Formulas May Predict Visual Acuity in Patients With Astigmatism

Researchers developed regression formulas that may be clinically beneficial for estimating visual prognosis after astigmatic correction and determining the surgical indication of astigmatic correction. Findings were published in Scientific Reports.

“Astigmatic correction plays an essential role in improving the good quality of vision (QOV) as well as the quality of life, even in a presbyopic population,” authors explained. Although several studies have explored the effect of astigmatism on visual acuity in phakic and pseudophakic subjects, necessity of astigmatic correction can be influenced by numerous factors including age, pupil size, higher-order aberrations (HOAs), status of the eyelid, and others.

To establish the estimation method for predicting visual acuity in daily practice, it is important to understand the overall relationship of refractive astigmatism with actual visual acuity in a large cohort of astigmatic subjects, the researchers said.

They conducted a retrospective review of clinical charts to assess the relationship between refractive astigmatism and distance visual acuity and to provide a prediction formula of visual outcomes according to the amount of astigmatism.

A total of 318 eyes from 318 consecutive patients (158 phakic and 160 pseudophakic) without any concomitant eye disease (excluding refractive errors) were included in the study. One eye from each patient was randomly selected for use in statistical analysis, and researchers also routinely measured visual acuity and spherical equivalent visual acuity (SEVA). All patients were 45 years or older.

In addition, “Based on the axis of the steep meridian, we also classified astigmatism as ‘with-the-rule’ (WTR) when the steep meridian was between 60° and 120°, and as ‘against-the-rule’ (ATR) when it was between 0° and 30° or between 150° and 180°. Otherwise, we classified the remaining astigmatism as oblique (OBL) astigmatism,” authors explained.

The devised regression formula was expressed as: y = 0.017x² + 0.125x − 0.116 (R² = 0.544), where y = logMAR SEVA, and x = astigmatism. “After normalization, the regression formula was expressed as follows: y = 0.021x² + 0.103x − 0.102 (R² = 0.629),” researchers wrote.

Analyses revealed:

• A significant correlation between the SEVA and the amount of astigmatism (r = 0.715; P< .001) in the entire study population
• Similar correlations in phakic eyes (r = 0.718; P < .001) and in pseudophakic eyes (r = 0.717; P < .001).
• No significant differences in SEVA for matched comparison among the WTR, ATR, and OBL astigmatism subgroups (P = .922)
• A weak but significant correlation between age and SEVA (r = 0.201; P < .001), and a moderate, significant correlation between astigmatism and SEVA/best spectacle-corrected visual acuity (r = 0.662; P < .001)
• The change rate of visual acuity was estimated at 0.16 to 0.21 logMAR per D of astigmatism (0.16 to 0.24 logMAR per D of astigmatism in pseudophakic subjects; 0.16 to 0.19 log MAR per D of astigmatism in phakic subjects).

Overall, “our results showed that there was a significant correlation between the amount of astigmatism and the SEVA in a presbyopic population, implying that distance visual acuity significantly worsens, as the amount of astigmatism increases, in such subjects,” researchers wrote.

The lack of evaluation of pupil size or HOAs in the current study marked a limitation, and researchers also did not examine the effect of astigmatism on near visual acuity or the impact of neuronal adaptation to astigmatism.

Results “showed that the predicting formula for visual acuity was made according to the amount of astigmatism in such population,” authors concluded. “We assume that it will be helpful for estimating the visual prognosis after astigmatic correction, as well as for determining the surgical indication of astigmatic correction in daily practice.”

Reference

Hoshikawa R, Kamiya K, Fujimura F, Shoji N. Prediction of distance visual acuity in presbyopic astigmatic subjects. Sci Rep. Published online March 26, 2021. doi:10.1038/s41598-021-85313-3