The effect of astigmatic axis on visual acuity measured with different alphabets in Roman alphabet readers

Astigmatism produces meridional variations in the retinal blur pattern, thus interacting with object spatial detail and altering visual performance as the axis changes. This study investigates the influence of astigmatic axis orientation on visual acuity (VA) for four alphabets used worldwide.

Visual acuity was measured monocularly in 25 Roman alphabet users (mean age: 25.6±7.5 years) using computer-presented logarithm of the minimum angle of resolution (log-MAR) charts with letters from four different alphabets (Arabic, Chinese, Roman, and Tamil). VA was assessed under the effect of four optical conditions: best distance correction and three astigmatic conditions (using a +2.00 cylindrical diopter trial case lens with its axis oriented at 180, 45, or 90 degrees). For each alphabet, single optotypes were presented on a monitor viewed from a distance of 4.0 m, and a matching technique was used to identify the letters.

The degradation in VA with astigmatic defocus was influenced by the alphabet used (p<0.001) and by the astigmatic axis (p<0.001). Interactions in VA degradation between astigmatic axes and alphabet (p<0.001) showed differences within 0.10 logMAR. These interactions were more pronounced in alphabets with higher dominance of curves and vertical (Tamil) and horizontal (Arabic) detail.

Interactions between alphabet and type of astigmatism indicate that the effects of meridional blur on letter discrimination differ between alphabets. These findings have relevance in the way VA is assessed in populations using different typographies, and ultimately in the impact of astigmatic axis on their visual performance.