BLOG: Do you need tomography to diagnose keratoconus?
Corneal topography, along with a good patient and family history, provides an excellent starting point for keratoconus diagnosis and management.
However, advanced anterior segment tomography systems provide us with much more information, particularly early in the disease.
Tomography has become more widely understood and valued in ophthalmology with greater awareness of the contributions of posterior corneal astigmatism for IOL planning and power calculation. Rather than thinking of the cornea as a single refracting element, cataract surgeons now understand it as a dual-surface entity. While the majority of the refractive power comes from the anterior aspect of the cornea, “hidden” information on the posterior cornea is relevant to both cataract surgery and keratoconus.
One need not be an imaging guru to glean new information from tomography or to utilize these diagnostic tools effectively in keratoconus management. Fortunately, a number of people who are imaging experts have created algorithms and displays that make it easier for the rest of us. I like to look at the Holladay Report on the Pentacam (Oculus). It shows both corneal thickness and anterior/posterior curvatures, making it easier to correlate elevation changes to the corresponding areas of corneal thinning. Some versions of the device also have a keratoconus grading output on this screen. I typically review the Holladay Report and the maximum keratometry values (Kmax) as an important indicator of progression.
Other cornea specialists like the Belin/Ambrosio display. Initially used for screening refractive surgery candidates for ectasia risk, this display now has an integrated ABCD classification system that creates a composite score of four different parameters important to keratoconus progression: anterior (A) radius of curvature; back (B) radius of curvature; minimum corneal (C) thickness; and best corrected distance (D) acuity. Other devices offer additional tools. Ophthalmologists can choose one or more that make the most sense and fit best into their workflow.
All of these tomographic measures have to be evaluated more carefully in eyes after cross-linking. A cross-linked cornea will be stiffer, but it is possible for some of the indices or even the Kmax to look worse initially, even when the cornea is actually much more stable. Scoring, grading and individual measurements should be evaluated in relationship to one another and to the patient’s vision.
Patient education tool
I also find tomography to be a great education tool for patients. I use the images to explain to patients and families what is happening to the cornea. An analogy I like to use is that the cornea is like a double-pane window. Keratoconus starts at the back pane. As it warps, the back pane pushes forward but doesn’t affect the front pane at first. I explain that I’m looking for the earliest changes on the back side and that those changes will eventually push the front pane forward, too, making it more difficult for them to see well.
I know we have more to learn from tomography as the technology matures. This is an exciting time to incorporate it into your practice, for multiple indications.
Collapse