A technique called Fourier series harmonic analysis can separately quantify regular and irregular corneal astigmatism and detect subtle shape changes from corneal disorders or anterior segment surgery. Recently, a case-control study used the analysis on data from both a Placido ring–based corneal topographer and a 3D anterior segment optical coherence tomographer (OCT) to better understand dry eye disease. The two sets measured a significantly higher amount of corneal irregular astigmatism in dry eyes than in normal eyes.

Researchers obtained topographical data from 44 subjects with dry eye and 20 normal control subjects. The team then compared spherical, regular astigmatism and irregular astigmatism (asymmetry and higher-order irregularity) refractive error components of the cornea. Because asymmetric values and higher-order irregularities from the topographer were significantly greater in dry eyes with moderate/severe central superficial punctate keratopathy (SPK) than in those with no or mild SPK, the findings suggest a relationship between central SPK and the severity of corneal irregular astigmatism in eyes with dry eye disease. The differences between mild and moderate/severe dry eye were greater than those between normal and mild dry eye.

The study noted that the Placido-based topographer uses the reflection of ring images projected onto the air–tear film interface, making it more capable of detecting tear film–induced changes in corneal topography. Still, Placido-based topography may overestimate tear film changes if digitizing errors occur because of remarkably distorted rings, particularly in moderate or severe dry eye. Researchers propose that OCT may be predictive of corneal shape after the tear film condition in a dry eye has improved.

While the two devices’ measurements were statistically different, the magnitudes of these differences were not clinically relevant. Nevertheless, the researchers advise not to use the two interchangeably for corneal irregular astigmatism measurements. Yet when a greater amount of corneal irregular astigmatism is detected with a Placido-based topographer than with OCT, they advise to suspect dry eye.