Assessing Progression of Keratoconus Using Corneal Topography
To estimate the variation in topography parameters due to inter session variations, diurnal variation and variability following pupillary dilation, to accurately identify progression in keratoconus.
Corneal topography was performed using the Orbscan IIz system on 100 normal eyes of 100 subjects and on 100 eyes of 100 subjects with clinically diagnosed keratoconus by the same examiner on two occasions on the same day. In the keratoconus groups, the examination was repeated in 18 eyes at 8am, 12pm and 4pm to assess any diurnal variation in topography. Topography was also repeated in 15 keratoconus eyes before and after dilation with tropicamide to assess any variation.
Excellent intersession repeatability was seen in the normal eyes with the astigmatism, maximum and minimum keratometry showing an interclass correlation between 0.985 and 0.995. Greater variation was found in the keratoconus eyes with a standard deviation (SD) of 0.4 ± 0.8 for astigmatism, 0.48 ± 0.55 for maximum keratometry, 0.43 ± 0.82 for minimum keratometry and 3.8 ± 6.2 degrees in the axis of astigmatism. There was no significant diurnal variation in any of the parameters (p = 0.938) and pupillary dilation with tropicamide showed no significant variation in maximum and minimum keratometry (p=0.732).
Care needs to be exercised in assessing progression on corneal topography in patients with keratoconus, as these eyes tend to show grater variability in parameters when compared to normal eyes. There seems to be no influence of the time of the measurement.