Corneal Topography Pearls

By Vipul Shah, MD

Corneal topography used to be solely the tool of the corneal specialist, only good for removing Penetrating Keratoplasty (PK) sutures, screening for LASIK, or fitting RGP’s. With the development of cataract surgery into refractive surgery, a working knowledge of topography is now necessary for performing LRI’s and implanting presbyopic and toric intraocular lenses. Here’s a little background on topographers and some tips on reading them:

1) Anterior float: plots elevation of the anterior surface above and below a best-fit sphere.
2) Posterior float: plots elevation of the posterior surface above and below a best-fit sphere.
3) Keratometry: corneal curvature in diopters.
4) Pachymetry: map of corneal thickness (the central number is not the thinnest point).
5) Miscellaneous data: includes K's, thinnest point, white-to-white, etc.
Note that each map has an adjustable color scale next to it.

Types of Topographers

Placido Disc
These machines use an extrapolation of keratometry by projecting placido discs onto the corneal surface, then reading the distance between them to measure curvature. While accurate, these measurements are less accurate for extreme curvatures. Such machines also require relatively clear corneas, so dryness or other surface disease can be a problem. Placido disc technology does not give posterior float or pachymetry data. Some machines use placido disc in addition to another technology type.

Slit-Scanning Topography
This method uses scanning-slit lamps projected at an angle to image the cornea. This allows measurement of curvature, anterior and posterior float, and pachymetry. The Orbscan machine is the most common example.

Scheimpflug Imaging
In addition to being the most recent technology, this type of topography has the coolest name (try to use it as much as you can in a sentence, you know you love saying “Scheimpflug”). As used in the Pentacam machine, the Scheimpflug camera rotates continually, providing multiple images of the eye from cornea to lens. This provides curvature, anterior and posterior float, pachymetry, as well as cross-sectional images of the anterior segment.

Topography Pearls

Know your machine. Different machines have different limitations, and knowing what technology you have and what it’s best for will help you know which scans to throw out, and which ones to believe.

Watch the color-scale. The color scale is adjustable on most machines, so that the full color spectrum can represent a large or small change in diopters. If it’s set too low, the machine will be too sensitive and everyone will be abnormal; if it’s too high, you’ll miss that subtle cone (keratoconus).

Remember, it’s about pattern recognition. While there are many helpful guidelines to follow regarding what are significant differences in curvature, pachymetry, or elevation, particularly when screening for LASIK patients, the most important pearl is to know abnormal from normal. A subtle drooping bowtie, decentered pachymetry, or irregular posterior float can all signal a problem cornea, even if the numbers are not that extreme.

Correlate the topography with the slit-lamp and/or refraction. Hopefully any of your patients that are getting a topography will also be examined under a slit-lamp shortly thereafter by someone (preferably you). If the topography is abnormal, be especially alert for causes such as scars, pterygium, dry eyes, or prior incisions. Also see if the topography and refraction are at least in the same ballpark axis.

While it takes a little practice, a corneal topographer can be an essential tool for not only diagnosis, but also in obtaining excellent surgical results.