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Refractive Surgery | Jul/Aug 2014

Parameters to Document Progression of Keratoconus

The ability to measure how a patient's disease changes is key.

When treating a patient with keratoconus, several aspects of care must be taken into consideration, including diagnosis, disease progression, and efficacy of treatment. It is difficult to isolate one of these factors from the totality of a patient’s care. However, it is important for us to consistently evaluate our approach to each of the aforementioned aspects, as doing so will ultimately improve our management of keratoconus.

When we talk about measuring the progression of keratoconus, there is a difference between showing progression in moderate to advanced disease and documenting progression in early or subclinical disease. It is key to be able to measure how a patient’s disease changes, and five parameters are used to document this progression. These include the anterior and posterior corneal surfaces, epithelial thickness, BCVA, and corneal thickness. This article aims to identify the best parameter for documenting disease progression, highlighting the advantages and disadvantages of each measure.


Parameter No. 1: Anterior corneal surface. Patients with early clinical keratoconus are typically prescribed rigid contact lenses. The problem with using the anterior surface as a measure of progression is that it can be altered by these lenses. Additionally, curvature can change with the axis of measurement, so, as the disease changes, the measurement of total curvature—not just anterior curvature but posterior curvature—does not really mimic the change in the disease. Additionally, subclinical disease (ie, true ectasia with a normal anterior surface) will not show changes on the anterior surface until later in the disease process (Figure 1).

Parameter No. 2: Epithelial thickness. Readily available commercial methods for determining a patient’s epithelial thickness are lacking, and, therefore, this measure cannot be used for screening patients. Although there has been some early work evaluating epithelial thickness in keratoconus, there is no published literature exploring the progression of disease with relation to epithelial thickness.

Parameter No. 3: BCVA. Patients with keratoconus have variable BCVA. It can change from day to day, depending on which part of the cornea is evaluated; it also changes dramatically depending on pupil size and varies with lighting. Due to this variability, BCVA is not a reliable measure of keratoconus progression.

Parameter No. 4: Corneal thickness. Single-point measurements of corneal thickness are also probably not suitable indicators of progression because, again, they can vary significantly with exam. A full corneal thickness map, however, does have good potential to document progression (Figure 2).

Parameter No. 5: Posterior surface. Of the parameters used to document keratoconus progression, the posterior surface is the least affected by outside forces. Some change in the posterior surface can be seen with contact lens wear, but not nearly as much as on the anterior surface. The posterior surface has strong potential for measuring disease progression (Figure 3).


As we all know, patients with keratoconus may have changes in their visual axes and lines of sight as well as changes in their corneal curvature that do not mimic the shape of the cornea.

In my experience, the best parameters for documenting the progression of keratoconus are corneal thickness and the status of the posterior surface. With a fullthickness corneal surface map, the change in corneal thickness can be seen not just from a single point, but over the entire map. Posterior elevation maps are also effective for evaluating change.

When elevation maps are used for following patients with keratoconus, one key is to keep the reference surface constant. By convention, the preoperative or initial exam is usually used as a baseline.

In my opinion, full-thickness corneal surface maps and posterior elevation maps are the best ways to measure disease progression in patients with keratoconus.

Michael Belin, MD, is a Professor of Ophthalmology and Vision Sciences at the University of Arizona in Tucson. Dr. Belin states that he is a consultant to Oculus Optikgeräte. He may be reached at e-mail: mwbelin@aol.com.