What is corneal topography?

What is corneal topography?

Corneal topography

Topography originally was used to describe corneal curvature. New technologies address both curvature and shape, or elevation of the cornea. Accordingly, corneal topography describes measurements of the curvature of the cornea. An evaluation of corneal topography is necessary for the accurate diagnosis and follow-up of certain corneal disorders, such as keratoconus.

Why is corneal topography necessary?

Corneal topography is a process for mapping the surface curvature of the cornea, similar to making a contour map of land. Corneal topography is an established technology that aids in the diagnosis, monitoring, and treatment of a variety of visual disorders. Corneal topography is not a routine test. Rather, it is used in diagnosing certain types of problems, in evaluating a disease's progression, in fitting some types of contact lenses, and in planning surgery. It is commonly used in preparing for refractive eye surgery. The corneal topography map is used in conjunction with other tests to determine exactly how much corneal tissue will be removed to correct the visual defect. Corneal topography is used in the diagnosis and management of various corneal curvature abnormalities and diseases such as:

  • Diagnosis of hidden astigmatism
  • Keratoconus, a degenerative condition that causes a thinning of the cornea
  • Corneal transplants
  • Corneal scars or opacities
  • Corneal deformities
  • Fitting contact lenses
  • Irregular astigmatism following corneal
  • transplantation
  • Planning cataract surgery
  • Planning refractive surgery
  • Techniques for corneal topography

    Computerized corneal topography: Recent advances in computerized corneal topography have made it much easier to establish a diagnosis in questionable cases and the clinicians can now establish the diagnosis earlier, and more reliably than was possible with conventional topography methods.

    Conventional (map-based) topography: Non-computerized corneal topography is considered part of the evaluation and management included in general ophthalmological services and is, therefore, considered INCIDENTAL to those services.

    How does corneal topography work?

    The corneal topographer is made up of a computer linked to a lighted bowl that contains a pattern of concentric rings. The patient is seated in front of the bowl with his or her head pressed against a bar while a series of data points are generated on a disk, which has been projected on the cornea. Computer software digitizes these data points to produce a printout of the corneal shape, using different colors to identify different elevations. The procedure itself is painless and brief. It is a noncontact examination that photographs the surface of the eye using ordinary light. The greatest advantage of corneal topography is its ability to detect conditions invisible to most conventional testing

    What does corneal topography show?

    Corneal topography is the most accurate tool available for measuring corneal curvature. It produces a map of the surface of the cornea and provides essential information about the cornea's shape, distortions, and astigmatism for patients considering refractive surgery or who are suffering from keratoconus. Similarly, computerized corneal topography uses digitized image analysis to provide quantitative corneal topographic data. With this procedure, a series of illuminated rings are projected on to the corneal surface. A video camera then captures measurements of the reflected light rings, which are digitized to create a three-dimensional map of the cornea.

    Why do I need post-surgery corneal topography done?

    Corneal topography is often needed / used for the evaluation of the cornea after refractive such as LASIK or cataract surgery and also for the postoperative management of penetrating keratoplasty. Patients may need a post-surgery corneal topography since it reflects the actual changes induced by the laser treatment and subsequent wound healing, accounting for pre-existing topography features (eg, astigmatism). It may also be used following cataract surgery to assess the effect of cataract incision placement and size. In short, the indications for performing corneal topography after refractive surgery can be listed as follows:

    • For documentation of immediate effects of surgery
    • For the assessment of healing
    • For the investigation of a poor outcome
    • For the planning of augmentation
    • For intraocular lens power calculation
    • For suture manipulation/removal

    What if my LASIK surgeon doesn’t want to do pre- and post-op topographies?

    At times, your LASIK surgeon would not want to perform corneal topography especially when it is contraindicated in patients with thin corneas that will not leave a residual stroma of 250 µm after the procedure. Similarly, if any of the contraindications for LASIK or PRK are present, you surgeon may not want to do corneal topography as a precaution. In such cases, one or more of the following tests are recommended:

    1) Differential pachymetry: This refers to measuring corneal thickness both centrally and inferiorly.

    2) Dilated retinoscopy: If a patient has early signs of keratoconus, the light reflex will be a scissoring reflex

    3) Check for sharp mires: When doing manual keratometry, irregular mires can be an early and very subtle sign of irregular astigmatism, often present in early keratoconus.

    4) Check for refraction stability

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