The differing impact of induced astigmatic blur on crowded and uncrowded paediatric visual acuity chart results

Vision screening programs often rely on acuity measures to detect significant refractive error and clinicians use measures of acuity to monitor refractive treatment. This study investigated the effects of induced astigmatism on visual acuity (VA) using several paediatric acuity tests: the Keeler LogMAR crowded and single letter acuity tests, the single letter Sheridan Gardiner chart, and the crowded and single Kay picture tests.

Six fully-corrected emmetropic adults had induced astigmatic blur imposed at different axes of astigmatism (90°, 180°, 135°, 45°). Astigmatic errors ranged from 0.75 to 3.50 DC. Monocular VA was measured with induced astigmatic blur for each acuity test. Participants and examiner were masked to the axis and power of the astigmatism.

All participants demonstrated similar variability in performance across acuity charts (p = 0.54). For all charts, there was a linear reduction in VA with increasing amounts of induced astigmatism. The Keeler crowded LogMAR acuity test required the least amount of astigmatic blur to exceed a 0.2 LogMAR threshold (1.17 D, for 90° induced astigmatism), while the crowded and single Kay pictures required the largest magnitudes (2.09 and 2.85 D for 90° induced astigmatism respectively). The most variable acuities were recorded within and between participants with the Sheridan Gardiner chart.

The present study demonstrated that none of the acuity charts under test are likely to detect uncorrected astigmatic refractive error unless the magnitude of astigmatism is large. Given the limitations of other paediatric acuity charts under test, it is important that practitioners use a crowded, LogMAR letter chart as soon as it is feasible.