Measuring corneal astigmatism is important for patients undergoing cataract surgery, and there are several different imaging techniques, but which is the most accurate? In this study, researchers assessed the repeatability and accuracy of corneal astigmatism measurement with a spectral-domain OCT system and compared them with Scheimpflug-based topography and swept-source optical biometry.
Sixty pseudophakic eyes with monofocal non-toric intraocular lens that previously had refractive surgery were analyzed. To assess accuracy, simulated keratometric (SimK) and net corneal astigmatism obtained from each device were compared with subjective manifest refraction astigmatism.
OCT net astigmatism showed the highest accuracy for manifest astigmatism prediction. OCT net corneal astigmatism measurement was more accurate than that of Pentacam HR in both post-LASIK/PRK eyes and post-RK eyes. The authors noted this showed the advantage of using OCT in both mildly and highly irregular corneas. OCT was also more accurate than biometry overall, though the difference was not significant in the post-LASIK/PRK subgroup.
“Astigmatism derived from OCT net readings was shown to have good test-retest repeatability with a coefficient of repeatability similar to that reported for manifest refraction, and significantly lower than that found for [topography],” the authors concluded in their study. “These results are in agreement with previous studies, where OCT astigmatism showed better repeatability than Scheimpflug imaging.”
One reason the OCT had better accuracy could be that the smaller number of meridians in the other technology could limit the precision of astigmatism measurement, the authors noted. Another could be the size of the analytical zone: the study used a 4mm zone for both OCT and topography but 2.5mm for biometry, the authors explained in their study. “The smaller measurement zone may be less precise,” they suggested, “because our post-refractive surgery study population have less uniform corneal curvatures.”
Llorens-Quintana C, Pavlatos E, Thaware O, et al. Accuracy of OCT-derived net corneal astigmatism measurement. Journal of Cataract and Refractive Surgery. [Epub ahead of print].