Do picture-based charts overestimate visual acuity? Comparison of Kay Pictures, Lea Symbols, HOTV and Keeler logMAR charts with Sloan letters in adults and children

Abstract

Purpose: Children may be tested with a variety of visual acuity (VA) charts during their ophthalmic care and differences between charts can complicate the interpretation of VA measurements. This study compared VA measurements across four pediatric charts with Sloan letters and identified chart design features that contributed to inter-chart differences in VA.

Methods: VA was determined for right eyes of 25 adults and 17 children (4-9 years of age) using Crowded Kay Pictures, Crowded linear Lea Symbols, Crowded Keeler logMAR, Crowded HOTV and Early Treatment of Diabetic Retinopathy Study (ETDRS) charts in focused and defocused (+1.00 DS optical blur) conditions. In a separate group of 25 adults, we compared the VA from individual Kay Picture optotypes with uncrowded Landolt C VA measurements.

Results: Crowded Kay Pictures generated significantly better VA measurements than all other charts in both adults and children (p < 0.001; 0.15 to 0.30 logMAR). No significant differences were found between other charts in adult participants; children achieved significantly poorer VA measurements on the ETDRS chart compared with pediatric acuity tests. All Kay Pictures optotypes produced better VA (p < 0.001), varying from -0.38 ± 0.13 logMAR (apple) to -0.57 ± 0.10 logMAR (duck), than the reference Landolt C task (mean VA -0.19 ± 0.08 logMAR).

Conclusion: Kay Pictures over-estimated VA in all participants. Variability between Kay Pictures optotypes suggests that shape cues aid in optotype determination. Other pediatric charts offer more comparable VA measures and should be used for children likely to progress to letter charts.

Fig 1. Key features of the acuity charts investigated.

Fig 2. Number of participants able to reach threshold acuity measurements on each of the pediatric tests for the different test distances shown.

Only the ETDRS chart allowed for threshold measurements at the standard (3 m) testing distance for all participants. The number of children able to provide results on the ETDRS chart was less.

Fig 3. Mean (± standard deviation) of VA measurements for adults (white) and children (light grey) in focused conditions and defocused (adults black and children dark grey) conditions.

Fig 4

Bland Altman comparisons of visual acuity measurements between ETDRS and pediatric charts in adults (A-D) and children (E-H and I-L with two outliers removed). A positive bias indicates the second chart gave better acuities.

Fig 5. Visual acuities provided by individual Kay Pictures optotypes compared with a Landolt C target.

All Kay Pictures were more discriminable than the Landolt C. Significant differences (p < 0.002) between individual Kay Pictures are indicated on the figure.

Fig 6. Demonstration of focused and defocused Kay Pictures optotypes (upper and lower left, respectively) compared with a reference Landolt C optotype (upper and lower right).

The stroke width, nominal acuity and amount of defocus are equal in the Kay Pictures and Landolt C targets; however, Kay Picture optotypes, because of their construction principles, are at least twice the size of the Landolt C. Images were constructed in Adobe Photoshop CS6 v13 using the Lens Blur filter.