Diagnosis and Imaging of Corneal Astigmatism

Abstract

Accurate diagnosis and measurement of corneal astigmatism is of vital importance for treatment. Refractive and corneal astigmatic power and axis values are correlated but not coincidental. Corneal power and astigmatism amount measurements have a considerable degree of variability, which may be due to systematic error of the devices employed. Test to test variations may be falsely estimated as surgically induced refractive or astigmatic changes. Manual and automated keratometry instruments sometimes yield non-coincidental corneal power and axis readings, with differing reproducibility or repeatability. Even among modern automated keratometry, Placido-disk based videokeratoscopy and advanced slit-scanning or Scheimpflug technology scanning, diverging corneal measurements have been reported, which are attributable to different underlying methodology, reliability and repeatability. In this chapter, we will summarize the basic functioning principles of the main devices used for the evaluation of corneal power and astigmatism. We will also review published studies about variations in corneal power measurements using different equipment, in astigmatic power and axis calculated values, and correlation obtained among measurements taken with different instruments. Results reported by different authors will be compared with our own data where available.