Keratoconus and contact lens-induced corneal warpage analysis using the keratomorphic diagram

Videokeratography of early keratoconus may be difficult to distinguish from contact lens-induced corneal warpage, even by experienced examiners. Furthermore, topographic irregularity may be judged inconsistently if quantitative standards are not applied. Quantitative measures based on videokeratographic data were developed and evaluated to determine if improved corneal topographic classification can be achieved.

The Corneal Irregularity Coefficient (CIC) and Corneal Power Coefficient (CPC) were derived from multiple measures of mean corneal power and its variance for 207 videokeratographs of normal, warped, keratoconus, and keratoconus-suspect corneas. CIC was plotted against CPC, creating a distribution of points representing all maps that tended to be grouped according to surface conditions (the Keratomorphic Diagram). Normal, steep, abnormal, and warped zones were defined by CIC and CPC cutoff values chosen to distinguish normal from keratoconus corneas graphically.

Seventy of 76 normal corneas were grouped in the normal zone and 6 in the steep zone; 84 of 84 keratoconus corneas were grouped in the abnormal zone; 35 of 35 contact lens-induced warpage cases were grouped in the warped zone; and 10 of 12 keratoconus-suspect corneas were grouped in the warped zone, with 2 in the abnormal zone. Serially plotted data of keratoconus progression and warpage regression demonstrated that the vector displacement of CIC and CPC values may provide a potentially useful means of distinguishing contact lens-induced warpage from keratoconus-suspect corneas.

The Keratomorphic Diagram aids in classifying and comparing corneal shape by plotting indices along axes with easily recalled scales. The diagram may become a useful tool to assess presurgical corneal surface instability and postoperative progression of corneal shape change due to healing.