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

Abstract

ObjectiveAstigmatism 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.MethodsVisual 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.ResultsThe 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.ConclusionInteractions 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.

Effects of Lens-Induced Astigmatism at Near and Far Distances

“…Due to the added positive spherical power of half the cylindrical power, the VA drop due to 1.00 diopters of cylindrical power in this work should be compared with the 0.50 diopters drop in Hughes et al 23 Therefore the values shown in Figure 3a at far (0.22±0.06 diopters −1 and 0.18±0.06 diopters −1 for the two axis orientations) are in very good agreement with the results of Hughes et al (from 0.26±0.09 to 0.45± 0.12). 23 On the contrary, a lower slope was found by Serra et al, 26 who studied the influence of 2.00 diopters lens-induced astigmatism on VA at far and found a VA worsening ranging from 0.19 to 0.28 logMAR. Since no spherical compensation was added, 26 the slopes in Figure 3a (0.22±0.06 diopters −1 and 0.18±0.06 diopters −1 for the two axis orientations) should be compared with ¼ of the drop reported by Serra et al (¼ of 0.19–0.28).…”

“… 23 On the contrary, a lower slope was found by Serra et al, 26 who studied the influence of 2.00 diopters lens-induced astigmatism on VA at far and found a VA worsening ranging from 0.19 to 0.28 logMAR. Since no spherical compensation was added, 26 the slopes in Figure 3a (0.22±0.06 diopters −1 and 0.18±0.06 diopters −1 for the two axis orientations) should be compared with ¼ of the drop reported by Serra et al (¼ of 0.19–0.28). 26 The agreement is relatively poor, the slopes in Figure 3a being three to four times greater.…”

“…Since no spherical compensation was added, 26 the slopes in Figure 3a (0.22±0.06 diopters −1 and 0.18±0.06 diopters −1 for the two axis orientations) should be compared with ¼ of the drop reported by Serra et al (¼ of 0.19–0.28). 26 The agreement is relatively poor, the slopes in Figure 3a being three to four times greater. Also, Casagrande et al 29 investigated the effect of astigmatism on VA.…”

Do reduced visual acuity and refractive error affect classroom performance?

“…In addition, some studies have demonstrated that oblique astigmatism (the least common type of astigmatism found in children) has the most detrimental effect on vision and functional performance in adults compared to WTR and ATR astigmatism . A recent study showed that the effects of astigmatic axis are also dependent on the typography of the alphabetic language used . These inconsistencies between studies are likely to be a result of differences in the methodologies employed, including factors such as the method of astigmatic simulation (cylindrical lenses with or without spherical equivalent compensation), functional outcome measures (visual acuity, reading or other specific task‐related performances), the age of participants (young or older adults), pupil size (natural or artificial) and the method of accommodative control (with or without cycloplegia).…”

Induced astigmatism biases the orientation information represented in multivariate electroencephalogram activities

“…Astigmatism is fairly widespread, with a prevalence of up to 60% in the general population (Attebo, Ivers, & Mitchell, 1999 ; Sorsby, Sheridan, Leary, & Benjamin, 1960 ). It impairs various levels of visual perception, including low‐level properties such as visual acuity (Atchison & Mathur, 2011 ; Kobashi, Kamiya, Shimizu, Kawamorita, & Uozato, 2012 ), contrast sensitivity (Bradley, Thomas, Kalaher, & Hoerres, 1991 ), and legibility of letters (Guo & Atchison, 2010 ) to high‐level cognitive functions, such as reading (Rosenfield, Hue, Huang, & Bababekova, 2012 ; Wiggins & Daum, 1991 ), alphabet judgment (Serra, Cox, & Chisholm, 2018 ), or even driving (Wood et al, 2012 ). Therefore, it is not surprising that ophthalmologists always include an indicator of astigmatism in eyeglass prescriptions.…”