), an eye with keratoconus (Fig 1B), an eye having undergone LASIK (Fig 1C), and an eye having undergone PK (Fig 1D). In a normal cornea (Fig 1A), the minimum amount of irregular astigmatism (asymmetry and higher order irregularity) is found. In keratoconus (Fig 1B), a prolate shape of the cornea is demonstrated in the spherical equivalent power map, and a large with-the-rule cylinder is shown in the regular

Discussion

In the current study, we defined the normal range of each Fourier index by adopting the mean±2×SD in 200 eyes of 100 normal subjects. Similar methodology has been applied in several other fields in medicine, such as systemic arterial pressure28, 29 and intraocular pressure. When the population conforms to the normal distribution, 96% of the samples are expected to lie within 2 SDs of the mean. In our normal group, the proportion of eyes that were included in the normal range was 96.0%, 98.5%,

References (36)

  • L.J Maguire et al.

    Corneal topography of early keratoconus

    Am J Ophthalmol

    (1989)
  • Y.S Rabinowitz et al.

    Videokeratography of the fellow eye in unilateral keratoconus

    Ophthalmology

    (1993)
  • N Maeda et al.

    Detection and classification of mild irregular astigmatism in patients with good visual acuity

    Surv Ophthalmol

    (1998)
  • J.A Staessen et al.

    Mean and range of the ambulatory pressure in normotensive subjects from a meta-analysis of 23 studies

    Am J Cardiol

    (1991)
  • M.G Mulhern et al.

    Topographical analysis of ablation centration after excimer laser photorefractive keratectomy and laser in situ keratomileusis for high myopia

    J Cataract Refract Surg

    (1997)
  • S.D Klyce

    Computer-assisted corneal topographyhigh-resolution graphic presentation and analysis of keratoscopy

    Invest Ophthalmol Vis Sci

    (1984)
  • M Guillon et al.

    Corneal topographya clinical model

    Ophthalmic Physiol Opt

    (1986)
  • L.J Maguire et al.

    Graphic presentation of computer-analyzed keratoscope photographs

    Arch Ophthalmol

    (1987)
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    Manuscript no. 230013.

    1

    None of the authors has a proprietary interest in any material or method mentioned.

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