Grating acuity overestimates Snellen acuity in patients with age-related maculopathy

In foveally fixating patients resolution acuity is approximately the same when measured with a Snellen chart (Snellen acuity) or when estimated from the high frequency cutoff of the contrast sensitivity function (CSF) (grating acuity). In contrast, with peripheral viewing, normal patients have better grating than Snellen acuity. This suggests that grating acuity deteriorates less rapidly with eccentricity than does Snellen acuity. If patients with bilateral age-related maculopathy (ARM) fixate with a single, healthy peripheral-retinal locus, grating and Snellen acuities should be similar to those in the normal periphery. As expected from the normal data, grating acuity was better than Snellen acuity in all 19 patients with ARM. Although some showed acuities which were similar to those of the normal periphery, others had Snellen acuities that were even worse than predicted from the normal peripheral acuity. Possible explanations for the superiority of grating over Snellen acuity are discussed.