Contrast sensitivity based orientation changes in refractive surgery patients.

It is well known that refractive surgery leads to an increase in higher-order aberrations. These can cause problems such as glare, starburst and halos in vision, particularly in low light levels when the pupil is large. The overall wavefront aberration of the eye is typically non-circularly symmetric due to the presence of aberrations such as coma and trefoil. These asymmetries have been shown to induce orientation-selective effects in grating-based contrast sensitivity tests. Classically, contrast sensitivity is assumed to be independent of orientation and is measured at only one orientation. Examining more orientations may give a better indicator of visual performance when abnormal amounts of aberration are present. To examine this, contrast sensitivity through a dilated pupil was determined for four orientations of a 12 cycles deg(-1) sine wave grating in 12 subjects both before and after refractive surgery. Surgery produced orientation-selective changes in contrast sensitivity in most patients (9 out of 12, p < 0.05). The correlation between contrast sensitivity and higher-order aberrations was found to improve when several orientations were tested compared with data from only one orientation (combined orientations, r(2) = 0.46, p = 0.007; single orientation r(2) = 0.55, p < 0 .05 to r(2) = 0.09, p > 0.05). These results indicate that grating orientation is an important factor when investigating the link between visual performance and higher order aberrations.