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Corneal Ectasia

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Corneal Topography

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Abstract

In primary corneal ectasias, noninflammatory stromal thinning causes the cornea to progressively protrude forwards. Secondary ectasia occurs as a result of thinning due to corneal surgery or inflammatory stromal melt. In both types, in the early stages, the classical clinical signs may be absent, so the diagnosis relies upon topographic findings alone.

In keratoconus the cornea has an exaggerated prolate shape with paracentral corneal steepening, which most commonly occurs inferiorly, with corneal flattening in the opposite hemisphere. This causes corneal irregularity which can only be improved by glasses in the earliest stages. Keratoconus can be classified according to the severity, location and shape of the cone.

In pellucid marginal degeneration, the thinning is further inferiorly, creating a “droopy bow tie” area of corneal steepening, associated with high orders of regular astigmatism. With further progression, the “bows” of the “tie” may meet in the inferior periphery, producing a pattern resembling a more inferior keratoconus. Keratoglobus has a more even thinning affecting the whole cornea, which expands more symmetrically, with relatively little effect on the refraction.

Topographic indices have been developed to detect very early stages of ectasia and quantify progression. These include the asymmetry and regularity indices, inferior-superior value, difference in central power, keratoconus predictability index, enhanced ectasia display and, in certain devices, neural networks. The differential diagnosis includes artefact, contact lens-induced corneal warpage and various types of corneal disease and surgery.

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Authors and Affiliations

  1. Imperial College Healthcare NHS Trust, London, UK

    Melanie Corbett

  2. University Hospital Coventry and Warwickshire, Coventry, UK

    Nicholas Maycock

  3. Manchester, UK

    Emanuel Rosen

  4. Department of Ophthalmology, St. Thomas Hospital, London, UK

    David O’Brart

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  1. Melanie Corbett
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  2. Nicholas Maycock
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  3. Emanuel Rosen
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  4. David O’Brart
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Corbett, M., Maycock, N., Rosen, E., O’Brart, D. (2019). Corneal Ectasia. In: Corneal Topography. Springer, Cham. https://doi.org/10.1007/978-3-030-10696-6_10

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  • DOI: https://doi.org/10.1007/978-3-030-10696-6_10

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