The Universal Standard Scale: Proposed improvements to the American National Standards Institute (ANSI) scale for corneal topography

Purpose: To evaluate the American National Standards Institute (ANSI) corneal topography scale (ANSI Z-80.23-1999) and to develop a Universal Standard Scale (USS) based on scientific principles and clinical usefulness. Design: Retrospective data analysis and computer modeling of frequency of corneal power distributions. Methods: Four variations of corneal topography color scales were constructed using the ANSI standard description. The scales used 21 contour intervals with step sizes of 0.5, 1.0, or 1.5 diopters (D). In example 1, hue was varied, whereas saturation and brightness were held constant. In example 2, hue was varied systematically, whereas luminance was varied to compensate for spectral sensitivity. In example 3, hue was varied, whereas luminance was adjusted to improve color contrast at the center of the scale. In example 4, the palette was generated by varying the red, green, and blue color intensity levels. In the USS, colors were defined by hue, brightness, and saturation, and appearance was optimized by using contrast. The contour interval was determined from the standard deviation of keratometry for 27 normal corneas. The scale range was based on corneal powers found in 388 topography maps of 12 different corneal conditions. Results: ANSI-derived maps were difficult to interpret because of poor color contrast and the inability to associate contours with specific colors in the scale. The 0.5 D interval slightly improved the appearance of some patterns but reduced the useful clinical range of the scale. ANSI colors did not match the user's expectations; abnormal patterns appeared in shades of green, which is associated with normality. The standard deviation of keratometry readings for the normal population was 1.59 D. Hence, the contour interval for the USS was set to 1.5 D. More than 99.9% of the corneal powers in 388 clinical maps were within the range of 30 to 67.5 D, which was chosen as the range for the USS. Conclusions: The ANSI scale for corneal topography is a series of scales that lead to clinical misinterpretations. The alternative USS would provide uniform presentation, unambiguous interpretation, and greater ease of comparison among maps produced by different topographers.